Mixing Soda Ash with water and feeding it with a pump to increase pH is a trial and error procedure. It's best to start with a small amount of your mixture so that you can make alterations as needed.
The amounts below are a suggestion made by one source:
Mix 2 lbs of Soda Ash with 5 gallons of water. Set your pump at 50% or 60% capacity. Let the unit run for a time, and use water as you normally would, then check the pH a few feet downstream from the injection point. If the pH isn't where you want it, make adjustments.
Adjustment can be made by adding Soda Ash, by adding water, or by adjusting the pump up or down. Keep adjusting until you get the pH exactly where you want it.
Be sure to keep records, and keep in mind if the solution gets too strong, you may have to dump your mix and start over.
If you aren't able to get the pH as high as you want it, you may have to switch to Caustic Soda.
Caustic Soda is about 30% stronger than Soda Ash. Ten pounds of Soda Ash in 20 gallons of water is equal to approximately 7.5 lbs. of Caustic Soda in ten gallons of water.
Caustic Soda is available in pool supply stores. It's trickier to use than Soda Ash, so proceed with caution.
Here are some helpful facts about Soda Ash injection:
1. 0.926 lbs. of soda ash mixed with one gallon of water makes a solution of 10% Soda Ash.
2. A 10% soda ash solution is 100,000 parts per million soda ash.
3. It is common to inject between 50 and 500 ppm soda ash to increase the pH of well water. The amount you inject depends on how much you need to raise the pH. It is a process that usually involves some trial and error.
(whole house & well units)