RICHARDS AND ROGERS. — ATOMIC WEIGHT OP ZINC. 



177 



centimeter of an equivalent solution of silver produced an evident 

 cloudiness. 0.40 cubic centimeter more also produced a cloud. Still 

 0.20 cubic centimeter gave a very faint indication of opalescence, after 

 a long time, and finally 0.21 gave no trace. On titrating back vpith 

 hydrobromic acid, it was necessary to add 1.21 cubic centimeters more 

 before the point was reached where an extra 0.20 produced no precipi- 

 tate. The total amount of silver solution added was then 1.41 cubic 

 centimeters, or 0.21 to precipitate the first amount of hydrobromic acid 

 added, 0.21 to show that all the bromine was precipitated, and 0.99 to 

 correspond to the true amount of silver to be added in order to attain 

 the end point ; while the total amount of hydrobromic acid added to 

 reach the end point in the other direction was 1.42 cubic centimeters, 

 an amount which must be subtracted from the amount of silver added. 

 Hence, since the true amount of silver corresponding to the zincic 

 bromide is the mean between the amounts found by titrating in oppo- 

 site directions,* we obtain it as follows : — 



Amount of Silver Required. 



Since it is hardly probable that the end point may be obtained by 

 this method more nearly than the tenth of a milligram, the last figure 

 is omitted below. 



If, as is probable, the action of silver and hydrobromic acid in pre- 

 venting the solution of silver bromide resembles the action of silver 

 and hydrochloric in precipitating solutions of silver chloride (Stas), 

 the difference between the end points (1.00 milligram of silver) cor- 

 responds to six times the amount of argentic bromide dissolved. That 

 is, this amount must be about '/^ or 0.29 milligram in 0.85 liter of a 

 solution containing two or three cubic centimeters of nitric acid and 

 about five grams of zincic nitrate. 



In order to precipitate all the bromine, four milligrams more of 

 argentic nitrate were added to the solution and the whole was very 



* These Proeeedings, XXVIII. 24, XXIX. 74, XXX. 384. 



VOL. XXXI. (n. 8. XXIII.) 12 



