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- From: Bruce Hamilton <B.Hamilton@irl.cri.nz>
- Newsgroups: sci.chem,sci.answers,news.answers
- Subject: Sci.chem FAQ - Part 5 of 7
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- Date: Thu, 15 Jan 2004 22:11:36 +1300
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-
- 18.6 What is the most bitter compound?
-
- Denatonium Benzoate = Bitrex, or even in some strange chemistry circles,
- N-[(2-[2,6-Dimethylphenyl)amino]-2-oxoethyl]-N,N-diethylbenzenemethan-
- aminium benzoate [3734-33-6]. It is added to toxic chemicals ( such as
- methylated spirits ) as a deterrent to accidental ingestion.
-
- 18.7 What is the sweetest compound?
-
- Most scales use sucrose as a sweetness of 1, and compare the relative
- sweetness of other sweeteners to sucrose.
-
- Name Relative Sweetness Category
- D-Glucose 0.46 Natural Food Product
- Lactose 0.68 " " "
- D-Fructose 0.84 " " "
- Sucrose 1 " " "
- Cyclamate 30 EC Permitted, USA Prohibited
- Aspartame 200 EC, USA Permitted.
- Saccharin 300 EC Permitted, USA Prohibited
- Sucralose 650 Au, Ca Permitted, trials elsewhere
- Alitame 2,000 Undergoing trials
- Thaumatin 3,000 EC permitted, US chewing gum only.
- Carrelame 160,000 Guanidine sweetener
- Bernardame 200,000 " "
- Sucrononate 200,000 " "
- Lugduname 220,000 " "
-
- The guanidine sweeteners are not expected to be approved for food use.
- There are several other important attributes of sweeteners, such as
- low toxicity, no after-taste, whether metabolised or excreted, etc.,
- that must also be considered.
-
- The potency scale is fairly flexible, and differing publications can
- assign different values. The August 1995 copy of the Journal of Chemical
- Education contained several papers from a symposium on sweeteners [3,4],
- and an article in Chemistry and Industry also discusses sweeteners from
- both natural and artificial sources [5], and Kirk Othmer has a monograph
- on sweeteners.
-
- The sweetener used in "diet" beverages is usually Aspartame, and they
- are usually required to display a warning for phenylketurics that the
- product contains a source of phenylalanine. As Aspartame slowly degrades
- in acid solutions, such products also have a "use-by" date.
-
- Although banned by the FDA in 1970 ( because a mixture of saccharin and
- cyclamate caused tumours in test animals ), saccharin has been still
- marketed under extensions of approval, Ironically, subsequent work
- implicated the saccharin, and the cyclamate was found not to be the
- tumour-causing agent, but it is still banned.
-
- 18.8 What salts change the colour of flames?.
-
- Both Vogel ( qualitative inorganic ) and the Rubber Handbook list details of
- flame tests for elements. The spectra of the alkaline earth compounds are
- relatively complex, so using filters to view the flame can change the colour
- observed as dominant lines are filtered out. In general, except for copper,
- any compound of an element can be used, however toxic salts ( such as
- cyanides ) should not be used. Halogen salts are usually readily available,
- and are reasonably volatile. In all cases, perform experiments in a
- well-ventilated area - preferably a fume hood. The emission spectra in the
- visible region is the sum of several emission lines, with dominant lines
- masking others. The visible spectrum is approximately :-
- Red 800 - 620 nm
- Orange 620 - 600 nm
- Yellow 600 - 585 nm
- Green 585 - 505 nm
- Blue 505 - 445 nm
- Violet 445 - 400 nm
-
- There are also the various bead tests employing borax ( sodium tetraborate
- Na2B4O7.10H2O ), Microcosmic salt ( NaNH4HPO4 ), or sodium carbonate
- (Na2CO3), using both oxidising and reducing flames. The bead test procedures
- are detailed in Vogel ( qualitative inorganic ), and similar texts.
-
- Element Colour Some of the contributing lines, and comments.
-
- Arsenic Light Blue 449.4 nm, 450.7 nm.
- ( Arsenic is highly toxic - only perform in fume hood under supervision )
- Barium Green-Yellow 553.6 nm, 539.1 nm, 536.1nm, 614.2 nm.
- Blue (faint) 455.4 nm, 493.4 nm.
- Cesium Red-Violet 852.1 nm.
- Calcium Orange 618.2 nm, 620.3 nm.
- Yellow-Green 530.7 nm, 559.5 nm.
- Violet (faint) 422.7 nm.
- Greenish with blue glass.
- Copper Emerald Green 521.8 nm, 529.2 nm, 515.3 nm.
- Not chloride, or in presence of HCl
- Azure Blue 465.1 nm.
- Copper chloride, or HCl present
- Lead Light Blue 500.5 nm.
- ( Lead is highly toxic - only perform in fume hood under supervision )
- Lithium Carmine Red 670.78 nm, 670.79 nm.
- Orange (faint) 610.1 nm.
- Violet with blue glass
- Potassium Red 766.5 nm, 769.9 nm.
- Violet 404.4 nm, 404.7 nm.
- Purple-red with blue glass
- Rubidium Violet 780.0 nm, 794.8 nm.
- Sodium Yellow 589.0 nm, 589.6 nm.
- Invisible when viewed with blue glass
- Strontium Scarlet Red 640.8 nm, 650.4 nm, 687.8 nm, 707.0 nm.
- Violet 460.7 nm, 421.5 nm, 407.8 nm.
- Violet with blue glass
- Tellurium Green 557.6 nm, 564.9 nm, 566.6 nm, 570.8 nm.
- ( Tellurium is highly toxic - only perform in fume hood under supervision )
- Thallium Green 535.0 nm.
- ( Thallium is highly toxic - only perform in fume hood under supervision )
- Zinc Whitish Green Large number of peaks between 468.0-775.8 nm.
- ( Zinc fumes are toxic - only perform in a fume hood under supervision )
-
- Impressive coloured flames have been obtained using chlorides and a methanol
- flame in a petri dish [6]. Even more spectacular results have been obtained
- by nitrating cellulose filter paper, and impregnating it with salts prior
- to ignition [7].
-
- 18.9 What chemicals change colour with heat, light, or pressure?.
-
- Compounds that visibly and reversibly change colour when subjected to a
- change in their environment are known as chromogenic materials. There are
- four major categories - electrochromic, photochromic, piezochromic, and
- thermochromic, all of which are extensively discussed in a recent, well
- referenced, monograph in Kirk Othmer [8].
-
- Electrochromic materials exhibit a change in light transmittance or
- reflectance induced by direct current at potentials of approximately one
- volt. The change usually is an oxidation-reduction reaction, using either
- inorganic or organic compounds, and the colour change can occur at either
- the anode or the cathode - which are usually thin films. There are two major
- classes, the ion-insertion/extraction type - such as tungsten trioxide, and
- the noninsertion group - such as the viologens, a family of halides of
- quaternary bases derived from 4,4'-bipyridinium. One viologen example is
- 1,1'-diheptyl-4,4'-bipyridinium bromide [6159-05-3], which changes from
- clear to bluish purple. The most common application of viologens has been
- the electrochromic interior rearview mirrors available for cars since 1988.
