METHOD VALIDATION FOR THE DETERMINATION OF EXCHANGEABLE CATIONS IN NATURAL ZEOLITES USING INDUCTIVELY COUPLED PLASMA OPTICAL EMISSION SPECTROMETRY

Abstract

The paper presents the validation of a method for the determination of exchangeable cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺) in zeolites by inductively coupled plasma optical emission spectrometry (ICP-OES), after extraction in ammonium chloride solution. Amongst the analytical techniques that can be used for exchangeable cations measurement, ICP-OES provides rapid, robust, multi-element analysis on a wide range of concentrations. The exchange capacity is one of the most important parameters of zeolites since it indicates their adsorption capacity in different processes. Method validation is an essential requirement for testing laboratories in order to provide trustworthy results in accordance with ISO/IEC 17025:2017 standard. The main figures of merit were studied, and the measurement uncertainty was assessed. The selectivity assay showed no significant non-spectral matrix effect. The linearity study was conducted for the calibration curves in the range of 0.02–20 mg L⁻¹ for each exchangeable cation. Limits of quantification were 0.005 mEq 100 g^-1 for Na⁺, 0.011 mEq 100 g^-1 for Mg²⁺, 0.002 mEq 100 g^-1 for K⁺, 0.003 mEq 100 g^-1 for Ca²⁺. Relative standard deviations of repeatability (RSDr) (n=6 parallel samples) were 4.50 % for Na⁺, 4.43 % for Mg²⁺, 6.55 % for K⁺ and 5.53 % for Ca²⁺. Recoveries (%) estimated using a certified reference material (CRM BCS-CRM 375/1), for total content of cation oxides were in the range 92–103 %. Fourteen zeolite samples from Racos deposit, Romania were analysed, and according to the chemical composition and X-ray diffraction the main mineral in zeolitic tuffs is clinoptilolite-type. The obtained figures of merit demonstrate that the method has a suitable level of precision and accuracy for the intended purpose.