pH factor a measure of the acidity or alkalinity of a solution. It is always spelt with a small p, which stands for ‘potential’, and a capital H, the chemical symbol for hydrogen. acids are substances which liberate hydrogen ions (charged atoms) when they dissolve in water. pH was originally defined as the logarithm to base 10 of the reciprocal of the concentration of hydrogen ions; now, however, it is defined by comparison with a standard solution. What all this means in practice is that pH values lie on a scale from 0 to 14. Values from 0 to 7 represent acidity; 7 is neutral; from there up to 14 the solution is alkaline.