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Even small changes in temperature can significantly impact pH measurements, as the two are linked. The temperature affects a pH measurement in several ways. This blog article will cover the reasons why and discuss how you can handle the different effects from temperature on pH measurements.

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–  Choice of an appropriate membrane glass

–  Correct positioning of the temperature sensor and the pH electrode

–  Combined pH electrodes with «Long Life» reference system

–  Calibration of the pH electrode

–  Temperature of the measuring solutions

### Why does temperature affect the pH value?

Temperature and pH value are related by the Nernst equation. This equation describes the ideal relationship between the activity aM of a measuring ion in solution and the potential measured between the reference electrode and the measuring electrode. The temperature influences the Nernst potential – more often called the slope in pH measurement.

U = measured potential

U0 = temperature-dependent standard potential of electrode

R = general gas constant 8.315 J mol-1 K-1

T = temperature in K

z = ionic charge including sign

F = Faraday constant 96485.3 C mol-1

aM = activity of measuring ion M

From this, the slope UN can be calculated:

A temperature change of 1 °C corresponds to a change of 0.2 mV. To put this in relative terms: a pH difference of 0.01 corresponds to 0.6 mV. Therefore, it is necessary to take the temperature into account in all pH measurements. Otherwise, no correct results will be obtained if calibration and measuring temperatures are not known.

The slope UN is different for different temperatures. At T = 298.16 K = 25 °C and z = 1, the slope UN is equal to 59.16 mV. For other temperatures, a different value for the slope UN is used in the Nernst equation. This is called temperature compensation. Table 1 lists the value of the slope for different temperatures.

Table 1. Temperature dependency of the slope.

Temperature T [°C] Slope UN
[mV/pH unit]
Temperature T [°C] Slope UN
[mV/pH unit]
0 54.20 50 64.12
5 55.19 55 65.11
10 56.18 60 66.10
15 57.17 65 67.09
20 58.16 70 68.08
25 59.16 75 69.07
30 60.15 80 70.07
35 61.14 85 71.06
37 61.54 90 72.05
40 62.13 95 73.04
45 63.12 100 74.03

Modern pH meters include a function for temperature compensation purposes. This means that as soon as a temperature sensor is connected to the pH meter, the temperature dependency of the slope UN is automatically considered and corrected. Measuring the temperature not only helps to ensure accurate pH measurements, it also ensures compliance with GLP/ISO guidelines which require recording of the temperature for all measurements.

### Temperature effects on pH measurement and how to handle them

The pH value is probably the most commonly measured parameter in analytical chemistry. It influences product characteristics, chemical and biochemical reactions, and physiological processes, among other things. Often, consistent ambient conditions are necessary for precise measurement results.

In some cases temperature changes cannot be avoided. For example, simply opening a door can cause a change in the ambient temperature. Even when working in air-conditioned environments, exothermic reactions may occur resulting in a temperature increase. The causes of temperature fluctuations could not be more varied—therefore, this section will give you some preparation tips. Follow them to minimize or even eliminate possible temperature-related effects before you start your calibration / pH measurement.

#### Choice of an appropriate membrane glass

To cover pH measurements in a wide range of samples, Metrohm offers pH electrodes with different membrane glass types.

Figure 1. Unitrode easyClean with integrated Pt1000 and green-colored «U» membrane glass.

At higher temperatures the pH electrode ages faster which leads to an increase of the membrane resistance. Thus, it becomes more difficult for hydronium ions to pass through the membrane. This can change the equilibrium potential of the electrode, causing a shift in the pH reading.

For pH measurements at higher temperatures, use a pH electrode with green-colored membrane glass «U» as they are more heat-tolerant.

Figure 2. Porotrode with blue-colored «T» membrane glass and filled with Porolyte reference electrolyte.

pH measurements at low temperatures show similar effects. At lower temperatures, the membrane becomes more rigid and ion transportation is more difficult as well. Additionally, the activity of hydrogen ions in the electrolyte solution decreases at low temperatures. Both effects result in an increase of the membrane resistance.

Roughly speaking, when the measuring solution is cooled by 10 K, the membrane resistance doubles [1,2].

For pH measurements at lower temperatures, an electrode with a blue-colored «T» membrane glass and a thickened reference electrolyte is recommended as the reference electrolyte contains solvents which act as an antifreeze.

#### Correct positioning of the temperature sensor and the pH electrode

Make sure that the temperature sensor is positioned in the immediate vicinity of the pH electrode’s glass membrane. If not, the temperature of the measuring solution cannot be measured correctly.

Furthermore, the pH compensation will be incorrect as the temperature and pH are not measured at the same location.

