Separation of mixtures

Distillation of a solution consisting of salt dissolved in water, (a) When the solution is boiled, steam is driven off. If this steam is collected and cooled, it condenses to form pure water, which drips into the collection flask, (b) After all of the water has been boiled off, the salt remains in the original flask and the water is in the collection flask. 

No chemical change occurs when salt water Is distilled. 

Separation of a solid from a liquid by using filter paper 

Filtration separates a liquid from a solid. The liquid passes through the filter paper, but the solid particles are trapped. 

We have seen that all matter exists as elements or can be broken down into elements, the most fundamental substances we have encountered up to this point. We will have more to say about the nature of elements in the next chapter. 

When we carry out the distillation of salt water, water is changed from the liquid state to the gaseous state and then back to the liquid state. These 

When water from the Great Salt Lake evaporates (changes to a gas and escapes), the salt is left behind. This is one commercial source of salt. 

Solubility differences of this type allow the separation of some quite complex mixtures to be carried out easily and quickly in the laboratory. All that is needed is a pair of immiscible solvents, a separating funnel and an understanding of the effects of pH on the solubility of drugs. An example of this type of separation is shown below. 

8 Describe how distillation and filtration rely on physical changes and properties to separate components of mixtures. 

If we collected natural water with some sediment in the sample, our first step would be to separate the sediment from the liquid by filtration (Fig. 2.14). A porous 

Iron and sulfur can be separated by stirring with a magnet. The first time the magnet is removed, much of the iron is removed with it. The sulfur still looks dirty because a small quantity of iron remains. 

Repeated stirrings eventually leave a bright yellow sample of sulfur that cannot be purified further by this technique. 

The filter paper is constructed so that when it is folded in half and then folded again, it opens into a conical shape that fits a filtration funnel. A piece is often torn from the corner to help seal the paper to the funnel, (b) The paper is placed into a filtration funnel, (c) The funnel is placed in a ring on a ring stand. The mixture is poured along a stirring rod, and the filter paper traps the solids as the liquid solution passes into a beaker. 

Although we say we can separate mixtures into pure substances, it is virtually impossible to separate mixtures into totally pure substances. No matter how hard we try, some impurities (components of the original mixture) remain in each of the pure substances.  

Mixtures can be separated into pure substances elements and/or compounds. 

For example, the mixture known as air can be separated into oxygen (element), nitrogen (element), water (compound), carbon dioxide (compound), argon (element), and other pure substances. 

When table salt is stirred into water (left), a homogeneous mixture called a solution forms (right). 

Sand and water do not mix to form a uniform mixture. After the mixture is stirred, the sand settles back to the bottom. 

As we previously learned, mixtures are not pure substances. In order to obtain one of the components in a mixture, we need to separate them. The separation of mixtures can only be possible when we use the physical properties of substances. 

For different types of mixtures, different methods are needed. Now let s see some of these methods used to separate mixtures. 

Water is condensed, and then collected in the receiving flask 

A mixture of a solid and a liquid is poured through filter paper. The liquid passes through the paper while the solid remains on the paper. 

Compounds a, b, and c are adsorbed to different degrees on the solid stationary phase 

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Separation of mixtures of condensable and non-condensable components. If a fluid mixture contains both condensable and noncondensable components, then a partial condensation followed by a simple phase separator often can give a food separation. This is essentially a single-stage distillation operation. It is a special case that deserves attention in some detail later. 

In general, the longer a chromatographic column, the better will be the separation of mixture components. In modem gas chromatography, columns are usually made from quartz and tend to be very long (coiled), often 10-50 m, and narrow (0.1-1.0 mm, internal diameter) — hence their common name of capillary columns. The stationary phase is coated very thinly on the whole length of the inside wall of the capillary column. Typically, the mobile gas phase flows over the stationary phase in the column at a rate of about 1-2 ml/min. 

As a rule of thumb, one can say that the efficiency of separation of mixtures and the simplicity of operating and maintaining apparatus are much greater for GC than for LC. Hence, other things being equal, GC is most often the technique of first choice and can be used with a very wide variety of compound types. However, for nonvolatile or thermally labile substances like peptides, proteins, nucleotides, sugars, carbohydrates, and many organometallics, GC may be ruled out completely... 

Prior separation of mixtures into individual components may not be needed. If the mass spectrometer is capable of MS/MS operation, one of the mass spectrometers is used to isolate individual ions according to m/z value (mass-to-charge ratio), and the other is used to examine their fragmentation products to obtain structural information. 

Solubility Properties. Fats and oils are characterized by virtually complete lack of miscibility with water. However, they are miscible in all proportions with many nonpolar organic solvents. Tme solubiHty depends on the thermal properties of the solute and solvent and the relative attractive forces between like and unlike molecules. Ideal solubiHties can be calculated from thermal properties. Most real solutions of fats and oils in organic solvents show positive deviation from ideaHty, particularly at higher concentrations. Determination of solubiHties of components of fat and oil mixtures is critical when designing separations of mixtures by fractional crystallization. 

Liquid pyridine and alkylpyridines are considered to be dipolar, aprotic solvents, similar to dimethylformarnide or dimethyl sulfoxide. Most pyridines form a significant azeotrope with water, allowing separation of mixtures of pyridines by steam distillation that could not be separated by simple distillation alone. The same azeotropic effect with water also allows rapid drying of wet pyridines by distillation of a small forecut of water azeotrope. 

More sophisticated GC columns and techniques perform more detailed separations of mixtures of hydrocarbons into discrete groups. Table 12-3... 

The separation of mixtures of immiscible liquids constitutes one of the important chemical engineering operations. This empirical design has proven satisfactory for many phase separations. 

Chromatography A method of analysis using separation of mixtures based on selective adsorption. 