- These utilise a substituted viologen as the cathode colouring material, with
- a compound like phenylene diamine as the anode colouring electrochromic
- material. The mechanism details, along with a description of the ingenious
- control system, are described in a recent comprehensive review of
- electrochromic materials [9].
-
- Photochromic materials undergo a reversible change in light absorption that
- is induced by electromagnetic radiation, however most common applications
- involve reversible changes in colour or transparency on exposure to visible
- or ultraviolet light. This is often seen as a change in the visible spectrum
- ( 400 - 700 nm ), and can be rapid or very slow. There are two major classes
- of photochromic materials, inorganic and organic.
-
- Examples of the inorganic type are the silver halides, which are suspensions
- of fine ( 10-20 nm ) silver halide crystals dispersed throughout a glass that
- has been slowly cooled. An alternative technique involves diffusion of the
- silver halide into the surface of the glass. The cuprous ion can catalyse
- both the photochromic darking and thermal fading reactions, and the colour
- can be shifted from grey to brown by the addition of gold or palladium -
- which may be added to the glass in trace amounts. The most popular current
- application for glass containing silver halide is for prescription eyewear.
-
- The organic photochromic systems can be subdivided according to the type of
- reaction. Geometric isomerism can result in different optical properties,
- eg azobenzene ( C12H10N2 [103-33-3] ) undergoes photoisomerization, and the
- cis form [1080-16-6] has higher absorbance than the trans form [17082-12-1].
- Cycloaddition can produce photochromism, such as the reversible formation of
- the colourless 4b,12b,endoperoxide ( C28H14O4 [74292-77-6] ) from the red
- parent compound dibenzo(a,j)perylene-8,16-dione ( C28H14O2 [5737-94-0] ).
- Dissociation, either heterolytic ( photolysis of triphenylmethyl chloride
- [76-83-5] ), or homolytic ( photolysis of bis(2,4,5-triphenylimidazole
- [63245-02-3] to form a red-purple free radical ), may also produce
- photochromism.
-
- UV can excite polycyclic aromatics, such as 1,2,5,6-dibenzacridene ( C21H13N
- [226-36-8] ), to their triplet state, which has a different absorption
- spectrum. Viologens may undergo redox reactions and exhibit photochromic
- behaviour when crystalline and subjected to UV. The most popular photochromic
- materials utilise reversible electrocyclic reactions, and are often indolino
- spiropyrans and indolino spiroxazines, however the mechanism also covers
- fulgide, stilbene, and dihydroindolizine examples. Details and structures
- are provided in the Kirk Othmer monograph [8], and the Journal of Chemical
- Education has published descriptions and preparation techniques for both
- inorganic [10] and organic photochromic compounds and sunglasses [11].
-
- Piezochromic materials change colour as they are compressed. There are three
- common types:- organic molecules ( such as N-salicylidene-2-chloroaniline
- [3172-42-7] ), metal cluster compounds ( such as the octahalodirhenates,
- (Re2X8)2-, where X=Cl,Br,I ), and copper (II) organic complexes with
- compounds like ethylene diamine. They are still being researched, and
- interested readers should investigate the references in the Kirk Othmer
- monograph [8].
-
- Thermochromic materials reversibly change colour as their temperature is
- changed. There are a very large number of systems, but one common example
- of thermochromic transitions in metal complexes is the transition between
- the blue tetrahedral and pink octahedral coordinations of cobalt (II) when
- cobalt chloride is added to anhydrous ethanol and the temperature changed.
- Examples of thermochromic transitions in inorganic compounds include
- Ag2HgI4 [12344-40-0] and VO2, and several inorganic sulfides also have large
- changes occurring in the infra-red range, and are being considered for IR
- imaging applications.
-
- There are thousands of organic thermochromic compounds, with well known
- examples including di-beta-naphthospiropyran [178-10-9] ( thermally-induced
- heterolytic bond cleavage resulting in ring opening), poly(xylylviologen
- dibromide [38815-69-9] ( charge transfer interactions resulting in hydration-
- dehydration changes ), and ETCD polydiacetylene [63809-82-5] ( thermally-
- induced transitions in the unsaturated backbone resulting in rearranged side
- groups ). Information on photochromism in organic and polymeric compounds is
- available in published reviews [12,13].
-
- ------------------------------
-
- Subject: 19. Physical properties of chemicals
-
- 19.1 Rheological properties and terminology
-
- Contributed by Jim Oliver
-
- RHEOLOGY
-
- What is RHEOLOGY ?
- RHEOLOGY describes the deformation of a material under the influence of
- stresses. Materials in this context can be solids, liquids or gases. In this
- FAQ we will be concerned only with the rheological properties of liquids.[1]
- Perry discusses the some aspects of the behaviour of gases, and Ullmann
- discusses elastic solids.
-
- When liquids are subjected to stress they will deform irreversibly and flow.
- The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids
- are very few, whereas non-ideal examples abound. Ideal liquids are : water
- and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour
- mixed with a little water. [2]
-
- What is VISCOSITY ?
- VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the
- conditions used to measure the VISCOSITY must be given. This is due to the
- fact that non-ideal liquids have different values of VISCOSITY for different
- test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4]
-
- A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a
- particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this
- curve reveals the particular type of VISCOSITY for the liquid being studied.
- [3]
-
- What is a NEWTONIAN LIQUID ?
- NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the
- origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids
- are mineral oil, water and molasses. (Isaac NEWTON first described the laws
- of viscosity) [1] All the other types are NON NEWTONIAN.
-
- What does NON NEWTONIAN mean ?
- a. PSEUDOPLASTIC liquids are very common. These display a curve starting at
- the origin again and curving up and along but falling under the straight
- line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results
- in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with
- increasing shear. Examples are toothpaste and whipped cream.
- b. DILATANT liquids give a curve which curves under then upward and higher
- than the straight line NEWTONIAN curve. (Like a square law curve) Such
- liquids display increasing viscosity with increasing shear. Examples are
- wet sand, and mixtures of starch powder with small amounts of water. A car
- may be driven at speed over wet sand, but don't park on it, as the car may
- sink out of sight due to the lower shear forces (compared to driving over)
- the wet sand.
-
- There are other terms used which include :
-
- THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this
- case the liquid shows a YIELD or PLASTIC POINT before starting to thin out.
- What this means is the curve runs straight up the y axis for a short way then
- curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve.
- This YIELD POINT is time dependant. Some water based paints left overnight
- develop a FALSE BODY which only breaks down to become useable after rapid
- stirring. Also: the curve describing a THIXOTROPIC liquid will be different
- on the way up (increasing shear rate) to the way down (decreasing shear rate).
- The area inside these two lines is a measure of it's degree of THIXOTROPY.
