Determination of cation exchange capacity and exchangeable cations in soils by means of cobalt hexamine trichloride. Effects of experimental conditions

Different ways of using solutions of cobalt-hexamine (Cohex) trichloride, [Co(NH3)(6)]Cl-3, to determine the cation exchange capacity (CEC) and the quantity of exchangeable cations in soil samples are compared. First, various techniques for measuring the residual concentration of Cohex ions left in solution after the exchange reaction with soil are examined. Cobalt can be determined by flame atomic absorption spectrometry and ammonium nitrogen by spectrocolorimetry. However, a direct spectrocolorimetric determination of the coloured Cohex ions at 425 nm proves to be easier to carry out with a good repeatability, provided that non-specific absorptions have been properly corrected in advance, That method was retained to compare three procedures of ion exchange: single extraction, successive extractions and percolation. The latter study allows us to chose a particular procedure with a knowledge of its limitations, advantages and disadvantages. In spite of a slightly lower reaction yield, results obtained with a single extraction remain quite appropriate to quantify the magnitude of the exchange reactions.