An evaluation of the toluidine blue test for the assessment of allophane and imogolite

Abstract

Toluidine blue (TB) is adsorbed on negatively charged colloids ana exhibits a characteristic color change from blue to purplish red (metachromasis). A procedure to test the meta- chromasis of TB was applied to two sets of soil samples derived from volcanic ash, which differed in their horizon, day mineral composition and nature and content of humus. The results showed that the absence of TB metachromasis can be used to diagnose the predominance of aliophane and imogolite with low SiO2/Al2O3 ratios in B horizon samples, and also in A1 or Ap horizon samples when these samples are pretreated with H2O2. The metachromasis of TB which was observed in the presence of relatively small amounts of layer silicates made the assessment of aliophane and imogolite difficult in a few Ap horizon samples. The absence of TB metachromasis can also be used as an indication of “andic” soil properties, when the test is applied to samples without H.O, pretreatment. © 1985 Taylor & Francis Group, LLC.