pH Calibration Techniques
The calibration of a pH electrode is crucial for accurate results, as the pH electrode can change its properties due to contamination of the reference electrolyte. Regular calibration is recommended for optimal results, with at least weekly calibrations recommended depending on the number of measurements and sample matrix. If the sensor is used often or if the sample matrix contaminates the sensor, daily or even more frequent calibrations are recommended. If the pH electrode is not used often, always calibrate it prior to a new set of measurements.
When selecting the correct buffers, it is essential to select the pH values that you would like to use for calibration. Use at least two different buffers, though it is even better to perform a multi-point calibration. Ensure that the pH of your sample is within the calibration range, as errors become large especially outside of the calibrated range.
The quality of your buffer solutions is also essential, as your calibration can only be as good as the buffers used. Never use expired calibration buffers, and do not reuse buffer solutions meant for single use only. Always mark your buffer solution bottle with the opening date, and especially ensure that alkaline buffers above pH 9 are not used for too long (<1 month), as CO2 will enter and change the pH value slowly. Moreover, never pour the standards back into the bottle, as they might have been contaminated.
Setting up your instrument correctly is also important, as pH measurements are sensitive to temperature, and the measured pH value can change with the temperature. Most instruments already include buffer table templates from various buffer manufacturers, which contain information about the exact pH value at various temperatures for a certain buffer. The instrument will then select the correct pH value according to the measured temperature. If your buffer is not available with a table, make sure you enter the correct pH value or use a custom buffer table to store the information.
Measuring the temperature when performing pH measurements is necessary because the Nernst equation describes the potential of an electrochemical cell as a function of concentrations of the ions taking part in the reaction. Modern pH meters will correct the slope for this temperature variation when the calibration and measurement are not performed at the same temperature. However, samples do not have the same pH value at different temperatures, and it is crucial to measure your samples at the same temperature to compare their pH values and carry out the pH calibration at the same temperature at which you are measuring your samples.
To perform calibration, prepare your electrode for calibration by opening the refilling plug, rinse the electrode well with deionized water, and place the sensor into the buffer solution. Position the electrode in the beaker for maximum reproducibility, especially when stirring. Do not place the sensor haphazardly into the beaker where the glass membrane is touching the glass of the beaker, as this can cause scratches on the glass membrane and lead to erroneous results.
Stir the solution between the buffer solutions, making sure not to stir so strongly that a vortex is formed and avoid any splashing of the solution. Most instruments autonomously decide when the reading is stable by monitoring the drift (mV change per minute). Sometimes, it is also possible to stop the buffer measurement after a fixed time interval, but this requires enough time for the electrode to reach a stable potential.