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Conductometric and Potentiometric Titration of Exchangeable Aluminum¹

ABSTRACT

Conductometric and potentiometric titrations of an AlCl₃ solution and AlCl₃-treated Dowex-50, trioctahedral vermiculite, and two montmorillonites were compared with the Al and acidity removed from these systems with neutral chloride salts. The endpoints of potentiometric titrations of AlCl₃ or AlCl₃-treated exchangers in neutral salt solution were lowered by these salts and adsorption of partially neutralized hydroxy-Al groups on the exchangers. Titrations of solutions to pH 8.0 gave better estimates of the acidity present than did inflection points. Hydrolysis of Al on the exchanger during exchange may produce H⁺ ions not originally present. Conductometric titrations with Na₂B₄O₇ and NaOH were confounded by incomplete exchange and reaction with Al. The formation of aluminate in the titration with NaOH and Ba(OH)₂ decreased the conductivity and caused "high" results. It is proposed that conductometric titrations should be continued until the change in conductivity with added base approaches that of a blank. A method for correcting for aluminate formation is proposed.

¹ Contribution from the Agronomy Department, Virginia Agr. Exp. Sta., Virginia Polytechnic Institute, Blacksburg. Financial support from Project S-60, Regional Research Fund, Hatch Act, is gratefully acknowledged.

² Professor of Agronomy.

Received for publication April 11, 1969. Accepted for publication September 23, 1969.