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C • • <br /> <br />ANALYTICAL -GENERAL <br />VIII Notes <br />1. A white precipitate of calcium carbonate might sepatat¢ from samples <br />containing large quantifies of calcium. This is dissolved by addition of 0.1-0.2 <br />g of EDTA disodium salt. <br />2. When heating the picric solutions they should reach a temperature close to the <br />boiling point but should not be allowed to boil for any length of time. <br />Alternately, the analysis is carried out in 250 mL conical flasks which are then <br />immersed in a boiling water bath for a period of 20 minutes. The third <br />alternative is to use the microwave heating directions. <br />3. The absorbance of the reagent blank usually varies between 0.006 - 0.009 (520 <br />nm, slit width 0.03 mm, 1-cm glass cell). By using the stpndard which is <br />darker than the sample the result will be biased slightly high. The converse <br />is also true. <br />4. ppm is short for part per million by weight for liquids and sdlids and by mol <br />% for gases. In the case of aqueous solutions a µg/ml is approximately a ppm. <br />Sources of Error in Cyanide Determination by the Picric Acid Method <br />See the Operating Procedures-Analytical Section of this manual for a discussion of this <br />topic. <br />11 <br />