- This property is extremely important in industrial products, e.g to prevent
- settling of dispersed solids on storage. [3]
-
- A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing
- viscosity with a constant shear rate over time. Again, time dependant but in
- this case _increasing_ viscosity.
-
- Why do some liquids become solid ?
- A few special liquids (dispersions usually) display extraordinary DILATANT
- properties. A stiff paste slurry of maize or cornflour in water can appear to
- be quite liquid when swirled around in a cup. However on pouring some out
- onto a hard surface and applying extreme shear forces (hitting with a hammer)
- can cause a sudden increase in VISCOSITY due to it's DILATANCY. The
- VISCOSITY can become so high as to make it appear solid. The "liquid" then
- becomes very stiff for an instant and can shatter just like a solid material.
-
- It should be noted that the study of viscosity and flow behaviour is
- extremely complex. Some liquids can display more than one of the above
- properties dependant on temperature, time and heat history.
-
- What are Electrorheological Fluids? ( added by Bruce Hamilton )
-
- Electrorheological (ER) fluids change their flow properties when an electric
- field is applied, and are usually dispersions of polarizable particles in an
- insulating base fluid [5]. Their apparent viscosity can change by orders of
- magnitude in milliseconds when a fews watts of electrical power are applied.
- The shear stress versus shear rate properties of ER fluids vary as a function
- of the applied electric field, When an electric field is applied, the fluid
- switches from a liquid to semisolid. The particles are usually irregularly-
- shaped 0.5-100um and present at concentrations of 10-40% by mass. ER fluids
- are dielectric particles in an insulating medium ( such as silicone oil ),
- along with additives ( such as surfactants, dispersants, and possibly a
- polar activator ). ER fluid effectively function as leaky capacitors. The
- electric field can be either AC, pulsed DC, or DC, with AC producing less
- electrophoresis of particles to electrodes.
-
- There are two categories of ER particulate materials, extrinsically
- polarizable materials ( which require a polar activator ), and intrinsically
- polarizable materials. Extrinsically polarizable materials can be polar
- nonionic compounds ( such as silica, alumina, or polysaccharides ), or polar
- ionic materials ( such as the lithium salt of polymethacrylic acid ),
- Intrinsically polarizable materials provide simpler systems - because a polar
- activator is not required, and they have a lower thermal coefficient of
- conductance. The most common examples are the ferroelectrics like barium
- titanate (BaTiO3 ) and polyvinylidene difluoride, however their performance
- has been poor, as has been that of metal powders ( such as iron and
- aluminium - even when coated with an insulating layer ), and research is
- concentrating on conducting polymers ( such as polyanilines and pyrolysed
- hydrocarbons ) [5,6].
-
- The ability to utilise computer-based electrical switching to control ER
- fluid properties has resulted in vehicle suspension and industrial vibration
- control as major target applications for ER fluids. Demonstration systems
- have been built, and they match performance predictions, however cost and
- durability issues still have to be solved [7].
-
- 19.2 Flammability properties and terminology
-
- There are several properties of flammable materials that are frequently
- reported. It should be remembered that most discussions concerning
- flammable liquids usually consider air as the oxidant, but oxygen and
- fluorine can also be used as oxidants for combustion, and they will result
- in very different values.
-
- The flammability limits in air are usually reported as the upper and lower
- limits ( in volume percent at a certain temperature, usually 25C ), and
- represent the concentration region that the vapour ( liquid HCs can not burn )
- must be within to support combustion. Hydrocarbons have a fairly narrow range,
- ( n-hexane = 1.2 to 7.4 ), whereas hydrogen has a wide range ( 4.0 to 75 ).
-
- The minimum ignition energy is the amount of energy ( usually electrical )
- required to ignite the flammable mixture. Some mixtures only require a very
- small amount of energy (eg hydrogen = 0.017mJ, acetylene = 0.017mJ ),
- whereas others require more (eg methanol = 0.14mJ, n-hexane = 0.29mJ,
- diethyl ether = 0.20mJ, acetone = 1.15mJ, dichloromethane = 133mJ @ 88C ),
- and some require significant amounts, (eg ammonia = >1000mJ ).
-
- The flash point is the most common measure of flammability today, especially
- in transportation of chemicals, mainly because most regulations use the flash
- point to define different classes of flammable liquids. The flash point of a
- liquid is the temperature at which the liquid will emit sufficient vapours
- to ignite when a flame is applied. The test consists of placing the liquid
- in a cup and warming it at a prescribed rate, and every few degrees applying
- a small flame to the air above the liquid until a "flash" is seen as the
- vapours burn. Note that the flame is not applied continuously, but is
- provided at prescribed intervals - thus allowing the vapour to accumulate.
-
- There are a range of procedures outlined in the standard methods for
- measuring flash point ( ASTM, ISO, IP ) and they have differing cup
- dimensions, liquid quantity, headspace volume, rate of heating, stirring
- speed, etc., but the most significant distinction is whether the space above
- the liquid is enclosed or open. If the space is enclosed, the vapours will be
- contained, and so the flash point is several degrees lower than if it is
- open. Most regulations specify closed-cup methods, either Pensky-Martens
- Closed Cup or Abel Closed Cup. It is important to remember that these methods
- are only intended for pure chemicals, if there is water or any other volatile
- non-flammable compounds present, their vapours can extinguish or mask the
- flash. For used lubricants, this may be partially overcome by using the TAG
- open cup procedure - which is slightly more tolerant of non-flammable
- vapours. A material can be flammable, but may not have a flash point if other
- non-flammable volatile compounds are present. For alkane hydrocarbons, flash
- point increases with molecular weight.
-
- There is an older measure, called the fire point, which is the temperature
- at which the liquid emits sufficient vapours to sustain combustion. The fire
- point is usually several degrees above the flash point for hydrocarbons.
-
- The minimum autoignition temperature is the temperature at which a material
- will autoignite when it contacts a surface at that temperature. The procedure
- consists of heating a glass flask and squirting small quantities of sample
- into it at various temperatures until the vapours autoignite. The only
- source of ignition is the heat of the surface. For the smaller hydrocarbons
- the autoignition temperature is inversely related to molecular weight, but it
- also increases with carbon chain branching. Autoignition temperature also
- correlates with gasoline octane ratings ( refer to Gasoline FAQ available in
- rec.autos.tech, which lists octane ratings and autoignition temperatures for
- a range of hydrocarbons.)
- Flash Point Autoignition Flammable Limits
- Temperature Lower Upper
- ( C ) ( C ) ( vol % at 25C)
- methane -188 630 5.0 15.0
- ethane -135 515 3.0 12.4
- propane -104 450 2.1 9.5
- n-butane -74 370 1.8 8.4
- n-pentane -49 260 1.4 7.8
- n-hexane -23 225 1.2 7.4
- n-heptane -3 225 1.1 6.7
- n-octane 14 220 0.95 6.5
- n-nonane 31 205 0.85 -
- n-decane 46 210 0.75 5.6
- n-dodecane 74 204 0.60 -
- n-tetradecane 99 200 0.50 -
-
- 19.3 Supercritical properties and terminology?
