The strongly coloured Cu-triethylenetetramine complex (Cu-Trien) provides a fast and easy way to measure the cation exchange capacity of clay samples, but the reliability of this method has been questioned in recent publications. This work identifies several reasons for the poor reliability, thus improving both the precision and accuracy of the CEC method. Pure Trien is shown here to intercalate as does the Cu-Trien complex. Hence, the purity of Trien and also the Cu/Trien ratio require special attention. Using cold Cu-Trien solutions produces systematic deviations in CEC determinations caused by errors in the pipetted volumes. The present study demonstrates, for the first time, that the Cu-Trien complex protonates and changes its effective charge beyond 2+. Based on titration experiments at three different temperatures, a correction function is derived for calculating the total charge of the Cu-Trien complex as a function of pH. Accounting for these pH-corrected charges indicates that Cu-Trien may well be insensitive to edge charges on smectite. Finally, it is shown here that the ratio of sample mass to the initial amount of Cu-Trien is critical because excess adsorption of Cu-Trien in addition to CEC sensu strictu was observed.

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