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Why should temperature be specified when doing weak acid pH calculations?
Kw varies with temperature, and the variation isn't small.
According to Whitfield,
log Kw = -4470.99/T + 6.0875 - 0.01706 T
Temperature affects both the ionization of water and
the ionization of the acid, so Ka will vary with temperature too. Depending on H for the
reaction, the Ka variation may be a more important temperature influence on weak acid
equilibrium calculations than the Kw variation.
Every numerical value of an equilibrium constant
should be accompanied by a temperature.
Most acid-base titrations are done at room temperature for convenience (glassware is usually calibrated
for 20°C), and when people say "Kw is 1.01×10-14" or "pH + pOH = 14",
they're talking about room temperature. These numbers are easy to remember and they're appropriate for
most situations you'll encounter in general chemistry. It's an i you'll often find undotted.
But consider calculating the pH of blood plasma inside a living organism. You'd have to take the temperature
variation of Kw into account (pKw is 13.60 at body temperature (37°C), and 13.996 at room
temperature (25°C)). It's a little more difficult to take into account the temperature
variation of the various Ka's in the plasma buffering system. The analysts just measure the
pH at 37°C rather than taking the measurement at a different temperature and bothering with
a temperature correction.
M. Whitfield, in Chemical Oceanography, J. P. Riley and Skirrow, Eds., Vol. 1, 2nd ed., Academic, New York, 1975, pp. 44-171.
Author: Fred Senese firstname.lastname@example.org