IDENTIFICATION OF SOLID PHASES _ I -
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BY I,. F. HAWLEY
I. Lead hydroxide and lead earbonate Salvadori' has shown that a salt of the composition, Pb0,H,.2PbC03, is formed when lead carbonate is boiled with water or when lead sulphate or chloride is boiled with aqueous sodium carbonate. These experiments seem to point to white lead being a definite compound having the formula PbO,H,. 2PbC0,. It is possible however that there might be a series of solid solutions of which the apparent compound was merely the last term. This possibility was the more to be considered because the obtaining of a definite composition does not seem to be altcgether easy when white lead is made electrolytically, It seemed desirable therefore to check the composition of white lead by another method, the solubility method. Neither lead carbonate nor lead hydroxide is soluble enough in water to make the usual form of the method applicable but the solubility in sodium acetate solutions is sufficient. If the ratio of water to sodium acetate be kept constant, the sodium acetate solution may be considered as a new solvent and we can treat the system as a three-component one. Lead carbonate and lead oxide were taken as the constituents to be added to the sodium acetate solution. Pare lead hydroxide is difficult to prepare and the oxide was used instead after it had been found that lead oxide hydrates readily when in contact with lead carbonate. Mixtures of lead oxide and carbonate in varying proportions were left in contact with 2 0 percent sodium acetate solutions a t 75' for from twelve to fourteen hours. The solutions were then allowed to stand at ordinary temperature for several hours, after which 50 cc portions were pipetted off and the lead in solution determined as sulphate. The undissolved solids were washed in water Gazz. chim. Ital., 24, I, 87 (1904;. Isenburg: Zeit. Elektrochemie, 9, 275 (1903). Miller and Kenrick: Jour. Phys. Chem., 7 , 2 5 9 (1903).
Jc?entz$catzon of Solid Phases
655
free from carbon dioxide, dried in a desiccator over potassium .hydroxide and analyzed for lead oxide, water and carbon dioxide. The results obtained are shown in Fig. I , the ordinates being the amount of lead in solution and the abscissas the molecular percentages of PbO and PbCO, in the solid phase or phases. '0
40
20
f
Fig. I
This diagram shows that there are no solid solutions and that white lead is a definite compound containing two of lead carbonate to one of lead oxide. The water determination shows that the formula is Pb0,H,.2PbC03. An interesting check on this was obtained in the case of mixtures rich in lead oxide. The amount of oxide changed to hydroxide is exactly equivalent to one-half the lead carbonate present. When mixtures of lead oxide and lead carbonate are treated with water a t ordinary temperatures, the possible solid phases are PbO, PbOZH,.2PbCO,, PbCO,. 11. Thallium sulphide and stannie sulphide When thallium sulphide and stannic sulphide are precipitated together, a solid is obtained which is only partially soluble in either dilute acids or alkaline sulphides and which varies in color from an orange-yellow when rich in stannic sulphide through a bright red to a very dark red when rich in
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L. F. H a w b y
thallium sulphide. Loczkal in I 898 mentioned this peculiar precipitate and promised to tell more about it ; but nothing has appeared since, so we ventured to take up the problem. This could have been worked out by the same method as the preceding, using a solution of an alkaline sulphide as solvent, with thallium sulphide and stannic sulphide as the other two components, and determining the dissolved tin for varying compositions of the precipitate. On account of the striking differences of color it was thought that the point at which there was a change in the solid phase could be determined more readily by the microscope. This method was tried with the following results : over the range of concentrations from pure T1,S to a point corresponding to the composition Tl,SnS,, there were distinguishable two phases, a black and a red, both with a i e r dency toward crystalline structure and both opaque ; at the point 2T1,S.SnS2 only one phase was present, the red crystalline opaque phase. From this point to 76 molecular percent SnS, two phases were present, the red crystalline opaque phase and an orange-yellow amorphous translucent phase; from 76 percent SnS, to pure SnS, the solid is entirely homogeneous and changes gradually in color from orangeyellow to the light yellow of pure SnS,. These observations show that besides the pure components two other phases are present in the series, a compound Tl,SnS, and a series of solid solutions running from 76 percent SnS, to pure SnS,. Another indication of the formation of the compound Tl,SnS, is noticed when precipitates containing an excess of SnS, are treated with sodium sulphide solution. Stannic sulphide will be dissolved until the residue has the composition Tl,SnS, and then no more will be dissolved even on increasing the concentration of Na,S to four mols. per liter and boiling. The compound TI,SnS, is decomposed, however, by a boiling solution of sodium sulphide when the concentration of the latter is about five mols. per liter. The similar insoluble precipitates formed with arsenic Chem. Centralblatt, ( 5 ) z I, 657 (1898).
Identzjfcation of Solid Phases
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and antimony sulphides and with stannous sulphide are now under investigation. The results of this paper are as follows: I . A general method has been described for determining the phases in mixtures of two insoluble salts. 2. An application of the method to mixtures of lead oxide and lead carbonate shows that white lead is a definite compound having the formula PbO,H,.2PbCO,. 3 . A colorimetric method has been used with mixtures of thallous and stannic sulphides. The solid phases are Tl,S, Tl,SnS,, and a solid solution running from 76 to roo molecular percents of SnS,. This work was suggested by Professor Bancroft and has been carried on under his direction. Cornell University.
