The Use of Color to Quantify the Effects of pH and Temperature on the Crystallization Kinetics of Goethite Under Highly Alkaline Conditions

Tetsushi Nagano1, Satoru Nakashima2, Shinichi Nakayama1 and Muneaki Senoo1
1 Environmental Geochemistry Laboratory, Japan Atomic Energy Research Institute Tokai, Naka, Ibaraki 319-11, Japan
2 Geological Institute, Faculty of Science, The University of Tokyo Hongo, Tokyo 113, Japan

Abstract: The crystallization kinetics of goethite were studied colorimetrically under highly alkaline conditions (pH 10.1–12.2) at temperatures from 40° to 85°C. Color changes during crystallization from fresh precipitates, plotted on a*–b* colorimetric diagrams, were used to discriminate between pure goethite and mixtures of goethite and hematite. Only the b* value increased as goethite crystallization proceeded, and even a minor increase in the a* value revealed the existence of hematite. The rate of goethite crystallization, estimated from the b* value, could be modeled by a pseudo-first-order rate law. This rate depended both on pH and on temperature. Apparent activation energies for the reactions of 56.1 kJ/mol at pH 11.7 and 48.2 kJ/mol at pH 12.2 were estimated from Arrhenius plots.

Key Words: Color • Colorimetry • Crystallization • Goethite • Hematite • Kinetics • L*a*b* color space

Clays and Clay Minerals; April 1994 v. 42; no. 2; p. 226-234; DOI: 10.1346/CCMN.1994.0420213
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)