The pH of a particular substance or solution is a figure that conveys its acidity or alkalinity. Accuracy in the matter of pH measurements is of the utmost importance when monitoring all types of processes in a range of industries. High accuracy measurements give the insight of a certain chemical’s condition or effect, for example, in the food industry, it informs of nutrient status, behaviour, and microbial activity. In the Pharmaceutical and chemical R&D, a highly accurate measurement can verify if a reaction has reached completion, or if the correct compound has been achieved. It can also affect yields and the quality of the end product.
Tips from Sentek to ensure Measurement Reliability
Measurement accuracy is not solely dependent on the use of a high-quality sensor, and the experimental set-up and methodology should also be considered. The tips below will ensure measurement reliability, repeatability and high accuracy.
- Ensure full temperature equilibration of the sensor and sample. If the sensors are not left long enough to stabilise, the readings will drift.
- It is best practise to perform both calibration and measurement at the same temperature. If the calibrating buffer temperature differs from the sample temperature with more than 15°C, an error as great as 0.1pH can occur.
- Temperature should not fluctuate during observations. Take measures to keep the temperature as stable as practically possible.
- Use temperature compensation (internal/external) for both calibration and sample measurements if not performed at room temperature.
- Ensure the temperature sensor is completely submerged in the sample and allow it to fully reach equilibrium.
- If the sensor is fitted with a fill hole, ensure the cover is opened during measurement. If the cover is tightly screwed, the electrolyte cannot flow/diffuse into the sample, giving drifty and unstable readings.
- Ensure sensors are periodically maintained (clean after each use, change fill solution regularly, ensure correct and continuous hydration of the pH membrane and junction).
- Remove air bubbles by shaking in a downward motion. Bubbles can form inside the sensor, or get trapped on the pH membrane and junction, giving erratic readings.
- Do not touch the pH membrane as this could contaminate with grease/sweat. If this happens, wipe with non-abrasive tissue and propanol, then rehydrate the membrane.
- Do not rub the sensor when drying, as it could create an electrical charge (polarisation), which can take up to a few hours to dissipate.
- Stir sample slowly during measurement.
- Condition the sensor in the sample for approximately 1 hour prior to measurement.
- Store the sensor in electrode storage solution to keep the pH glass membrane hydrated. Never store in tap water or deionised water, as this will dehydrate the pH membrane.
- Calibrate in buffers that bracket the expected pH of the sample.
- For the most accurate results, it is recommended to perform the tests in enclosed vessels to avoid contamination (for example, the sample or buffers can absorb CO2, thus, lowering the pH, or water can evaporate).
Steps to Ensure Accurate Measurement
To take high accuracy pH measurements, we must take the following steps to prepare the instruments for the measuring process:
- Use a water bath. Set at 25°C and place both the sample and calibration buffer(s) into the bath and allow them to reach temperature. Cover the containers to avoid contamination.
- Remove the soaker boot and rinse the sensor in deionised water, then blot the membrane dry (do not rub). Open the fill hole and leave for 30-60 mins in the first buffer (buffer 7 for example) to completely equilibrate.
- Calibrate in the first buffer and rinse the sensor in deionised water, then place the sensor in the next buffer. Allow a few minutes to stabilise and calibrate.
- Place the sensor into the sample, then start to slowly stir. Leave the sensor in the sample for 15-30 mins to equilibrate.
- While the sample is stirring, take the measurement of the sample.
Follow These Steps Every Time
It may be troublesome to observe these steps every time, however, this exercise helps to ensure accuracy of measurement every time.
Here at Sentek, we specialise in a wide range of products, including dissolved oxygen electrodes, ion-selective electrodes, and electrodes designed for education purposes and for use within food and industrial sectors. Please contact us today for more information on our product range.
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