Solutions of Metallic Salts. 119 



sulphate to a pressure of 20 atmospheres in the digester. It 

 usually required about 40 minutes to raise the pressure to the 

 limit set ; but this limit once reached, the digester was allowed 

 to cool slowly. The duration of an experiment was about two 

 hours. 





Table 



III. 





Alum taken 



Amount of 







as A1 2 3 . 



Na 2 S 2 3 used. 



A1 2 3 found. 



Error. 



grms. 



grms. 



grms. 



grms. 



0565 



5 



•0633 



•0068 + 



•1132 



10 



•1154 



•0022 + 



•1153 



5 



•1186 



•0033 + 



•1128 



3 



•1129 



•0001 + 



•1126 



3 



•1142 



•0016 + 



•1128 



2 



•1120 



•0008 — 



•1136 



2 



•1121 



•0015 — 



•1128 



2 5 



•1136 



•0008 + 



•1124 



2-5 



•1127 



•0003 + 



•1134 



2-25 



•1133 



•0001 — 



This table shows that sodium thiosulphate precipitates alumi- 

 num completely as the hydroxide when pressure is employed. 

 The high results seen in some of the experiments appear to be 

 due to the difficulty of removing by ignition the large amounts 

 of sulphur formed in the action, as well as to the salts mechani- 

 cally included in the precipitate. The amounts of sulphur and 

 contaminating salts present depend upon the amount of thio- 

 sulphate taken ; therefore this should be as small as possible, 

 2 to 3 grms. being sufficient to precipitate all the alumina in a 

 gram of alum. When the amount of thiosulphate is reason- 

 ably restricted the weights of alumina accord fairly well with 

 the theory. 



J£xperiments with a Salt of Chromium. 



Up to the time of the completion of this work nothing 

 appears to have been done upon the quantitative precipitation 

 of chromium as the hydroxide by means of sodium thiosul- 

 phate. Slater* and Rosef make mention of the action of sodium 

 thiosulphate upon chromic acid, bichromates and neutral chro- 

 mates, but give no quantitative data. Recently, however, 

 F. Faktor;£ has studied the action of sodium thiosulphate on 

 chromium compounds. This investigator has found that if 

 aqueous solutions of potassium bichromate and sodium thio- 

 sulphate are boiled together, a brown precipitate of hyd rated 

 Cr 2 3 , Cr0 3 separates out and the liquid turns yellow owing to 



* Loc. cit. fTraite de Chimie Analytique, vol. i, p. 479. 



JZeitschr. anal. Chem., 1900, xxxix, 345. 



