pH 4 and pH 9 buffer solutions
for slope adjustment

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.

Why slope adjustment is required

In theory, 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.

slope adjustment

To 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 additional point.

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

The 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%.

slope value

As a thumb of rules, if the slope value is:

  • between 95-102% (-56,2 and -60,4 mV) => Go ahead and calibrate.

  • between 90-95% (-53,3 and -56,2 mV) => First clean the electrode and then calibrate.

  • between 102-105% (-60,3 and -62,1 mV) => First clean the electrode and then calibrate.

  • < 90% (53 mV) or > 105% (62 mV) => You need to change the electrode, it is no longer usable.

Manufacturers of pH electrodes often specify the slope value for an electrode in good conditions.

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Download the new release ISO 14001:2015