The estimation of lanthanum by air–acetylene atomic absorption spectrophotometry using an indirect procedure

Abstract

A method for measuring the release of lanthanum from some ceramic dental materials was required using air–acetylene atomic absorption spectrophotometry (AA-AAS), and an analytical procedure was devised based on the release of calcium in the presence of phosphate by lanthanum addition. The extent of phosphate interference in the determination of calcium by AA-AAS was assessed, and it was shown that the absorbance of a 10 ppm Ca standard was reduced by 45% in the presence of 20 ppm or more of phosphate (as PO₄). Analysis of standards containing 10 ppm Ca, 20 ppm PO₄, and lanthanum added at concentrations up to 100 ppm showed rapid increase of calcium absorbances from 10 to 40 ppm La, after which absorbance increased slowly to a constant value at 90 ppm La. This corresponded to the value of a 10 ppm Ca standard solution containing no phosphate. Closer examination of solutions containing 10–40 ppm La revealed a quantitative relationship between lanthanum levels and calcium absorbances which deviated slightly from Beer’s law. Consequent analysis of solutions containing various amounts of lanthanum in the presence of 10 ppm Ca and 20 ppm PO₄ followed by repeated analysis of standards demonstrated good precision and reproducibility. The relative standard deviation for repeated standard analyses was 4.2%, and the detection limit was 0.6 ppm La representing an increase of sensitivity of approximately 100 times over lanthanum determinations using nitrous oxide–acetylene techniques. The method indicates that similar procedures may be used to estimate elements which exercise similar release effects.