Soda ash raises the pH of pool water. While water has a neutral pH, chemicals used to clean the pool are more acidic and lower the water's pH. Soda ash keeps the water from becoming too acidic.
When water becomes too acidic, it causes skin irritation in swimmers who use the pool. It also damages the finishes of fixtures such as ladders, drains and diving boards; damages the pool liner; and stains plaster finishes used on pool equipment. Too much soda ash results in a pH that is too high, which causes cloudy water and mineral deposits. It also lowers the effectiveness of chlorine, making the pool water less safe.