354 Allen and Crenshaw — Sulphides of Zinc, 



produce recognizable crystals. It is a notable fact that sphal 

 eritc only urns obtained by this method, never tourtzite. 



Zinc sulphide from acid solutions. 



When zinc sulphide is precipitated by hydrogen sulphide 

 from solutions which are either neutral or acid at the start, at 

 temperatures ranging from the ordinary up to 200°, the 

 product is always amorphous so far as our observation goes. 

 The double tube method, which proved so satisfactory in the 

 formation of some other sulphides, did not generate hydrogen 

 sulphide at a rate slow enough to give crystals. True, the 

 products were frequently, though not always, doubly refract- 

 ing, and for some time we were misled into the belief that they 

 were wurtzite, but the measurement of their refractive index 

 later on showed that they were in all probability amorphous 

 and that the double refraction was caused by strains produced 

 in the hardening of precipitates originally gelatinous. (See 

 microscopic part.) Experience in qualitative analysis might 

 lead one to believe that zinc salts could not be precipitated by 

 hydrogen sulphide from solutions containing much free acid. 

 As a matter of fact precipitates are obtained from rather strong 

 acid solutions, provided only sufficient time is allowed, as 

 Glixelli* has proved. 



Table VI shows our own results on this point. In it are 

 given the times which elapsed after hydrogen sulphide was 

 introduced, before a precipitate was observed, and the com- 

 position of solutions after ten days' standing. The concentra- 

 tion of the zinc was 02 N at the beginning in all the solutions 

 except the last ; the hydrogen sidphide was kept at a pressure 

 of approximately one atmosphere, by uncorking the flasks and 

 passing in fresh gas from time to time, while the concentration 

 of the sulphuric acid varied as stated in the table. The experi- 

 ments were carried out at room temperature. It will be noted 

 that a partial precipitation was obtained in the above manner 

 from solutions which contained about 5 per cent sulphuric acid 

 at the start, and nearly 6 per cent at the end of the experi- 

 ment. With a higher initial concentration of zinc or a higher 

 pressure of hydrogen sulphide, precipitation would naturally 

 be obtained from solutions containing still more acid. The 

 text-books commonly assume equilibrium in these systems and 

 use them as examples of the mass law. Preliminary work on 

 the subject has convinced us that such is not the case. In fig. 

 4 are piotted the concentrations of zinc in solution as they vary 

 with the concentration of sulphuric acid, after the precipitation 

 with one atmosphere of hydrogen sulphide has apparently 

 ceased. If one begins with the same precipitate, adding dif- 

 ferent concentrations of acid and maintaining the hydrogen 

 sulphide at one atmosphere, different results are obtained. 

 * Zs. anorg. Ch., lv, 297, 1907. 



