86 Gooch and Weed — Estimation of Chromium. 



solution of silver nitrate in considerable excess, and the mix- 

 ture was again brought to the boiling point. Ammonium 

 hydroxide was added until the clear liquid became colorless 

 and turned litmus paper blue ; then acetic acid was added 

 cautiously until the reaction of the solution was distinctly 

 acidic to litmus. After standing for at least a half hour the 

 precipitate was filtered off on asbestos in a perforated crucible, 

 washed first with a dilute solution of silver nitrate and then 

 with 20 cul3 to 30 cm3 of distilled water applied in small portions, 

 and dried with gentle heating to a constant weight. In exper- 

 iments (1) to (10), the drying was done in an air bath heated 

 to 135° ; in experiments (11) and (12), the crucible and pre- 

 cipitate were heated gently over the free flame. In experi- 

 ments (7) and (8) the precipitation was made in presence of 

 5 grms. of ammonium nitrate, and in (9) and (10) in presence 

 of 5 grms. of sodium nitrate, to test the effect of each of 

 these substances upon the process. In no case did the filtrate, 

 with the washings, show by the lead acetate test the presence 

 of a chromate. 



The Precipitation of Silver Chromate. 







AgN0 3 













K 2 Cr 2 7 



Used in 

 precipi- 



Volume at 

 precipi- 





Ag 2 Cr0 4 















taken 



tation 



tation 



Found 



Theory 



Error 





grm. 



grm. 



cm 3 . 









(0 



0-0921 



0-4248 



100 



0-2072 



0-2076 



— 0-0004 



(2) 



0*0921 



0-4248 



100 



0-2073 



0-2076 



— 0-0003 



(3) 



0-0921 



0*4248 



100 



0-2075 



0-2076 



—o-oooi 



(4) 



00921 



0-4248 



100 



0-2074 



0-2076 



— 0-0002 



(5) 



0-0921 



0'4248 



100 



2075 



0-2076 



—o-oooi 



(6) 



0-0921 



0-4248 



100 



0-2073 



0-2076 



— 0-0003 



(7) 



0-0921 



0-4248 



100 



0-2073 



0-2076 



— 0-0003* 



(8) 



0-0921 



0-4248 



100 



0-2075 



0-2076 



—o-oooi* 



(9) 



0-0921 



0-4248 



100 



0-2080 



0-2076 



+ 0-0004 f 



(10) 



0-0921 



0-4248 



100 



0-2070 



0-2076 



— 0-0006 } 



(11) 



0-5801 



3- 



150 



1-3087 



1-3082 



+ 0-0005 



(12) 



0-7352 



3* 



200 



1-6573 



1-6574 



-o-oooi I 



*The precipitation was made in presence of 5 grm. of NH 4 N0 3 . 

 fThe precipitation was made in presence of 5 grm. of NaN0 3 . 

 |An excess of l cm3 of 40% acetic acid was added before filtering. 



From the results of these experiments it is apparent that 

 accurate determinations of chromium taken as the chromate or 

 dichromate may be secured by precipitating silver chromate in 

 presence of an excess of silver nitrate, making the solution 

 ammoniacal and then faintly acid with acetic acid, transferring 

 the precipitate to the filtering crucible, washing with a dilute 

 solution of silver nitrate, and, after other soluble impurities 

 have been removed, finishing the washing with small amounts 

 of water applied in successive portions. 



