Dye solubility testing
Solubility is the amount of dye that will dissolve in a given volume of water under stated conditions before it stops dissolving. It is not a fixed property of a dye: it changes with temperature and falls sharply as salt and alkali are added to the bath. A reactive dye that dissolves freely in plain water can reach its limit in a padding liquor carrying 60 g/l of Glauber salt and 20 g/l of soda ash, and dye that will not dissolve does not simply disappear. It settles on fabric as speckles, blocks filters, and produces the unlevel results that a dyehouse then tries to correct by adding more dye.
What dye solubility means, and why it decides the recipe
That is why solubility is measured under both conditions and reported as two figures rather than one. A buyer padding a deep shade, running a long liquor in a jig or dyeing yarn in package form needs to know the dye will stay in solution at the concentration the recipe requires. Solubility is a formulation constraint, settled before the shade is chosen, not a property that can be corrected afterwards.
The reference Nuchem follows
Solubility is measured by a Nuchem laboratory method rather than by a published standard. Two figures come from one procedure: salt-soda solubility, measured in a solution containing Glauber salt and soda ash, and plain water solubility, measured without either. Both are tested at temperatures that rise as the test proceeds, because solubility is temperature dependent.
| Method | Medium | Conditions |
|---|---|---|
| Salt-soda solubility | Water with 60 g/l Glauber salt and 20 g/l soda ash | Starts at room temperature, raised to 60 °C |
| Plain water solubility | Water only | Same procedure, without salt and soda |
| End point | A ring appearing at the centre of the spot on filter paper | Read against Whatman 3 Chr paper |
How the test is run
The procedure adds dye in instalments to a fixed volume of liquor, taking a spot sample on filter paper after each addition. The end point is a ring at the centre of the spot, which shows the liquor can no longer hold the dye and undissolved material is being deposited.
- 100 ml of water is taken, and 60 g/l Glauber salt with 20 g/l soda ash is added and dissolved under stirring.
- 0.5 to 2 g of dye powder is added at room temperature and stirred for 10 minutes.
- A spot of the solution is taken on Whatman 3 Chr paper.
- The solution is heated to 60 °C.
- The next instalment of dye is added, stirred at 60 °C for 10 minutes and spotted again.
- Additions are repeated until the spot begins to show a ring at its centre.
- The total solubility of the dye at that point is calculated from the quantity added.
How the result is read
The result is a figure in grams per litre at the stated conditions, not a pass or fail. It is read against the concentration the recipe requires: a padding liquor prepared at a given g/l needs a dye whose solubility at that temperature and salt content sits comfortably above the working concentration, because a recipe that sits at the limit will deposit dye when the liquor is held, cooled or recirculated.
Salt-soda and plain water figures are both published for each COTFIX grade on the fastness and solubility data page, so a grade can be compared against the liquor it will actually be used in rather than against pure water. Where a recipe sits close to a published limit, the laboratory can measure that specific condition.
Frequently asked questions
Salt reduces the ability of water to hold the dye in solution, which is the same effect that makes the dye fix to the fibre during dyeing. That effect also limits how much dye a salted liquor can carry.
Because solubility rises with temperature. Dye is added at room temperature and further additions are made at 60 °C, so the figure reflects the way a real dye bath is prepared and heated.
It shows the liquor has reached its limit. Undissolved dye particles travel differently through the paper from dissolved dye and collect at the centre, producing the ring that marks the end point.
The salt-soda figure, because a padding liquor contains salt and alkali. Where the recipe is unusually concentrated, the laboratory should measure the working condition directly.
Yes. A liquor that has to pass repeatedly through a wound package under pressure is less tolerant of undissolved dye than an open bath, so solubility matters more rather than less.
Need a solubility figure for a concentrated recipe?
Give the COTFIX grade and the liquor concentration, salt content and temperature the recipe specifies, and the laboratory will measure it for that condition.