Determination of Alkalinity of Water-EE Lab
The alkalinity of water is its quantitative capacity to neutralize strong acid to designated pH. The measured value may vary with the endpoint pH used in the determination.
Therefore the determination of alkalinity of water is important in the field of environmental engineering.
Alkalinity is important in treatments of natural waters and wastewaters. The alkalinity of natural waters primarily results from salts of weak acids. Weak and strong bases may also contribute to alkalinity.
The alkalinity of many natural waters is primarily due to carbonates, bicarbonates and hydroxides. The values may also include contributions from borates, phosphates and silicates.
Bicarbonate represents the major form of alkalinity since they are formed in a considerable amount from the action of CO2 upon basic materials in the solution.
CO2 + H2O + CaCO3 =Ca(HCO3)2.
Natural waters may also contain an appreciable amount of CO3 and OH alkalinity, particularly surface waters blooming with algae. The algae take up CO2 for its photosynthetic activities and raise the pH.
The carbonate alkalinity may be present with either hydroxide or bicarbonate alkalinity, but hydroxide and bicarbonate alkalinity cannot be present together in the same sample.
For samples whose initial pH is above 8.3, the titration is conducted until phenolphthalein indication turns from pink to colourless. The second step of titration is conducted with the aid of methyl orange to a pH of about 4.5.
When the pH is less than 8.3, a single titration is made using methyl orange as the indicator. At pH 8.3 all the hydroxide alkalinity will be neutralized and all carbonates converted bicarbonates.
This alkalinity is shown as phenolphthalein (or hydroxide). If the titration of a sample that originally contained both carbonate and hydroxide alkalinity, is continued beyond the phenolphthalein endpoint, the bicarbonates react with the acid and are converted to carbonic acid. This reaction is complete when the pH is lowered to about 4.5.
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Hydroxyl ions, present in a sample as a result of dissociation or hydrolysis of solutes are neutralized by titration with a standard acid. Thus the alkalinity depends on the endpoint pH used.
The endpoint pH may be 4.5 to 5.1 for total alkalinity and 8.3 for phenolphthalein alkalinity.The determination of acidity of water can be found by using reagents standard H2SO4 of 0.02 N, phenolphthalein indicator and methyl orange indicator.
Apparatus used for measuring water alkalinity
- Burette
- Pipette
- Erlenmeyer flasks
Reagents
- Standard H2SO4 of 0.02 N
- Phenolphthalein indicator
- Methyl orange indicator
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Determination of Alkalinity of water
- Pipette 20 mL of sample into a clean Erlenmeyer flask (V ml).
- Add two drops of phenolphthalein indicator. If the pH is above 8.3 the colour is pink.
- Titrate this sample against standard acid, 0.02 N H2SO4 in the burette till the colour just disappears. Note the volume of the titrant used (V1 ml).
- Then add two drops of methyl orange indicator. The colour turns yellow.
- Titrate this again against the acid in the burette, till the yellow colour just turns orange-yellow.
- Note the volume of titrant used (V2 ml).
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Below video shows how to find alkalinity of water