SOP for Determination of Alkalinity
Scope
These methods are applicable to determine alkalinity in water and wastewater in the range of 10 to 500 mg/l alkalinity as CaCO3.
Principle
Alkalinity of water is the capacity of that water to accept protons. It may be defined as the quantitative capacity of an aqueous medium to react with hydrogen Ions to pH 8.3 (phenolphthalein alkalinity) and then to pH 3.7 (total alkalinity or methyl orange alkalinity).
Reagents
Sulphuric Acid (0.02N)
Dilute 0.56 ml of concentrated Sulphuric acid (relative density 1.84) to one litre with distilled water. Standardization of Sulphuric acid by 0.02N sodium carbonate.
Phenolphthalein Indicator
Dissolve 0.5 g of phenolphthalein in 100 ml, 1;1 (v/v) alcohol water mixture.
Mixed Indicator solution
Dissolve 0·02 gm methyl red and 0.01 gm bromocresol green In 100 ml 95 percent. Ethyl or isopropyl alcohol.
Procedure
P-alkalinity Determination
- Pipette 20 ml or a suitable aliquot of sample into 100-ml beaker. If the pH of the sample is over 8.3.
- Add 2 to 3 drops of ‘phenolphthalein Indicator and titrate with standard sulphuric acid solution till the pink colour observed by indicator just disappears (equivalence of pH 3).
- Record the volume of standard sulphuric acid solution used.
Total Alkalinity or M alkalinity Determination
- Add 2 to 3 drops of mixed indicator to the solution in which ‘the phenolphthalein alkalinity has been determined.
- Titrate with the standard acid to light pink Colour (equivalence of pH 3-7)-Record the volume of standard acid used after phenolphthalein alkalinity.
Calculations:-
Phenolphthalein Alkalinity, (as mg/l CaCo3) = A x N x 50,000/V
Total Alkalinity (as mg/l CaCo3) = (A+B) x N x 50,000/ V
Where,
A = ml of standard Sulphuric acid used to titrate to pH 8.3,
B = ml of standard Sulphuric acid used to titrate from pH 8.3 to pH 3.7,
N = normality of acid used, and
V = volume in ml of sample taken for test.
Alkalinity Relationship
The following table gives the stoichiometric classification of three principle forms of alkalinity present in water.
| Result of Titration | Hydroxide
Alkalinity (as CaCO3) | Carbonate
Alkalinity (as CaCO3) | Bicarbonate
Alkalinity (as CaCO3) |
|
P = 0 P < 1/2m P = 1/2m P >1/2m P = m | 0
0 0 2p-m M | 0
2p 2p 2(m-p) 0 |
M m-2p 0 0 0 |
Where:
P = Phenolphthalein Alkalinity.
m = m-Alkalinity (or total alkalinity).
According to the above mentioned classifications:
Carbonates (Co3)– alkalinity is present when phenolphthalein alkalinity is not zero but less than total alkalinity.
Hydroxide (OH)– alkalinity is present if phenolphthalein alkalinity is more than half the total alkalinity.
Bicarbonate (HCO3)– alkalinity is present if phenolphthalein alkalinity is less than half the total alkalinity .These relationships may be calculated by the following schemes and where P is the phenolphthalein alkalinity and m is the total alkalinity.
Safety guidelines
Proper safety & health precaution are taken while doing the analysis by using safety goggles, apron, safety shoes & using fume chamber for fume generating chemicals.
Reference
IS 3025 Part-23:1986 RA 2019