Arsenic by Coulometric Titration

No practical apphcations are given in this area to cover food analysis, although the reader is referred to the References section, additional recent references portion, for some suggestions. The paper by Kalbus and Lieu represents, for example, a laboratory experiment in the coulometric titration determination of the iodine number of fats and oils. For a practical experiment the following determination of arsenic by coulometric titration is suggested. It is of interest since it presents to alternatives in the final estimation procedure. 

Prepare a solution 0.00200 M (0.00400M) to AS2O3. Dissolve 0.1978 g of previously-dried AS2O3 in 10 mL of 10% NaOH, dilute to 50 mL with distilled water (DW) and add 2 drops of 0.1% phenolphthalein in 80% ethanol. Add 1 1 HCl until the pink colour disappears and the solution is just acid. Transfer quantitatively to a 500 mL volumetric flask, dilute to the mark with DW and mix well. Keep stoppered in the flask. 1.00 mL =0.3 mg As. 

Place in the generating ceU of a constant current coulometer 50 mL of DW, 2.0 g of KI and 2.0 g of Na2C03. Add 5 mL of soluble starch solution (0.4%), insert the generating electrodes (twin platinum sheets 0.5 cm ) and a stirring bar. Start stirring and apply a constant current of 10 mA. Stop the timer as soon as 

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