Whether in food and beverage, pharmaceutical, chemical or environmental monitoring, pH measurements are relied on to ensure product quality and compliance with safety standards. However, one factor that affects the pH readings yet can be easily overlooked is temperature. A slight difference in temperature can change the sample’s pH values, thereby causing significant measurement errors.

Imagine a food testing laboratory scenario where the pH of brine is being measured. If the pH meter used is calibrated at room temperature but the product is tested after being heated, the readings would vary, not reflecting the right acidity levels in the sample. Such errors can have adverse implications.

Let's explore how temperature impacts pH readings, why it's important to understand the connection between pH and temperature compensation.

The relationship between pH and temperature

To understand the connection between pH and temperature, it is important to start with the definition of pH. pH is an indicator of acidity or alkalinity. The pH scale ranges from 0 to 14 levels, where 7 is considered neutral, for example, pure water. The lower the values are (0 to 6.9), the higher the acidity, while the higher pH values (7.1 to 14) indicate higher alkalinity.Source: Mettler ToledoSource: Mettler Toledo

How does temperature affect pH?

One of the fundamental properties of pH is that it is temperature dependent. As temperature changes, the behavior of the molecules in a solution also changes. With higher temperatures, the dissociation of water molecules and the mobility of ions also increase, leading to shifts in pH values.

For instance, at 25° C, the neutral point is pH 7. However, at higher temperatures, the neutral pH value decreases, and this change can lead to misinterpretation of results if not properly accounted for.

Temperature changes also impact the performance of the pH-measuring instruments. The pH electrodes respond differently depending on the temperature.

In extreme cases, if the sample is cold, it will be necessary to use an electrode whose membrane is made of low-temperature glass (LoT) with an electrolyte working temperature down to -30° C. The same applies to high temperatures — up to 130° C.

Compensating for temperature variations

There are many temperature compensation techniques used to maintain the accuracy of pH readings. Here are the most common.

Automatic temperature compensation (ATC)

Advanced pH meters usually come with ATC. With this feature, the calibration readings are adjusted based on the current temperature of the buffer solutions used for calibration. The ATC feature is crucial in ensuring that the pH readings are accurate across a range of temperatures.

It is important to note that most pH meters, however, allow ATC only for calibration and not for the sample measurements. The result of a pH measurement consists of the pH value and the sample temperature. To analyze the results meaningfully, all measurements should be carried out at the same temperature.

Manual temperature correction

For pH meters that do not have ATC, users need to manually correct for temperature. This involves applying a correction factor to the pH readings based on the temperature at which the measurement is taken. Standard correction tables or formulas can adjust the pH value according to the difference from the calibration temperature.

Temperature-stabilized buffers

When calibrating pH sensors, it is advisable to use temperature-stabilized buffer solutions that match the temperature of the sample being tested. By ensuring that the calibration buffers are at the same temperature as the sample, users can minimize the risk of introducing errors due to temperature discrepancies.

Regular maintenance and calibration

Regular maintenance of pH sensors, including frequent calibration checks at different temperatures, can help identify drift and other issues that may lead to inaccurate readings. Users should follow manufacturer guidelines for calibration frequency and procedures, adjusting as necessary based on their specific operational conditions.

Why temperature compensation matters

Recognizing how temperature affects pH is crucial in various fields.

Measurement precision: If temperature fluctuations are not compensated, they may lead to incorrect readings, resulting in poor decisions about overall product specifications and failure to meet regulatory compliance or environmental assessments.

Quality control: Pharmaceuticals, food processing and agriculture rely on specific pH levels. Any minor deviation from these values can change the product quality. As such, they must ensure that their measurements reflect the actual conditions of their products or processes.

Safety concerns: Poor pH readings, in some cases, can lead to unsafe situations, such as the incorrect treatment of wastewater or improperly formulated consumer products.

Summary

Understanding the relationship between pH and temperature is not just an academic exercise; it's an essential aspect of accurate measurement in many practical applications. Whether one is working in the lab or on a food production line, understanding temperature's influence on pH measurements helps in making informed decisions.

To contact the author of this article, email pHmatters@mt.com