The chemical action of the acid is a very simple one, and is expressed by the formula--
Ca(OH)_{ 2} + 2HCl = CaCl_{ 2} + 2H_{ 2}O 74 + 73·1 = 110·9 + 36[7]
[7] From the molecular weights it will be seen that 74 grm. of calcium hydrate, equivalent to 56 grm. of lime (CaO), require 73 grm. of HCl gas for neutralization. This quantity of gas is contained in 265 grm. of commercial hydrochloric acid of 18° Bé., or about 230 c.c.--that is, for 100 grm. CaO, 410 c.c. of 18° Bé. acid are required; hence for the 100 kilos wet grain containing 400 grm. CaO, 1640 c.c. of acid are required for _complete neutralization_. It is impracticable to use this amount of acid, for the reasons stated above.
It will be noted that the quantity of acid used is only about one-tenth of that required to neutralize completely the lime in the skin. If a greater amount of acid be used the skins begin to absorb acid before the lime in the interior is neutralized; but, provided the lime on the surface of the skins is removed, that remaining in the interior is forced out during the course of the bating process (see Chapter III.). If more acid were used, then it would require to be added so slowly that time would be wasted unnecessarily in doing the work.
With regard to the use of other acids for deliming, Lamb considers formic acid preferable to hydrochloric, and states that the removal of lime is more effectively accomplished by its use. He attributes a pulling down action to the calcium formate produced, so that less formic acid is required than would be equivalent to hydrochloric, merely regarded as a solvent for lime. We shall refer to this point again in Chapter II. It may be stated here that the author has obtained better results by the use of a mixture of formic and acetic acid, in equal proportions, than from either acid alone, but that the cost of working is considerably greater than with hydrochloric acid.
Lactic acid is in fairly common use as a deliming agent. In practice 1 per cent. lactic acid (50 per cent. strength) calculated on the wet weight of pelt, is used, added gradually in small quantities. This does not remove the whole of the lime, but sufficient for the skins to puer quickly.
Some tanners measure their acid per dozen skins. In a case which came under my notice, 1200 c.c. 90 per cent. formic acid were being used for ten dozens medium goat skins at a temperature of 35° C., time 1-1/4 hours.