Store the dried sample in the desiccator until it has cooled to room temperature. Place the uncovered weighing bottle in its upturned cap in a small beaker, cover the beaker with a dry watch glass, identify the beaker with a graphite pencil and dry the sample in a 105-110 oC oven for at least one hour. Transfer the unknown sample to a clean dry weighing bottle. Cover each beaker with a watch glass, and store the equipment until needed. Number the beakers on the ground areas for identification using a graphite pencil. Wash three 100 mm watch glasses, three stirring rods and three 400 mL beakers rinse them thoroughly with tap water and then with distilled water.
The presence of ferric iron can produce errors as high as 2% in the determination. These are substitutional impurities, and the magnitude of the error depends upon the differences between the weight of the foreign sulfate or oxide and the weight of an equivalent amount of barium sulfate. Since nitrate and chlorate interfere even at low concentrations, they should be removed from the solution before precipitation.įoreign cations such as ferric iron, calcium and, to a lesser extent, the alkali metals are coprecipitated as the sulfates. The coprecipitation of chloride can be decreased by slow addition of the precipitant. Foreign anions such as nitrate, chlorate and chloride are coprecipitated as their barium salts, and the ignited precipitate contains the salt or oxide as an additive impurity. Variations in the acidity, temperature, manner of addition of the precipitant and time of digestion markedly affect the filterability of the barium sulfate precipitate and the extent to which various foreign ions are coprecipitated.
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