| (1) | 3CrO3 | + | 6HCl | + | 6Na2S2O3 | = | 3Na2S4O6 | + | 6NaCl | + | 3OH2 | + | Cr3O6. |
Further addition of hydrochloric acid brightens the colour of the skins, while the liquid still remains clear, and chromium chloride is formed instead of chromic chromate, the main reaction being:
| (2) | 2CrO3 | + | 12HCl | + | 6Na2S2O3 | = | 3Na2S4O6 | + | 2CrCl3 | + | 6NaCl | + | 6OH2. |
On still further addition of hydrochloric acid, sulphur is separated according to the following equation, and is deposited partly in the skins, and partly in the bath:
| (3) | 2CrO3 | + | 6HCl | + | 3Na2S2O3 | = | 3Na2SO4 | + | 3S | + | 2CrCl3 | + | 3OH2. |
After complete reduction, and consumption of the free hydrochloric acid, further reactions take place at the expense of the excess of thiosulphate which should be present, resulting in the production of basic chromic salts, and the further deposition of sulphur, mostly within the skin, as shown in the following equations:
| (4) | Cr2(SO4)3 | + | Na2S2O3 | + | OH2 | = | 2CrOH.SO4 | + | SO2 | + | S | + | Na2SO4. |
| (5) | 2CrCl3 | + | Na2S2O3 | + | OH2 | = | 2CrOH.Cl2 | + | SO2 | + | S | + | 2NaCl. |
The thiosulphate bath therefore not only reduces, but precipitates sulphur in the skin, and reduces the chromic salt to a basic state. In boiling solution, thiosulphate precipitates the whole of the chromium as chromic oxide, but in the cold, and in presence of free sulphurous acid, it only reduces to a basic salt. Eitner does not consider the possibility, which certainly requires investigation, that instead of basic salts, sulphite-sulphates are formed at least in the first instance. Such salts of one base and two acids are quite possible, and it is very probable that in the use of chroming baths containing organic acids, they have considerable influence on the tannage.
The free sulphur which is liberated is partially deposited on and among the fibres of the leather, and adds to its softness, and also acts chemically on the oils used in “fatliquoring,” so that it is probably one of the main causes of difference between the products of the Schultz or “two-bath” method, and the “one-bath” processes subsequently to be described.