146 ANIMAL PROTEINS 



Each c.c. N/io thiosulphate corresponds to 0-0033 gram 

 CrO 3 or 0-0049 gram K 2 Cr 2 O 7 . The same volume of 

 liquor should also be titrated with N/io caustic soda and 

 phenolphthalein. Potassium chromate is neutral to this 

 indicator, i.e. chromic acid acts as a dibasic acid. Any 

 excess of hydrochloric acid is also titrated. More indicator 

 should be added towards the end of the titration, as it is 

 often oxidized. % Each c.c. N/io soda corresponds to 0*005 

 gram CrO 3 , om gram " half-bound " CrO 3 (i.e. present 

 as dichromate), 0-0147 gram K 2 Cr 2 O 7 , or 0-00365 gram 

 HC1. If a c.c. N/io thiosulphate and b c.c. N/io soda 

 be needed the type of chroming bath may be seen at a 

 glance 



If 10 c.c. chrome liquor require a and b c.c. of thiosulphate 

 and soda respectively 



I. 10 c.c. of a Schultz bath contain (b \a) Xo'Oi gram 

 Cr0 3 



and {(tfX'0033) [(b la) xo-oi]} Xi'47 grams K 2 Cr 2 O 7 



II. 10 c.c. of an acid bath contain (0x0-0033) grams 

 Cr0 3 



and {(&#) X 0-00365} grams HC1 



III. 10 c.c. of a neutral bath (0X0-0033) gramsip ~ 



or (6x0-005) grams/ 3 



The second bath of the two-bath chrome tannage 

 consists of a solution of sodium thiosulphate acidified with 

 hydrochloric acid. The reactions in this bath are some- 

 what complicated, several occurring simultaneously. 

 Broadly speaking, the final result is due to (i) the reduction 

 of the chromic acid to a chromic salt by the sulphurous 



