Temperature plays a key role in pH measurements. When temperature levels increase, the pH level drops. However, this does not mean that the sample becomes more acidic at higher temperatures. A solution is considered acidic only if there are more hydrogen ions than hydroxide ions.
Both temperature and pH are essential parameters to gain an understanding of water quality in different solutions. When measuring the pH of a solution, you must also measure the temperature of the solution, as pH varies with temperature. Temperature and pH are also measured at the same time because a pH value without a temperature value is incoherent.
One type of temperature compensation when working with pH measurements is that many pH sensors have an Automatic Temperature Compensator (ATC) sensor built in. So, how does temperature alter pH, and why does it have such a large effect?
In this article, we will look at the relationship between temperature and pH, and how temperature affects pH readings and pH solutions.
What’s The Relationship Between Temperature & pH?
To jump straight in, the pH level of a solution is inversely proportional to the temperature. So, when the temperature increases within a solution, molecular vibrations rise, which results in ionization and the formation of hydrogen ions (H+). For those that skipped chemistry class, the more hydrogen ions in a solution, the more acidic it becomes. Therefore, the pH level decreases as temperature increases in a solution.
To put it into perspective, if we were to increase a solution’s temperature by 50 °F, the pH of the solution would drop by 0.2. However, if the temperature were to decrease, the opposite will happen, and the pH level will increase very slightly. While that may not sound like a large decrease in pH, imagine the difference between the winter and summer months and the effect it has on a plant.
Imagine being a plant during the winter – happy in your soil with the correct pH. A few months later, things start to warm up outside as it’s summer, and the temperature has increased from winter’s 50 °F to 86 °F – that’s a pH decrease of 0.6, enough to cause issues with your plant soil.
Using Le Chaterlier’s Principle
According to Le Chaterlier’s principle, if you alter the conditions of a reaction in dynamic equilibrium, the point of equilibrium shifts to counteract the changes made.
This is a perfect explanation of what happens when temperature affects the chemical solution and pH level in equilibrium. But, how do we get there?
So, Le Chaterlier says that changes in temperature, pressure, volume, or internal concentration can shift the equilibrium state. This is how temperature can shift the equilibrium state and the pH of a solution.
So, when the temperature increases, the equilibrium is forced to lower the temperature by absorbing more heat. This results in the formation of H+ ions, which lowers the pH of a solution. The dissociation of ions when heat is increased inside a solution is displayed in the equation below:
H20 (L) = H+ (aq) + OH- (aq)
Are pH & Acidity The Same?
Often confused is the relationship between pH and acidity. A decrease in pH does not mean that the solution’s water has become more acidic at a higher temperature. A solution can only become more acidic when the number of hydrogen ions increases beyond the number of hydroxide ions.
If we were to look at purified water, for example, the concentration of hydrogen and hydroxide ions never changes, and therefore the pH of pure water is always neutral with a pH value of 7.0 at room temperature.
But, if you were to increase the room temperature by 167 °F, the pH meter would read the pH value as 6.14. Despite it being still neutral on the pH scale, that is a significant decrease in pH which may result in someone making the wrong choices, simply because of a temperature change – thinking that the chemical properties of the sample have changed, when they haven’t.
How Does Temperature Affect pH Readings?
The effects of temperature can be divided into two main categories:
Temperature effects on the accuracy and speed of response of a pH electrode
Temperature Coefficient of the solution (either buffer or sample) being measured by the pH sensor
Temperature Effects On pH Electrode
Temperature affects pH electrodes both physically and chemically. To understand how we must cover the following effects that temperature has.
Electrode Slope Effects
As temperature increases, the electrode slope increases. This can be fully compensated by a pH meter that uses an ATC.
Most modern pH meters have an ATC to measure the temperature of the sample. If your pH meter has an ATC, your pH readings will automatically be adjusted to account for any temperature changes. Alternatively, a temperature sensor can be used with the pH meter, but this can be inconvenient for people measuring small applications and it requires the use of conversion tables.
Isothermal Point Effects
In an ideal electrode world, plots are made on a graph to compare pH, intersecting at a pH of 7.0 and 0 mV (pure water). Usually, the plots on the graph intersect at different points for each pH electrode.
This ‘reality’ doesn’t normally happen, as the temperature gets in the way. When calibration and sample tests are taken at different temperatures, the displayed pH readings on the plot can change as much as 0.1 pH.
Unfortunately, ATC systems cannot compensate for this effect. So, environmental conditions which could result in Isothermal Point effects should be avoided. But, what does this mean?
Well, it is not as complicated as it may sound. All you need to do is calibrate and measure the sample being tested at the same temperature (or as close as you can get it).
Saying that, if you are working with an application that requires extremely high-accuracy work, it is recommended to work with a laboratory water bath. You can then set the water bath to a specific temperature and place both the samples and calibration buffers inside the water.
Most electrodes will state a minimum and maximum operating temperature, so keep this in mind if you are working with a water bath, as the resistance of the membrane glass and pH probe response time can shift in response to temperature.
Temperature Effects On pH Samples
In addition to temperature changing the working principle of the pH electrode, it can change the sample too.
Samples have a specific relationship between pH and temperature levels. This relationship is commonly known as the temperature coefficient. The temperature coefficient of a sample differs between solutions and samples, so this makes it tricky to compensate.
Imagine testing a sample at 77 °F and the pH meter displays the value as 14.0. Then you heat the sample to 140 °F without changing the chemical properties of the solution, but the pH value has changed by 1.0.
Usually, this shift in pH as a response to a temperature increase happens more often with alkaline solutions than acidic ones.
So, can we compensate for this?
Yes, but it’s not simple. The only way to compensate for the temperatures’ effect on the pH of samples is to retest, ensuring that the buffers and samples are all tested at the same temperature, or as close as possible to minimize error.
Measuring pH & Temperature
The easiest way to measure pH in a solution is with a pH probe. A pH probe is used to detect the potential hydrogen (pH), which has a usual range from 0-14. Measuring pH tells us how much hydrogen is in a solution. It can also tell us how active hydrogen ions are. If a solution has a high amount of hydrogen-ion activity, it is an acid, but if a solution has a high amount of hydroxide ion activity, it is a base.
A pH meter works by measuring the electrical potential (voltage) produced by the solution being tested, using the potential difference to determine the pH. An acidic solution has more positively charged hydrogen ions than an alkaline solution. Therefore, the solution has a greater electrical potential to produce an electrical current.
How To Prevent Temperature-Related Errors When Testing pH?
The good news is that pH reading errors due to temperature fluctuations can be eliminated using a pH sensor with a built-in ATC system. If your pH sensor does not have an ATC system, then temperature must be measured and compensated when taking pH measurements.
This is why we do not recommend using pH litmus paper (Old School pH Test Kits), as they do not compensate for potential temperature changes.
As previously mentioned, measuring samples in a controlled water bath helps compensate for temperature fluctuations, as you can place both the buffer solutions and the sample being tested inside the water.
Utilizing a Temperature Correction Calculator
The easiest way to work out the temperature compensation during calibration is to use a pH temperature correction calculator. This is only necessary if your pH meter does not feature an ATC system. The temperature correction calculator attempts to correct the reading of the pH meter.
Typically, pH levels decrease with an increase in temperature. However, this does not mean that the solution becomes more acidic at higher temperatures. Only when a solution has an excess of hydrogen ions over hydroxide ions, is it considered acidic. If you have any questions regarding temperature or pH, or what testing kits we have to offer, do not hesitate to contact our world-class team at Atlas Scientific.
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