Dyes
1. Solubility of powder dyes
The solubility of leather powder dyes in water is measured at 60°C. This is an internal TFL test method.

Method :
An exact amount of dye is pasted with a small amount of deionised water. The rest amount of deionised water is added with stiriing. The solution is heated to at least 90°C with stirring and kept at this temperature for approx.2 minutes. The solution is allowed to cool to 60°C and then filtered at 60°C with a pre-heated vacuum filter using a fast qualitative filter paper.
The filter paper is inspected for any undissolved residue. If a residue is present then the method is repeated at a lower concentration.

Result
There should be no significant residue of undissolved particles at the specified solubility concentration.

General Comments
In the tannery application there are several factors that can influence the solubility of the dye are the float temperature and pH, as well as the other chemicals in the float and the water hardness.

2. Stability of dyes to:
- acid (IUF 202 & IUF 203)
- alkali
- hard water (IUF 205)
The stability and colour fastness of dyes to acid, alkali and hard water is sometimes reported by dye suppliers

Methods：
Acid
A 5 g/l solution of dye is prepared. 2 samples of this solution (each 10ml) are warmed to 60°C and acidified as follows:
- 0.5ml of 10% formic acid and
- 0.5ml of 10% sulphuric acid.

The stability and change in colour of the solutions are evaluated using a 5, 4, 3, 2, 1 scale (5 = very good, 1= very poor) in comparison with an unacidified solution. To help with the evaluation some drops are spread on a filter paper and after drying assessed.

Alkali:
Same procedure as for the acid, but made with a 10% ammonia solution (or sodium carbonate).

Hard Water
A 2 g/l solution of dye is prepared and tested at room temperature for its stabilty in water of 3 differing degrees of hardness, namely 0°dH, 20°dH and 40°dH (dH = German degrees of hardness).
1 ml of the dye solution is added to each of the 19ml hard water solutions. After 10 minutes and 1 hour any propensity to flocculate or precipitate is noted and evaluated using a 5, 4, 3, 2, 1 scale (5 = very good, 1= very poor).
(20°dH = 200 mg CaO/l = 360 mg CaCO3/l)

3. General comments relating to impact of pH, hard water and cationic fixing agents on dye stabilit
Acid
pH Practically all leather dyes are stable down to pH 3.5. However, at very low pH values, like pH 2.5, some dyes in water are not stable and some can change colour considerably. Important note, once a dye is bound to the leather it is less susceptible to undergoing a change colour at low pHs.

Alkaline
pH At high pH values some dyes change colour, for example, Acid Black 210 can be noticeably greener at alkaline pH values.

Hard
water Some dyes are not stable in very hard water. In areas of very hard water the dyes are best dissolved in soft water before addition to the drum.
Additional care must be taken when using sequestering agents for water softening or inhibiting metal staining in vegtable tannages/retannages. If they are used in the same bath as dyes, they could remove Fe from iron complex brown dyes.

Cationic fixing agents
Catioinc fixing agents can complex with the anionic dyes typically used to colour leather, causing a precipitate in the application drum. The cationic fixing agent is best applied to the coloured leather in a fresh float at the end of the wet treatment process.