In practice it is! Even when measuring in alkaline solutions.
First of all, pH is a linear function of the measured potential by the electrode and a pH electrode itself is a linear detector between at least pH=2 and pH=12. A straight line is perfectly defined by two points.
People often want to use a buffer of pH 9 or 10 or even 12 when measuring in alkaline solutions. From the pure mathematical point of view, they are completely right. No doubt about that. But, all alkaline solutions, including buffer solutions, have a much bigger temperature dependency (temperature coefficient) than neutral or acidic solutions. This means that a small temperature error during calibration with alkaline buffers results in a bigger error compared to using acid buffers.
Furthermore alkaline solutions tend to dissolve acid vapours like CO2 or worse, HCl, HNO3, etc. In some industrial or laboratory environments these acid vapours are present. When they dissolve in the buffers, there is a risk that the pH value of the buffer solution is lowered.
To conclude, the risk of having calibration errors by using alkaline buffer solutions is relatively high. Especially in industrial and inline measurements. Therefore we recommend using the most standard buffer solutions of pH 4 and 7. If for some reason you really need to use alkaline buffer solutions, make sure your pH electrode is completely in thermal equilibrium with the buffer (also the part which is not immersed) and that you use a correct alkaline buffer solutions.
To be more sure always use a fresh buffer!10/04