Equilibria in Solutions Containing Mixtures of Salts. I—The System Water and the Sulfates and Chlorides of Sodium and Potassium
Journal of Industrial & Engineering Chemistry
If this is true then t h e increase in yield due t o activated sludge over ordinary barnyard manure is very striking and a t once places a high monetary value on this material, for the value of fertilizers must be directly proportional to the crop returns yielded by them. The various processes suggested for increasing the production of potassium-containing compounds involve the separation of t h a t element from associated salts b y fractional crystallization. The only satisfactory method of
... actory method of obtaining a clear understanding of the possibilities of making such separations is a study of the phase-rule diagrams representing the equilibria which exist in aqueous solutions between the salts t o be separated. Unfortunately much of the data necessary for the preparation of such diagrams is lacking, and the present paper represents the first of a series which have been planned by members of the Department of Chemistry of the University of California for the purpose of supplying what is believed t o be much-needed information. In carrying out t h e work here reported assistance in the large amount of analytical work involved has been rendered by students of the Department, and special acknowledgment should be made t o Messrs. The system designated in the sub-title consists of four components, namely, water and any three of the four salts concerned, which constitute a "reciprocal salt pair." I n addition t o t h e four simple anhydrous salts the only solid phases t o be considered are ice, the decahydrate of sodium sulfate, which will be called Glauber's salt, and the double sulfate of sodium and potassium generally known as glaserite. This name was first used by Penny t o represent a compound corresponding to the formula K3Na(S04)2, but double sulfates containing somewhat different proportions of the constituent salts were subsequently reported by other investigators and different names applied t o them. Van't Hoff2 was able t o show that it was possible t o prepare a series of solid solutions in which the percentages of potassium sulfate varied between 78.6 and 61.8, which justifies treating all of these compounds as a single solid phase. It was also shown by van't Hoff and Reichera that it was possible t o prepare a solution saturated with 1 This work has been supported by the Council of Defense of the State of California. 2 "Unters. U. Bildung der ozeanischen Salzablagerung," p. 22G. a Z. fihysik. Chem., 3 (1889), 482. respect t o glaserite, Glauber's salt, potassium chloride and sodium chloride at a temperature of 3.7", and t h a t therefore this temperature represents the "transition temperature" for the equilibrium 3KC1 + 2NazS04.1oH~O KsNa(SO& + 3NaC1 + 20 HzO, in which all t h e formulae, except that of water, represent solid phases. A consequence of this fact is t h a t one of the two pairs of salts (in this case potassium chlorideand Glauber's salt) can exist assolidphasesinequilibrium with solutions of the four salts at temperatures below 3.7' only, whereas the other pair, i. e., glaserite and sodium chloride, can exist as solid phases in equilibrium with such solutions only above this temperature. If the temperature of the system is limited t o the interval between 0" and IOO", which represents the limits of practical importance, so that ice is eliminated, nine different univariant systems in which three solid phases are present are theoretically possible. T h e univariant systems which can be actually realized experimentally were first studied by Meyerhoff er and Saunders,' who fixed the transition temperature discovered by van't Hoff a t 4.4' instead of 3.7",and worked out the phase-rule diagrams for the system a t temperatures of o", 4.4", 16" and 2 5 " . In taking up the work at this point it was thought desirable t o repeat the determinations upon which the diagrams for o o and 2 5 " were based, and t o obtain data necessary for the preparation of similar diagrams a t temperatures of soo, 75' and 1 0 0 ' . EXPERIMENTAL METHODS USED Saturation of solutions with respect t o the different salts was effected by stirring in an apparatus similar t o t h a t used by Meyerhoffer until its composition remained constant, which required from one t o 4 days. The tubes for the determinations made at 0" were kept in a large thermostat capable of holding sufficient ice t o last for 5 days. For saturation a t the four other temperatures the necessary heat was supplied to the thermostats b y electric lamps immersed in the water or oil of the bath; a large mercury regulator kept the temperature of the 2 5 ' and the 50' baths constant t o within 0 . 2 " , and of the 7 5 O and 100" baths t o within 1.0~. The composition of the saturated solution was ascertained by removing portions of i t in a weight pipette previously heated t o the temperature of the bath, weighing and analyzing the solution. The chlorine ion was determined by titrating a fractional part of the solution with a standard solution of silver nitrate, the SO1 ion by precipitating and weighing as BaS04, and the potassium ion by separating and weighing as As it seemed probable that the control of any processes based on these diagrams could be most easily effected by means of a hydrometer the specific gravities of most of the solutions were also determined. T h e method consisted in removing and weighing, by means of a pipette which had been drawn out t o a capillary a t t h e mark on its stem,a definite volume of the solution, K2P t Cle. 12. physik. Chem. 28 (18991, 453.