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At 103-105° C positive bias may result from physically occluded water and the retention of waters of crystalization. This is especially true for waters high in calcium, magnesium, chloride or sulfate. The problem of physically occluded water may be exacerbated by the presence of large amounts of residue (i.e., more than 200 mg). On the other hand, some conversion of bicarbonate to carbon dioxide and subsequent volatilization may cause a small amount of negative bias. At 180± 2° C occluded water is removed, but some water of crystallation may still remain, especially in the presence of sulfates. Furthermore, at this higher temperature additional volatilization of certain inorganic salts (e.g., carbonate, some chlorides and nitrates) will occur. Overall, drying to 180± 2° C seems to give results for TDS that are closest to what one would calculate from a complete chemical analysis. For this reason, it is included as an option for the determination of TDS. In general, either temperature may be used for waters of low color and low alkalinity. Hard waters, alkaline waters, or highly-colored waters should probably be dried at the higher temperature.
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