Common Problems with Ag, As, S, Ba, Pb and Cr

Sulfur (S)

Conventionally, sulfur measurements are made using combustion techniques coupled with measurement of the SO₂ combustion gas by infrared, micro-coulometric, or titrimetric (iodometric) techniques. Since 1974, techniques involving ion chromatography (speciation) and X-ray fluorescence have become very popular. More recently, ICP-OES has become a viable measurement technique for sulfur due to the availability of affordable radial view instrumentation with measurement capability in the vacuum UV spectral region and the relative freedom of spectral interferences. Popular emission lines with IDLs measured in our laboratory are shown in Table 13.2:

The following tips may prove useful in the preparation and solution chemistry of samples for sulfur analysis using ICP-OES:

• Loss during sample preparation is a significant issue. Preparations using closed vessel systems are recommended. Parr bomb fusions, Schöniger Flask combustions, and closed vessel microwave digestions should be considered depending upon the sample matrix, sulfur compound type(s), sulfur levels and sample size requirements needed to make quantitative measurements.
• Preparations including sulfate, Ba and Pb should be avoided. The molecular form of the sulfur may have compatibility issues with other chemical species in the sample solution preparation. Sulfate (SO4⁼) sulfur is a common molecular form resulting from oxidative sample preparations. Even though the preparation promises to deliver sulfite (SO3⁼) sulfur this species quickly air oxidizes in aqueous solution to the sulfate form. Sulfate readily precipitates with solutions containing Pb or Ba.
• Water soluble samples known to contain sulfur as sulfate, sulfite or low molecular weight water soluble sulfonic acids (RSO₃H) may need no sample preparation but samples known to contain sulfur in other forms such as sulfides (S⁼), elemental (S^?), polysulfides ( Sn⁼), thiols (RSH), organic sulfides and disulfides (R-S-R and R-S-S-R), thiolesters (R-CO-SR) etc. should undergo oxidative sample preparation to avoid possible compatibility issues with other solution components. In addition, the addition of acid to sulfide containing samples will emit H₂S.

• S elemental data

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