pH calibration challenge
To calibrate pH measuring systems you must use a solution with a known pH value, also known as pH reference or buffer pH solution. The accuracy of your subsequent pH measurements is dependent on how accurately the pH measuring system is calibrated, so particular attention must be paid to this step. Because there are a great number of different buffer pH solutions available many people are uncertain about how many and what pH calibration solutions should be used.
What is a buffer pH solution and how many pH calibration points are reasonable?
A buffer pH solution is composed of either a weak acid and the conjugated base or a weak base and the conjugated acid. The main characteristic of a buffer pH calibration solution is that the pH value of the solution will not alter when a small amount of acid or a base is added. The components and their concentration can cause the pH value of the calibration solution to vary across a large range e.g. with HCl and sodium citrate (pH 1-5), citric acid and sodium citrate (2.5 – 5.6), acetic acid and sodium acetate (3.7-5.6), Na2HPO4 and NaH2HPO4 (6-9) or borax sodium hydroxide (9.2-11).
The pH value of a calibration solution does not only alter with its composition but with temperature changes. Please see Table below Temperature behavior of reference pH buffer. The thermal characteristics of these buffer pH calibration solutions have been determined by metrological institutes.
|Temperature in °C||pH|
In contrast to reference pH calibration solutions the composition of technical buffer pH solutions is not regulated. So it is important to note that the temperature reaction of those pH calibration solutions can vary, even if the same nominal pH value is specified at 25°C. In particular at a calibration temperature other than 25°C considerable errors can occur with the pH measurement results. In addition to different kinds of buffer pH solutions the calibration procedure plays a major role in determining the accuracy of the pH measurement. The following pH calibration procedures are identical for any pH measurement.
One-point-calibration: A one-point-calibration is accomplished using one reference pH calibration solution. Here only the zero point of the pH electrode is verified and it is assumed that the increasing pH values measured by the pH electrode approximately match the Nernst effect. This method of pH electrode calibration is the fastest.
Two-point-calibration: This calibration method is accomplished using two reference pH calibration solutions, with a minimum pH difference of two units. Here the maximum measurable pH value and zero point are determined by a linear slope cutting through the measuring points. Because pH electrodes usually have very good linearity, this form of calibration is sufficient enough to provide high measurement accuracy.
Multipoint-calibration: A multipoint calibration is accomplished with three or more reference pH calibration solutions. The difference between pH solutions should be greater than 0.5 pH units. The pH electrode calibration curve is determined by either linear regression through all measuring points or built from segments between neighbored buffers in which the zero point and transconductance can be calculated. To evaluate the certainty of the calibration procedure the stability index (R2) could be consulted. It shows whether the theory correlates with the results and should have a value around 1. Often alkaline buffer solutions are used to accomplish a multipoint calibration. These should be checked for freshness and their percentage error effect has to be estimated.
Three-point-calibration with Dawn buffer pH calibration solutions 4 ,7 and 10increase the accuracy and for most pH measurement