The Dyeing of Wool with Chrome Dyes
Authors: CARLENE P. W. , ROWE F. M. , SPEAKMAN J. B. 
Published: July 2010

The titration curve of wool with chromic acid at 0°c. has been determined, a low temperature being chosen so as to minimise the rate of reduction of the chromic acid by the wool. Some reduction does occur, even at 0°c. in absence of light, and the amount of combined chromic acid was determined by extracting the wool with a pH 8 buffer solution. Since the second dissociation constant of chromic acid is very small (pKa = 6‐4), it tends to behave as a monobasic acid in reaction with wool, but at low pH values the acid appears to behave as a weak monobasic acid. Instead of the acid‐combining capacity reaching a limit of about 160 c. c. N‐acid per 100 g. dry wool, the titration curve shows an inflexion at a combining capacity of 200 c. c. N‐acid and pH 2. Below this pH value, the combining capacity increases rapidly with change of pH, just as in the case of concentrated solutions of weak acids like monochloroacetic acid. The most interesting feature of the titration curve is, however, that it lies far to the right of the hydrochloric acid curve on the pH scale, indicating that the anions of chromic acid have a marked affinity for wool.

With this information as basis, attention was turned to the reaction between wool and boiling solutions of potassium dichromate. The latter ionises in solution to give K+ and HCrO4 ions, and since the HCrO4‐ ion has a marked affinity for wool, chromium is rapidly absorbed when wool is immersed in a solution of potassium dichromate. Combination between the basic side‐chains of wool and the HCrO4‐ ions involves the combination of a corresponding number of hydrogen ions with the acid side‐chains. In consequence, the pH value of the solution in contact with the wool rises, and disulphide bond attack is likely to be severe at b. p. Through such attack, reducing side‐chains are formed in the fibres, with liberation of hydrogen sulphide, and the chromium in both the fibres and the bath is rapidly reduced from the hexavalent to the trivalent state. The course of combination and reduction of the chromium was followed by determining the total amount of chromium absorbed by the wool, and the proportions of hexavalent and trivalent chromium in the residual chroming baths and on the fibres, after various times of chroming with three concentrations of potassium dichromate.

Besides disulphide bond hydrolysis, reduction of chromate ions may be brought about by direct attack on the disulphide bond, with formation of, say, the disulphoxide, and on side‐chains, such as that of tyrosine. If disulphide bond hydrolysis plays the more important part, the rate of reduction of chromate ions should be minimised by mordanting wool with potassium dichromate in presence of ammonium sulphate, because the rise in pH of the bath during chroming is thereby minimised. This expectation was found to be justified, but the possibility of direct attack on disulphide bonds could not be excluded. If such attack does occur, its extent should be minimised by mordanting wool with potassium dichromate in presence of ammonium formate, oxalate or tartrate, i. e. the salts of readily oxidisable acids, hydrolytic attack being also minimised by the use of ammonium salts. Although the ammonium salts of readily oxidisable acids may prevent direct oxidation of disulphide bonds, it was found, from determinations of the sulphur contents of wools mordanted under various conditions, that the loss of sulphur in presence of ammonium sulphate and ammonium oxalate was only a little less than with potassium dichromate alone; and the loss in presence of ammonium tartrate was the same as in its absence.

In an attempt to discover whether ammonium salts do have a significant protective effect on wool fibres during mordanting with potassium dichromate, the elastic properties of fibres chromed under various conditions were examined. The results were, however, inconclusive. Within the limits of the experimental error of the load‐extension curves, there was no difference between fibres chromed in presence and absence of ammonium salts.

The nature of the wool‐chromium complex was then examined by studying the elastic properties of chromed fibres before and after removal of chromium. Chromium was removed by extracting the fibres with pH 1 oxalic acid, and the results indicated that the chromium deposited in the fibres does increase their resistance to extension in water, thus masking part of the damage brought about during mordanting. Although the supercontraction of chromed fibres was less than that of untreated fibres in boiling 5% sodium metabisulphite solution, the supercontraction of chromed fibres, after all the chromium had been removed with oxalic acid, was the same as that of unextracted chromed fibres. There is thusno evidence that chromium‐containing cross‐linkages are formed between the peptide chains of wool during mordanting. Without further evidence, it must be concluded that the effect of chromium compounds in increasing the reaistance to extension: of mordanted wool is due simply to their mechanical interference with the unfolding of the main peptide chains.