-
- Supercritical fluids have some very unusual properties. When a compound is
- subjected to conditions around the critical point ( which is defined as
- the temperature at which the gas will not revert to a liquid regardless how
- much pressure is applied ), the properties of the supercritical fluid become
- very different to the liquid or the gas phases. In particular, the solubility
- behaviour changes. The behaviour is neither that of the liquid or that of the
- gas. The transition between liquid and gas can be completely smooth.
-
- The pressure-dependant densities and corresponding Hildebrand solubility
- parameters show no break on continuity as the supercritical boundary is
- crossed. Physical properties fall between those of a liquid and a gas.
- Diffusivities are approximately an order of magnitude higher than the
- corresponding liquid, while viscosities are an order of magnitude lower.
- These properties ( along with low surface tension ) allow SCFs to have
- liquid-like solvating power with the mass transport characteristics of
- a gas.
-
- Potential Supercritical Fluids
- Compound Critical Critical Density
- Temperature Pressure
- ( C ) ( bar ) (g cm^-3)
- Ammonia 132.4 112.8 0.235
- Carbon dioxide 30.99 73.75 0.468
- CFC-12 111.8 41.25 0.558
- Dimethyl ether 126.9 52.7 0.271
- Ethane 32.4 49.1 0.212
- HCFC-22 96.15 49.90 0.524
- HCFC-123 183.68 36.62 0.550
- HFC-116 19.7 29.8 0.608
- HFC-134a 101.03 40.57 0.508
- Methanol 240.1 83.1
- Nitrous oxide 36.4 72.54 0.453
- Propane 96.8 42.66 0.225
- Water 374.4 227.1
- Xenon 16.6 58.38 1.105
-
- Nitrous oxide is seldom used because early researchers reported explosions.
- Note that using liquid CO2 at pressure ( as for the commercial extraction
- of hops ) is still just liquid CO2 extraction, not supercritical CO2
- extraction. There are several good general introductions to supercritical
- fluids [8,9,10]
-
- 19.4 Formation of gaseous bubbles in liquids
-
- Discussions about the behaviour of dissolved gases in liquids, especially
- when discussing carbonated beverages, are usually more appropriate in
- sci.physics and/or sci.mech.fluids, and there is a good text available [11].
-
- Section 23.9 of this FAQ lists the change in solubility with temperature
- for common atmospheric gases in water at near-ambient pressure. As the
- temperature increases, the solubility decreases, creating a supersaturated
- solution that can result in bubble formation. A similar effect occurs if the
- pressure is reduced. The formation of bubbles can be understood in
- thermodynamic terms using the Gibbs free energy of the bubble.
-
- Gibbs free energy = -n * R * T ln(C/Cs) + gamma * A
-
- A = Surface area of the bubble.
- C = Concentration of gas in the liquid,
- Cs = Concentration of gas in the liquid at saturation,
- gamma = Interfacial tension between the gas and the liquid
- n = Number of moles of gas in the bubble
- = (P*V)/(R*T), where P = pressure, and V = volume of a sphere.
- R = Gas Constant
- T = Temperature
-
- After inserting the expressions for the surface area of a sphere (r = radius)
- and number of moles, and differentiating, then we obtain:-
-
- r(mininum) = 2 * gamma / ( P * ln(C/Cs))
-
- This describes the size of a bubble that would continue to grow under the
- existing conditions, rather than redissolve. Of course, the expression
- assumes homogeneous precipitation of the bubble, and in real life most
- bubbles are created heterogeneously. Statistics and kinetics are also
- required to determine the rate of formation of bubbles, and predict the
- effect of changing parameters such as temperature. As the liquid is warmed,
- bubbles may be created faster, as the higher temperatures overcome the
- activation barrier - which is the difference between the Gibbs free energy
- when r is less than r(minimum), and the Gibbs free energy at r(minimum).
-
- The formation of a bubble also dramatically perturbs the system, even
- causing secondary bubbles to form. Secondary bubble formation may be
- implicated in the production of copious quantities of froth from shaken,
- quickly-opened, carbonated drink containers. The sites for gaseous bubble
- formation in supersaturated drinks are typically small particles, or minor
- flaws on the smooth surface of the container.
-
- 19.5 Why is Mercury a liquid at room temperature?.
-
- First, let's look at the melting points of some of the elements surrounding
- mercury in the periodic table ( in degrees C ) :-
- Period IB IIB IIIA
- 4s3d4p Cu 1083 Zn 419.5 Ga 29.8
- 5s4d5p Ag 960.8 Cd 320.9 In 157
- 6s(4f)5d6p Au 1063 Hg -38.4 Tl 304
-
- The interesting comparison is between Hg and Au, as their properties differ
- dramatically, although their electron structures are similar:-
- 14 10 1
- Au(g) : Xe | 4f , 5d , 6s
- 79 54
- 14 10 2
- Hg(g) : Xe | 4f , 5d , 6s
- 80 54
-
- Very few chemistry textbooks discuss relativistic effects on chemical
- properties, despite the availability of a comprehensive review by P.Pyykko
- [12]. There several good introductory articles on the derivation and
- calculation of various relativistic effects in molecules and atoms, so I'm
- not going to include details [13,14,15]. Suffice to say, that whilst
- smaller elements can treated simply, larger elements need treatment based
- on the Dirac equation, which shows that the s electrons are approaching
- the speed of light, consequently relativistic effects are important.
- If we take the relativistic mass of mercury (m);-
-
- Mo where
- m = -------------------- c (speed of light) = ~137 atomic units
- _____________ v = Z = 80
- / ( v ) 2 Mo = rest mass
- / 1 - ( - )
- \/ ( c )
-
- The masses of the 1s electrons are increased by approximately 20% over
- their rest masses, which means that the radius is decreased by 20% - since
- mass appears in the denominator of Bohr radius calculations. All the other
- s shells also contract, with the 6s contracting ~14%, because their electron
- speeds near the nucleus are comparable, and the contraction of the inner part
- of the wave function also pulls in the outer tails. The p orbitals also
- contract a similar amount, and these contractions also results in increases
- the screening for d and f orbitals, which may then expand - about 3% for the
- 5d orbital of mercury.
-
- In mercury, the relativistically-contracted 6s2 orbital is full, thus the
- the two electrons do not contribute much to the metal-metal bond, which is
- not the situation for gold. The bonding in mercury is believed to be mainly
- van der Waals forces with a contribution from 6p orbital interaction. The
- relativistic contraction of the filled 6s2 orbital, when added to the
- contraction across the sixth row of the periodic table, results in relatively
- weak Hg-Hg bonds that are responsible for mercury being a liquid at room
- temperature. For those curious to know more, a recent article in J.Chem.Ed.
- provides much more detail and several good references [16]. Relativistic
- effects are also responsible for the colour of gold ( partially explained
- by the 5d -> 6s transition in gold requiring less energy than the 4d -> 5s
- transition in silver, resulting in a smaller d-s gap ) [12,14,16].
