Acid salts are also used for deliming, and Procter suggested sodium bisulphate NaHSO4 (“Principles,” p. 155). More recently, sodium bisulphite (NaHSO3) has been recommended;[11] the skins are tumbled for half an hour with a dilute solution, which combines with the lime according to the following equation—
(1) Ca(OH)2 + NaHSO3 = CaSO3 + NaOH + H2O
if sulphides are present the reaction is—
(2) Na2S + 2NaHSO3 = H2S + 2 Na2SO3
After this treatment the theoretical quantity of hydrochloric acid, diluted with plenty of water, is run in through the axle of the tumbler, and the skins run another half hour. The reactions are—
(1) CaSO3 + 2HCl = CaCl2 + SO2 + H2O
(2) Na2SO3 + 2HCl = 2NaCl + SO2 + H2O
The sulphurous acid evolved in this way slightly swells the skins, and also removes stains due to the alkaline polysulphides; it is also said to remove salt stains.
