THE COMPOSITION OF SOLID PHASES I N FOURCOMPONENT SYSTQMSl -~ BY JAMES M.
BELL
A method for the determination of the composition of solid phases which cannot be completely separated from the mother-liquor by washing with water on account of the instability of the solid phase on the addition of pure water has been proposed by Schreinemakers2 and later by Bancroft. This method consists in the use of a triangular diagram, on which are plotted the composition of the mother-liquor and that of the solid with some of the adhering mother-liquor. The line joining the two points passes through the point representing the composition of the solid phase. By determining several such lines, the intersection gives the composition of the solid phase. In this way an indirect analysis has been accomplished of solid phases without separation from the solution. The method has hitherto been employed only for three-component systems, where the solid phase which is stable along any boundary curve was to be analyzed. In the case of fourcomponent systems, one solid phase exists over a field and so long as only one solid is present, a modification in the above triangular diagram method may be employed to determine the composition of the solid phase. The present method involves the use of two such diagrams, Analyses are made as before for all four components in the mother-liquor and in the solid with adhering mother-liquor, at two different points taken'.on the same field. The results are then plotted on the first of the diagrams, in which the distances from the sides of the triangle represent the percentage of A, of B, and of the sum of C and D, respectively. The point of intersection of two lines gives the percentage of A, of B, and of the sum of C and D in the solid phase. The 1
Published by permission of t h e Secretary of Agriculture. Zeit. phys. Chem., IT, 81 (1893). Jour. Phys. Chem., 6, 178 (1902).
SoZid Phases iiz Four-Conlrponeizt Systems
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same results are then plotted in another diagram in which the coordinates are the percentages of C, of D, and of the sum of A and B, respectively. From the two diagrams the percentage composition of all four components may be determined graphically . This method has been applied in the following paper on the solubility of some alkali salts, to determine the composition of a double compound which was encountered. The first of the two figures on page 418 shows that there was no sodium chloride in the compound and also that there was 61 percent sodium sulphate. Owing t o the very acute angle at which the lines happen to meet, in the second figure, the present instance does not afford a good example of the application of this method. However, assuming that the lines meet as they are drawn-and it is shown by a different method that they do-the second figure shows that neither sodium chloride nor water is present in the solid phase, and that the solid phase consisted of 39 percent calcium sulphate and 61 percent sodium sulphate, or 2CaS0,.3Na2SO,. Of course, if the combination of components should happen to be chosen originally as in the second diagram, it would be unnecessary to draw another diagram. In other words, if the solid consists of only two of the components, and if the proper combination should happen to be chosen, a second diagram becomes unnecessary. This method is subject to the same limitation as in the case of a single diagram. If two lines cross at a very small angle, a very slight error in analysis causes a very great error in the determination of the intersection of the lines. Bureau of Soils, U. S. Department of Agm’culhre, Washington, D. C.
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