Wednesday, March 12, 2008

Concept of pH in Dyeing

A variety of factors can influence the quality of dyeing and batch-to-batch reproducibility, but pH ranks as one of the most important factors. The pH of dye bath solution is most critical because of its effect on the dyeing cycle with respect to level and reproducible dyeing5. Even if the pH of the dye bath has been adjusted prior to the dyeing process, it may then be affected by various factors such as the absorption of acid by the fiber itself, increased alkalinity by boiling temporarily hard water and reduce alkalinity by loss of ammonia where ammonium salts are present in the bath at elevated temperatures in open-dyeing systems.

The control of pH in dyeing is ensured by three fundamentally different techniques.

A) Maintenance of a relatively high degree of acidity or alkalinity.
B) The control of pH within narrow tolerance mainly near the neutral region.
C) The gradual shifting of pH as dyeing proceeds.

Approach A is normally easiest to control and is used in the application of leveling type of acid dyes and 1:1 metal complex dyes to wool and nylon, and of the reactive, sulphur, vat dyes to cellulose. The agents traditionally used are the acids such as sulphuric, hydrochloric, formic acid, and alkalies like sodium bicarbonate and caustic soda.

Approach B needs a greater awareness of the factors that not only determine pH, but also stabilize it against interferences. Most of the dye-fibre systems requiring this approach are operated in the near neutral region (pH 4-9) and are much more sensitive to minor changes in pH. The pH of water supply may vary, or drift, during heating.

Approach C is particularly useful for non-migrating acid dye on wool and nylon. More recently, similar systems have been proposed for reactive dyes on cellulosics. In this case, pH control involves a deliberate shift of pH during processing in a consistent direction, rather than randomly.

pH control has received considerable attention in dyeing processes because of its critical role in quality assurance8,9. In dyeing, pH exhibits strong nonlinearity and time varying behavior. Some dyeing processes such as acid dyes on nylon, etc., are very dependent on pH. In dyeing process, pH not only responds to the addition of acids or base, but also varies as the temperature increases.

Thursday, March 6, 2008

General Concept of pH and Buffer


Pure water contains, in addition to H2O molecules, a very small proportion of electrically charged ions formed by electrolytic dissociation, according to the equation

Where H + is a hydrogen ion, hydrion or proton, produced from a hydrogen atom by the loss of an electron and hence carrying positive charge; and OH - is a hydroxyl ion, which is negatively charged.

In pure water the two ions are present in equal numbers, since the water is electrically neutral, and at 250C. their concentration is 10-7 gm-ions per litre. It can be shown (from the law of mass action) that, in any dilute aqueous solution, the product of concentrations of the hydrogen ions and hydroxyl ions is constant and is equal to 10-14 gm-ions per litre. i.e.,

[H +] X [OH -] = 1.0 X 10-14

When an acid is added to water it introduces hydrogen ions. Hydrochloric acid, for example, introduces hydrogen ions and chloride ions through electrolytic dissociation

Consequently, hydroxyl ions in the water are reduced so that the product of the hydrogen ion concentration and hydroxyl ion concentration remains equal to 10-14. For example, if the concentrations of hydrogen ions is increased to 10-3, the concentration of hydroxyl ions must fall to 10-11. Similarly, alkalies such as caustic soda introduce hydroxyl ions.

and so cause a reduction in the number of hydrogen ions present in the water. Solutions are acid when the number of hydrogen ions exceeds the number of hydroxyl ions, and alkaline when the hydroxyl ions are in excess of the hydrogen ions. The degree of acidity or alkalinity depends entirely on the relative proportions of the two ions.

In pure water at 250C, the hydrogen ion concentration (H +) = 10-7gm-ions per litre, or –log10 (H+) = 7. The pH value of water is therefore 7, the relation between pH and hydrogen ion concentration being expressed by the equation –

i.e., the pH value is equal to the logarithm of the hydrogen ion concentration with negative sign.