324 M. G. Lea — Silver Hemisulphate. 



A weighed quantity of silver nitrate is precipitated with an 

 excess of alkaline carbonate and washed. The carbonate, as 

 well as all the other reagents employed must be absolutely free 

 from chlorides, otherwise the product becomes contaminated 

 with silver chloride which cannot be removed. The silver 

 carbonate is then treated with a solution of alkaline hypophos- 

 phite acidified with sulphuric acid. All the alkaline hypo- 

 phosphite of commerce contains much more than a trace of 

 chloride : this is best got rid of by adding to its solution a 

 little solution of silver nitrate, stirring well at intervals, letting 

 stand for twenty-four hours and filtering. This filtrate with 

 addition of sulphuric acid is to be poured over the moist silver 

 carbonate and constantly stirred. The reaction is complete in 

 twenty or twenty-five minutes, when a bluish-black film of 

 reduced silver begins to form on the surface. Further ac- 

 tion is then cut short by neutralizing the liquid with alkaline 

 carbonate. The precipitate is next to be washed several times 

 by clecantation. Yery pure distilled water is, of course, needed 

 throughout. 



Convenient proportions are : 40 grams silver nitrate pre- 

 cipitated with excess of alkaline carbonate. Of sodium hypo- 

 phosphite, 100 grams, dissolved in 650 c. c. of water are 

 treated with a little silver nitrate, and after standing and filter- 

 ing, 4 c. c. of sulphuric acid are to be added and the liquid 

 poured over the silver carbonate. After a few minutes, 6 c. c. 

 more of sulphuric acid, diluted with a little water, are added 

 by degrees. With this second quantity of sulphuric acid the 

 characteristic reddish-brown color of the substance first ap- 

 pears. 



This process may be varied by precipitating with disodic 

 phosphate (which must be perfectly free from chloride) instead 

 of alkaline carbonate. The action is much slower, about 24 

 hours being needed. Silver nitrate itself may be used, but 

 the action is too rapid and the product is less in quantity. 



The crude product obtained in either way is to be purified 

 with nitric acid. Acid of 1*42 is diluted with three times its 

 volume of water, and of this dilute acid a quantity is taken 

 about double in volume to that of the precipitate and of the 

 water left after decanting closely. After a time some efferves- 

 cence takes place, but the mixture does not become warm. 

 After standing for three or four hours over the precipitate, it 

 is to be poured off and the precipitate washed. This treat- 

 ment with acid is applied three times : the first removes a good 

 deal of silver, the second a little, the third a trace. Each time 

 the acid is left three or four hours in contact. The product is 

 then washed by pouring on it a large quantity of boiling 

 water. This is repeated four or five times, each time (except 



