Maintaining proper alkalinity and pH levels in your pool water is vital, as these are factors that can cause burning of your eyes when swimming. It is also important to understand that alkalinity and pH are not the same thing.
Alkalinity is the capacity of water to neutralize acids. This capacity is caused by the water’s content of carbonate, bicarbonate, hydroxide and occasionally borate, silicate and phosphate. On the other hand, pH is an expression of the intensity of the basic or acid condition of a liquid. Furthermore, alkalinity and pH are different because water does not have to be strongly basic (high pH) to have a high alkalinity.
However, confusion often comes from the fact that some of the chemicals commonly used in swimming pools will simultaneously raise and lower both readings – causing people to have problems maintaining the proper levels of both alkalinity and pH. Fortunately, I can offer you an easy (and inexpensive) remedy. To raise pH and alkalinity, I recommend using plain old ordinary baking soda (which also happens to be significantly cheaper then the pH or alkalinity increaser the pool store will try to sell you). To lower either of these, use muriatic acid. The key to adjusting these levels is to add half of the amount required for the change you want – then wait 24 hours and retest before adjusting further. Remember this is not brain surgery – there is nothing super critical here, so don’t be in a hurry for instantaneous results. By waiting 24 hours and retesting, you greatly reduce the risk of overdoing it and then going though the whole yo-yo, up and down thing. Also remember that it is far better to err on the side of higher pH than lower. A 6.8 pH will burn your eyes more than 7.8 – with 7.2 to 7.4 being ideal.