Effect of pH in Textile Wet Processing Industry | Determination of pH of Water

Define pH:
pH is a measure of the acidic or alkaline condition of water. It is a way of expressing the hydrogen ion concentration, or more preciously, the hydrogen ion activity. 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.

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

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: Reagent: 
Standard pH solution for calibration of PH Meter

Procedure:
 1) Perform calibration of the PH meter using standard PH solutions. The calibration procedure would depend on the PH range of interest.

2) 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.

3) Insert PH meter in the sample. Allow sometime for attainment of equilibrium. Turn on the PH meter and take reading.