DETERMINATION OF THE SILVER NEEDED FOR PRECIPITATION. 41 



THE DETERMINATION OF THE SILVER NEEDED FOR PRECIPITATION. 



The silver titration method of Gay-Lussac, as developed by Pelouze, 

 Mulder, Stas, and more especially by the recent work at Harvard, yields 

 very consistent and accurate results, if the proper conditions are carefully 

 observed. The reading of the end-point is rendered easy by the use of 

 the nephelometer. 1 In the case of the somewhat soluble chloride, various 

 precautions must be strictly heeded ; but with argentic bromide, which is 

 almost insoluble, the matter is a simpler one, and a very slight excess of 

 either bromide or silver can be easily determined. 



The method used in the present case is easily inferred from previous 

 Harvard work of the same kind. From the weight of a piece of the purest 

 silver, the equivalent amount of potassic bromide was calculated ; slightly 

 more than this amount of substance was then fused as before in a plati- 

 num-iridium boat placed in the porcelain tube of the bottling apparatus. 

 From the weight of fused bromide, the equivalent quantity of silver was 

 calculated; the greater part of the difference between this calculated 

 weight and that of the original piece of silver was added in the shape of 

 pure silver wire, 2 and any final difference of 0.1 or 0.2 mg. was made up 

 with a dilute solution of silver nitrate. The silver was dissolved in nitric 

 acid and the nitrous acid expelled as usual, and the solution was then 

 diluted to about tenth normal. 



To the dilute bromide solution, with continual agitation, was added this 

 dilute argentic solution; the total volume including the water used in 

 rinsing usually amounted to about 1.5 liters. After shaking steadily for 

 15 minutes, and occasionally for a day, the mixture was allowed to settle 

 during another day. When clear above, about 0.05 liter of the aqueous 

 solution was withdrawn and tested in the nephelometer, one tube being 

 treated with an excess of bromide, the other with an excess of argentic 

 nitrate. If any difference in the opalescence was noticeable after due time 

 had been allowed for the very faint clouds to attain their maxima, the 

 slight deficiency was made up in the flask by means of solutions containing 

 approximately 1 milligram of argentic nitrate or of potassic bromide per 

 milliliter. These additions were continued till equality in the opalescence 

 was attained. From the sum of the original weights and subsequent addi- 

 tions, the total amounts of bromide and silver were obtained ; from these, 

 reduced to vacuum, the ratio was calculated. All the analyses which were 

 performed are given in the table, excepting No. 2, which was rejected for 

 just cause before it was finished. Vacuum corrections of -f- 0.00029 for 

 every gram of potassic bromide and 0.00003 for every apparent gram 



iRichards and Wells, Am. Chem. Journ., 31, 235 (1904). 

 2 Richards and Parker, Proc. Am. Acad., 32, 60, 1896. 



