This International Standard specifies a method for the determination of the pH of an aqueous extract of paper, board and pulps, in which the soaking and the measuring are performed in salted water (0,1 M KCl solution).

This International Standard is applicable to all kinds of paper, board and pulps, but is specifically applicable to bleached pulps from virgin fibres and to pulp samples having a low ionic strength for which the pH-value using this International Standard will give more realistic results related to mill conditions than those obtained with ISO 6588-1 or ISO 6588-2.

When deionized water is used and the sample has a low ionic strength, the suspension will have a very low ionic strength and the pH-value obtained in the measurement will not be that of the sample but that of the distilled water. When salted water is used in the soaking, there will be no concentration difference between the fibre wall and the surrounding water, and the pH-value measured will be that of the sample (in the fibre wall).

The use of salted water when dealing with samples having a low ionic strength also promotes more stable and more accurate pH-readings.