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GUIDE TO ACCURATE AND RELIABLE pH MEASUREMENTS

Introduction

Obtaining accurate and reliable pH measurement is essential in research and development and quality control environments of many industries, including food and beverage, pharma, environmental monitoring, biological research and quality control in manufacturing. However, several factors can affect pH measurement accuracy, including electrode maintenance, sample preparation, buffer solution preparation and pH meter calibration.

To ensure accurate and consistent results, it is essential to use different measurement and maintenance practices. Here are some tips:

1.        Choose the correct electrode for your sample

pH electrodes play a critical role in performing correct pH value determinations, and selecting the right electrode based on your application is crucial.

Different types of electrodes and key differences for each

Epoxy vs Glass body Electrodes

Epoxy body electrodes are more durable, tend to be the more economical choice, and are ideal for environments where rough handling is expected. However, the maximum temperature limit for most epoxy body electrodes is approximately 176ºF (80ºC).

Glass body electrodes are capable of withstanding much higher temperatures, up to 230ºF (100 to 110ºC) depending on the specific electrode, and also offer chemical resistance to highly corrosive materials or solvents. Glass body electrodes are also easier to clean after use. Either electrode type will require care when handling, as even a small fracture to the bulb or body of the electrode can lead to erroneous readings.

Sealed vs Refillable Electrodes

Sealed or gel-filled electrodes are virtually maintenance free and ideal for most applications. They also tend to be a more economical choice. However, once the inner fill solution level is low (or if it dries out), the electrode will have to be replaced. This is why sealed electrodes may have a shorter life span than a refillable electrode.

Refillable electrodes have a port near the top of the electrode which allows them to be refilled once the inner fill solution is low or depleted. For many applications, this can significantly prolong the life of the electrode. Refillable electrodes allow the user to change the filling solution if it becomes contaminated. Users can also change the filling solution for specialized applications if necessary.

2.        Use correct calibration buffers

The buffers used for calibration must be selected according to the sample’s pH. For instance, if a sample is expected to have a pH of 7.45, the calibration must include pH buffers 7.00 and 9.21 (or similar). It is crucial to use a minimum of two fresh buffers for calibration and up to five calibration ranges.

Fresh buffers must be used every time to ensure accurate calibration.

The risk of contamination can be eliminated by using single-use buffer sachets

3.        Calibrate the pH electrode daily

Regular calibrations lead to more accurate results. Some applications might require a calibration before every measurement, but it is generally enough to calibrate every 24 hours.

Use fresh buffers for calibration

Check the expiry date of the calibration buffers before use.

Never calibrate the sensor directly in the buffer bottle.

Do not reuse calibration buffers, and never pour them back into the bottle.

Close the bottles immediately after use and store them at room temperature.

4.        Check your calibration results

The electrode calibration provides valuable data on the electrode condition. The slope of the calibration curve should lie between 95% and 102% of the theoretical value.

5.        Regular cleaning of the electrode

Frequent cleaning of the electrode is necessary to remove any residuals and prevent contamination from the previous sample.

After rinsing the electrode with distilled water, dab it dry with a tissue, but never wipe it. The rough surface of the paper tissue can scratch the pH-sensitive glass membrane or create an electrostatic charge that might cause the measured signal to become very unstable.

6.        Stir the sample

When measuring pH, gently stir the sample to ensure that it is homogeneous. It is essential that calibration and measurement are done under the same stirring conditions. Do not use the electrode to stir.

7.        Measure the temperature of the sample

Every sample has a different pH behavior depending on the temperature, and therefore, the temperature should be measured.

The temperature probe is either integrated into the pH electrode or an external one can be used.

If the electrode and the sample have different temperatures, give the system enough time to equilibrate. To compare the pH of different samples, they should always be measured at the same temperature.

8.        Keep the electrode cleaned

Using a dirty electrode is one of the typical sources of error in pH measurement. Make sure that the electrode is always clean and well-maintained.

9.        Store the electrode in a proper solution

Electrodes should always be stored in aqueous and ion-rich solutions.

The electrode should never be stored dry or in distilled water as this will affect the pH-sensitive glass membrane and thus shorten the electrode’s lifetime. If you are unsure which storage solution to use, check the electrode’s manual.

In conclusion, obtaining accurate and reliable pH measurements ensures high-quality data and reproducible results in many scientific fields. By following the tips and hints provided in this article, including selecting the correct electrode for your sample, using the correct calibration buffers, rinsing the electrode without wiping, checking calibration results, stirring the sample, daily calibration, measuring the temperature of the sample, using fresh buffers, keeping the electrode cleaned, and storing the electrode in a proper solution, you can improve your pH measurement accuracy and obtain consistent results.

Reference

pH 101: A beginner’s guide to accurate and reliable pH measurements | GlobalSpec

pH Electrode Selection Guide for Component & Style Types – Cole-Parmer (coleparmer.com)

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