FIGURE 4.7 Separation of mixture of proteins and peptides on a TSK-GEL G2000SWxl column using Wyatt/Optilab DSP and miniDawn detectors. 

The separation of mixtures involving N-methyl-JLtetrahydropyridines into their pure components by means of gas-liquid chromatography was discussed in a report by Holik et al. (87). They found that, using tris(/3-cyanoethoxymethyl)-y-picoline as the stationary phase, the primary factors involved in the specific retention volumes of these enamines is the electronic effect of a methyl substituent and the nitrogen atom on the carbon-carbon double bond. It was observed that 1,3-dimethyl-Zl -tetrahydropyridine (141) has a smaller specific retention volume and, hence, is eluted before... 

Figure 15.4 Separation of mixtures of beta-blockers by using micellar HPLC, employing the following mobile phases (a) 0.12M SDS, 5% propanol, 0.5% tiiethylamine (b) 0.06 M SDS, 15% propanol (c) 0.1 IM SDS, 8% propanol. Adapted from Journal of Chromatographic Science, 37, S. Carda-Broch et al., Analysis of urine samples containing cardiovascular drugs by micellor liquid chromatography with fluorimetric detection , pp. 93-102, 1999, with permission from Preston Publications, a division of Preston Industries, Inc.

The use of heart-cuts to analyze a sample of 90% evaporated gasoline is illustrated in Figure 15.9. Each of the lettered chromatograms illustrates the further separation of mixture components which is not possible on a single column. The initial... 

Click Chemistry Interactive for a self-study module separation of mixtures. 

Chromatography is a separation process employed for the separation of mixtures of substances. It is widely used for the identification of the components of mixtures, but as explained in Chapters 8 and 9, it is often possible to use the procedure to make quantitative determinations, particularly when using Gas Chromatography (GC) and High Performance Liquid Chromatography (HPLC). 

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

H- and 3//-Azepines are generally unstable in aqueous acid solution and the few examples of simple azepinium salts, namely perchlorates,77 bromides,105 picrates35201 and a solitary iodide,105 have been prepared under nonaqueous conditions. The fractional crystallization of oxalate salts has been used for the separation of mixtures of 4- and 6-substituted 3f/-azepines,66 and 3,6-di-tm-butyl- and 2,5-di-tert-butyl-3//-azepine, on treatment with tetrafluoroboric acid in acetonitrile, are converted quantitatively into their crystalline tetrafluoroboratc salts.70... 

The separation of mixtures of these structural isomers has proven to be possible.96,104 ... 

Finally an example is included of the use of a Cl 8 reverse phase column. The packing is also a silica based but is contained in a short column 3.3 cm long, 4.6 mm in diameter and packed with particles 3 Jim in diameter. The example of its use is in the separation of mixture of growth regulators which is shown in figure 9. 

Ion chromatography can be used in unique ways and by appropriate modification can often be applied to the separation of mixtures where the components themselves do not ionize or do not normally produce interactive ions in aqueous solution. A good example of this type of separation is afforded by the analysis of saccharide mixtures using ion exchange interactions. An illustration of such a separation is given in figure 15. 

Separation by adsorption chromatography takes place preferentially as a result of hydrogen bonding or dipole-dipole interactions. Hence, separation of mixtures of substances on silica gel layers by lipophilic solvents primarily takes place according to polarity differences. Further separation within a polarity group can then be achieved either two-dimensionally or off-line by partition chromatography on anotho TLC plate (Fig. 4). 

Figure 6.2.2 shows the separations of mixtures of standards on a monomeric C,g column and also on a polymeric C30 column. The elution order on the monomeric C18 column is, as expected, first the dihydroxy xanthophyUs (lutein and zeaxanthin), followed by the monohydroxy compounds (rubixanthin and P-cryptoxanthin), and finally by the carotenes (y-, a-, and p-carotene). However, on the C30 column, rubixanthin and y-carotene, both with 1 acyclic /-end group, eluted after a- and P-carotene, with two cyclic end groups. 

HPLC has proved to be fast and sensitive for the analyses of phenolic plant constit-nents, and is especially useful for the analysis of anthocyanins. The first application of HPLC to anthocyanin analyses was in 1975 by Manley and Shubiak and it has now become the method of choice for the separation of mixtures of anthocyanins and anthocyanidins. HPLC is now used for anthocyanin qualitative, quantitative, and preparative work, offering improved resolution compared to chromatographic procedures previously employed. It also allows for simultaneous rapid monitoring of the eluting anthocyanins. ... 

The same principle is used for the preparative separation of mixtures of biological materials, the extraction of different individual components from these mixtures, and their purification. In this case one uses an electrophoretic method with continued introduction of individual portions of the mixture and withdrawal of portions of pure fractions. There have been reports that such processes were accomplished in spacecraft where, since gravitational forces are absent, the liquid solutions can be used without the danger of natural convection. 

Examples of separation of mixtures in the fine chemicals industry... 

The most important current problem of planar chromatography is the elaboration of theoretical and experimental methods for predicting the conditions of mixture separation in order to achieve better results. Planar chromatography is an analytical chemistry technique for the separation of mixtures that involves passing of solutes in the mobile phase through the stationary phase. Usually, each component has a... 

One of the most important problems of planar chromatography is that of the optimization of solvent systems for the separation of mixtures of different samples. An analyst is interested in obtaining the expected result using a minimum number of experiments. Snyder has introduced a new system for solvent classification that permits a logical selection of solvents both in term of polarity indices (F ) and selectivity parameters (Xj), proving theoretically the validity of such universal solvent systems [18,38,41,42]. 

The window diagram method can also be used to optimize the separation of mixtures when the number and identity of the components are unknown [421-423]. Two liquid phases, A and S, of different selectivity are chosen. Trial chromatograms are run on... 

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