Abstract

Oxyanion impurities in reagent grade chemicals may be detected and measured by Raman spectrophotometry. During recent studies of NaNO₃, samples were prepared from a bottle of certified reagent grade chemical. A weak band at 981 cm⁻¹ was detected in the Raman spectrum when high sensitivity was employed—conditions that cause the 1051 cm⁻¹ line of NO₃⁻ to be much off scale (Fig. 1). The origin of the 981 cm⁻¹ band was traced to SO₄²⁻ impurity, listed by the manufacturer as 0.002 wt %. Phosphate ion also contributes intensity in this spectral region, but its molar intensity is considerably less than that of the symmetric stretch of sulfate, and interference in this case was not considered significant.

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