Determination of Exchangeable Cations of Ca, Mg, K and Na in Bentonite by Inductively Coupled Plasma-Atomic Emission Spectrometry

Bentonite exchangeable cations and cation exchange capacity is one of the main indicators of the evaluation of bentonite mineral. The traditional measurement method of bentonite exchangeable cations was made by using either atomic absorption spectrometry or the volumetric method and involved complicated operating procedures and took a long time. In this paper, a description of how the exchangeable cations of Ca, Mg, K and Na were extracted by NH₄Cl-ethanol exchange liquid is given. The obtained solution was evaporated to wet salt and dissolved by 5% (by volume) of HCl acid solution, which was ready to be determined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) for exchangeable cations of Ca, Mg, K and Na. The optimal conditions for the exchange were achieved by adding 25 mL exchange liquid, stirring for 30 min and washing the precipitate twice, then studied by orthogonal experiment. The analysis spectral lines of 315.887 nm, 279.079 nm, 766.490 nm and 589.592 nm were selected for Ca, Mg, K and Na, respectively. Based on the working curves, K and Na have a good linear relationship from 0 to 0.6 mmol/L, Ca and Mg from 0 to 3.0 mmol/L. The method detection limit was 0.012-0.048 mmol/100 g. Verified with national standard materials, the measured values and the certified values were in good agreement. Precision (RSD, n=11) ranged from 0.5% to 2.5%, and accuracies (RE) ranged from -6.1% to 10%. The recoveries with spike were from 97.0% to 100.0%. The proposed method overcame the disadvantages of the traditional analysis, as mentioned above. This simple and fast sample handling method before measurement with good reproducibility is applicable to the analysis of bulk samples.