To completely avoid this effect, use a pH electrode with an integrated temperature sensor. In this case, the temperature sensor is located within the electrode in the immediate vicinity of the glass membrane (Figure 3).

Figure 3. pH electrode with A: separated and B: integrated Pt1000 temperature sensor.
Figure 4. With the Metrohm «Long Life» reference system, the dissolved AgCl is retained in the cartridge and cannot block the diaphragm.

#### Combined pH electrodes with «Long Life» reference system

Most pH electrodes available on the market are combined pH electrodes with the Ag/AgCl reference system. The solubility product of silver chloride depends on the temperature.

The solubility product of silver chloride in water is very small at approximately 10-10 mol2/L2. However, silver chloride dissolves very easily under complex-formation. Increasing temperature favors this effect, resulting in a change of the equilibrium between solid and dissolved silver chloride. Therefore, if the temperature changes, it is necessary to wait until a stable equilibrium is reached again, as this equilibrium determines the potential of the reference electrode.

Thanks to the «Long Life» reference system used in Metrohm pH electrodes (Figure 4), the thermodynamic equilibrium between silver, silver chloride (solid), and silver chloride (dissolved) is established very quickly, and the potential of the reference electrode becomes stable after a very short time.

Figure 5. Isothermal intersection point for the calibration of a pH electrode at two different temperatures.

#### Calibration of the pH electrode

Metrohm pH electrodes are constructed according to DIN 19263. These electrodes exhibit a 0 mV potential reading (zero point) around pH 7. As previously explained, according to the Nernst equation, the electrode slope and (under certain circumstances) also the electrode zero point shift when the pH electrode is exposed to a temperature change.

Considering the calibration curves (isotherms) of pH electrodes at different temperatures under ideal conditions, one would expect them to intersect at the electrode zero point. Unfortunately, this is not the case with pH electrodes in real life. An isothermal intersection point is formed (Figure 5) close to the electrode zero point. How close depends on the condition of the electrode.

To minimize such effects, the calibration of the pH electrode should be carried out at the same temperature as will be used for subsequent pH measurements.

#### Temperature of the measuring solutions

The pH value of pure water at 25 °C is 7.00. In this case, there are equal numbers of hydronium and hydroxide ions present in the water. Due to the temperature dependence of the ionic product of water, this equilibrium shifts towards a higher pH at lower temperatures and vice-versa. These equilibrium shifts are known for buffer solutions and for well-known acids and bases (see Table 2 for examples), but not for all kinds of sample solutions.

Table 2. Three examples showing how changes in temperature can affect the pH value of the sample [3].

pH of solutions measured at different temperatures 0 °C 25 °C 50 °C
H2O 7.47 7.00 6.63
c = 0.001 mol/L HCl * 3.00 3.00 3.00
c = 0.001 mol/L NaOH 11.94 11.00 10.26
##### * Temperature effects are weaker regarding pH determination of acidic substances. There is a general trend of increasing pH value with increasing temperature in these cases.

Even the most modern pH meters can only correct for the temperature behavior of the electrode, but never that of the solutions to be measured. For correct pH measurements, it is essential to always measure the pH value of your samples at the temperature at which they were sampled. As an example, if a sample is taken at 10 °C, then the pH electrode calibration and sample measurement should also be done at 10 °C. Following this protocol helps to avoid unwanted thermal equilibrium effects and results in a faster pH electrode response.

### Conclusion

Due to their optimized construction, the real behavior of high-quality Metrohm pH electrodes deviate only slightly (asymmetry potentials of maximum +/- 15 mV) from the ideal values. However, like most things, more than one factor is at play.

The checklist below can help you to achieve precise measurement results during your calibration and pH measurement. If you can answer all listed points with a YES, most effects caused by changes in temperature are considered.

YES / NO
(    )

I’ve chosen an appropriate pH glass electrode considering its membrane glass type for my application.

(    ) My combined pH glass electrode is equipped with a «Long Life» reference system.
(    )

My temperature sensor is positioned close to the glass membrane of my pH electrode.

OR

I’m using a combined pH glass electrode with integrated temperature sensor for my calibration / pH measurement.

(    )

My pH meter has integrated temperature compensation.

(    )

My calibration is carried out at the same temperature as all subsequent pH measurements.

(    ) All sample solutions to be measured are at the same temperature.

#### References

[1] Degner, R.; Leibl, S. PH Messen: So Wird’s Gemacht!; Wiley, 1995.

[2] Galster, H. PH-Messung: Grundlagen, Methoden, Anwendungen, Geräte; VCH, 1990.

[3pH und Temperatur – zwei un­trenn­bare Größen. Wiley Analytical Science. https://analyticalscience.wiley.com/do/10.1002/was.00050234 (accessed 2023-02-09).