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Why does a pH meter need calibrating?
Firstly - it's important to understand that all pH meters need to be calibrated to a pH electrode, not just models from our AWE Instruments range, but all pH meters.
Before we can answer the question of why we calibated a pH meter - first we need to understand what's happening inside our pH electrode.
What is happening inside a pH electrode?
BNC Connector
The industry standard connector type for a pH electode which transmits the millivoltage signal through very low resistance connecting cable.
pH Glass
The pH glass on an electrode is hydrogen ion selective, only allowing the H+ ions to permeate through.
Reference Electrode
Silver Silver-Chloride reference electrode which allows the electrode to generate the millivolt signal for use with a pH electrode.
Salt Bridge Solution
Salt bridge solution within the electrode which is contact with the measured solution via a reference junction.
We've included a lot in our knowledge base about pH electrodes and pH measurements which runs into a fair amount of detail about the inner working of a pH electrode including pH itself, pH theory, and temperature compensation in pH measurement.
For a more abridged version of the inner workings of a pH electrode we can say that most pH electrodes comprise of a number of common elements.
This includes a hydrated gel layer (glass membrane) which combines with a salt bridge solution, electrode and a reference junction which work in combination to produce an electrical potential at the glass tip. The design of pH electrodes follow this basic principal although the design and manufacture of each of the elements is a closely guarded secret between different manufacturers of pH electrodes.
For instance we offer a variety of glass types, polymerised derivative solutions and annular type references to give our electrodes different properties, including:-
- Long lasting general purpose pH electrodes
- High temperature pH electrodes
- Low sodium Ion pH electrodes
- Low Conductivity pH electrodes
Despite the different methods and materials used in the construction of the pH electrodes, the premise is the same with each of the different glass types - hydrogen ions migrate into or out of the hydrated gel layer and the resultant electrochemical or potentiometric signal is proportional to the pH of the measured solution.
pH Electrode Signal
The potentimetric or electrochemical signal that is produced is a millivoltage. A pH electrode can be thought of a little like a battery with a voltage that varies with the pH of the measured solution.
The millivoltage that is generated is ±59.16mV per pH decade at 25˚C. The voltage that is generated when the pH is 7.00pH is 0.00mV (regardless of temperature), and at 25˚C the millivoltage that's generated at 6pH is +59.16mV and at 8pH it's -59.16mV - we've created a tool below which illustrates the millivolt signal that's generated per pH decade.
pH Electrode Signal Simulator
As we can see from the above indicator at 7.00pH the signal generated is 0.00mV. The output signal from the pH electrode can be calculated as 7.00pH minus the measured pH multiplied by 59.16mV (at 25˚C).
Calculating mV values from pH Electrodes
- Calculate the mV signal generated at 6.00pH
- 7.00pH less our measured 6.00pH is equal to 1.00pH. So 1.00 multiplied by 59.16mV gives us + 59.16mV.
- Calculate the mV signal generated at 10.00pH
- 7.00pH less our measured 10.00pH is equal to -3.00pH. So -3.00 multiplied by 59.16mV gives us -177.48mV.
Why is knowing the mV signal from a pH Electrode important?
Knowing the theoretical output of the pH electrode to the pH meter helps us understand why pH meters need to be calibrated to pH electrodes. As the output of the pH electrode is linearly aligned and proportional to the measured pH value - we know what our pH meter is looking for.
So in a pH 7.00 buffer solution out pH meter is looking for 0mV.
At 6.00pH our pH meter is expecting +59.16mV and at 8.00pH our pH meter is expecting -59.16mV.
If our electrode is not outputting this exact signal then we're going to generate a pH reading that is under or or over the expected value.
pH electrodes and a changing mV output
The pH electrodes that we supply are high quality devices used for process control applications where the pH measurement must be accurate. We're proud to say that our pH electrodes will often out perform most other manufactures devices due to the stringent quality control checks and testing that each electrode adheres to prior to supply.
