This part of ISO 6588 specifies a method for the determination of the pH-value defined by the electrolytes extractable by hot water from a sample of paper, board or pulp.

This part of ISO 6588 is applicable to all kinds of paper, board and pulp.

As the quantity of extractable ionic material approaches zero, as in the case of highly purified pulps, the precision of the method becomes poor because of the difficulties encountered in making pH measurements on water containing little electrolytic material.

Since the extraction in this part of ISO 6588 is performed with distilled or deionised water, the pH-value measured will sometimes be different (e.g. fully bleached pulp) from the pH-value measured under mill process conditions in which various types of process waters, e.g. chemically treated river water containing electrolytes, are used.

It is necessary to be aware that the results will not be the same when measuring pH according to this part of ISO 6588 when compared to ISO 29681. The differences can be significant especially when measuring pulps having a low ionic strength.

ISO 6588-1 differs from this part of ISO 6588 only as regards the extraction conditions. No general guidance can be given as to which of the two procedures (hot or cold) is best suited in a particular situation.

For cellulosic papers used for electrical purposes, the method used should be that given in IEC 60554-2.