THE NATURE OF CHROME LEATHERS 135 



the chrome tannage. This he has found to be the case. 

 Thus the chrome tannage is reversible in solutions of 

 ammonium or potassium oxalate and of ammonium acetate. 

 With these salts the full effect of multivalent anions is not 

 attained, so that somewhat strong solutions are necessary. 

 A 10 per cent, solution of ammonium acetate shows some 

 stripping effect after a few days, but a 40 per cent, solution 

 after a few hours. Saturated ammonium oxalate is only 

 a 4 -2 per cent, solution, but shows a stripping effect in 

 2-3 days. Potassium oxalate (33 per cent.) shows distinct 

 stripping in 24 hours. Potassium acetate and sodium 

 acetate show only slight action, because the solution is 

 too alkaline, but strip if acetic acid be added until litmus 

 is just reddened. It is noteworthy from a theoretical 

 point of view that a 40 per cent, solution of ammonium 

 acetate is distinctly acid, and indeed smells of acetic acid. 

 There can be little doubt that such stripping actions are 

 also connected with the solubility of the stripping agent 

 in the gel, for the liquid must pass through the walls of 

 the gel to dilute the liquid in the interior. This view fits 

 in with the facts that hydroxy acids and ammonium salts 

 are particularly efficient, for the tendency of chrome to 

 form ammonia-complexes as well as hydroxy complexes 

 is well known. From this point of view we should not 

 expect a stripping action from a salt such as disodium 

 phosphate, which would form an insoluble substance. 

 Actually sodium phosphate does not strip, and indeed 

 reduces the stripping power of ammonium acetate. Simi- 

 larly, we might expect some stripping action by ammonia 

 and ammonium chloride, with the formation of chrome 

 ammonia complexes. This actually occurs, a pink solution 

 bein;^ obtained. Sodium sulphite does not strip, possibly 

 partly on account of its too great alkalinity, but is interest- 

 ing theoretically to observe that sodium sulphite as well 

 as Rochelle salt will strip salt stains (see Yocum's pat 

 Collegium (London), 1917, 6; also Procter and Wilson, 

 loc. cit.). This points to the formation of a negative sol, and 

 suggests many other substances for removing salt stains. 



