PH is a measure of how acidic or basic the water is. pH is a scale from 0-14, with 7 being neutral. Below 7 and a substance is defined as being acidic, while levels above 7 are said to be basic or alkaline. Everything that enters your pool has a pH value. Ever heard of acid rain? This is rainfall with a very low pH. The human eye at a pH value of 7.35, is just slightly basic. This is, coincidentally, in range with proper pH levels for your pool. If your testing of the water shows a pH value below 7.2 the water is in a corrosive (acidic) condition and you will need to add a base to bring the pH into a more basic range to prevent corrosion. Conversely, if the pH is above 7.8, we are in a scaling (basic) condition and must add an acid to bring down the pH to prevent the formation of scale. The level of alkalinity in the water is a measurement of all carbonates, bicarbonates, hydroxides, and other alkaline substances found in the pool water. pH is alkaline dependent; that is, alkalinity is defined as the ability of the water to resist changes in pH. Also known as the buffering capacity of the water, alkalinity keeps the pH from "bouncing" all over the place. Low alkalinity is raised by the addition of a base (similar to pH). High levels of alkalinity are lowered by the addition of an acid (similar to pH). Adding an acid will lower both pH and alkalinity. A very important component of water balance, alkalinity should be maintained in the 80-120 ppm range.