-
- ------------------------------
-
- Subject: 20. Optical properties of chemicals
-
- 20.1 Refractive Index properties and terminology
-
- When light passes between media of different density, the direction of the
- beam is changed as it passes through the surface, and this is called
- refraction. In the first medium, the angle between the light ray and the
- perpendicular is called the angle of incidence (i), and the corresponding
- angle in the second medium is called the angle of refraction (r). The
- ratio sine i / sine r is called the index of refraction, and usually the
- assumption is that the light is travelling from the less dense (air) to more
- dense, giving an index of refraction that is greater than 1. Although the
- theoretical reference is a vacuum, air ( 0.03% different ) is usually used.
- The refractive index of a compound decreases with increasing wavelength
- ( dispersion ), except where absorption occurs, thus the wavelength should
- be reported. The D lines of sodium are commonly used.
-
- The refractive index of a liquid varies with temperature and pressure, but
- the specific refraction ( Lorentz and Lorentz equation ) does not. The molar
- refraction is the specific refraction multiplied by the molecular weight,
- and is approximately an additive property of the groups or elements
- comprising the compound. Tables of atomic refractions are available in the
- literature, as are descriptions of the common types of refractometers [1].
-
- 20.2 Polarimetry properties and terminology
-
- Supplied by: Vince Hamner <vinny@vt.edu>
-
- Polarimetry is a method of chemical analysis that is concerned
- with the extent to which a beam of linearly polarised light is rotated
- during its transmission through a medium containing an optically active
- species.[2] Helpful discussions regarding polarised light may be found
- elsewhere.[3,4] In general, a compound is optically active if it has
- no plane of symmetry and is not superimposable on its mirror image.
- Such compounds are referred to as being "chiral". Sucrose, nicotine,
- and the amino acids are only a few of these substances that exhibit
- an optical rotary power.
-
- A simple polarimeter instrument would consist of:
-
- 1). a light source -- typically set to 589 nm (the sodium "D" line)
- 2). a primary fixed linear polarising lens (customarily called the
- "polariser")
- 3). a glass sample cell (in the form of a long tube)
- 4). a secondary linear polarising lens (customarily called the
- "analyser") and
- 5). a photodetector.[5]
-
- Biot is credited with the determination of the basic equation
- of polarimetry.[6,7] The specific rotation of a substance (at a given
- wavelength and temperature) is equivalent to the observed rotation (in
- degrees) divided by the path length of the sample cell (in decimeters)
- multiplied by the concentration of the sample (for a pure liquid,
- -density- replaces concentration). Influences of temperature,
- concentration, and wavelength must always be taken into consideration.
- If necessary, it is possible to apply corrections for each of these
- variables.[8] A few early contributors to our understanding of optical
- activity and polarimetry include: Malus, Arago, Biot, Drude, Herschel,
- Fresnel, and Pasteur.
-
- ------------------------------
-
- Subject: 21. Molecular and Structural Modelling
-
- Supplied by: Dave Young (young@slater.cem.msu.edu)
-
- 21.1 What hardware do I need to run modelling programs?
-
- There are two types of programs that are referred to as molecular
- modeling programs. This first is a program which graphically displays
- molecular structures as Lewis structures, ball & stick, etc. The second
- is a program which does a calculation to tell you something about the
- molecule, such as it's energy, dipole moment, spectra, etc.
-
- For an introductory description of various types of computations,
- see http://www.cem.msu.edu/~young/topics/contents.html
-
- There are many programs of both sorts available for a large range
- of machines. The speed, memory, graphics and disk space on the machine
- will determine the size of molecules that can be modelled, how accurate
- the model is, and how good the images will look. There are a few programs
- that will run on a 286 PC with Windows. There are some fairly nice things
- that can be done on a 386 with about 8 MB of RAM and Windows. The
- professional computational chemists are generally using work stations and
- larger machines.
-
- Currently many computational chemists are using machines made by
- Silicon Graphics (SGI) ranging from the $5,000 Indy to the $1,000,000
- power challenge machines. These are all running Irix, which is SGI's
- adaptation of Unix. SGI is popular for two reasons; first that the power
- is very good for the price, second that SGIs run the largest range
- of chemical software. However, you will find some computational chemistry
- software that can run on almost any machine.
-
- As far as graphics quality, the SGI Onyx (about $250,000) is about
- the top of the line. Even if you find a machine that claims to have better
- graphics than this, chances are you won't find and chemistry software that
- can utilise it.
-
- For chemical calculations there is no limit to the computing
- power necessary. There are some calculations that can only be done
- on the biggest Crays or massively-parallel machines in the world. There
- are also many calculations which are too difficult for any existing
- machine and will just have to wait a few years or a few centuries.
-
- 21.2 Where can I find a free modelling program?
-
- The single best place for public domain modelling software
- is probably the anonymous FTP server at ccl.osc.edu in the directory
- pub/chemistry/software. "ccl" stands for "computational chemistry
- list server" and is a list frequented mostly by professional
- computational chemistry researchers. This machine contains their
- archives with quite a bit of information as well as software.
-
- For work stations and larger, the program GAMESS (General Atomic
- and Molecular Electronic Structure System) can be obtained as source
- code from Mike Schmidt at mike@si.fi.ameslab.gov GAMESS is a quantum
- mechanics, ab initio and semi-empirical program. It is powerful. but
- not trivial to learn how to use.
-
- The COLUMBUS program for work stations and larger can be obtained
- by anonymous FTP from ftp.itc.univie.ac.at It is a HF, MCSCF and
- multi-reference CI program. This is probably the most difficult program
- to use that is in use today since it requires the user to input EVERY
- detail manually. However, because you control everything there are some
- calculations that can only be done with COLUMBUS.
-
- CACAO is an extended Huckel program available by anonymous FTP
- at cacao.issecc.fi.cnr.it
-
- 21.3 Where can I find structural databanks?
-
- 21.4 Where can I find ChemDraw or ChemWindows
-
- For ChemDraw (Macintosh, Windows, UNIX)
- CambridgeSoft Corporation
- 875 Massachusetts Avenue
- Cambridge, MA 02139
- Phone: (800) 315-7300 or (617) 491-2200
- Fax: (617) 491-7203
- Internet: info@camsci.com
- http://www.camsci.com
-
- For ChemIntosh or ChemWindows
- SoftShell
- 1600 Ute Avenue
- Grand Junction, CO 81501
- Phone: (970) 242-7502
- Fax: (970) 242-6469
- Internet: info@softshell.com
- http://www.softshell.com
-
- ------------------------------
-
- Subject: 22. Spectroscopic Techniques
-
- All of these are covered in texts on instrumental Analysis [1-4], and I
- will eventually include a paragraph about each.
