Sodium hydroxide's main effect is to raise the pH of pool water in pools with chemical feed pumps. The pH is a measure of how many hydrogen ions are in the water. The lower the number, down to zero, the more acidic the water, and the higher the number, up to 14, the more basic the water.
Effects of Low pH
Pool water ideally should be between a pH of 7.4 and 7.6 and should not fall outside the range of 7.2 to 7.8. If it falls below 7, the water may cause eye irritation in swimmers. Chlorine will be less effective in acidic water. Pieces of metal and parts of the pump may corrode over time. If this happens, sodium hydroxide added to the water can counter the problem.
Effects of High pH
When pH rises above 8, chlorine becomes less effective and scaly white calcium carbonate or magnesium carbonate may form around the pool. The water may also seem cloudy. The filter will have to work harder, and eye irritation could also result, though less severely than in acidic water. Adding too much sodium hydroxide can have these effects.
Total alkalinity is a measure of how easily a solution's pH changes when acids or bases are added. Total alkalinity should be between 80 and 150 ppm. If total alkalinity is too high, it will be difficult to adjust pH. If it is too low, pH may fluctuate too much and be hard to adjust without overshooting.
Effect of Total Alkalinity on Sodium Hydroxide
Sodium hydroxide doesn't affect total alkalinity, but total alkalinity does change how the sodium hydroxide will affect the pool pH. If total alkalinity is too high, it could be difficult to increase the pH with sodium hydroxide. If it is too low, sodium hydroxide will have a more pronounced effect on pH per amount added.