The term "pH" is a scale of measurement to tell us how acidic (corrosive) or basic (alkaline) the water is. It essentially ranges from 0 to 14, although the extremes of 0 or 14 are never experienced with pool water. pH of 7 is neutral, that is, the water is not acidic nor is it a base. The ideal range for a pool or hot tub is 7.4 - 7.6, slightly on the base side which assists with bather comfort, as the pH of the human eye is about 7.5.
Low pH causes; corrosive water, pitting of concrete, metals dissolve and staining of pool or spa walls. It also affects or increases; chlorine loss, vinyl wrinkles, skin and eye irritation. High pH causes; Scaling water, mineral precipitate out of the water (namely calcium, copper, iron etc...), plugged filters, cloudy water, reduced circulation. It also affects or increases chlorine inefficiency along with skin and eye irritation.
How to adjust pH;
REMEMBER: the alkalinity level must be correct or the pH will bounce back to the original level before the adjustment
When conditioning your water it is always best to adjust alkalinity first, calcium hardness second (if desired or used) and finally the pH. Amounts of chemical vary due to such things as buffering effect of the alkalinity of the water etc...However most containers do give a guideline amounts to use, based on a correct water balance and alkalinity. If still unsure then consult your local dealer. To increase pH :
Use soda ash (sodium carbonate) or caustic soda (sodium hydroxide), to name a few. Increasing pH using baking soda (sodium bicarbonate) is not recommended as this also increases alkalinity which may not need to be raised, but can be done if your careful.
To decrease pH:
Use dry acid (sodium bisulfate). Mix in a5 gal container and slowly add to spa with jets on high
REMEMBER: The spa must be allowed to rest for 6 to 8 hours before a accurate water test can be made!
Battery Acid, Caution! (Only service tech should use this product)
Note Muriatic acid is used to both lower pH and Total Alkalinity. The technique of adding this acid determines which will occur. To maximize the lowering of the alkalinity and minimize the pH drop, slug the acid, i.e. pour the acid in columns into the water. To maximize the pH drop and minimize the alkalinity drop, the acid should be walked, i.e., spread about the pools surface as much as possible. Total amount of acid given from the tables must be applied over time to avoid low pH. Initially slug 40%, 25%, 15%, and then the final 15%, waiting a few days between each addition to allow for pH recovery.