FAS622en_pH measurement

2 Measurement
2.2.5 Buffer solutions
pH buffer solutions are used as a means of calibrating pH electrodes. They are aqueous solutions
with known pH values. Buffer solutions are categorized into primary reference buffer solutions, secondary
reference buffer solutions and technical buffer solutions, according to their properties.
Primary reference buffer solutions show the lowest uncertainty in pH values (U(pH) = 0.003). They
are used mainly in metrological institutes and are not available commercially.
Secondary reference buffer solutions have the same composition as primary solutions. The uncertainty
of the pH values is around U(pH) = 0.006. These solutions are needed by manufacturers of
technical and working reference buffer solutions, and by control and quality assurance laboratories.
Technical buffer solutions are solutions for practical use; their uncertainty is in the range from U(pH)
= 0.01 to U(pH) = 0.05. Technical buffer solutions are the robust solutions. They are relatively immune
to contamination and dilutions, and so are best suited for calibration of plant and hand-held
meters.
There are pH buffer solutions for almost the entire range of the pH scale. For routine work, two solutions
with pH values of approx. pH = 7 and pH = 4 are adequate for a pH range from pH = 2 to
pH = 10. Basic buffer solutions are often very unstable and many pH electrodes react very sluggishly
in them. Because of this, the slopes in basic solutions are shallow in most cases. Practical calibrations
with basic buffer solutions can only be achieved by excluding air from the solutions and
allowing a relatively long settling time. This effort is only worthwhile for measurements in the laboratory.
The exclusion of air prevents any change in the pH of the buffer solution as a result of atmospheric
carbon dioxide being absorbed.
People often talk about the traceability of the buffer solutions. The traceability concerns the pH value
of the pH buffer solution. It means that the pH value was tested by the manufacturer either directly
against a primary reference buffer solution, or via intermediate solutions (e. g. secondary reference
buffer solution).
Example
technical buffer solution r sec. reference buffer solution r prim. reference buffer solution
or technical buffer solution r prim. reference buffer solution
The traceability of the pH value forms a basis for calculating the uncertainty.
Note
During calibration, please take into consideration that the pH of the buffer solution alters with temperature.
24 2 Measurement JUMO, FAS 622, Edition 04.07