Temperature Compensation for pH Instruments

Oakton Tech Tips

Temperature Compensation for pH Instruments

Tech Tip #11 ©1997

Although it is widely advertised, the need for temperature compensated pH measurements is not always explained except in technical books and articles. This Tech-Tip will give a brief explanation of the major characteristics of temperature compensation in pH instruments.

1. The Solution Temperature Effect

When temperature changes, the actual pH of the solution being measured can change. This change is not an error caused by the change in temperature. It is the true pH of the solution at the new temperature. Since this is not an error, there is no need to correct or compensate for this temperature effect.

2. The pH Electrode Temperature Effect

There is only one major temperature effect in pH measurement that can cause errors in readings.  This is the change in the electrode’s response (or sensitivity) to pH that results from a change in temperature. It is the only reasonably predictable error due to changes in temperature and is the only temperature-related factor that pH instruments with temperature compensation can correct.

This temperature error is very close to 0.003 pH/ºC/pH unit away from pH 7. In a perfect pH electrode—one that is zeroed at exactly pH 7—there is no temperature effect on the electrode sensitivity at pH 7 no matter how much temperature changes. (Remember though, the temperature of the solution may actually change the pH of the solution to be different from pH 7 at the new temperature). Most pH electrodes are not perfect, but the errors from changes in temperature are still very minute when near pH 7, plus or minus one or two-tenths of a pH, and can be disregarded. However, the further from pH 7 the solution is and the greater the temperature change, the greater the measurement error due to changes in the electrode's sensitivity.

The errors from changes in electrode sensitivity due to changes in temperature are the only errors that can be corrected by , meters with temperature compensation. In both cases, a correction factor based on 0.003 pH/ºC/pH unit away from pH 7 is applied to the final reading you see on your meter.

There are Two Variations for Temperature Compensation:

    Automatic: Where a temperature sensor signals the meter what the solution temperature is, and the meter automatically corrects the pH readings for changes in the electrode sensitivity.

    Manual: Where the user must dial or key in the solution temperature, and the meter then corrects the pH readings for changes in the electrode sensitivity.

Temperature compensation eliminates most of the temperature-dependent measurement error from the electrode.  Further minimization of this error in both temperatures compensating and non-temperature compensating pH instruments can be achieved by calibrating the instrument and electrode in pH buffers that are close to the expected measurement values for pH and at (or near)the same temperature as the solution to be measured. This technique also minimizes other temperature-related errors that occur in all electrodes but which cannot be predicted or compensated for when temperatures change.

These unpredictable pH measurement errors due to temperature changes are somewhat complex in nature and vary from the electrode to electrode. They can result in small but noticeable pH measurement errors. Unfortunately, there is not much that can be done about these in a manner that is economical or practical for most users of pH instruments. Therefore, as a practical matter, they are mostly ignored in everyday pH applications.
 

Information courtesy of Oakton® Instruments

Changes in temperature affect pH. Therefore, when the temperature of a solution changes, it will likely change the solution’s pH. When this happens while measurements are being performed, it does not indicate an error. It simply means that the temperature of the solution has changed.

electrode and buffer

However, it should be acknowledged that this temperature effect is particularly important in applications where precise pH measurement is critical, such as chemical reactions, biological systems, and industrial processes.

For example, a small change in pH can affect the rate of a chemical reaction or the function of an enzyme. In biological systems, changes in pH can affect the activity of proteins and other biomolecules. In industrial processes, pH can affect the quality of a product, the efficiency of a process, and the safety of workers.

To account for the effect of temperature on pH, it is essential to measure pH at a known temperature and to correct the pH measurement for the temperature of the solution. This correction can be done using a temperature compensation factor that accounts for the change in the dissociation constant of water with temperature.

General pH Equipment

Modern pH meters use automatic temperature compensation (ATC) to adjust for the temperature dependency of pH. The pH meter's temperature sensor measures the temperature of the solution being tested, and the meter applies a temperature compensation factor to the pH reading based on the temperature of the solution.

Automatic temperature compensation is essential for accurate pH measurement in many applications, especially when temperature changes are frequent or significant. It allows pH measurements to be made consistently and reliably, regardless of the temperature of the solution being tested.

How It Works

Automatic

This option occurs when a temperature sensor signals to the meter what the solution temperature is, and the meter automatically corrects the pH readings for changes in the electrode sensitivity.

Manual

This option requires the user to dial or key in the solution temperature. The meter then corrects the pH readings for changes in electrode sensitivity.

Temperature Compensation Explained

Temperature compensation eliminates most of the temperature-dependent measurement error potential from an electrode. Further minimization of this error in both temperature-compensating and non-temperature-compensating pH instruments can be achieved by calibrating the instrument and electrode in pH buffers that are close to the expected measurement values for pH and at (or near)the same temperature as the solution to be measured. This technique also minimizes other temperature-related errors that occur in all electrodes but cannot be predicted or compensated for when temperatures change.

These unpredictable pH measurement errors due to temperature changes are somewhat complex in nature and vary from electrode to electrode. They can result in small but noticeable pH measurement errors.

Unfortunately, not much can be done about these kinds of errors in an economical or practical manner for most pH instrument users. Therefore, as a practical matter, they are mostly ignored in everyday pH applications.

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