* The role of alkali in reactive dyeing

When dyeing with reactive dyestuffs, soda ash is often used as the alkali in dyestuff manufacturers' recipes. Recently, however, a synthetic alkali has appeared. What is the role of alkalis in reactive dyeing and what should be considered when choosing one?

In the dyeing of cellulose with reactive dyes, alkali is necessary because it acts as a catalyst in the reaction between the dyestuff and the fiber. The important point is not the type or amount of alkali but rather the pH of the dyebath, which must be closely supervised.
The most suitable pH for dyeing varies with the temperature, being approx. 11.5 for common warm dyeing (dyeing at approx. 60°C), 10-11 for hot dyeing (80°C) and 12.5 for cold dyeing (40°C). The relationship between temperature and the optimum pH is shown in the following graph.

Soda ash is often used because 20g/l usually produces a pH of around 11.5. However, it can be used with sodium phosphate or caustic soda when a higher pH is required, or with sodium bicarbonate (baking soda) when a lower pH is required. Caustic soda is not often used because it is a strong alkali and achieving a pH in the range of 10-12 is difficult. Nothing prevents it from being used, however, if the pH can be adjusted accurately with automatic equipment.
Recently introduced alkaline agents specifically for use with reactive dyes adjust the pH of the dyebath to a level suitable for dyeing with reactive dyestuffs and are easy to use because they come in liquid form, but the alkaline agent to be used should be chosen in consideration of the optimum pH for the temperature at which the dyestuff is to be used. The pH of the first bath is often checked to determine the pH for the dyeing, but attention should be paid to the pH after dyeing because if alkali is not added in sufficient quantity, the pH at the conclusion of dyeing may be considerably lower.