PREPARATION OF REAGENTS--_Wijs Iodine Solution_--Dissolve 13.0 g. of resublimed iodine in one liter of C. P. glacial acetic acid and pass in washed and dried chlorine gas until the original thiosulfate titration of the solution is not quite doubled. The solution is then preserved in amber glass-stoppered bottles, sealed with paraffin until ready for use.
Mark the date on which the solution is prepared on the bottle or bottles and do not use Wijs solution which is more than 30 days old.
There should be no more than a slight excess of iodine, and no excess of chlorine. When the solution is made from iodine and chlorine, this point can be ascertained by not quite doubling the titration.[32]
The glacial acetic acid used for preparation of the Wijs solution should be of 99.0 to 99.5 per cent strength. In case of glacial acetic acids of somewhat lower strength, the Committee recommends freezing and centrifuging or draining as a means of purification.
_N_/10 _Sodium Thiosulfate Solution_--Dissolve 24.8 g. of C. P. sodium thiosulfate in recently boiled distilled water and dilute with the same to one liter at the temperature at which the titrations are to be made.
_Starch Paste_--Boil 1 g. of starch in 200 cc. of distilled water for 10 min. and cool to room temperature.
An improved starch solution may be prepared by autoclaving 2 g. of starch and 6 g. of boric acid dissolved in 200 cc. water at 15 lbs. pressure for 15 min. This solution has good keeping qualities.
_Potassium Iodide Solution_--Dissolve 150 g. of potassium iodide in water and make up to one liter.
_N_/10 _Potassium Bichromate_--Dissolve 4.903 g. of C. P. potassium bichromate in water and make the volume up to one liter at the temperature at which titrations are to be made.
The Committee calls attention to the fact that occasionally potassium bichromate is found containing sodium bichromate, although this is of rare occurrence. If the analyst suspects that he is dealing with an impure potassium bichromate, the purity can be ascertained by titration against re-sublimed iodine. However, this is unnecessary in the great majority of cases.
_Standardization of the Sodium Thiosulfate Solution_--Place 40 cc. of the potassium bichromate solution, to which has been added 10 cc. of the solution of potassium iodide, in a glass-stoppered flask. Add to this 5 cc. of strong hydro-chloric acid. Dilute with 100 cc. of water, and allow the _N_/10 sodium thiosulfate to flow slowly into the flask until the yellow color of the liquid has almost disappeared. Add a few drops of the starch paste, and with constant shaking continue to add the _N_/10 sodium thiosulfate solution until the blue color just disappears.