15F3 Cation exchange capacity by 0.01 M AgTU⁺ [CEC_AgTU]

Searle (1986) found that the determination of CEC by measuring residual monovalent Ag⁺ in solution (following AgTU⁺ extraction) was accurate and rapid. Moreover, results should be similar to those obtained by Gillman’s (Gillman 1979) compulsive exchange procedure.

The method, based on that reported by Blakemore et al. (1987), uses portion of the original supernatant soil extract from Method 15F1. Residual Ag⁺ is measured by ICPAES, or AAS in the presence of Cs⁺. It should not be used on soils where CECs >30 cmol_c/kg are expected.

Reagents

0.01 M AgTU⁺ Extracting Solution

As for Method 15F1 (same batch used for soil extracts).

Caesium Chloride Solution

1 L contains 1.0 g Cs⁺

Dissolve 1.267 g caesium chloride (CsCl) and dilute to 1 L with deionised water. Store in plastic or borosilicate bottle.

Thiourea Solution

1 L contains 15 g thiourea

Dissolve 15.0 g thiourea [CS(NH₂)₂] and make to volume to 1 L with deionised water.

AgTU⁺ Working Standards

Accurately dispense 0.01 M AgTU⁺ and Thiourea Solution into 100 mL volumetric flasks as detailed in Table 15.16. Store these solutions in the dark and discard should any precipitate develop with time.

Table 15.16. Examples of dilutions for AgTU⁺ Working Standards for CEC_AgTU

*Values of CEC_AgTU apply, provided both these Working Standards and original soil extracts are diluted equally.

Procedure

Dilute identical volumes of both original supernatant soil extracts from Method 15F1 and AgTU⁺ Working Standards (Table 15.16) ≈80-fold with CsCl solution (1.0 g Cs/L); i.e. the same dilution should apply to soil extracts and standards. This wide dilution is optional for analysis of Ag⁺ by ICPAES and necessary to achieve a linear calibration when determining Ag⁺ by AAS. Provided standard solutions are prepared from dilutions of Extracting Solution, no reagent blank should be necessary. Run method blanks occasionally to cover for the possibility of other sources of contamination.

Determination and Reporting of CEC_AgTU

Measure Ag⁺ in the diluted Working Standards and extracts by ICPAES (preferred; spectral line = 328.0 nm) or by AAS. Follow manufacturer’s recommendations with respect to instrument parameters. An air/C₂H₂ flame and the 328.1 nm spectral line are preferred for AAS, although the 338.3 nm line approximately doubles the linear range.

On completion of each batch of the standards/extracts, aspirate water for ≥5 min; also thoroughly clean burner and mixing chamber. These steps are necessary to prevent accumulations of Ag actylides, which are potentially hazardous.

Report CEC_AgTU(cmol_c/kg), expressed on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to convert to an oven-dry concentration. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.