Introduction to Analytical Separations
Separations are extremely important in synthesis, in industrial chemistry, in the biomedical sciences, and in chemical analyses. Analytical separations occur on a much smaller laboratory scale than the industrial-scale distillation shown in the photograph. The separation methods introduced in this chapter include precipitation, distillation, extraction, ion exchange, and various chromatographic techniques
A substance that affects an analytical signal or the background is called an interferenceor an interferent. An interferent is a chemical species that causes a systematic error in an analysis by enhancing or attenuating the analytical signal or the background. Several methods can be used to deal with interferences in analytical procedures, as discussed in Section 8D-3. Separations isolate the analyte from potentially interfering constituents. In addition, techniques such as matrix modification, masking, dilution, and saturation are often used to offset the effects of interferents. The internal standard and standard addition methods can sometimes be used to compensate for or to reduce interference effects.
The goals of an analytical separation are usually to eliminate or reduce interferences so that quantitative analytical information can be obtained from complex mixtures. Separations can also allow identification of the separated constituents if appropriate correlations are made or a structurally sensitive measurement technique, such as mass spectrometry, is used.
Separation Techniques
Separation principles. In (a), a mixture of four components is completely separated so that each component occupies a different spatial region. In (b), a partial separation is shown. In the partial separation, species A is isolated from the remaining mixture of B, C, and D. The reverse of the separation process shown is mixing at constant volume.
Separation by Precipitation
Separations by precipitation require large solubility differences between the analyte and potential interferents. The theoretical feasibility of this type of separation can be determined by solubility calculations such as those shown in Section 11C. Unfortunately, several other factors may preclude the use of precipitation to achieve a separation.
Separation Based on control of acidity
There are enormous differences among the solubilities of the hydroxides, hydrous oxides, and acids of various elements. Moreover, the concentration of hydrogen or hydroxide ions in a solution can be varied by a factor of 1015 or more and can be easily controlled by the use of buffers. As a result, many separations based on pH control are in theory possible. In practice, these separations can be grouped in three categories: (1) those made in relatively concentrated solutions of strong acids, (2) those made in buffered solutions at intermediate pH values, and (3) those made in concentrated solutions of sodium or potassium hydroxide.
Sulfide Separations
[S22] 5 1.2 3 10222 [h3O1]
The exception of the alkali metals and alkaline-earth metals, most cations form sparingly soluble sulfides whose solubilities differ greatly from one another. Because it is relatively easy to control the sulfide ion concentration of an aqueous solution of H2S by adjustment of pH, separations based on the formation of sulfides have found extensive use. Sulfides can be conveniently precipitated from homogeneous solution, with the anion being generated by the hydrolysis of thioacetamide.
Separation by Other Inorganic Precipitants
No other inorganic ions are as generally useful for separations as hydroxide and sulfide ions. Phosphate, carbonate, and oxalate ions are often used as precipitants for cations, but they are not selective. Because of this drawback, separations are usually performed prior to precipitation. Chloride and sulfate are useful because of their highly selective behavior. Chloride can separate silver from most other metals, and sulfate can isolate a group of metals that includes lead, barium, and strontium.
Separation by Organic Precipitants
Selected organic reagents for the isolation of various inorganic ions were discussed in Section 12C-3. Some of these organic precipitants, such as dimethylglyoxime, are useful because of their remarkable selectivity in forming precipitates with only a few ions. Other reagents, such as 8-hydroxyquinoline, yield slightly soluble compounds with many different cations. The selectivity of this sort of reagent is due to the wide range of solubility among its reaction products and also to the fact that the precipitating reagent is usually an anion that is the conjugate base of a weak acid. Thus, separations based on pH control can be realized just as with hydrogen sulfide.
Separation of Species Present in Trace Amounts by Precipitation
A problem often encountered in trace analysis is that of isolating from the major components of the sample the species of interest, which may be present in microgram quantities. Although such a separation is sometimes based on a precipitation, the techniques required differ from those used when the analyte is present in large amounts. Several problems can accompany the quantitative separation of a trace element by precipitation even when solubility losses are not important. Supersaturation often delays formation of the precipitate, and coagulation of small amounts of a colloidally dispersed substance is often difficult. In addition, it is common to lose an appreciable fraction of the solid during transfer and filtration. To minimize these difficulties, a quantity of some other ion that also forms a precipitate with the reagent is often added to the solution. The precipitate from the added ion is called a collector and carries the desired minor species out of solution. A collector is used to remove trace constituents from solution.
Separation by Electrolytic Precipitation
Electrolytic precipitation is a highly useful method for accomplishing separations. In this process, the more easily reduced species, either the wanted or the unwanted component of the sample, is isolated as a separate phase. The method becomes particularly effective when the potential of the working electrode is controlled at a predetermined level.
Salt-Induced Precipitation of Proteins
A common way to separate proteins is by adding a high concentration of salt. This procedure is termed salting out the protein. The solubility of protein molecules shows a complex dependence on pH, temperature, ionic strength, the nature of the protein, and the concentration of the salt used. At low salt concentrations, solubility is usually increased with increasing salt concentration. This salting in effect is explained by the Debye-Hückel theory. At high concentrations, protein solubility, S is given by the following empirical equation: log S 5 C 2 Km
where C is a constant that is a function of pH, temperature, and the protein; K is the salting out constant that is a function of the protein and the salt used; and m is the ionic strength.
