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Analytical Chemistry Division (V)


Number: 2004-016-2-500

Title: Guidelines for potentiometric measurements in suspensions

Task Group
S.F. Oman

Members: Filomena Camões, Raj Rajagopalan, and Petra Spitzer

Completion Date: 2007 - project completed

A) To state the causes and natures of the Suspension Effect in
potentiometric measurements in suspensions

B) On the base of A) to (i) give an unambiguous, experimentally
verified definition and interpretation of the Suspension Effect and (ii)
recommend procedures for pH (pIon) measurements in suspensions

The term "suspension effect" (SE) was introduced seventy five years ago. Originally it was defined as the difference between the potentiometrically determined hydrogen ion concentration of a suspension and of the medium pertaining to it. Many interpretations of this difference were published until now but no consensus about its origin or explanation was accepted, though an enormous experimental and theoretical study of these problems was done. For the present it is possible to find two different definitions even on the web sites of IUPAC. On the base of an early SE definition and using a detailed analysis of the suspension galvanic cells the cause and the nature of the SE phenomena can be explained and an unambiguous definition of SE proposed.

One of the current definitions states the SE as "the difference in emf obtained in the conventional determination of the pH in a suspension and in its equilibrium solution" (Overbeek, 1953.) This is the base of the new revised definition. This will be in accordance with the "IUPAC Recommendations 2002" (on pH) and interpreted with regard to its causes and natures.

Taking into account the revised definition and its interpretation, described above, guidelines for practical routine pH (and pIon) measurements in suspensions will be given. The electrodes and the instructions for their use will be described and illustrated with some examples.

April 2006 - A manuscript is being prepared for publication in Pure Appl. Chem. A final document is submitted to public review comments until 30 Sep 2006. > see provisional recommendations

Jan 2007 - Project completed
Both a technical report and recommendatiosn have published been:

Guidelines for potentiometric measurements in suspensions
Part A. The suspension effect (IUPAC Technical Report)
Pure Appl. Chem. 79(1), 67-79, 2007

Part B. Guidelines for practical pH measurements in soil suspensions (IUPAC Recommendations 2006)
Pure Appl. Chem. 79(1), 81-86, 2007


Last Update: 9 January 2007

<project announcement published in Chem. Int. Jul-Aug 2005 >

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Page last modified 8 February 2007.
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