Effect of sample-related sources of error on the analysis of pulp and paper mill process liquids by ion chromatography. I. Determination of sulphur species.
Douek M. | Sullivan J. | Ing J.
Several factors which could affect the accuracy of measurements of sulphur species by ion chromatography have been identified. Analyses of sulphide, sulphite, sulphate and thiosulphate were conducted on samples of kraft liquors, spent sulphite liquor and newsprint white water. Erroneously high results were observed in the determination of sulphide in kraft white and green liquors, presumably due to shifts in equilibrium between elemental sulphur and polysulphide ions during sample dilution. Another problem was encountered during the analysis of sulphite in kraft liquors. Sulphite concentrations in diluted liquors rose rapidly with increase in time between dilution and injection into the ion chromatograph. Evidence presented in this paper suggests that, in diluted kraft liquors containing polysulphide and/or elemental sulphur, the sulphite concentration increases rapidly, possibly due to hydrolysis of elemental sulphur. Determination of thiosulphate was also subject to positive errors, due to oxidation of sulphide to thiosulphate. The effect of storage conditions on the stability of sulphur species was also determined. Room-temperature storage was the most deleterious for white water samples containing low levels of sulphur ions. Long-term stability in all samples was much improved by refrigeration or freezing. Kraft liquors were very stable even when stored at room temperature. Various recommendations are made to maintain sample integrity and minimize analysis errors.
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