Temperature plays a significant role on pH measurements. As the temperature rises, molecular vibrations increase which results in the ability of water to ionise and form more hydrogen ions. As a result, the pH will drop. The dissociation of water into hydrogen and hydroxide ion can be represented as:
H2O (l) ⇌ H+ (aq) + OH− (aq)
Every solution will undergo a change in their pH value through changes in temperature. A difference in pH measurements at different temperatures is NOT an error! The new pH level simply tells about the true pH for that solution at that specific temperature.
The value of Kw (Water ionisation constant) and pH with increasing temperature
It is clearly evident from the table that the pH of water at 0oC is 7.47, but the same water at 100oC will have a pH of 6.14*.
Typical pH values for solutions at different temperatures
From the table, we can conclude that the effect of temperature is greatest for highly basic solutions.
Common Oversight with pH testing
A common oversight is when you take a sample from a process tank and make the pH measurement in the laboratory. At that time, you are probably not measuring pH at the same temperature as the temperature in the process tank, this means that you will not have the correct pH value for the sample. Thus a pH value without a temperature value is meaningless. This can be simply overcome by testing pH on site and at the source of the sample.
Types of Temperature Compensation
There are two common types of temperature compensation when working with pH measurements.
- Automatic Temperature Compensation (ATC) compensates for the fluctuating milli-volt output from the electrode. ATC is commonly built into today’s pH meters for quick and accurate results.
- Solution Temperature Compensation (STC) (whether this is needed will depend on the pH accuracy required) converts the pH at the measurement temperature to the pH at a reference temperature. The reference temperature is generally 25°C. Only pH values taken at the same temperature or converted using solution temperature compensation can be compared to each other.
Remember that solution temperature effect and electrode temperature effect are different. To get your best results in pH measurement, always remember to calibrate and measure at the same temperature.
- The pH value of a solution is directly dependent on the temperature.
- A pH value without a temperature value is incoherent.
- Solution temperature compensation (STC) converts measured pH to the pH at 25°C.
- pH values taken at the same temperature or converted using solution temperature compensation can be compared to each other.
- To achieve highest accuracy, calibrate and measure at the same temperature.
*pH decreases with increase in temperature. But this does not mean that water becomes more acidic at higher temperatures. A solution is considered as acidic if there is an excess of hydrogen ions over hydroxide ions. In the case of pure water, there are always the same concentration of hydrogen ions and hydroxide ions and hence, the water is still neutral (even if its pH changes). At 100oC, a pH value of 6.14 is the New neutral point on the pH scale at this higher temperature.