Sodium Carbonate is used to raise the pH of swimming pools.
The water's pH is a measure of its total acid-alkalinity balance -- the relative proportion of acids and alkalis in the water. Simply put, water that is either too acidic or too alkaline will cause undesirable chemical reactions. If the water is too acidic, it will corrode metal equipment, cause etching on the surface materials and cause skin irritation. If the water is too alkaline, it can cause scaling on the pool surface and plumbing equipment and can cloud the water. Additionally, both high acidity and high alkalinity alters the effectiveness of the chlorine. The chlorine won't destroy pathogens as well if the water is too alkaline, and it will dissipate much more quickly if the water is too acidic.
On the pH scale, zero indicates extreme acidity, 14 indicates extreme alkalinity and 7 indicates a neutral state. We recommend a pool pH between 7.2 and 7.8. To raise or lower pH, simply adds acids or alkalis into the water. Sodium Carbonate will raise the pH, and sodium bisulfate will lower the pH.
Maintaining the proper balance of chemicals in the pool is a continual process, because any new element - oils from a swimmer's body, chlorine, debris that falls in the water - shifts the water's total chemical makeup.