pH and pH buffers

pH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. definition: pH is the negative logarithm of the hydrogen ion concentration.  pH is a measure of the concentration of hydrogens ions ( H⁺ ions or protons) in a solution.

pH scale

The pH scale was defined by Sören Sörensen, Danish biochemist, in 1909The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic and a pH more than 7 is alkaline.

Measuring pH

Measuring pH is essential not only in finding the chemical characteristics of a substance but also as the first step toward managing chemical reactions.

Measuring pH involve either the use of pH measuring electrodes or indicators whose colors are dependent on pH. A pH meter measures the difference in potential between a reference electrode insensitive to changes in pH and an electrode sensitive to such changes.

pH indicators based on color changes are normally used in the form of pH papers. The paper is wetted with the solution being measured and the resulting color is compared with color standards to determine the pH.

In the textile industry, measuring pH is important in product testing, pretreatments,dyeing and finishing processes. 

pH buffers

A pH buffer is a substance that resists a change in pH when small amounts of an acid or a base are added to it. The pH of a buffer changes very little when small amounts of  an acid or a base is added to the buffered solution. A buffer consists of approximately equal amounts of conjugate weak acid/base pair in equilibrium with each other . Strong acids and their conjugate bases do not produce a buffer since in  strong acid ionization is complete and there is no equilibrium.

Why pH changes during a Textile dyeing process

1. Water quality
2. Reaction products
3. Additives during the process
4. Time
5. Temperature
6. Contaminants in the substrates.

Some pH  buffers systems useful in textile pretreatment , dyeing and finishing.

HCl and sodium citrate                                     pH 1-5

Citric Acid and sodium Citrate                          pH 2.5-5.6

Acetic Acid and Sodium Acetate                       pH 3.7-5.6

K₂HPO₄ and KH₂PO₄                                      pH 5.8-8.0

Na₂HPO₄ and NaH₂PO₄                                  pH 6-7.5

Borax and NaOH                                              pH 9.2-11

pH in textile coloration

Most of the dyeing processes requires a controlled pH , such as slightly alkaline in direct dyes , strong alkaline in reactive , vat and azoics , acidic in disperse and basic dyes strongly acidic in acid dyes etc.

The control of pH in textile processing is ensured by fundamentally three different techniques , such as

1. The maintenance of a relatively high degree of acidity or alkalinity .
2. The control of pH within fairly narrow tolerances mainly in near neutral regions.
3. The gradual shift  of pH as dyeing proceeds.

Many processes of textile processing are pH dependent.

1. Scouring of cotton in highly alkaline conditions
2. Bleaching  of different substrates where pH has to be maintained for proper bleaching action.
3. Solubilising the dyestuffs.
4. Exhaustion and fixation
5. Oxidation
6. Stripping
7. Finishing 

further reading
pH of textiles by water extraction method