Does Temperature Affect pH?

Does Temperature Affect pH

The pH (pondus Hydrogenium) indicates the acidic or basic nature of a solution and is a measure of a number of free hydrogen ions present in an aqueous solution.

A solution with higher hydrogen ion concentration has a lower pH and that which has a lower hydrogen ion concentration has a higher pH.

The temperature change in a solution increases or decreases with the changing flow of heat and the pH values are inversely proportional to the temperature of the solution.

The temperature affects the activity and concentration of ions in a solution which again is related to the pH of a solution. When the temperature of a solution rises, the molecular vibrations in the solution rise resulting in the ionization and formation of H+ ions. This happens due to a decreased tendency of hydrogen bond formation. Owing to temperature changes, the pH value of the solution changes.

The molecular equilibrium of a solution undergoes changes in the state due to temperature changes thereby affecting the nature of the solution. If we increase the temperature from 0°C to 10°C, the pH of a solution or a substance will drop by 0.2.

How do the pH values of the solutions change with varying temperatures? Let us understand the science behind this.

Relationship between temperature and pH

Le Chaterlier’s principle explains how factors like temperature influence the chemical solution in equilibrium. The principle says that changes in temperature, pressure, volume or concentration of a reacting system at equilibrium allow the system to change its state in order to nullify the effect of stress.

This is achieved by a shift in the equilibrium state as it undergoes a series of opposing changes. In order to counteract the temperature variation, exothermic or endothermic reactions occur and reverse the changes impacted by stress.

As mentioned in the statement, the temperature can have an influence on the equilibrium state of a system and it affects the pH of a solution.

For instance, if a temperature change is applied to water, the rise in temperature will cause the equilibrium to lower the temperature and that will occur by the absorption of additional heat. This results in the formation of hydrogen ions and hydroxide ions which lower the pH of water.

The dissociation of water into hydrogen and hydroxide ions takes place.

H20 (L) = H+ (aq) + OH- (aq)

 

What are pH electrodes and how do they work?

Does Temperature Affect pH

A pH meter consists of a pair of probes to which the electrodes are attached. The two electrodes are –a glass electrode and a reference electrode. The electrode is a small tube that is tied to a pH-meter using a cable.

The electrode consists of a special fluid called the 3M-fluid (3M Kalium Chlorine) that has silver and platinum wires. The small membranes are delicate.

When placed in a solution, the H+ and OH- ions pass into the electrode crossing the membrane. A slight negative and positive charges are created at each end of the electrode.

The potential of both the glass electrode and the reference electrode is used to determine the pH of the solution being tested. The charge potential will determine the H+ and OH- ion density.

This density will determine the pH of the solution which will appear on the pH meter. The temperature of the solution affects the charge potential in the electrode.

The pH meters, therefore, show the temperature of the solution. Any change in temperature leads to an increase in electrode sensitivity. With an increase in electrode sensitivity, there is a change in electrode potential and thus the pH changes.

This can be exemplified using the Nernst equation. The Nernst equation is used to determine the equilibrium potential and equilibrium constants. The equation determines the voltage required to oppose the net movement of an ion down the concentration gradient.

E(cell) = E° (cell) – RT/zF ln Q

The above equation implies that E(cell) decreases as T increases.

How does pH change with the change in temperature?

The temperature affects the pH readings in mainly two ways:

By impacting the pH meter or pH electrodes

The major effect of temperature on the pH values of aqueous solutions leads to errors in readings and pH measurements.

This happens due to change in the response of electrode to pH which happens as a result of variation in temperature.

It is a predictable error and can be corrected using a pH meter with temperature compensation. The errors from changing electrode response or sensitivity with respect to changes in temperature can be corrected using pH controllers.

By impacting the sample solution

When the solution to be tested for pH value is heated, the activity of the hydrogen ions increases and they move faster. The voltage increases with increased movement of ions.

When the temperature of the solution is brought down, the activity of hydrogen ions decreases and their movement becomes slow. This results in a drop in voltage. The measured potential gives a new pH value on the pH meter.

The temperature variations that occur in the sample solution are a result of chemical activity and thus the temperature is monitored using the ATC (Automatic Temperature Compensation) systems.

 

How does temperature affect the pH electrodes?

The effects of temperature on the electrode are categorized into three main types.

Electrode Slope Effects

The relationship between the electrode response and temperature variation remains constant.
The electrode behavior with changing temperatures can be monitored using ATC systems.

Isothermal Point Effects

Isothermal point changes produce errors in the pH readings as the ATC system fails to work when the pH electrodes are placed in two different temperatures. This can be avoided by carrying out calibration and sample measurement at the same temperature.

Electrode Response Time Effects

The resistance of membrane glass and the response time of the electrode changes with a change in temperature. Thus, some electrodes mention the operating minimum and maximum temperature levels for proper results of pH value.

 

How to eliminate temperature-related errors in pH readings?

The errors in the pH readings due to temperature changes can be minimized or eliminated by using a pH controller. For the pH readings to be accurate, the calibration ( checking if the pH electrodes are correctly determining the pH using voltage or potential) is carried out carefully.

The pH electrodes are calibrated by inserting them into buffer solutions ( solutions of known pH) and are adjusted according to the buffer solution.

The temperature factor that serves as a barrier while obtaining accurate readings is eliminated by employing pH meters that have built-in thermometers. These thermometer-assisted pH meters work by correcting the pH values that alter due to temperature fluctuations. ATC systems are designed to account for changes that occur in the electrode.

 

pH electrodes with automated temperature compensation:

This pH electrode is built with a temperature sensor placed on the top of the electrode which is connected to a pH controller. Both the sensor and the controller work simultaneously to adjust the readings and avoid errors that occur due to temperature variation and electrode sensitivity.

pH controllers with manual temperature compensation:

In this case, the laboratory technician has to manually enter the temperature of the solution into the pH controller. The meter automatically corrects the pH readings for the variation in the electrode sensitivity.

 

What impact does temperature have on the pH of different samples?

There is a definite relationship between temperature and pH for every sample. This is called the temperature coefficient that varies for different samples.

For example, a sample tested at 30°C may have a pH of 14 and the same sample when tested after heating at 80°C may show a slight change in the value.

The effect of temperature variation is relatively higher in the case of alkali or basic solutions than in acidic solutions.

At high temperatures, the chemical equilibrium shifts towards the endothermic direction and at low temperatures the equilibrium shifts towards the exothermic direction. In easy words, when the temperature rises, the reaction absorbs more heat and when the temperature drops, the reaction produces more heat.

 

Does pure water become acidic with an increase in temperature?

Drop in the pH values of pure water does not make it acidic because in pure water the concentration of hydrogen and hydroxide ion remain constant.

There is an equal increase in the -OH ions as that of H+ ions which balance the overall nature of the water. This maintains the neutral pH of water regardless of changes in the pH levels.

The pH of water at room temperature is 7 and when it is subjected to a high temperature of 100°C, the pH drops to 6.14 which is neutral but slightly lower than 7.

Water consists of OH- and H3O+ ions whose pH will be denoted as log{c(H3O+)}. With changes in temperature, the chemical equilibrium of water will also change. However, the concentration of H3O+ and OH- in the water remain equal. Water undergoes autoionization and is amphoteric (behave either as acid or a base) in nature.

 

Conclusion: Does Temperature Affect pH?

Temperature impacts the pH of samples in two ways, either by affecting the electrode or by changing the nature of samples. Most of these effects can be eliminated using pH meters with built-in ATC systems.

Not all ATC systems are reliable when working with temperature variant samples. Suitable pH electrodes for a particular temperature of the sample are to be employed.

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