Interlaboratory quality control study no. 7 : major cations and anions / D.J. McGirr and R.W. Wales.: En36-508/30E-PDF

"Two synthetic samples and one natural water sample were distributed to twenty-two participating laboratories for determination of calcium, magnesium, sodium, potassium, alkalinity, chloride, sulfate and nitrate. The results for calcium, magnesium and alkalinity were good even though the natural sample was unstable for these three parameters. The precision for calcium and total hardness was much better by titration than by atomic absorption. Precision and accuracy were good for sodium and potassium, and precision was about the same whether atomic absorption or flame emission photometry was used. Precision for nitrate at low levels was only fair by any of the variety of methods used, although the cadmium reduction method was satisfactory at higher levels. Sulfate was also determined by a variety of methods, but precision was satisfactory for most of them. Chloride was determined with acceptable precision. The natural sample was supersaturated with carbonates and therefore was unstable with respect to calcium, magnesium and alkalinity. It was stable, however, with respect to the other parameters and the synthetic samples were stable with regard to all parameters tested over a three-month period of storage. Precision and accuracy were not seriously adversely affected by either sample instability or long-distance shipment"--Summary, page v.

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