The pH scale is a measure of the acidity or alkalinity of a solution. A pH of 7 is a neutral reading; it is neither acidic nor alkaline. Readings of 1-2 are strongly acidic, while 13-14 indicates a strong alkaline or caustic media.
Note: A pH reading alone without the names of the chemicals involved is not enough to select a gasket. Also, since the pH scale is quite limited in range, a reading of "1" or "14" does not fully describe the concentration. We need the concentration expressed as a percentage. For example, sodium hydroxide at a concentration of around 4% will "peg" the pH scale at 14, the same reading produced by a 40% concentration.