152 ANIMAL PROTEINS 



Of the first type the most common is that in which 

 chrome alum (a bye-product of the dyeing industry) is 

 the starting-point. To a solution of this a solution of 

 washing soda is gradually added, with constant stirring, 

 until the salt corresponding with the formula Cr(OH)SO 4 

 is obtained. 



Now : 



K 2 SO 4 Cr 2 (SO 4 ) 3 24H 2 O3 -f Na 2 SO 4 +NaCO 23 , ioH 2 O 



998 286 



= 2Cr(OH)S0 4 -f K 2 S0 4 + CO 2 + 33H 2 O 



Hence, in practice, for every ten parts of chrome alum 

 2*86 parts of soda crystals (or 1*06 parts anhydrous soda) 

 are used. A convenient " stock solution " is of 10 per 

 cent, strength. Thus 10 Ibs. of chrome alum is dissolved, 

 made basic, and made up to 10 gallons. To dissolve the 

 alum a mechanical stirrer is necessary, for the water must 

 not be more than warm. The disadvantage of this liquor 

 is the limited solubility of chrome alum and the nee.d for 

 its solution in the cold. I/iquors may be also made by 

 dissolving chromium hydrate in hydrochloric acid, and 

 making basic to correspond to the formula Cr 2 (OH) 3 Cl 3 . 

 Many preparations are on the market containing both 

 chlorides and sulphates with appropriate basicity. Chrome 

 alum liquors have been less often used in Britain of recent 

 years owing to the high price of chrome alum, caused in 

 part by the presence in the salt of potassium, all the salts 

 of which have been scarce and dear under war conditions. 



Of the second type Procter's " glucose liquor " is a 

 good example. Use 5 Ibs. sulphuric acid, 6 Ibs. sodium 

 dichromate, and 7 Ibs. of glucose, or quantities in similar 

 proportion. The dichromate is first dissolved, and the 

 acid added gradually. The glucose is then added cautiously 

 on account of the brisk effervescence of carbon dioxide. 

 A glucose of good quality is necessary, and the proportion 

 to be used is not quite definite, for sufficient only is needed 

 to effect the reduction, and this amount is influenced by 

 the rate of addition and temperature of the mixture. The 



