An explanation of the origin and interpretation of the suspension effect (SE) is presented in accordance with "pH Measurement: IUPAC Recommendations 2002" [Pure Appl. Chem.74, 2169 (2002)]. It is based on an analysis of detailed schemes of suspension potentiometric cells and confirmed with experimental results. Historically, the term "suspension effect" evolved during attempts to determine electrochemically the thermodynamically defined activity of H+ (aq) in suspensions. The experimental SE arises also in determining other pIon values, analogous to pH values.The SE relates to the observation that for the potential generated when a pair of electrodes (e.g., reference electrode, RE, and glass electrode) is placed in a suspension, the measured cell voltage is different from that measured when they are both placed in the separate equilibrium solution (eqs). The SE is defined here as the sum of: (1) the difference between the mixed potential of the indicator electrode (IE) in a suspension and the IE potential placed in the separated eqs; and (2) the anomalous liquid junction potential of the RE placed in the suspension. It is not the consequence of a boundary potential between the sediment and its eqs in the suspension potentiometric cells as is stated in the current definition of the SE.

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With the aid of a membrane osmometer, the osmotic coefficients, @, of aqueous solutions of polystyrenesulfonic acid and its salts with mono-and bivalent counterions were measured at molalities from 1.10-to less than 1.10-mol/kg. The results were inconsistent with published "limiting laws" and a dependence of @ on the nominal molecular weight of the macroions was observed. The obtained @-values were compared with the data compiled from other studies. The @-values for solutions containing H and Ca counterions were compared with the corresponding @-values obtained cryoscopically to establish the validity of the method used. ZUSAMMENFASSUNG:Mit einem Membranosmometer wurden die osmotischen Koeffizienten (@) waBriger Losungen von Polystyrolsulfonsaure und ihrer Salze mit mono-und divalenten Gegenionen im Konzentrationsbereich von 1.10-bis 1.10-mol/kg und niedriger gemessen. Es wurde keine Ubereinstimmung der Resultate rnit "limiting laws" aus der Literatur gefunden, und es wurde eine Abhangigkeit der osmotischen Koeffizienten von den nominalen Molekulargewichten der Makroionen beobachtet. Die erhaltenen @-Werte wurden mit den aus anderen Arbeiten entnommenen Ergebnissen verglichen. Die @-Werte von Losungen, die H und Ca Gegenionen enthielten, wurden rnit den entsprechenden @-Werten aus kryoskopischen Messungen verglichen, um die Gultigkeit der benutzten Methode zu bestatigen.

Eight sodium salts of polystyrenesulfonic acid (poly[ 1 -(sulfophenyl)ethylene]) of different molecular weights were prepared similarly and purified in the same way. Their aqueous solutions were thus chemically equivalent, and differed only in their molecular weights, which were determined viscosimetrically.The osmotic coefficients (@) of the acid and its sodium, thallium, calcium and cadmium salt solutions were measurea by a membrane osmometer for counterion molalities m,= to 5.10-"mol/kg,at 25°C. In the same concentration range, the activity coefficients of hydrogen counterions (YH) of acid solutions were calculated from electromotive force measurements and the osmotic coefficients of the acid solutions were determined by the cryoscopic method for two different molecular weights. The values obtained are presented graphically as functions of the measured average molecular weight for m, = and lo-* mol/kg and are in good mutual agreement within experimental error.This agreement of results could be assumed to verify experimentally that the colligative properties of polystyrene-sulfonate polyelectrolytes at low concentrations are dependent on the molecular weight of the polyion.The obtained results together with some literature data, allow us to evaluate very approximately the activity coefficients of monovalent counterions (;,J for m, = 1 O-' mol/kg as a function of the degree of polymerization (M) of linear oligoions or polyions, for values of n up to lo4.

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