There is a common misconception that performing a 2-point calibration across both the acid and alkaline range (e.g. say pH4 and pH 10) will result in correct measurement anywhere in this range.
This practice of crossing the pH 7 boundary is entirely incorrect and although this is common practice there are several problems with this method.
Firstly, slope derived from a 2-point pH4 and pH10 calibration is actually incorrect throughout the entire range because it averages the acidic and alkaline slope to yield a slope that is in fact not accurate for the acidic measurement, nor the alkaline measurement. With this practice you will get poor results anywhere in the measurement range.
By omitting pH 7 buffer (or pH 6.86) it means there is no true asymmetric potential (AP) calibration and you will not really know what the pH sensor reads and pH7. Since for pH measurement the isopotential point is pH7 for the purposes of automatic temperature calibration you will induce errors in the temperature compensation by having an incorrect AP. By utilising the 2-point pH4 & pH10 calibration approach you are essentially forcing the AP to be an erroneous value. The larger the deviation of the actual AP from this erroneous value, the greater the introduced error.
If only a two-point (single slope) style calibration is to be done then one of the buffers must be pH7 (or pH6.86) to avoid this incorrect AP issue described above. This typically means pH4 (or pH1.86) and pH7 for applications that are almost always acidic or pH7 and pH9.18 (or pH10) for applications that are almost always alkaline.
3 Point Calibration – Dual Slope
For applications that cross the pH7 boundary commonly best practice is to enable the three point (3-point) calibration (dual slope feature) and perform the AP/Offset calibration at pH7 (or pH6.86) followed by the acidic slope calibration with pH4 (or pH1.86) and then alkaline slope calibration with pH9.18 (or pH10, pH12.45). Please note that 3 point calibrations are not supported by all analysers. This function is supported by our TT-MA and TT-MAD series analysers.
- The alkaline pH buffers are much more temperature sensitive and generally unstable as compared to the neutral or acidic pH buffer. Many factors are the cause of this but the result is the same. If you have only two pH buffers employed (two-point single slope calibration) and one of the two buffers used is likely to be inaccurate this makes for a very poor calibration.
- In increasing order of sensitivity to aging (decomposition), air (mainly intrusion of carbon dioxide), light, contamination and temperature induced changes are the pH9.18, pH10 and the pH 12.45 buffers.
- Only the TT-MAD-pH analyser in the automatic calibration mode corrects for the temperature induced change to find the exact value of the pH buffers.
- The temperature dependence of the pH buffers can be accounted for in manual calibration mode by reading the value of the pH buffer at the current temperature as obtained from the connected pH sensor & adjusting to this exact value. This correction is performed automatically by the TT-MAD-pH analyser improving efficiency by saving time and ensuring an accurate pH buffer value at any temperature.
- If the procedure for the facility states that they must use pH4 buffer and pH10 buffer for their calibration then this can only be done in proper way by enabling the three-point calibration mode. Using the the intelligent digital TT-MAD-pH analyser this is quite easy to do with the automatic calibration mode.