December 4, 2016 1:25 pm
Alkalinity is capacity of aqueous solution to neutralize an acid.
Definition
Alkalinity test 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).
Range
| Drinking Water Standards of BIS (IS: 10500: 2012) | |||
| Characteristic | Required (Acceptable Limit) | Permissible Limit in Absence of Alternate Source | Method of test reference to |
| Total alkalinity as calcium carbonate, mg/l, Max | 200 | 600 | IS 3025 (Part 23) |
Interferences
Free available residual chlorine markedly affects the indicator colour response. The addition of minimal volumes of sodium thiosulphate eliminates this interference. Substances such as salt of weak organic or Inorganic acids present in large amount may interfere. OiIs and greases may also interfere by coating the electrode. Coloured or turbid samples may interfere in end point. Analyse such samples by potentiometric titration.
Indicator Method
Pipette 20 ml or a suitable aliquot of sample into 100-ml beaker.
If the pH of the sample is over 8.3, then 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 8.3).
Record the volume of standard sulphuric acid solution used.
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.
Calculation
Conclusion
Alkalinity is thus an important factor in determining the quality of water which helps in determining the treatment method & the facilities needed.
Disclaimer
The details given are for general understanding. References should be taken from respective authorized agencies.