Phewwww. That is a ton of good information there. Thanks for sharing.
In your first post with steps 1-4, can this be performed with any pH probe or just Hanna Instrument ones?
The offset check (pH 7.01) will work with any meter with a BNC connector. Once the connector is shorted then a o mV signal is being sent to the meter.
Many meters, including testers, that have automatic calibration have the ability to clear the calibration data that has been stored. A meter with a default calibration will be set to 0 mV = pH 7.01 and +178 mV = pH 4.01. From a default calibration it is then possible to determine both offset and slope.
The original post was for a meter that was calibrated manually. Since it has a BNC connector it was at least possible to check the offset. The slope would be difficult to check since a +178 mV signal from a simulator would have to be driven to the meter to adjust the pH 4 trimmer. Since more than likely a simulator is not available that the slope check was left out.
The offset is very helpful since a high offset value is an indicator that the probe needs to be cleaned.
If you have a meter with auto calibration and it can be cleared to a default then knowing the readings in buffers will allow to calculate both offset and slope.
Feel free to message me and I can walk you through how perform the calculation. It is based on the Nernst equation in which theoretically a pH of 7.01 is 0 mV and each pH unit generate 59.16 mV at 25
oC. Acids the pH increases positive and bases the increase is negative. A pH electrode with 100% slope will generate +59 mV in pH 6 while in pH 5 will generate +118 mV. It would be recommended to make sure that the probe has better than 90% slope or 54 mV/pH.
For example, on default calibration my pH probe reads pH 7.25 in pH 7.01 buffer and pH 4.5 in in pH 4.01 buffer.
*pH 7.25 is 1/4 pH unit away on the base side so the voltage being generated is -15 mV. Well within +/-30 mV for an offset.
* pH 4.5 is 2.5 pH units away from pH 7.00 which in mV that would equal 59 * 2.5 = 147.5 mV. But since the offset is -15 mV then the mV difference is 147.5 - (-15) = 162.5 mV. 162.5/178 x 100 = 91.3% slope. The slope is above 90%.
* If in the same example the pH meter read pH 6.75 in pH 7 for a default cal then the voltage would be +15 mV (more H
+ = increase in + voltage). The slope would be 147.5 - 15 mV = 132.5 mV. 132.5/178 x 100 = 74.4% slope. Most meters will not calibrate less than 85% slope but if it did then you definitely should not use the probe since the slope is too low.
The numbers above are approximate since Nernst dictates 59.16 mV/pH at 25
oC. I have rounded to 59 for convenience. Either way, knowing how to approximate the offset and slope will help you understand the health of the pH electrode. To achieve an accuracy >0.1 pH it is critical to know.
The accuracy statement of the meters stated by the manufacturers is the meter itself and not the system which includes the probe. I can achieve a higher accuracy from a $50 tester than from a $600-1K benchtop if the probe of the tester has a good offset/slope and the benchtop does not.
General rule:
New pH electrodes: +/- 10 mV offset and 95-105% slope
User tolerance: +/- 30 mV offset and slope greater than 90%
Meter limits: +/- 60 mV offset and slope greater than 85%
It is possible to calibrate a probe that is outside the limits of use since the window for calibration is pretty wide. At that point it is like throwing darts and were hitting the dart board. To hit a triple 20 you want to be well within the user tolerance range.
Topic: pH probes (Read 4455 times)
