Importance of pH in Textile Wet Processing Industry
Apu Kumar Das
B.Sc. in Textile Technology
Chittagong Textile Engineering College
Email: akdas.textile13@gmail.com
What is pH?
pH is a measure of the acidic or alkaline condition of water. The values range from 0-14. 7 means neutral, below 7 means more acidic, and above 7 means more alkaline. The pH value of textiles refers to the acid and alkaline components of the fabric residue, which is one of the limited indicators of harmful substances affecting human health. It is a way of expressing the hydrogen ion concentration, or more preciously, the hydrogen ion activity. Every chemical reaction works best at a certain pH, or degree of acidity.
pH is defined as follows:
pH = -log {H+}………………………………………. (1)
Where {H+} is the concentration of hydrogen ion (proton) in moles per liter (M).
Water dissociates to form hydrogen ion (H+) and hydroxyl ion (OH-) according to the following equation:
H2O =H+ +OH- ……………………………………….. (2)
At equilibrium, we can write,
Kw = {H+} {OH-}/ {H2O}…………………………… (3)
But, since concentration of water is extremely large (Approximately 55.5 mol/ L) and is diminished very little by the slight degree of ionization, it may be considered as a constant and its activity is taken as 1.0 Thus Eq. 3 may be written as:
Kw = {H+} {OH-} (4)
Where, Kw = equilibrium Constant
For pure water at 25oC, Kw = 10-7 × 10-7 = 10-14. This is known as the ion product of water or ionization constant for water. In other words, water (de- ionized or distilled water) at 25oC dissociates to yield 10-7mol /L of hydrogen ion and 10-7 mol /L of hydroxyl ion. Hence, according to Eq. 1 pH of de-ionized water is equal to 7.0
The pH is usually represented by a scale ranging from zero to 14, with 7 being neutral. Groundwater is often found to be slightly acidic due to the presence of excess carbon- di-oxide. Aeration removes carbon dioxide and hence causes a rise in pH value. Some natural waters are sometimes found to be slightly alkaline sue to the presence of bicarbonate and, less often, carbonate. Water with pH outside the desirable neutral range may exhibit sour taste and accelerate the corrosion of metallic plumbing fittings and hot water services.
Effect of pH in Textile Wet Processing Industry:
Textile wet processing is a very complicated process with many different phenomena occurring simultaneously. It covers different areas of science including chemistry, physics, mechanics, physical chemistry, fluid mechanics, thermodynamics and others. Controlling of pH is an important factor in textile wet processing. pH has great significance on every steps of textile wet processing such as pre-treatment, dyeing, printing and finishing. pH control in some textile coloration processes can be crucial, and an unsuitable value may adversely affect the quality of the product or may even result in the destruction of the fabric. pH influences the ‘exhaustion at equilibrium’, and the higher this value, the higher is the rate of dyeing at that moment. The value of the initial exhaustion (strike) in a pH-controlled process must be selected in such a way that the initial unlevelness is no more than can be rapidly corrected by the migration of the dye.
pH has great effect on dyeing process. Such as, the uptake of basic dyes on polyacrylonitrile fibers increases with increase in pH. The dyeing of PAN fibers with cationic dyes is usually carried out in the pH range 4–5.5 in order to secure optimum stability of both dye and fiber, pH often being achieved using a buffer system. Equilibrium dye exhaustion decreased with decrease in pH as a result of a decrease in the ionisation of sulphonate groups in the fiber. The effect of pH on dye uptake has also been explained in terms of the change in zeta potential of the fiber and also competition between protons and dye cations on neutralisation of the negative zeta potential; thus, the rate of dyeing decreases with increasing pH, this effect being most noticeable for fibers that contain only weakly acidic groups.
In the case of acid dyeing, a low pH helps to form the hydrogen bonds that attach acid dyes to protein fibers, such as silk and wool, as well as nylon. Silk can be dyed at low pH (acidic) or high pH as with cellulose fibers; wool can only be dyed at low pH because it is damage by high pHs. Cellulose fibers, such as cotton, linen, rayon, etc., cannot be dyed at all at low pHs. In the case of most popular fiber reactive dyes, a high pH actually activates the cellulose (cotton) fiber, forming a cellulosate anion, which can then attack the dye molecule, leading to a reaction that produces a strong, permanent covalent bond. Without a high pH, the dye will not fix permanently to the cellulose fiber. Dyeing polyester with disperse dyes requires an acidic pH. When the leveling chemicals are added to the dyeing bath, the pH should be between 4.5-4.7. And, after the addition of colors 4.2-4.3 pH.
Environmental Significance of pH:
A controlled value of pH is desired in water supplies, sewage treatment and chemical process plants. In water supply pH is important for coagulation, disinfection, water softening and corrosion control. In biological treatment of waste water, pH is an important parameter, since organisms involved on treatment plants are operative within a certain pH range. According to Bangladesh Environment Conservation Rule (1997). Drinking water standard for pH is 6.5- 8.5
Table: Limiting pH values and its effects
| Minimum | Maximum | Effects |
| 3.8 | 10.0 | Fish eggs could be hatched, but deformed young were often produced |
| 4.0 | 10.1 | Limits for the most resistant fish species |
| 4.1 | 9.5 | Range tolerated by trout |
| 4.3 | – | Carp died in five days |
| 4.5 | 9.0 | Trout eggs and larvae develop normally |
| 4.6 | 9.5 | Limits for perch |
| 5.0 | – | Limits for stickleback fish |
| 5.0 | 9.0 | Tolerable range for most fish |
| – | 8.7 | Upper limit for good fishing waters |
| 5.4 | 11.4 | Fish avoided waters beyond these limits |
| 6.0 | 7.2 | Optimum (best) range for fish eggs |
| 1.0 | – | Mosquito larvae were destroyed at this pH value |
| 3.3 | 4.7 | Mosquito larvae lived within this range |
| 7.5 | 8.4 | Best range for the growth of algae |
Determination of pH of Water:
A pH meter is often incorporated in modern dyeing vessels to determine changes in pH during the dyeing process. Determination of water pH is described below.
Reagent: Standard pH solution for calibration of pH Meter
Procedure:
- Perform calibration of the pH meter using standard pH solutions. The calibration procedure would depend on the pH range of interest.
- Take about 100ml of the sample in a beaker. Make sure not to agitate the sample in order to avoid exchange of gases between sample and atmosphere.
- Insert pH meter in the sample. Allow sometime for attainment of equilibrium. Turn on the pH meter and take reading.
You may also like:
- List of Chemicals and Auxiliaries Used in Textile Wet Processing
- Function of Salt in the Textile Wet Processing
- Typical List of Chemicals Used in Dyeing Mill
- List of Garment Washing Chemicals and Their Functions
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