-
- 22.1 Ultra-Violet/Visible properties and terminology
- 22.3 Nuclear Magnetic Resonance properties and terminology
- 22.4 Mass Spectrometry properties and terminology
- 22.5 X-Ray Fluorescence properties and terminology
- 22.6 X-Ray Diffraction properties and terminology
- 22.7 Fluorescence/Phosphorescence properties and terminology
-
- ------------------------------
-
- Subject: 23. Chromatographic Techniques
-
- There are chromatography mailing lists and WWW sites available that provide
- comprehensive introductions and access to chromatography experts. The
- following are simple introductions to popular techniques.
-
- 23.1 What is Paper Chromatography?
-
- Paper chromatography was the first analytical chromatographic technique
- developed, allegedly using papyrus (Pliny). It was first published by Runge
- in 1855, and consists of a solvent moving along filter or blotting paper.
- The interaction between the components of the sample, the solvent, and the
- paper, results in separation of the components. Most modern paper
- chromatography is partition chromatography, where the cellulose of the
- paper is the inert support, and the water adsorbed ( hydrogen bonded ) from
- air onto the hydroxyl groups of the cellulose becomes the stationary phase.
-
- If the mobile phase is not saturated with water, then some of the stationary
- phase water may be removed from the cellulose - resulting in a separation
- that is a mixture of partition and adsorption. Paper chromatography remains
- the method of choice for a wide range of coloured compounds, and is used
- extensively in both natural and artificial pigment research. The technique
- is suitable for any molecules that are significantly less volatile than the
- solvent, and many examples and references are provided in Heftmann [1].
-
- 23.2 What is Thin Layer Chromatography?
-
- Thin layer chromatography involves the use of a particulate sorbant on an
- inert sheet of glass, plastic, or metal. The solvent is allowed to travel
- up the plate with the sample spotted on the sorbant just above the solvent.
- Depending on the sorbant, the separation can be either partition or
- adsorption chromatography ( cellulose, silica gel and alumina are commonly
- used ). The technique came to prominence during the late 1930s, however it
- did not become popular until Merck and Desaga developed commercial plates
- that provided reproducible separations. The major advantage of TLC is the
- disposable nature of the plates. Samples do not have to undergo the
- extensive clean-up steps required for HPLC. The other major advantage is the
- ability to detect a wide range of compounds cheaply, using very reactive
- reagents ( iodine vapours, sulfuric acid ) or indicators. Non-destructive
- detection ( fluorescent indicators in the plates, examination under a UV
- lamp ) also means that purified samples can be scraped off the plate and
- be analysed by other techniques. There are special plates for such
- preparative separations, and there are also high-performance plates that can
- approach HPLC resolution. The technique is described in detail in Stahl [2]
- and Kirchner [3].
-
- 23.3 What is Gas Chromatography?
-
- Gas chromatography is the use of a carrier gas to convey the sample ( as a
- vapour ) through a column consisting of an inert support and a stationary
- phase that interacts with sample components, thus it is usually partition
- chromatography. There are also a range of materials, especially for permanent
- gas and light hydrocarbon analysis that utilise adsorption. The simplest
- partition systems consisted of a steel tube filled with crushed brick that
- had been coated with a hydrocarbon that had a high boiling point, eg
- squalane. Today, the technique uses very narrow fused silica tubes ( 0.1 to
- 0.3mm ID ) that have sophisticated stationary phase films ( 0.1 to 5um )
- bonded to the surface and also cross-linked to increase thermal stability.
-
- The ability of the film to retard specific compounds is used to ascertain
- the "polarity" of the column. If benzene elutes between normal alkanes
- where it is expected by boiling point ( midway between n-hexane and
- n-heptane ), then the column is "non-polar" eg squalane and methyl silicones.
- If the benzene is retarded until it elutes after n-dodecane, then the column
- is "polar" eg OV-275 ( dicyanoallyl silicone ) and 1,2,3-tris (2-cyanoethoxy)
- propane. In general, polar columns are less tolerant of oxygen and reactive
- sample components, but the ability to select different polarity columns to
- obtain satisfactory peak resolution is what has made GC so popular.
-
- The column is placed in an oven that has exceptional temperature control,
- and the column can be slowly heated up to 350-450C ( sometimes starting at
- -50C to enhance resolution of volatile compounds ) to provide separation of
- wide-boiling range compounds. The carrier gas is usually hydrogen or helium,
- and the eluting compounds can be detected several ways, including flames
- ( flame ionisation detector ), by changes in properties of the carrier
- ( thermal conductivity detector ), or by mass spectrometry. The availability
- of "universal" detectors such as the FID and MS, makes GC a popular tool in
- laboratories handling organic compounds. There are also columns that have a
- layer of 5-10 um porous particulate material (such as molecular sieve or
- alumina ) bonded to the inner walls ( PLOT = Porous layer open tubular ),
- and these are used for the separation of permanent gases and light
- hydrocarbons. GC is restricted to molecules ( or derivatives ) that
- are sufficiently stable and volatile to pass through the GC intact at the
- temperatures required for the separation. Specialist books on the production
- of derivatives for GC are available [4,5].
-
- There are several manufacturers of GC instruments whose catalogues and
- brochures provide good introduction to the technique. (eg Hewlett Packard,
- Perkin Elmer, Carlo Erba ). The catalogues of suppliers of chromatography
- consumables also contain explanations of the criteria for selection of the
- correct columns and conditions for analyses, and they provide an excellent
- indication of the range of applications available. Well-known suppliers
- include Alltech Associates, Supelco, Chrompack, J&W, and Restek. They also
- sell most of the standard GC texts, as do the instrument manufacturers.
- Popular GC texts include "Basic Gas Chromatography" [6], "High-Resolution
- Gas Chromatography" [7], and "Open Tubular Column Gas Chromatography" [8].
- There are Standard Retention Index Libraries available [9], however they
- really only complement unambiguous identification by mass spec. or
- dual-column analysis.
-
- 23.4 What is Column Chromatography?
-
- Column chromatography consists of a column of particulate material such as
- silica or alumina that has a solvent passed through it at atmospheric or low
- pressure. The separation can be liquid/solid (adsorption) or liquid/liquid
- (partition). The columns are usually glass or plastic with sinter frits to
- hold the packing. Most systems rely on gravity to push the solvent through.
- The sample is dissolved in solvent and applied to the front of the
- column. The solvent elutes the sample though the column, allowing the
- components to separate based on adsorption ( alumina, hydroxyapatite) or
- partition ( cellulose, diatomaceous earth ). The mechanism for silica
- depends on the hydration. Traditionally, the solvent was non-polar and the
- surface polar, although today there are a wide range of packings including
- bonded phase systems. Bonded phase systems usually utilise partition
- mechanisms rather than adsorption. The solvent is usually changed stepwise,
- and fractions are collected according to the separation required, with the
- eluted solvent usually monitored by TLC.
-
- The technique is not efficient, with relatively large volumes of solvent
- being used, and particle size is constrained by the need to have a flow of
- several mls/min. The major advantage is that no pumps or expensive equipment
- are required, and the technique can be scaled up to handle sample sizes
- approaching a gram in the laboratory. The technique is discussed in detail
- in Heftmann [1].