Separation of Species by Distillation
Distillation is widely used to separate volatile analytes from nonvolatile interferents. Distillation is based on differences in the boiling points of the materials in a mixture. A common example is the separation of nitrogen analytes from many other species by converting the nitrogen to ammonia, which is then distilled from basic solution. Other examples include separating carbon as carbon dioxide and sulfur as sulfur dioxide. Distillation is widely used in organic chemistry to separate components in mixtures for purification purposes.
Types of Distillation:
Vacuum distillation is used for compounds that have very high boiling points. Lowering the pressure to the vapor pressure of the compound of interest causes boiling and is often more effective for high boilers than raising the temperature.
Molecular distillation occurs at very low pressure (,0.01 torr) such that the lowest possible temperature is used with the least damage to the distillate.
Pervaporation is a method for separating mixtures by partial volatilization through a nonporous membrane.
Flash evaporation is a process in which a liquid is heated and then sent through a reduced pressure chamber. The reduction in pressure causes partial vaporization of the liquid.
Separation by Extraction
The extent to which solutes, both inorganic and organic, distribute themselves between two immiscible liquids differs enormously, and these differences have been used for decades to separate chemical species. This section considers applications of the distribution phenomenon to analytical separations.
Extracting Inorganic Species
An extraction is often more attractive than a precipitation method for separating inorganic species. The processes of equilibration and separation of phases in a separatory funnel are less tedious and time consuming than conventional precipitation, filtration, and washing.
Separating Metal Ions as Chelates
Equilibria in the extraction of an aqueous cation M21 into an immiscible organic solvent containing 8-hydroxyquinoline.
Many organic chelating agents are weak acids that react with metal ions to give uncharged complexes that are highly soluble in organic solvents such as ethers, hydrocarbons, ketones, and chlorinated species (including chloroform and carbon tetrachloride).3 Most uncharged metal chelates, on the other hand, are nearly insoluble in water. Similarly, the chelating agents themselves are often quite soluble in organic solvents but of limited solubility in water.
Extracting Metal Chlorides and Nitrates
A number of inorganic species can be separated by extraction with suitable solvents. For example, a single ether extraction of a 6 M hydrochloric acid solution will cause better than 50% of several ions to be transferred to the organic phase, including iron(III), antimony(V), titanium(III), gold(III), molybdenum(VI), and tin(IV). Other ions, such as aluminum(III) and the divalent cations of cobalt, lead, manganese, and nickel, are not extracted. Uranium(VI) can be separated from such elements as lead and thorium by ether extraction of a solution that is 1.5 M in nitric acid and saturated with ammonium nitrate. Bismuth and iron(III) are also extracted to some extent from this medium.
Solid-Phase Extraction
Liquid-liquid extractions have several limitations. With extractions from aqueous solutions, the solvents that can be used must be immisicible with water and must not form emulsions. A second difficulty is that liquid-liquid extractions use relatively large volumes of solvent, which can cause a problem with waste disposal. Also, most extractions are performed manually, which makes them somewhat slow and tedious.
Solid-phase extraction, or liquid-solid extraction, can overcome several of these problems. Solid-phase extraction techniques use membranes or small disposable syringe-barrel columns or cartridges. A hydrophobic organic compound is coated or chemically bonded to powdered silica to form the solid extracting phase. The compounds can be nonpolar, moderately polar, or polar. For example, an octadecyl (C18) bonded silica (ODS) is a common packing. The functional groups bonded to the packing attract hydrophobic compounds in the sample by van der Waals interactions and extract them from the aqueous solution.
Chromatographic Separations
Chromatography is a technique in which the components of a mixture are separated based on differences in the rates at which they are carried through a fixed or stationary phase by a gaseous or liquid mobile phase.
Chromatography is a widely used method for the separation, identification, and determination of the chemical components in complex mixtures. No other separation method is as powerful and generally applicable as is chromatography.7 The remainder of this chapter is devoted to the general principles that apply to all types of chromatography. Chapters 32 through 34 deal with some of the applications of chromatography and related methods for analytical separations.
The stationary phase in chromatography is a phase that is fixed in place either in a column or on a planar surface.
The mobile phase in chromatography is a phase that moves over or through the stationary phase carrying with it the analyte mixture. The mobile phase may be a gas, a liquid, or a supercritical fluid.
Planar and column chromatography are based on the same types of equilibria.
Mikhail Tsvet (1872-1919)
The Russian botanist Mikhail Tswett (1872–1919) invented chromatography shortly after the turn of the twentieth century. He used the technique to separate various plant pigments, such as chlorophylls and xanthophylls, by passing solutions of these species through glass columns packed with finely divided calcium carbonate. The separated species appeared as colored bands on the column, which accounts for the name he chose for the method (Greek chroma meaning “color” and graphein meaning “to write”).
(Skoog, 2013)
(https://www.chemicool.com/definition/Analytical Separations.html, 2017)
Fundamentals of Analytical Chemistry 