Sodium Bicarbonate is used to raise the Total Alkalinity in swimming pools.
The total alkalinity is a measure of how much alkaline substances there are in the water. In swimming pool water, we are concerned with bicarbonate alkalinity, which should be between 80 ppm and 120 ppm.
When the total alkalinity is within this range, it prevents rapid pH changes and stabilises the pH level.
If the total alkalinity is too low, plaster walls will become etched, metals corrode, the pool's walls and floor can stain, the water can turn green, eyes burn along with pH bounce (pH rapidly going up and down, seemingly at random).
It is recommended that you test the Total Alkalinity regularly. To raise the level of Total Alkalinity, we use sodium bicarbonate; it is the only chemical which will do this without increasing the pH too much. Note that you should only add sodium bicarbonate at the rate of 1kg per 50,000 litres of water every 4 days.