-
- 23.5 What is High Pressure Liquid Chromatography?
-
- HPLC is a development of column chromatography. it was long realised that
- using particles with a small particle size ( 3, 5, 10um ) with a very narrow
- size distribution would greatly improve resolution, especially if the flow
- rate and column dimensions could be adjusted to minimise band-broadening.
- Pumps were developed that could handle both the chemicals and pressures
- required. Traditional column chromatography ( nonpolar solvent and
- polar surface ) is described as "normal" and, as well as silica, there are
- columns with amino, diol, and cyano groups. If the system uses a polar
- solvent ( water, methanol, acetonitrile etc. ) and a non-polar surface it
- is described as "reverse-phase". Common surface treatments of silica include
- octadecylsilane ( aka ODS or C18), and it has been the development of
- reverse-phase HPLC that has experienced explosive growth. Reverse-phase HPLC
- is the method of choice for larger non-volatile biomolecules, however it is
- only recently that a replacement "universal" detector ( evaporative
- light-scattering ) has emerged. The most popular detector (UV), places
- constraints on the solvents that can be used, and the refractive index
- detector can not easily be used with solvent gradients. There are several
- excellent books introducing HPLC, including the classic "Introduction to
- Modern Liquid Chromatography" [10]. HPLCs can be a pain to operate, and
- novices should borrow "Troubleshooting LC Systems" by Dolan and Snyder [11].
- There is also a handy basic primer on developing HPLC methods by Snyder and
- Kirkland [12], however, unlike GC, you also need to search the journals
- ( Journal of Chromatography, Journal of Liquid Chromatography ) to find
- relevant examples to assist with method development.
-
- 23.6 What is Ion Chromatography?
-
- Ion chromatography has become the method of choice for measuring anions
- ( eg Cl-, SO4=, NO3- ) in aqueous solutions. It is effectively a development
- from ion-exchange systems ( which were extensively developed to deionise
- water and soften aqueous process streams ), and brings them down to HPLC
- size. IC uses pellicular polymeric resins that are compatible with a wide pH
- range. The sample is eluted through an ion-exchange column using a dilute
- sodium hydroxide solution. The eluant is passed through self-regenerating
- suppressors that neutralise eluant conductance, ensuring electrochemical
- detectors ( conductivity or pulsed amperometric ) can detect the ions down
- to sub-ppm concentrations. The major manufacturer of such systems is Dionex,
- who hold several patents on column, suppression, and detection technology.
- There are several books covering various aspects of the technique [13,14].
-
- 23.7 What is Gel Permeation Chromatography?
-
- Gel Permeation chromatography ( aka Size Exclusion chromatography ) is based
- on the ability of molecules to move through a column of gel that has pores of
- clearly-defined sizes. The larger molecules can not enter the pores, thus
- they pass quickly through the column and elute first. Slightly smaller
- molecules can enter some pores, and so take longer to elute, and small
- molecules can be delayed further. The great advantage of the technique is
- simplicity, it is isocratic ( single solvent - no gradient programming ),
- and large molecules rapidly elute. The technique can be used to determine
- the molecular weight of large biomolecules and polymers, as well as
- separating them from salts and small molecules. The columns are very
- expensive and sensitive to contamination, consequently they are mainly used
- in applications where alternative separation techniques are not available,
- and sample are fairly clean. The best known columns are the Shodex
- cross-linked polystyrene-divinylbenzene columns for use with organic solvents,
- and polyhydroxymethacrylate gel filtration columns for use with aqueous
- solvents. "Modern Size Exclusion Chromatography" [15], and Heftmann [1],
- provide good overviews, and there are some good introductory booklets from
- Pharmacia.
-
- 23.8 What is Capillary Electrophoresis?
-
- Capillary electrophoresis uses a small fused silica capillary that has been
- coated with a hydrophilic or hydrophobic phase to separate biomolecules,
- pharmaceuticals and small inorganic ions. A voltage is applied and the
- analytes migrate and separate according to their charge under the specific
- pH conditions, as also happens for electrophoresis. The capillary can also
- be used for isoelectric focusing of proteins. The use of salt or vacuum
- mobilisation is no longer required.
-
- 23.9 How do I degas chromatographic solvents?
-
- One major problem with pressurising chromatography systems using liquid
- solvents is that pressure reductions can cause dissolved gases to come out
- of solution. The two locations where this occurs are the suction side of the
- pump ( which is not self-priming, consequently a gas bubble can sit in the
- pump and flow is reduced ), and at the column outlet ( where the bubbles
- then pass through the detector causing spurious signals). Note that the
- problem is usually restricted to solvents that have relatively high gas
- solubilities - usually involving an aqueous component, especially if a
- gradient is involved where the water/organic solvent ratio is changing.
- As water usually has a higher dissolved gas content, then a gradient
- programme may cause the gases to come out of solution as the mobile phase
- components mix.
-
- There are three traditional strategies used to remove problem dissolved
- gases from chromatographic eluants. Often they are used in combination to
- lower the dissolved gases.
- a. Subject the solvent to vacuum for 5-10 mins. to remove the gases.
- b. Subject the solvent to ultrasonics for 10-15 mins. to remove the gases.
- c. Sparge the solvent with a gas that has a very low solubility compared
- to the oxygen and nitrogen from the atmosphere. Helium is the preferred
- choice - 5 minutes of gentle bubbling from a 7um sinter is usually
- sufficient, although maintaining a positive He pressure is even better.
- Note that most aqueous-based solvents usually have to be degassed every
- 24 hours. Also remember that solubility of gases increases as temperature
- decreases, so ensure eluants are at instrument temperature prior to
- degassing. Helium is preferred as the degassing solvent because it has
- relatively low solubility in water, and the solubility is less affected by
- temperature.
-
- The following data is from Kaye and Laby, 13th edition, and the units are
- the number of cm3 of gas at 0C and 760 mmHg which dissolve in 1 cm3 of water
- at the temperature stated ( when the gas is at 760 mmHg pressure and in
- equilibrium with the water ).
-
- Temp.(C) 0 10 20 30 40 50 60
- Helium 0.0098 0.0091 0.0086 0.0084 0.0084 0.0086 0.0090
- Hydrogen 0.0214 0.0195 0.0182 0.0170 0.0164 0.0161 0.0160
- Nitrogen 0.0230 0.0185 0.0152 0.0133 0.0119 0.0108 0.0100
- Oxygen 0.047 0.037 0.030 0.026 0.022 0.020 0.019
- Argon 0.054 0.041 0.032 0.028 0.025 0.024 0.023
- CO2 1.676 1.163 0.848 0.652 0.518 0.424 0.360
-
- I've no explanation for the aberrant trend for helium at higher temperatures,
- but I assume it's real - but it's irrelevant for HPLC solvents that are
- usually stored at ambient temperature. Points to note - the lower solubility
- of helium over the range of concern, *and* the lower rate of change of
- decreasing solubility with increasing temperature. There is heat generated
- in the compression of the solvent, along with friction in HPLC pump heads
- and, more importantly, HPLC columns are often heated - thus the solvent
- could outgas and form bubbles in UV detector cells that are at ambient.
