When you add Soda Ash to the pool, the carbonate anion (CO32-) wants to become bicarbonate (HCO3-). To do so, it needs to get a Hydrogen (H+) from somewhere. When carbonate takes its Hydrogen atom, the pH goes up.

How does soda ash affect pH?

Soda ash raises pH and alkalinity. Usually, if you try to use it to raise pH, you will raise the alkalinity too high and have to add acid to adjust it back down.

How does soda ash increase alkalinity?

Fill a container with soda ash and walk around the circumference of the pool, distributing the soda ash into the middle of the pool. Add 1.6 ounces of soda ash for every 1,000 gallons of water in the pool and for every 10 parts per million of increase in the total alkalinity that is needed.