When we need precise pH measurements, we must take into account the potential sources of error. In this entry, we will consider the effects of temperature on the pH measurement. The temperature has effects both on the solutions we need to measure and on the measuring instruments themselves. The pH is a measure of the acidity or alkalinity of a solution and is defined concerning the concentration of hydrogen ions present (less logarithm in base 10 of the activity of hydrogen ions).
The temperature can affect the pH measurement because it modifies the activity of the ions in the sample solution, the buffers (in the calibrations) and the solubilities of the weak acids or bases and also the behavior of the membranes and ions of the electrodes of measurement. We will discuss some of these effects and the best way to avoid possible errors.
Effects on the electrode
When we separate from the standard conditions (1 M, 1 atm, 298 k or 25 ºC) the Nernst equation defines the response of the electrode: E = E 0 -2.3 (RT / nF) log to H +
Although the discussion of this fundamental equation is beyond the purpose of this entry, it can be seen that any change in temperature (T) modifies the value of the measurement. For this reason, equipment with ATC (automatic temperature compensation) has been offered for some time, for which a precise measurement of the temperature is necessary. This measurement can be done by a temperature probe independent of the electrode, or integrated electrodes capable of simultaneously measuring potential and temperature can be used.
For exact measurements, an essential aspect that we must take into account is the thermal balance between the calibration solutions, sample solutions and electrodes. We must make the calibrations at the temperature specified by the buffers and we should measure the samples at that same temperature whenever possible.
The lack of thermal equilibrium caused by variations in the temperature of the solutions or between these and the electrode we are using (by introducing it successively in different samples at different temperatures) will cause drifts and slow or unstable response. In a laboratory ideal to avoid this possible error is to calibrate and measure the samples in a bath of thermostatic water at a controlled temperature.
Effects on solutions
An increase in temperature will increase the solubility and dissociation of salts, acids and bases (especially weak acids and bases) by increasing the concentration of ions in the solution. Also, increasing the temperature will decrease the viscosity and increase the mobility of the ions. Since pH is a measure of the concentration of protons, its modification by the effect of temperature will modify the pH measurement.
We have already discussed the direct effect of temperature on the electrode potential; if we look again at the Nernst equation, we see that the variations in hydrogen ion activity (logan H + ) also affect its response, thus modifying the measurement of pH.
The temperature has effects on the pH measurement because its variation affects all the elements involved, calibration buffers, samples, and electrodes, which we must know and take into account when we need precision measurements.
To prepare the calibration solutions (buffers), we must respect the working temperature indicated by the manufacturer; this point is critical to avoid measurement errors.
For the measurement in the samples, the best option is to perform it at the calibration temperature, if this is not possible, use ATC equipment or use tables that allow us to correct the value. Thermostatic baths can also be a good choice for high precision measurements.
Whenever we measure at a different temperature than the calibration one, we should write it down next to the pH value obtained so that it is reproducible.
Concerning the electrode, we have already stated that temperature is a variable that directly affects the potential measured by the wire and can also cause a kinetic effect by modifying the mobility of the ions in the membranes causing instability, drifts, or slow responses.