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I'm using a Thermo Scientific Orion Dual Star pH/ISE Dual Channel Benchtop Meter. The manual says that a $\mathrm{pH}~7$ buffer should read about $\pu{0 mV},$ and a $\mathrm{pH}~4$ buffer should read about $\pu{177 mV}.$

We use a two-point calibration with the $\mathrm{pH}~4.00$ and $\mathrm{pH}~7.02$ buffers at $\pu{20 °C}.$ I'm getting $\pu{-41 mV}$ for $\mathrm{pH}~7$ and $\pu{133 mV}$ for $\mathrm{pH}~4,$ i.e. my $\pu{mV}$ readings seem to be offset by about $\pu{-40 mV}.$ The slope is approx. $99\,\%,$ which is fine. Our internal methods require a voltage range of $\pu{\pm30 mV}$ for the $\mathrm{pH}~7$ buffer as validity criteria.

When I calibrate as is, I can test a $\mathrm{pH}~6.00$ buffer I use for verification and get $\mathrm{pH}~6.00$ consistently. The $\mathrm{pH}~6.00$ buffer is not part of the calibration, it's an independent test to see if the calibration is valid.

The buffers are purchased from a certified supplier. The CoA says they are made of $\ce{KH2PO4},$ $\ce{Na2HPO4},$ $\ce{NaHCO3},$ $\ce{Na2CO3}$ in DI $\ce{H2O}.$

Why would this happen? Can I recalibrate the voltage or enter an offset somehow?

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    $\begingroup$ I am sure you will have another electrode from Thermo. Replace the electrode and see if the values change. Also it does not make sense that you are so off and the slope is 99% of the theoretical? $\endgroup$
    – ACR
    Commented Aug 6, 2023 at 14:35

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Electronic components and electrodes are aging and their parameters drifting. Or the pH electrode is off because of its bad conditions.

As pH meters are usually also mV meters, check its voltage reading by independent electronic device.

Check the pH electrode by using other one, if both their reading on the same device match.

That will help to determine the cause.

As temporary workaround, one can technically always use fallback solution of measuring the voltage and recalculation to pH. Sure, if the standard operation procedure does not allow to continue, it will not help.

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The voltage measured is the potential difference between the glass electrode and the reference electrode. This depends on the construction of both and is set to zero by the calibration control of the meter. My procedure in using a meter was to record mv readings at pH 7 and 4 daily, monitoring the trend. If there was a change, determine the reason. Generally, there was a slow progression in voltages over months with little change in slope. This is with double junction flowing reference electrodes. An offset of 41 mv seems large and suggests a reference electrode effect, Check it out and either match the electrodes or simply monitor the voltages daily to see if everything is constant. Does the meter read to 0.1 mv?

The criterium for 30 mv: what is that based on? I have found that the voltage is constant to fractions of a mv with a slow drift over time if an electrode system is working properly. A side note I have found Fisher Scientific color-coded pH buffers to be extremely precise bottle to bottle. Different brands were generally agreeing within fractions of a mv. Also a better check is a pH 10 buffer that will stress the system a bit more. [Do not set the slope at 10 though]

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