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Old 05-07-2013, 09:20 PM   #11
ajdelange
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I don't see anything about calibration in that statement and I don't see any calibration parameters in their formula whatever you put into it.

Fortunately, there is only one requirement for success with the formulas at http://wetnewf.org/pdfs/ph-meter-calibration.html and that is that the reading, however it is scaled or offset, be a linear function of the voltage produced by the electrode. If you get actual millivolts the electrode voltage is
E = Eo - s*(R*T/F)(pH(T) - pHi)
with Eo being a few mV depending on the electrodes aging and s a number close to (a bit less than) 1 again depending on age and RT/F = 58.167 mV/pH at 293.15 K (20 °C). pHi is assumed to be 7 (though it actually isn't but should be close to it for most electrodes). The formula you are looking at assumes this but does not incorporate the other two calibration parameters :Eo and s. As such it can only produce approximate answers.

If you are going to write code I don't see much point in using an approximate formula when a much more robust one is readily available to you. Yes, you will have to also write code (doesn't have to be on the micro computer) that takes two mV readings (or 2 scaled mV readings) and two temperature readings (one temperature/mV pair for each buffer) and solves for s and Eo but that isn't rocket science. The formulas are there with the others.

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