Reserve Alkalinity in Swimming Pools

The reserve bicarbonate alkalinity of the water is expressed chemically as the equivalent amount of calcium carbonate in the water. Experience has shown that the reserve bicarbonate alkalinity should be kept at about 150 - 200 ppm for public pools using gaseous chlorine. However, for private pools using other chlorine products this figure will increase considerably.

The reserve alkalinity in a pool system is not susceptible to rapid change as in the free chlorine reading or in most cases the pH. Once this reserve has been built up a test once per week should be sufficient. However, should there be any mal-operation of plant or sudden change in pH level a test for reserve alkalinity should be carried out immediately.

Tests for free and total chlorine and pH should be carried out each day, preferably early morning before the sunlight reaches the pools, and also in late afternoon before further addition of chemicals.

The estimation of reserve alkalinity is carried out by means of simple titration requiring the following apparatus and chemicals;

• 10mL burette (automatic type)
• 25mL graduated cylinder
• White basin or evaporating dish
• Glass stirring rod
• Dropper bottle for methyl orange indicator solution
• N/50 sulphuric acid solution
• Methyl orange indicator or bromo-cresol green indicator solution (0.04% strength)
• Supply of A.R. grade sodium thiosulphate

Procedure

1. Measure out 25mL of sample water into the evaporating basin.
2. Add one small crystal of sodium thiosulphate and stir until dissolved. Alternatively add 2 drops of thiosulphate solution (0.13% strength) to destroy any free chlorine.
3. Add 2 drops of methyl orange or 6 drops of bromo-cresol green indicator solution, by means of a dropper bottle, and stir with a glass rod. Sample will turn a light yellow colour with methyl orange or from deep blue to apply green for bromo-cresol green.
4. Fill the burette to the zero mark with N/50 sulphuric acid.
5. Add the acid slowly to the basin, stirring gently with a glass rod until the colour changes to orange. (Do not continue adding acid until the colour changes to pink, stop at orange). Read the number of mL of acid that is required to reach this point and multiply by 40. This will give the bicarbonate alkalinity of the pool water expressed as "calcium carbonate in parts per million".