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Net Charge Characteristics of Dystrandept B and Theoretical Prediction¹

ABSTRACT

The net charge characteristics of five Dystrandept B samples containing components with variable charge were described by combining the two empirical equations that relate the retention of NH₄⁺ and Cl^- with the pH (4 to 8) and NH₄Cl concentration (0.005 to 0.1M) of the solution. There were two characteristic pH values; the point of zero charge (PZC) and the point of constant charge (PCC). The variation of PZC with NH₄Cl concentration was appreciable, whereas that of PCC was very small or insignificant. The PZC was found in the range from pH 4.4 to 6.3, and the PCC pH was 4.9 to 6.0. The small net charge at PCC was positive for the soils containing allophane and imogolite and negative for the soil containing 2:1 layer silicates and gibbsite but not containing allophane and imogolite. The effects of the pH and NH₄Cl concentration on the magnitude of the net charge are compatible with those calculated using the Stern theory of the electric double layer with an assumption that the thickness of the Stern layer (0.3 to 0.5 nm) decreases with increasing electrolyte concentration. The order of magnitude of the net charge measured is in agreement with that calculated.

¹ Contribution from the Faculty of Agriculture, Kyushu Univ., Fukuoka 812, Japan.

² Professor and formerly Graduate Student, Dep. of Agric. Chem., Kyushu Univ., Fukuoka 812, Japan. The Second Author is now Lecturer, Nagoya Shiritsu Hoiku Tanki Daigaku, Owariasahi 488, Japan.

Received for publication August 31, 1982. Accepted for publication April 22, 1983.