On Sunday my pH calibration solutions were 20C. These solutions are 4, 7, and 10 pH at 25C. When calibrating should I set the value to 4, 7, and 10 or adjust for temp and set the pH to the adjusted pH?
If you know what the buffers should be at 20C then use those values
Are there tables that specify the pH at different temps? I have seen tables for NIST pH calibration solutions but not these 4, 7, 10 pH solutions.
There are tables for standard buffers. If you know what the buffer is, you should be able to find the corresponding table.
My pH meter came with the corresponding pH table vs temps in the box. That is what I refer to when calibrating.
My meter came with a table for “NIST” calibration solutions (4.18, 6.86, 9.18 pH at 25C). But the calibration solutions I purchased (from the same company) are (4, 7, 10 pH at 25C). So the table in the manual is not useful for me.
Just for the record, I have this meter: ThermoWorks 8689 pH Meter
and this set of calibration solutions: ThermoWorks pH Calibration Buffer Solutions
I admit I am confused because the manual says if you don’t have NIST solutions you will have to adjust the reading during calibration. But, the solutions I bought are “NIST-certified reference materials”.
With pH calibration there are two very different issues that occur with regards to temperature.
-
The science behind the measurement is based on the Nernst equation which is for membrane potentials. The equation is temperature dependent in which a value must be used.
-
The pH of the buffer solution is temperature dependent. That is a pH 7.01 buffer is pH 7.01 at 25 oC. At 20 oC the pH of the solution is pH 7.03.
A meter with ATC will correct for the Nernst equation. The Nernst equation (when all values are plugged in) dictates that there will be 59.16 mV/pH unit at 25 oC. The mV/pH will change with temperature and we correct for this.
The pH of the solution is not compensated and it is for this reason that pH is reported with a temperature value. For example my Mash is pH5.5 at 50 oC. The actual pH of the mash can be different at different temperatures. We do not know to what extent since the composition changes. Unlike a pH buffer based on NIST standards in which the actual pH values are known at different temperatures.
For Hanna meters with automatic calibration it is the reason that the meter displays a different value from the pH value after calibration. For example I calibrate in pH 7.01 buffer at 20 oC. After calibration the meter will display pH 7.03. That is the pH of the solution that we know for 100%.
For meters with manual calibration adjust the pH value to match the value on the tables found on the bottles of buffer with regards to temperature. That is a best practice.
I hope this helps. If not feel free to message me with any questions.