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Cl :MnO = 71:86.9. After canceling the common factors, there remains 2Na S O :MnO = 316.4:86.9, and the factor for the conversion of thiosulphate into an equivalent of manganese dioxide is 86.9/316.4.
The solution of potato starch is less stable than the soluble starch. It is regarded as a "solid solution" of iodine in starch. Run out 40 cc. of the thiosulphate solution into a beaker, dilute with 150 cc. of water, add 1 cc. to 2 cc. of the soluble starch solution, and titrate with the iodine to the appearance of the blue of the iodo-starch.
As the thiosulphate in crystalline form has the formula Na S O .5H O, this weight is 248.12 grams. Tenth-normal or hundredth-normal solutions are generally used. !Approximate Strength, 0.1 N! PROCEDURE. Weigh out on the rough balances 13 grams of commercial iodine. Place it in a mortar with 18 grams of potassium iodide and triturate with small portions of water until all is dissolved.
This reaction between iodine and sodium thiosulphate, resulting in the formation of the compound Na S O , called sodium tetrathionate, is quantitatively exact, and differs in that respect from the action of chlorine or bromine, which oxidize the thiosulphate, but not quantitatively.
PROCEDURE. Weigh out into 500 cc. beakers two portions of about 0.150-0.175 gram of potassium bromate. Run in thiosulphate solution from a burette until the color of the liberated iodine is nearly destroyed, and then add 1 cc. or 2 cc. of starch solution, titrate to the disappearance of the iodo-starch blue, and finally add iodine solution until the color is just restored.
Sulphide sulphur was determined in a sample of reduced barium sulphate by the evolution method, in which the sulphur was evolved as hydrogen sulphide and was passed into CdCl solution, the acidified precipitate being titrated with iodine and thiosulphate. Sample, 5.076 grams; cc. I = 20.83; cc. Na S O = 12.37; 43.45 cc. Na S O = 43.42 cc. I ; 8.06 cc. KMnO = 44.66 cc. Na S O ; 28.87 cc.
Boil until the deep blue is displaced by a light bluish green coloration, or until brown stains form on the sides of the flask. Add a solution containing about 3 grams of potassium iodide, as in the standardization, and titrate with thiosulphate solution until the yellow of the liberated iodine is nearly discharged.
From the data obtained, calculate the relation of the thiosulphate solution to a normal solution, and subsequently calculate the similar value for the iodine solution. It is necessary to determine the amount of thiosulphate which reacts with the iodine thus liberated by making a "blank test" with the iodide and acid alone.
The liberated iodine is then determined by titration with sodium thiosulphate, as described on page 78. The titration of iodine against sodium thiosulphate, with starch as an indicator, may perhaps be regarded as the most accurate of volumetric processes. The fundamental reaction upon which iodometric processes are based is the following: I + 2 Na S O > 2 NaI + Na S O .
Both potassium bromate and cupric salts in solution will liberate iodine from an iodide, which is then titrated with the thiosulphate solution. Two methods for the direct standardization of the sodium thiosulphate solution are here described, and one for the direct standardization of the iodine solution. !Method A!
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