Cadmium iodide-starch solution

Procedure A mixture of the test substance and calcium oxide is prepared and ignited as described hi Procedure II. The cooled ignition residue is treated with a drop of 1 1 sulfuric acid and the contents of the crucible cooled by setting it in cold water. A drop of cadmium iodide-starch solution is then introduced. A blue color that appears within 30 seconds indicates arsenic. 

Schnepfe [83] has described yet another procedure for the determination of iodate and total iodine in seawater. To determine total iodine 1 ml of 1% aqueous sulfamic acid is added to 10 ml seawater which, if necessary, is filtered and then adjusted to a pH of less than 2.0. After 15 min, 1 ml sodium hydroxide (0.1 M) and 0.5 ml potassium permanganate (0.1M) are added and the mixture heated on a steam bath for one hour. The cooled solution is filtered and the residue washed. The filtrate and washings are diluted to 16 ml and 1ml of a phosphate solution (0.25 M) added (containing 0.3 xg iodine as iodate per ml) at 0 °C. Then 0.7 ml ferrous chloride (0.1 M) in 0.2% v/v sulfuric acid, 5 ml aqueous sulfuric acid (10%) - phosphoric acid (1 1) are added at 0 °C followed by 2 ml starch-cadmium iodide reagent. The solution is diluted to 25 ml and after 10-15 min the extinction of the starch-iodine complex is measured in a -5 cm cell. To determine iodate the same procedure is followed as is described previously except that the oxidation stage with sodium hydroxide - potassium permanganate is omitted and only 0.2 ml ferrous chloride solution is added. A potassium iodate standard was used in both methods. 

Analytical Characters.—1, To a solution of starch a few drops of cadmium iodide solution are added, then a small quantity of the fluiblue color is produced in the presence of hydrogen peroxide, even if the solution contain only 0.05 milligi ara per litre. 

Reagent 5 % solution of cadmium iodide in 1 % cadmium sulfate solution (Starch solution is added just before use.)... 

Procedure. A drop of the acid test solution is treated with a drop of bromine water in a depression of a spot plate. If the color is discharged, the addition is continued until a permanent yellow is obtained. The excess bromine is removed by adding a little solid sulfosalicylic acid. A drop of starch-cadmium iodide solution is added to the decolorized solution. Depending on the amount of thallium present, a deep or a light blue appears immediately. 

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