Alkalinity is a measure of the buffering capacity of water. It is determined by titrating the sample against a standard strong acid using specific indicators.
Principle
The titration is carried out in two successive stages:
- Phenolphthalein Alkalinity (P): Measured by titrating to pH 8.3. This accounts for all hydroxide and half of the carbonate present.
- Methyl Orange Alkalinity (M): Measured by titrating to pH 4.5. This accounts for all carbonate, bicarbonate, and hydroxide, representing Total Alkalinity.
Reagents Required
- Standard Sulfuric Acid ($H_2SO_4$) or Hydrochloric Acid ($HCl$) - usually 0.02N.
- Phenolphthalein Indicator.
- Methyl Orange Indicator.
Procedure
- Pipette 50 mL or 100 mL of the water sample into a conical flask.
- Add 2-3 drops of Phenolphthalein. If the solution turns pink, titrate with acid until it becomes colorless. Record volume as VP.
- To the same sample, add 2-3 drops of Methyl Orange. The solution turns yellow.
- Continue titration until the color changes from yellow to faint orange/red. Record the total volume of acid used from the start as VM.
Calculation
Alkalinity as $CaCO_3$ (mg/L) = $\frac{A \times N \times 50000}{V_{\text{sample}}}$
Where:
- A: Volume of acid used (mL).
- N: Normality of the acid used.
- 50000: Equivalent weight of $CaCO_3$ $\times$ 1000 (to convert to mg/L).
Interpretation of Results
| Condition | OH- | CO32- | HCO3- |
|---|---|---|---|
| P = 0 | 0 | 0 | M |
| P = M | M | 0 | 0 |
| P = ½M | 0 | 2P | 0 |
| P < ½M | 0 | 2P | M - 2P |
| P > ½M | 2P - M | 2(M - P) | 0 |