ISO 23496:2019 pdf download – Determination of pH value – Reference buffer solutions for the calibration of pH measuring equipment
4.1 General
Reference buffer solutions are prepared using primary or secondary reference materials. The pH values of the reference buffer solutions are determined using the methods described The pH value of a reference buffer solution and the associated uncertainty of measurement are documented in a calibration certificate as the result of calibration. pH values of primary reference buffer solutions form the basis of pH measurements in practice and are generally det erm ined by nationa l metrology instit utes. Secondary reference buffer sol utions are commercially available. The reference buffer sol utions prepared according to the methods described as a material measure of the pH value.
4.2 pH va lue s of prim ary reference buffer solutions
The pH value based upon the activity of single ions according to the formula cannot be measured. The pH value is assigned to reference buff er solutions outlined in Iab.J.e…l . This is achieved using an el ectroc hemical method of m eas ur e ment that is based on the thermodynamic dependence of the potential of the platinum/hyd rogen electrode on the h ydrogen ion activity. Through the use of cells without transference, diffusion voltages arising from liquid junctions do not appear and the refore do not need to be considered when calculating the vol tage of the cell.
pH values of primary reference buffer solutions are often referred to as pH(R) (R = Reference)]. The expanded uncertainty of measurement U(pH] when determining a pH(R) value has been shown to be 0,002 to 0,005 at 25°C depending on the primary reference buffer solution. As agreed, this uncertainty of measurement does not take into consideration the uncertainty of the Bates-Guggenheim convention for estimating the ionic activity of the chloride ion [see Formula (2)][2]. The expanded uncertainty of measurement of many pH(R) value measurements of primary reference buffer solutions prepared according to Clause 5 is U(pHj = 0,005 at 25。C and U(pH) = 0,008 at 50 °C, due to the method of measurement and the purity and homogeneity of the materials used for different batches depending on the buffer solution. Accordingly, the pH(R) values of each batch can vary. Therefore, the pH(R) value is only valid for an individual batch with a corresponding calibration certificate. For buffer solution G, the expanded uncertainty of measurement of the pH(R) values is U(pH) = 0,006 at 25 °C and U(pH) = 0,01 for temperatures higher than 25 °C(4). Contributions to uncertainty are a result of the method of measurement used as well as the purity and homogeneity of the material used. Typical pH values of reference buffer solutions for the temperature range of 0 °C to 50。C are given in Table 1.