The practical slope k' usually differs from the theoretical slope (Nernst coefficient) k = 2.3026 RT/F  (R is the universal gas constant, 8.31451 J·mol-1·K-1; T is the Kelvin temperature, F is the Faraday constant, 96485.309 Coulombs·mol-1).
The pH value to be determined should lie between the pH values of two adjacent buffer solutions, i.e. the pH value of one buffer solution pHS1 shall be greater than that of the sample to be measured, while the pH value of the other buffer solution (pHS2) shall be less.
Similar to experimental determination of other physical and chemical properties, pH measurement is affected by the limited accuracy of the measurement process. Some factors, such as liquid junction potential of cell (2), asymmetry potential of glass electrode, clogging of glass electrode diaphragm etc., lead to systematic errors in measured results, which can not be compensated by the adjustment procedure. Therefore, the accurate pH measurement requires a calibration of the pH meter- electrodes assembly .
Calibration is a set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring system, or values represented by reference materials, and the corresponding values realized by standards .
To achieve international comparability IUPAC recommended in 2002 to apply the traceability concept to pH measurements that will allow to connect pH measurement results from the national metrological institution level down to field, laboratory and industry measurements . Traceability is a property of the result of a measurement or the value of a standard, whereby it can be related to stated references - usually national or international standards - through an unbroken chain of comparisons, all having