Boiling Mixtures

In some cases, liquids with different boiling points cannot be separated in this way, owing to the formation of constant boiling mixtures. Such mixtures cannot, therefore, be separated by distillation. The excess of each constituent beyond the constant boiling proportion would, of course, pass over, until the composition reached that of the constant boiling mixture, which has either a maximum or minimum boiling point compared with any other mixture of the substances. 

The tables given below, which are taken from A Laboratory Course of Organic Chemistry (A. W. Titherley), show the boiling point and composition of some such constant boiling mixtures, consisting of two substances, A and B. 

The boiling points are given for 760 mms. pressure. In Type I. the boiling point of the constant boiling mixture is below that of either constituent, while in Type II. the boiling point of the constant boiling mixture is above that of either constituent. 

References.—For theoretical considerations of this subject the student is referred to — 

Reilly, Distillation. Robinson, Elements of Fractional Distillation. Young, Stoichiometry.  

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The convergence rate depends somewhat on the problem and on the initial estimates used. For mixtures that are not extremely wide-boiling, convergence is usually accomplished in three or four iterations,t even in the presence of relatively strong liquid-phase nonidealities. For example, cases 1 through 4 in Table 1 are typical of relatively close-boiling mixtures the latter three exhibit significant liquid-phase nonidealities. 

Calculations for wide-boiling mixtures are a little more difficult to converge, especially for mixtures having very light or noncondensable components together with relatively nonvolatile components and lacking components of intermediate volatility. 

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

Figure A2.5.5. Phase diagrams for two-eomponent systems with deviations from ideal behaviour (temperature T versus mole fraetion v at eonstant pressure). Liquid-gas phase diagrams with maximum (a) and minimum (b) boiling mixtures (azeotropes), (e) Liquid-liquid phase separation, with a eoexistenee eurve and a eritieal point.

The acid which comes over is a constant boiling mixture containing about 47 hydrogen bromide (density = 1.46 g cm... 

Properties—Hydrogen iodide is a colourless gas. It is very soluble in water and fumes in moist air (cf. hydrogen chloride), to give hydriodic acid. Its solution forms a constant boiling mixture (cf. hydrochloric and hydrobromic acids). Because it attacks mercury so readily, hydrogen iodide is difficult to study as a gas, but the dissociation equilibrium has been investigated. 

As a guide to the probable occurrence of a constant-boiling mixture, it should be noted that such mixtures most frequently occur when one of the components contains an hydroxyl (— OH) group. Only aqueous and alcoholic mixtures therefore are likely to have a constant boiling-point. 

Liquids are occasionally purified by removing impurities as constant-boiling mixtures, or by shaking with concentrated sulphuric acid and subsequently separating the dried liquid from the acid the second method is therefore limited to liquids which are insoluble in, and chemically unaffected by, the strong acid e.g., benzene, anhydrous chloral). 

Place 56 g. of finely-powdered, anhydrous sodium sulphide ( fused sodium sulphide) and 100 ml. of rectified spirit in a 500 ml. round-bottomed flask equipped with a reflux condenser. To the boiling mixture... 

Table 5.11 Binary Azeotropic (Constant-Boiling) Mixtures 5.58...

Each olefin is more soluble than the paraffin of the same chain length, but the solubiHty of both species declines as chain length increases. Thus, in a broa d-boiling mixture, solubiHties of paraffins and olefins overlap and separation becomes impossible. In contrast, the relative adsorption of olefins and paraffins from the Hquid phase on the adsorbent used commercially for this operation is shown in Figure 2. Not only is there selectivity between an olefin and paraffin of the same chain length, but also chain length has Httie effect on selectivity. Consequentiy, the complete separation of olefins from paraffins becomes possible. 

Hexane refers to the straight-chain hydrocarbon, C H branched hydrocarbons of the same formula are isohexanes. Hexanes include the branched compounds, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, and the straight-chain compound, / -hexane. Commercial hexane is a narrow-boiling mixture of these compounds with methylcyclopentane, cyclohexane, and benzene (qv) minor amounts of and hydrocarbons also may be present. Hydrocarbons in commercial hexane are found chiefly in straight-mn gasoline which is produced from cmde oil and natural gas Hquids (see Gasoline AND OTHER MOTOR fuels Gas,natural). Smaller volumes occur in certain petroleum refinery streams. 

Hydrogen chloride and water form constant boiling mixtures. The properties of these mixtures, determined with great accuracy, and often used as analytical standards (16), are summarized in Table 6 and graphically depicted in Figure 3. 

Fig. 1. Indirect hydration process for the manufacture of isopropyl alcohol CBM = constant boiling mixture (61,62).

The analytical chemistry of titanium has been reviewed (179—181). Titanium ores can be dissolved by fusion with potassium pyrosulfate, followed by dissolution of the cooled melt in dilute sulfuric acid. For some ores, even if all of the titanium is dissolved, a small amount of residue may still remain. If a hiU analysis is required, the residue may be treated by moistening with sulfuric and hydrofluoric acids and evaporating, to remove siUca, and then fused in a sodium carbonate—borate mixture. Alternatively, fusion in sodium carbonate—borate mixture can be used for ores and a boiling mixture of concentrated sulfuric acid and ammonium sulfate for titanium dioxide pigments. For trace-element deterrninations, the preferred method is dissolution in a mixture of hydrofluoric and hydrochloric acids. 

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