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Analytical Properties of the Soluble, Metal-Complexing Fractions in Sludge-Soil Mixtures: IV. Determination of Carboxyl Groups in Fulvic Acid¹

ABSTRACT

A detailed experimental examination of the titration behavior and reaction with calcium acetate of a series of model organic acids led to the conclusion that the standard Ca(OAc)₂ method for the determination of COOH groups in organic matter would be ineffective for sludge-derived fulvic acid. In particular, it was shown that positive interferences from SO₃H groups and acidic OH groups could be expected. A modified Ca(OAc)₂ method was developed, consisting of: (i) conversion of fulvic acid to K/H fulvate, (ii) reaction with 1N calcium acetate, (iii) distillation of the acetic acid product, and (iv) titration of the acetic acid with 0.1N NaOH, in which these positive interferences do not occur. The modified method was applied successfully to two representative sludge-derived fulvic acids, a soil-derived fulvic acid, and two model humic polymers, all of which exhibited positive interferences with the standard Ca(OAc)₂ method.

¹ Contribution from the Dep. of Soil and Environmental Sciences, University of California, Riverside 92521.

² Research Associate IV and Professor of Soil Science, respectively.

Received for publication August 28, 1978. Accepted for publication November 20, 1978.