Calibration buffers pH 4.00 and pH 9.21 are common buffers to use to
perform the slope adjustment calibration. To adjust the slope of your pH
electrode is absolutely necessary to make accurate pH measurements.
a pH electrode output slope at 25°C should be 59.2 mV/pH unit. In
practice the output of a new and good working pH sensor can be, at best,
close to the theoretical value. However, with time, the slope will
decrease so that the response will diminish further from the theoretical
value. As already stated the characteristic of a pH electrode will
change with time due to electrode coating and aging.
Just a reminder, as mentioned on "The pH versus temperature relation" page of this site the slope of a pH electrode output is temperature dependent. The slope increases with increasing sample temperature, and decreases with decreasing sample temperature.
determine the slope of a straight line you have to know at least two
points. Since you have performed the zero point adjustment you need an
If the measured value is above the zero point (pH 7.0) the second calibration point should be slightly above the expected pH of the sample. If instead the measured value is below the zero point (7.0 pH) the second calibration point should be slightly below the expected pH of the sample. However, for practical reasons, the use of buffers with a greater differential in pH values has become the convention. This is the reason the common calibration buffers are pH 4.00 and pH 9.21.
What is an acceptable slope range
slope value is specified as mV/pH or as a percentage of the theoretical
value (100% = 59.2 mV/pH). Depending on electrode coating and aging
it's more common to have a slope value below 100% than above 100%.
As a thumb of rules, if the slope value is:
Manufacturers of pH electrodes often specify the slope value for an electrode in good conditions.
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