ISO 6588-2:2021 紙、板紙、パルプ—水性抽出物のpHの測定—パート2：熱抽出 | ページ 2

Foreword

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This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps.

This fourth edition cancels and replaces the third edition (ISO 6588-2:2020), of which it constitutes a minor revision. The changes compared to the previous edition are as follows:

• an incorrect reference in 7.1 has been deleted.

A list of all parts in the ISO 6588 series can be found on the ISO website.

Introduction

Kraft fibre is well known to contain ionisable groups that are fixed to or in the fibre wall. In order to fulfil the electro-neutrality, these groups are balanced by an equivalent number of positive charges, which can be either protons or various metal ions. Especially in pulp suspensions at low ionic strengths, this can give rise to a marked uneven distribution of mobile ions between the volume held by the fibre wall and the bulk suspension liquor. This means that the fibre acts as an ion exchanger. These ion-exchange phenomena can be modelled very well with the Donnan theory^[6][7].

If a relatively clean pulp fibre sample, as for example bleached dried pulp, is diluted in deionised water, the result will be a pulp suspension with a very low ionic strength. In such a system, most of the cations present, including protons, will be concentrated in the water volume held by the fibre wall. If the pH is measured, it will be measured in the bulk suspension liquor. By adding salt to this kind of system, the ion exchange phenomena will be decreased and the concentration of different cations will be the same in the water held by the fibre wall and in the bulk suspension liquor. Since the process waters always contain a certain amount of ions, such a salt addition will give a more realistic environment when measuring the pH of relatively clean pulp samples.

Based on the foregoing, two ISO Standards, this document and ISO 29681, are available for determination of the pH of hot aqueous extracts of paper, board, or pulps. In ISO 29681, a salt solution is added prior to extraction; however, in this document, the extraction is carried out with distilled or deionized water. ISO 29681 is specifically applicable to bleached pulps from virgin fibres and to pulp samples having a low ionic strength for which the pH-value will give more realistic results related to mill conditions than those obtained with this document.

It is necessary to be aware that the results will not be the same when measuring pH according to this document and to ISO 29681. The difference can be significant especially when measuring pulps having a low ionic strength.

ISO 6588-1 differs from this document, in relation to 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.

It should be noted that in this document, addition of a salt solution is performed after extraction and filtration only for the purpose of obtaining a more stable and accurate pH measurement.

1 Scope

This document 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 document 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 document is performed with distilled or deionised water, the pH-value measured can sometimes be different (e.g. fully bleached pulp) from the pH-value measured under mill process conditions in which various types of process waters, such as chemically treated river water containing electrolytes, are used. In such cases, ISO 29681 can be used instead, as it is specifically applicable to bleached pulps from virgin fibres and to pulp samples having a low ionic strength for which the pH value gives more realistic results related to mill conditions than those obtained with this document. For cellulosic papers used for electrical purposes, the method used can be that given in IEC 60554-2^[5].

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

3 Terms and definitions

No terms and definitions are listed in this document.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

Bibliography