Reciprocal salt-pairs

Figure 4 Janecke diagram for the reciprocal salt pair KCl-NH. Oj in water. Calculated two-salt saturation lines and experimental two-salt saturation points...

Components Two salts without a common ion (so-called reciprocal salt pairs such as sodium chloride and ammonium bicarbonate) and water in addition This particular system is that occurring m the ammonia soda or Solvay process of formation of sodium carbonate Possible Phases several solids—solution—vapour... 

Since one pair of salts—NaCl + NH4NOJ3—is formed from the other pair—NH4CI + NaNOg—by double decomposition, the two pairs of salts are known as reciprocal salt-pairs It is with systems in which the component salts form reciprocal salt>pairs that we have now to deal. 

Since, then, two reciprocal salt-pairs constitute only three components or independently variable constituents, another component is necessary in order to obtain a four-component system. As such, we shall choose water. 

Transition Point.—In the case of the formation of double salts from two single salts, we saw that there was a point—the quintuple point—at which five phases could coexist. This point we also saw to be a transition point, on one side of which the double salt, on the other side the two single salts in contact with solution, were found to be the stable system. A similar behaviour is found in the case of reciprocal sal -pairs. The four-component system, two reciprocal salt-pairs and water, can give rise to an invariant system in which the six phases, four salts, solution, vapour, can coexist the temperature at which this is possible constitutes a sextuple point. This sextuple point is also a transition point, on the one side of which the one salt-pair, on the other side the reciprocal salt-pair, is stable in contact with solution. 

The sextuple point is the point of intersection of the curves of six univariant systems, viz. four solubility curves with three solid phases each, a vapour-pressure curve for the system two reciprocal salt-pairs — vapour and a transition curve for the condensed system two reciprocal salt-pairs—solution. If we omit the vapour phase and work under atmospheric pressure (in open vessels), we find that the transition point is the point of intersection of four solubility curves. 

See especially Meyerhoflfer, Sitzungsber. Wien. Akad, 1895,104, II. b, 840 MeyerhofFer and Saunders, Z. physikal. Chem. 1899, 28, 453 31, 370 Uyeda, Mem. CoU. Set. Eng. Kyoto 1902,2, 245. The investigation of the equilibria between reciprocal salt-pairs alone (three-component systems) is of great importance for the artincial preparation of minerals, as also in analytical chemistry for the proper understanding of the methods of conversion of insoluble systems into soluble by fusion (see MeyerhofFer, Z. physikal, Chem., 1901,38, 307 J necke, ibid. 1908,64, 305 343 1912, 80, I 1913, 82, I). 

Just as in the case of three-component systems we saw that the presence of one of the single salts along with the double salt was necessary in order to give a univariant system, so in the four-component systems the presence of a third salt is necessary as solid phase along with one of the salt-pairs. In the case of the reciprocal salt-pairs mentioned above, the transition point would be the point of intersection of the solubility curves of the systems with the following groups of salts as solid phases. Below the transition point ... 

Transition Interval.—double salt, we learned (p. 242), when brought in contact with water at the transition point undergoes partial decomposition with separation of one of the constituent salts and only after a certain range of temperature (transition interval) has been passed, can a pure saturated solution of the double salt be obtained. A similar behaviour is also found in the case of reciprocal salt-pairs. In the case of each salt-pair there will be a certain range of temperature, called the transition interval, within which, if excess of the salt-pair is brought into contact with water, interaction will occur and one of the salts of the reciprocal salt-pair will be deposited. For the salt-pair which is stable below the transition point, the transition interval will extend down to a certain temperature below the transition point and for the salt-pair which is stable above the transition point, the transition interval will extend up to a certain temperature above the transition point. Only when the temperature is below the lower limit or above the upper limit of the transition interval, will it be possible to prepare a solution saturated only for the one salt-pair. In the case of ammonium chloride and sodium nitrate the lower limit of the transition interval is 5 5 , so that above this temperature and up to that of the transition point (unknown), ammonium chloride and sodium nitrate in contact with water will give rise to a third salt by double decomposition, in this case to sodium chloride. ... 

Graphic Representation.—For the graphic representation of the isothermal equilibria in systems formed by two reciprocal salt-pairs and water, two methods are employed, one due to Lowenherz and the other due to J necke. In these systems, it may be recalled, an isothermal can be represented completely only by a three-dimensional model. 

Compare the reciprocal salt-pair NaCl—NH4HCO8 (p. 290). In this case the upper limit of the transition interval was found by extrapolation of the solubility curve for NaHCOg + NH4CI + NH4HCO3 and NaHCOj + NH4Q -f- NaCl to be 32 (Fedotieff, Z, physikal, Ckem. 1904, 49, 179). 

Conversion Saltpetre.—To illustrate the use of Janecke s method of representation we may consider briefly the equilibria formed by the reciprocal salt-pairs (Na,K)—(NOgjCl) and water, which are of importance for the manufacture of conversion saltpetre, and which have been studied by Uyeda and by Reinders/ In this system no compounds or salt hydrates are formed. 

