Ionization of water
Ionization depends on temperature,
and at 25 °C, Ki for water is 10-14. At neutrality,
H+ = OH¯ = 10-7. It is important to know that
neutrality equates to different concentrations at temperatures other
than 25 °C. Nevertheless, this is very seldom stated
explicitly. More usually pH meters are calibrated with reference
buffer solutions at the temperature of interest or the calibration
at one temperature is converted to another by a knob on the meter.
This minimizes the error of assuming that pH 7 represents neutrality
regardless of the temperature.