How can mV results for pH be used for troubleshooting?

Document ID

Document ID TE8285

Published Date

Published Date 18/08/2020
Question
How can mV results for pH be used for troubleshooting?
Summary
mV troubleshooting for pH.
The way that pH measurements work is that the probe measures a potential difference between the sample and reference electrode, measured in units of milivolts (mV). The mV are compared to the mV calibration curve to calculate ph of the sample.
See these articles for more information: Is there information on pH Theory? and How does pH measurement with an electrode work?

The raw mV measurement can be displayed if it is not by default and therefore is a better option for troubleshooting over using the pH result because the pH result is biased by the temperature and the calibration.

Given by the Nernst equation it can be shown that a perfect theoretical slope is -59.16 mV/pH which can be used to calculate an expected mV potential for various known pH values on buffers.

For Ag/AgCl reference systems (which almost all Hach pH electrodes have) a pH of 7 is expected to have a mV reading of 0 mV. Using a theoretical slope, the expected mV reading can be calculated. The 4 pH is 3 pH lower than 7 pH, therefore it could be expected for it to read 177 mV (59 x 3) and pH 10 buffer would be expected to read -177 mV by the same argument. This is different, however, for Red Rod electrodes (which use a Saturated KCl reference system instead of a Ag/AgCl reference system). For these electrodes, a pH of 7 is expected to read -26 mV. Therefore, a pH of 4 would have a potential of 151 mV, and a pH of 10 would have a potential of -203 mV.

Over time it is expected that the mV potential offset will drift as the probe ages, therefore an acceptable tolerance for any given buffer on any given electrode is ±30 mV. This is how auto-recognition calibrations are done. For example if in calibration the mV potential measurement is between -30 mV and +30 mV, the meter will recognize that buffer as 7 pH.

If a reading is taken on pH 4, 7, and 10 buffers and the mV measurements are recorded, this information can be used to identify many common issues with pH. See below for examples.

Example 1: If on an Ag/AgCl electrode the results seem sluggish but the probe does calibrate and the following mV results were recorded:
pH 4: 202 mV
pH 7: 23 mV
pH 10: -153 mV

Note that all three buffers have an offset of ~25 mV. This is within the acceptable tolerance of ±30 mV, but is on the higher end of that tolerance. Since all three have the same offset there is not a slope issue, so although the results may be sluggish, they are likely accurate results. This often indicates the probe is either approaching the end of it's useful life if non-refillable (over 6 months old) or if refillable probe (over a year). If the probe is refillable, replacement of the filling solution may help with the sluggishness ( What is the procedure for replacing the filling solution in a refillable pH probe?). If it is less than 6 months old and non-refillable, then the probe has most likely been stored incorrectly or may need to be replaced under warranty. ( Why is it important to use the correct storage solution for a pH probe?, What is the correct storage (and filling) solution for an IntelliCAL pH probe?. Why is it so critical that the Intellical PHC101 in particular be stored properly?) Sluggish measurement can also be caused by microbiological contamination of the reference which results from infrequent or not properly cleaning the probe, not changing the storage solution regularly, not storing the probe correctly, or being left in sample for too long.

Example 2: If on an Ag/AgCl electrode the calibration fails and the following mV results were recorded:
pH 4: 222 mV
pH 7: 45 mV
pH 10: -133 mV

This is similar to Example 1, but more advanced. In this case the offset is 45 mV which is over the tolerance, and therefore the calibration fails. The causes are the same as those described in Example 1. If the probe in example 1 were to continue to be used in the same way for a few more days, calibration failures would likely also occur. If the cause of the offset is identified to be improper maintenance, most commonly improper storage, then a replacement probe would need to be purchased even if the probe is within the warranty period. The warranty only covers manufacturing defects, not failures caused by misuse.

Example 3: If on an Ag/AgCl electrode the calibration fails and the following mV results were recorded:
pH 4: 110 mV
pH 7: 5 mV
pH 10: -110 mV

In this case the 7 buffer is reading fine, well within specifications, but the other two have an offset. You'll notice however that the offsets are in opposite directions, both biasing to 7. pH 4 has an offset of -77 mV and the pH 10 has an offset of +77 mV. The causes are the same as those described in Example 1, improper storage (though a loss of slope could also indicate a fouled glass indicator electrode and probe cleaning could be attempted). In this example the probe has likely been stored in pH 7 buffer. If you see one buffer reads well, but the other buffers are shifted towards the buffer that reads well there are indications that the probe has been stored in buffer. As with the other examples, if the cause of the offset is identified to be improper maintenance, most commonly improper storage, then a replacement probe would need to be purchased even if the probe is within the warranty period. The warranty only covers manufacturing defects, not failures caused by misuse.

Example 4: If on an Ag/AgCl electrode the calibration fails and the following mV results were recorded:
pH 4: 192 mV
pH 7: 17 mV
pH 10: -142 mV

In this example the pH 4 and 7 buffers are both reading within tolerance with an offset of ~15 mV, but the pH 10 buffer has an offset of ~35 mV which is outside of tolerance. This is likely caused by a problem with the pH 10 buffer. Whenever two buffers are reading within tolerance, and the third isn't that generally is an indication of a buffer issue. This is most common for pH 10 buffer because it has the shortest shelf life of the three ( What is the shelf life for liquid pH buffers?). If all the buffers were purchased together (which is common) then this can often be the cause of the problem. Loose caps, reuse of old buffer (poured out from the bottle on a different day), contaminated buffers can cause this symptom. In this example the symptom would be that it wouldn't recognize the pH 10 buffer. But this issue can also cause the slope to be out of range if the buffer with the highest offset is still within tolerance, but still much higher relative to the other buffers. This commonly can often be resolved by simply replacing the buffers.

Example 5: If on an Ag/AgCl electrode the calibration fails and the following mV results were recorded:
pH 4: 202 mV
pH 7: 10 mV
pH 10: -190 mV

In this example all three buffers have an offset within tolerance, but all three have different offsets. This will lead to failed calibrations since the slope is out of range. When the different buffers have different offsets, that is what causes bad slopes. The worse the slope (the further it is from theoretical) the worse the accuracy. This can be caused by either a dirty probe, in which the recommendation would be to perform a cleaning. ( How should pH and conductivity electrodes be cleaned?, How often is it recommended to clean laboratory pH electrodes?) This can also be caused by storing the probe improperly, such as by storing the probe in pH buffer. It will read the buffer that it has been stored in well, but the other buffers will have readings biased in the direction of the buffer that it was stored in.

Example 6: If on an Ag/AgCl electrode the calibration fails and the following mV results were recorded:
pH 4: 5 mV
pH 7: 0 mV
pH 10: 3 mV

If all packaging tape was removed, and if refillable the filling hole is open ( Why is there Orange/Amber colored tape on the IntelliCAL probe?, When should the filling hole on a refillable electrode be open or plugged?), then this probe is physically broken and a replacement probe would need to be purchased (Unless it was received in this condition out of box). See: If a pH probe becomes physically damaged within it's warranty period, can that probe be replaced under warranty?