[56] The
iodine is measured by titrating with a solution of sodium
hyposulphite,[57] using starch paste as indicator. The iodine is soluble
in the excess of potassium iodide, forming a deep brown solution; the
hyposulphite is added until this brown colour is almost removed. Starch
paste is then added, and strikes with the remaining iodine a dirty blue
colour. The addition of the "hypo" is continued until the blue colour is
discharged. The end reaction is sharp; a drop is sufficient to complete
it.
As regards the titration, the process leaves little to be desired; the
quantity of "hypo" required is strictly proportional to the copper
present, and ordinary variations in the conditions of working are
without effect. The presence of salts of bismuth masks the end reaction
because of the strong colour imparted to the solution by the iodide of
bismuth. Under certain conditions there is a return of the blue colour
in the assay solution after the finishing point has apparently been
reached, which is a heavy tax on the patience and confidence of the
operator. This is specially apt to occur when sodium acetate is present,
although it may also be due to excessive dilution.
~The standard "hypo" solution~ is made by dissolving 39.18 grams of the
crystallised salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to
one litre.
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