Lower quality pH electrodes may have variations in the glass thickness, the surface area, the flow rate through the reference junction, the electrolyte composition or how the electrolyte responds to changing temperatures. The result is that the mV signal generated from the pH electrode may well be different to what the pH meter is expecting.
In addition - all pH electrodes consume the electrolyte material over time. We delay this consumption time period by using double reference junctions, proprietary formulations of electrolyte and polymerised electrolytes for elevated temperatures. We are delaying the consumption - so over time, the electrolyte is still consumed and with this comes a change in the mV output signal.
As the electrolyte is consumed the output mV signal trends towards 0.00mV - in much the same way that a battery goes flat over time.
In a hypothetical situation - if a pH electrode is working a number of years old - where we might expect to see 59.16mV @ 25˚C, we might only be generating 55mV. per pH decade.
So - from our calculator we can see that in 4.00pH buffer the pH meter is expecting a signal of +177.48mV.
If our electrode is only generating 55.00mV per pH decade, then in a 4.00pH buffer we would only be generating 165.00mV.
With a signal of 165.00mV an uncalibrated pH meter would display a pH value of ~ 4.22pH.
What happens when we calibrate a pH meter and electrode?
When we calibrate the pH electrode to the pH meter we're effectively ammending the ±59.16mV per pH decade rule and bringing the theoretical output of the pH electrode inline with the actual output of the currently connected pH electrode.
This is why we conduct a wet calibration using pH calibration solutions rather than using an electrical pH simulator such as the APS2.
The wet calibration means that the pH electrode is immersed into a solution of known value. (Which is why it's important to use fresh uncontaminated buffer solutions) This way we know that the pH electrode is seeing a precise calibration value - so the only variable is the output mV figure from the electrode.
There are limitations on how much variance we can have with our pH meter - so each of instruments is listed with the acceptable operating parameters. So for example the P7635 pH controller has a zero point of ± 2.00pH - so we can offset the 0pH point by this much.
The Span point is the difference between the expected mV figure and the actual mV figure.
Each of our pH controllers is listed with a tolerance for the Span or Slope of the pH electrode. When the incoming mV from the pH electrode and the expected pH electrode are within the tolerance limits, the pH controller will allow the pH electrode to be calibrated to the desired ranges.
Again using the P7635 pH controller as an example - the listed Span or Slope of the electrode is from 80 - 110% of the default value, which is ±59.16mV per pH decade.
So at 80% of the expected value our pH electrode is generating 47.33mV per pH decade. If the output signal from the pH electrode is lower than this, then the pH controller will reject the calibration of the pH electrode and it's probably time for it to be replaced.
For perspective the P7687 pH controller with full P&ID control has the same span range of 80 - 110%.
Notice that the pH controller can also be spanned or the slope adjusted to a range that's higher than the ±59.16mV too - by upto 110% of the expected value.
This is due to the manufacturing differences in pH electrodes. Not all electrodes are made equal, some may have an inferior filling material or larger or smaller diameter glass with an un-even thickness. In these instances it may be possible for the pH electrode to generate an erroneous figure that is higher than the expected value. In this instance we can calibrate out the error and work with the pH electrode even though the signal is not where we expect it to be.
So How Often Should We Calibrate a pH Electrode and pH Meter?
There really isn't a definitive answer to this question, however - we've created an FAQ page which should satisfy most queries regarding the frequency of calibrating pH electrodes and pH meters.
Regular Calibrations Ensure Accurate pH measurements
So running through the theory on how a pH electrode works and the expected signals from a pH electrode that our pH meter is expecting.
Understanding that due to the way that pH is measured means that we have to periodically update our pH meter to ensure that the pH meters see a different mV signal per pH, yada, yada, yada.
For more information on how to construct a calibration schedule or profille then pleased don't hesitate to get in touch. In addition - we're able to offer an on-site calibration and certification service which to provide a 3rd party check to your in house calibrations. We can also provide training and support on pH electrode maintenance and operation.