Measuring pH of paper

We measured the pH of a sheet of paper with two different methods:

        Surface pH measurement of paper
        TAPPI 529 om-04

        Paper, board and pulp--Determination of pH aqueous extracts
        ISO 6581:1994

With the first method we had 5.5 pH but with the second we had pH
7.5 What could explain  we have such different pH measurements for
the same sheet of paper?

Dipl.-Rest. Sonia Anllo
GSK mbH
Gesellschaft zur Sicherung von schriftlichem Kulturgut
Archivcenter West
Von-Werth-Str.  4a
50259 Pulheim
Brauweiler, Germany
+49 2234 98787 30
Fax: +49 2234 98787 66


                                  ***
                  Conservation DistList Instance 25:29
                 Distributed: Sunday, December 18, 2011
                       Message Id: cdl-25-29-016
                                  ***