In a recent blog post, we discussed how to avoid the most common mistakes in pH measurement:
Here, I want to discuss in a bit more detail how you can correctly calibrate your pH electrode and what you have to consider to obtain the best measurement results afterwards by answering some of your most frequently asked questions.
Let’s get right into it! If you want to jump directly to a question, click on one of these links:
When do I have to calibrate my pH electrode?
Performing regular calibration of your pH electrode is important to get accurate results. The pH electrode can change its properties (e.g., by contamination of the reference electrolyte) which then leads to deviating calibration results. If you do not freshly calibrate your electrode, you obtain precise but inaccurate results of your pH measurement. Therefore, the more accurate the results need to be, the more often you have to calibrate.
Depending on the number of measurements and the sample matrix, I recommend calibrating at least weekly. If the sensor is used often, or if the sample matrix contaminates the sensor, then you should calibrate daily or even more frequently. If the pH electrode is not used often, then always calibrate it prior to a new set of measurements. Also make sure that you always calibrate your sensor if you have received a new one, or after maintenance.
How do I select the correct buffers?Any time you perform a calibration, it is essential that appropriate buffers are used.
First, you have 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. Furthermore, make sure that the pH of your sample is of course within the calibration range! For example, if you want to measure a sample at pH 9, your calibration should not be within pH 4 and 7, but at least up to pH 10. In the graph, you can see that errors become large especially outside of the calibrated range.
In addition, the quality of your buffer solutions is essential, as your calibration can only be as good as the buffers used. Never use expired calibration buffers! If the buffer solutions are meant for single use only, do not reuse them. Microbial growth in the buffer can alter its properties quickly. 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!
How should I set up my instrument?
Not only is the right choice of calibration buffers essential, it is also very important that you set up your instrument correctly. It’s not only the pH measurement that is sensitive to temperature, pH buffers are as well, and the measured pH value can change with the temperature. This temperature dependency of the pH buffers is usually depicted with buffer tables.
Most instruments already include buffer table templates from various buffer manufacturers. Several tables are available that contain the information about the exact pH value at various temperatures for a certain buffer. These tables are unique for each manufacturer.
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. As seen here, a temperature change of only 5 °C can have an influence of > 0.04 pH units.
Therefore selecting the manufacturer of your buffer solutions within the calibration parameters is important to obtain an accurate calibration.
Why do I have to measure the temperature?
You might wonder why you should always measure the temperature when you perform pH measurements. Most pH electrodes used for pH measurement have a temperature sensor directly included. This is because the pH value is temperature-dependent. Let me digress for a moment:
In 1889 the Nernst equation was established, describing the potential of an electrochemical cell as a function of concentrations of the ions taking part in the reaction. The relationship between potential and pH [-log(H+)] is given by the formula:
Where U is the measured potential, U0 the standard electrode potential, R the universal gas constant, T the absolute temperature, n the charge (here, +1), and F the Faraday constant. The central term
is called the Nernst slope and gives the mV change per pH unit. As you can see, this term includes the absolute temperature, meaning the slope of your calibration is temperature-dependent. The higher the temperature, the steeper the slope.
Modern pH meters will correct the slope for this temperature variation when the calibration and measurement are not performed at the same temperature.
However, there is an effect that cannot be corrected by the instrument: samples do not have the same pH value at different temperatures! This can already be seen when looking at the example buffer table above. This temperature dependence is different for each sample. Therefore: Always measure your samples at the same temperature if you want to compare their pH values. Also be sure to carry out the pH calibration at the same temperature at which you are measuring your samples. This will greatly reduce the error of your pH measurement.
How do I perform my calibration?
First, prepare your electrode for calibration: open the refilling plug to ensure proper electrolyte outflow, rinse the electrode well with deionized water, and place the sensor into the buffer solution. An important note: both glass membrane and diaphragm must be covered with the buffer solution.
Additionally, assure that you position the electrode in the beaker for maximum reproducibility, especially when stirring. Never place the sensor haphazardly into the beaker where the glass membrane is touching the glass of the beaker; this can cause scratches on the glass membrane, leading to erroneous results.
Do you even have to stir at all? No, you do not! However, as there can be effects on the measured potential depending on the stirring speed, make sure that you always choose the same stirring speed among all buffer solutions, but also for calibration and subsequent measurements. Also, make sure that you do not stir so strongly that a vortex is formed, and avoid any splashing of the solution.
Now, you can start your calibration. Most instruments decide autonomously 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. However, this requires enough time for the electrode to reach a stable potential as otherwise the calibration will be biased.
Between the buffer solutions, the electrode is rinsed with deionized water. Never dry the electrode afterwards with a tissue, paper towel, or a cloth! This can lead to electrostatic charges on the electrode or even scratches on the glass membrane. Both will lead to longer response times, and in the latter case – to irreversible damage.
What do «slope» and «offset» mean?
Once you’ve finished the calibration, the instrument will display the calibration result. The calibration results usually consists of a slope and offset value. In this section, I want to explain their meaning.
The slope is normally given in % and is calculated from
the measured slope of the calibration divided by the theoretical slope (Nernst Slope) which is equal to 59.16 mV per pH unit at 25 °C. This is done in order to be able to correct the slope for temperature differences between calibration and measurement.
The second parameter that is evaluated is the pH(0), which is the pH value measured at 0 mV. In an ideal case, 0 mV corresponds to a pH value of 7. However, reality usually does not stick to the ideal case. Sometimes, the offset potential (Uoff) is also given, which corresponds to the potential at pH 7.
After calibration, always check the slope and the pH(0). The slope should fall between 95 and 103% and the pH(0) should lie between pH 6.8 and 7.2 (Uoff within ± 15 mV).
If you would like to get more information about your pH electrode, you can either perform a pH electrode test, which is implemented in some instruments from Metrohm, or a test according to Metrohm application bulletin AB-188.
If the pH(0) is outside the recommended range, this can be caused by a contaminated electrolyte or your probe may require a general cleaning.
If the slope is lower than 95%, this can be related to expired or contaminated buffer solutions. However, old and slow electrodes can also exhibit slopes outside of the limits. Therefore, always use fresh buffers.
If the slope is still too low even with fresh buffer, or the pH(0) is outside the recommended range after cleaning and subsequent reconditioning, it is time to replace the electrode.
- Select the calibration frequency and buffer types according to your samples.
- Make sure that you always use fresh, high quality, and certified buffers as your calibration can only be as good as your buffers.
- Set up your instrument correctly and use a fixed electrode position for the best reproducibility.
- Measure the temperature for calibration and subsequent measurement. Moreover, only compare pH values of samples measured at the same temperature.
- After calibration, check that your data for slope and pH(0) are within the optimal limits.
Would you like to learn even more about pH measurement? Come to our website and check out our informative webinars!
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Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) at Metrohm International Headquarters, Herisau, Switzerland.