We are dealing here, therefore, with reciprocal salt-pairs, the behaviour of which has just been discussed in the preceding pages. Since the study of the reaction is rendered more difficult on account of the fact that ammonium bicarbonate in solution, when under atmospheric pressure, undergoes decomposition at temperatures above 15 , this temperature was the one chosen for the detailed investigation of the conditions of equilibrium. Since, further, it has been shown by Bodlander that the bicarbonates possess a definite solubility only when the pressure of carbon dioxide in the solution has a definite value, the measurements were carried out in solutions saturated with this gas. This, however, does not constitute another component, because we have made the restriction that the sum of the partial pressures of carbon dioxide and water vapour is equal to i atmosphere. The concentration of the carbon dioxide is, therefore, not independently variable (p. 7). 

The equilibrium conditions for the two salts with a common ion, KCl and MgCl2, have already been discussed (p. 249), and we shall consider briefly here the conditions for equilibrium in the systems formed by the reciprocal salt-pairs (K, Mg) — (Cl, SO4). Besides the four single salts there exist the double salts carnallite (KCl. MgCl2. 6H2O) and schoenite (K2SO4, MgS04, bHgO). 

Figure 12. Calculated phase diagrams of (a) the Ti-Mo-C-N [64] system for 1600°C with experimental data of [67] and of (b) the Ti-W- -N [65] system for 1423°C with experimental data of [68]. Both for 50at-% nonmetal in the representation of a reciprocal salt pair.

For the expression of crystallization kinetics there is some theoretical justification for recording compositions on a molar basis, e.g. as kmolm (i.e. molL ), while mole fractions are most frequently used for thermodynamic calculations. Mass fractions are commonly used in the eonstruction of phase diagrams, although the use of mole fractions is recommended for the representation of reciprocal salt pair systems (section 4.7.2). 

The four salts in each of the above systems form what is known as a reciprocal salt pair . Although all four may be present in aqueous solution, the composition of any mixture can be expressed in terms of three salts and water. Thus, from the phase rule point of view, an aqueous reciprocal salt pair system is considered to be a four-component system. 

Reciprocal salt pair solutions may be represented on an isothermal space model, in the form of either a square-based pyramid or a square prism. Figure 4.30a indicates the pyramidal model the four equilateral triangular faces stand for the four ternary systems AX-AY-W, AY-BY-W, BY-BX-W and AX-BX-W (W = water) for the salt pair represented by the equation... 

The square-prism space model Figure 4.30b) illustrates another way in which a quaternary system of the reciprocal salt pair type may be represented. The vertical axis stands for the water content, and the points on the diagram are the same as those marked on Figure 4.30a. In both diagrams all surfaces formed between the internal curves represent solutions of three salts in water saturated with one salt, all internal curves solutions of three salts in water saturated with two salts, and the two points P and Q solutions of three salts in water saturated with the three salts. The section above the internal curved surfaces denotes unsaturated solutions, the section below them mixtures of liquid and solid. 

In order to simplify the interpretation of the phase equilibria in reciprocal salt pair systems, the water content may be excluded. The curves of the space model can then be projected on to the square base to give a two-dimensional graph, called a Janecke diagram as described in section 4.7.1. A typical projection is shown in Figure 4.31a the lettering is that used in Figure 4.30. The enclosed areas, which represent saturation surfaces, indicate solutions in equilibrium with one salt, the curves solutions in equilibrium with two salts, points P and Q solutions in equilibrium with three salts. 

Molar or ionic bases must be used in this type of diagram for reciprocal salt pairs. The four corners of the square represent 100 mol of the pure salts AX, BX, BY and AY. Any point inside the square denotes 100mol of a mixture of these salts its composition can always be expressed in terms of three salts. The... 

Figure 4.31. Interpretation of the Jdnecke diagram for reciprocal salt pairs a) projection of the surfaces of saturation on to the base b) method of plotting...

Figure 4.32. Jdnecke projections for aqueous solutions of a reciprocal salt pair, showing a) two congruent points, fb) congruent and incongruent points...

Figure 4.32 shows Janecke diagrams for solutions of a given reciprocal salt pair at different temperatures. These two simple cases will be used to demonstrate some of the phase reactions that can be encountered in such systems. Both diagrams are divided by the saturation curves into four areas which are actually the projections of the surfaces of saturation (e.g., see Figure 4.32b). Salts AX and BY can coexist in solution in stable equilibrium the solutions are given by points along curve PQ. Salts BX and however, cannot coexist in solution because their saturation surfaces are separated from each other by curve PQ. Thus AX and BY are called the stable salt pair, or the compatible salts, BX and A Y the unstable salt pair, or the incompatible salts. In Figure 4.32a the AX-BY diagonal cuts curve PQ which joins the two quarternary invariant points, while in Figure 4.32b curve P Qj is not cut by either diagonal. These are two different cases to consider.

The phase reactions occurring on the removal of water from a reciprocal salt pair system will first be described with reference to Figure 4.32a. Point a which lies on the BY saturation surface represents a solution saturated with salt BY. When water is removed isothermally from this solution, the pure salt 5T is... 

So far in the discussion of Janeeke projections for reciprocal salt pair systems the water content has been ignored. This is not too serious, because much information can be obtained from the projection before consideration of the quantity of water present. One way in which the water content can be represented is shown in Figure 4.33a the plan shows the projection of the saturation surfaces, the elevation indicates the water contents. To avoid unnecessary complication, the elevation only shows the horizontal view of the particular saturation curve concerned in the problem. 

The four salts AX, BY, AY and BX constitute a reciprocal salt pair. One of these pairs, AX, BY or AY, BX is a stable pair (compatible salts), which can coexist in solution, and the other an unstable salt pair (incompatible salts) which cannot (section 4.7.2). 

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