- By using helium, there is less chance of that happening. For example, if the
- temperature increased from 10C to 40C, the undissolved gas volume would be
- 0.0007 cm3 for helium, and 0.0066 cm3 for nitrogen.
-
- Modern HPLCs are sold with a "solvent degassing module" that removes
- undissolved gases in the solvent automatically. These usually consist of
- a tube made from gas-permeable membrane that passes through a vacuum
- chamber.
-
- 23.10 What is chromatographic solvent "polarity"?
-
- There are four major intermolecular interactions between sample and solvent
- molecules in liquid chromatography, dispersion, dipole, hydrogen-bonding,
- and dielectric. Dispersion interactions are the attraction between each pair
- of adjacent molecules, and are stronger for sample and solvent molecules
- with large refractive indices. Strong dipole interactions occur when both
- sample and solvent have permanent dipole moments that are aligned. Strong
- hydrogen-bonding interactions occur between proton donors and proton
- acceptors. Dielectric interactions favour the dissolution of ionic
- molecules in polar solvents. The total interaction of the solvent and
- sample is the sum of the four interactions. The total interaction for a
- sample or solvent molecule in all four ways is known as the "polarity" of
- the molecule. Polar solvents dissolve polar molecules and, for normal
- phase partition chromatography, solvent strength increases with solvent
- polarity, whereas solvent strength decreases with increasing polarity
- in reverse-phase systems. The subject is discussed in detail in Snyder
- and Kirkland [10].
-
- ------------------------------
-
-
- Subject: 24. Extraction Techniques
-
- 24.1 What is Solvent Extraction?
-
- Solvent extraction is usually used to recover a component from either a solid
- or liquid. The sample is contacted with a solvent that will dissolve the
- solutes of interest. Solvent extraction is of major commercial importance
- to the chemical and biochemical industries, as it is often the most efficient
- method of separation of valuable products from complex feedstocks or
- reaction products. Some extraction techniques in involve partition between two
- immiscible liquids, others involve either continuous extractions or batch
- extractions. Because of environmental concerns, many common liquid/liquid
- processes have been modified to either utilise benign solvents, or move to
- more frugal processes such as solid phase extraction. The solvent can be a
- vapour, supercritical fluid, or liquid, and the sample can be a gas, liquid
- or solid. There are a wide range of techniques used, and details can be found
- in Organic Vogel, Perry, and most textbooks on unit operations.
-
- 24.2 What is Solid Phase Extraction?
-
- Solid Phase Extraction (SPE) is an alternative to liquid/liquid extraction,
- and has become the method of choice for the separation and purification of
- a wide range of samples in the laboratory. The sample is usually dissolved
- in an appropriate solvent and passed through a small bed of adsorbent of
- very consistent particle size and shape to maximise separation efficiency.
- The compounds are eluted with step changes of small volumes of solvents.
- The major advantage is that solvent volumes are greatly reduced. There is
- a newer, modified technique that is used in analytical laboratories, called
- Solid Phase Micro Extraction. This immerses a fused silica fibre coated with
- a stationary phase into the sample solution for several minutes, The analytes
- adsorb onto the stationary phase, which is subsequently pushed into a hot GC
- injector to rapidly desorb the sample for analysis.
-
- 24.3 What is Supercritical Fluid Extraction?
-
- Refer to Section 19.3 for some critical data on common supercritical fluids.
- Supercritical fluids have been investigated since last century, with the
- strongest commercial interest initially focusing on the use of supercritical
- toluene in petroleum and shale oil refining during the 1970s. Supercritical
- water is also being investigated as a means of destroying toxic wastes, and
- as an unusual synthesis medium [1]. The biggest interest for the last decade
- has been the applications of supercritical carbon dioxide, because it has
- a near-ambient critical temperature (31C), thus biological materials can
- be processed at temperatures around 35C. The density of the supercritical
- CO2 at around 200bar pressure is close to that of hexane, and the solvation
- characteristics are also similar to hexane, thus it acts as a non-polar
- solvent. Around the supercritical region CO2 can dissolve triglycerides at
- concentrations up to 1% mass. The major advantage is that a small reduction
- in temperature, or a slightly larger reduction in pressure, will result in
- almost all of the solute precipitating out as the supercritical conditions
- are changed or made subcritical. Supercritical fluids can produce a
- product with no solvent residues. Examples of pilot and production scale
- products include decaffeinated coffee, cholesterol-free butter, low-fat meat,
- evening primrose oil, squalene from shark liver oil. The solvation
- characteristics of supercritical CO2 can be modified by the addition of an
- entrainer, such as ethanol, however some entrainer remains as a solvent
- residue in the product, negating some of the advantages of the "residue-free"
- extraction.
-
- There are other near-ambient temperature supercritical fluids, including
- nitrous oxide and propane, however there are safety issues with some of them.
- There are several introductory texts on supercritical fluid extraction,
- including some the ACS Symposium series [2-4]. There are also a large
- number of articles on applications of the technique, including processing [5],
- extraction of natural products [6], and chemical synthesis [7]. The major
- concentration of information occurs in the various proceedings of the
- International Symposium on Supercritical Fluids [8]. There is also a Journal
- of Supercritical Fluids.
-
- 24.4 What traditional process extracted perfume from flower petals?
-
- The traditional cold-fat extraction process is known as " enfleurage".
- It is a very interesting, historical process used to obtain the essential
- oils and perfume components from rose, jasmine, and other flowers. The
- rose and jasmine flowers continue to produce perfume during the long
- process. Thus the technique can obtain more perfume from those flowers than
- if they were just macerated and extracted by hot fat, solvent or steam
- when they were picked - as happens to many other plant perfume sources.
- The process uses a fat comprised of 40 parts of beef tallow and 60 parts
- of lard. The two fats are melted together, and repeatedly beaten under
- cold water and alum solutions to purify them. Benzoin is added to the
- fat mixture to prevent biological degradation.
-
- The fat is spread about 4mm thick on both sides of 0.5 x 0.5 metre glass
- plates in wooden frames. Flowers are pressed into the fat on one side of
- the frame only, and the frames stacked vertically so that the flowers are
- very close to the layer of fat on the frame above. After 1-3 days, the
- flowers are stripped off and fresh flowers added to the other layer of fat
- that had not been used, and the frame are again stacked. The cycle is
- repeated about 30 - 35 times, or until the fat is saturated with perfume.
- The saturated fat is known as "pomade". The fat is removed from the frames
- and extracted with alcohol to collect the perfume. the alcohol is cooled
- and filtered to remove most of the dissolved fat. The alcohol solution
- is called the "extract", and the residue after evaporation of the solvent
- is known as the "enfleurage absolute".
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- Subject: 25. Radiochemical Techniques
-
- 25.1 What is radiochemistry?
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