[Footnote 1:  It will be found more economical to have a considerable quantity of the solution prepared by a laboratory attendant, and to have all unused solutions returned to the common stock.]

Weigh out 25 grams of sodium thiosulphate, dissolve it in water which has been previously boiled and cooled, and dilute to 1000 cc., also with boiled water.  Transfer the solution to a liter bottle and mix thoroughly (Note 2).

[Note 1:  Iodine solutions react with water to form hydriodic acid under the influence of the sunlight, and even at low room temperatures the iodine tends to volatilize from solution.  They should, therefore, be protected from light and heat.  Iodine solutions are not stable for long periods under the best of conditions.  They cannot be used in burettes with rubber tips, since they attack the rubber.]

[Note 2:  Sodium thiosulphate (Na_{2}S_{2}O_{3}.5H_{2}O) is rarely wholly pure as sold commercially, but may be purified by recrystallization.  The carbon dioxide absorbed from the air by distilled water decomposes the salt, with the separation of sulphur.  Boiled water which has been cooled out of contact with the air should be used in preparing solutions.]

INDICATOR SOLUTION

The starch solution for use as an indicator must be freshly prepared.  A soluble starch is obtainable which serves well, and a solution of 0.5 gram of this starch in 25 cc. of boiling water is sufficient.  The solution should be filtered while hot and is ready for use when cold.

If soluble starch is not at hand, potato starch may be used.  Mix about 1 gram with 5 cc. of cold water to a smooth paste, pour 150 cc. of !boiling! water over it, warm for a moment on the hot plate, and put it aside to settle.  Decant the supernatant liquid through a filter and use the clear filtrate; 5 cc. of this solution are needed for a titration.

The solution of potato starch is less stable than the soluble starch.  The solid particles of the starch, if not removed by filtration, become so colored by the iodine that they are not readily decolorized by the thiosulphate (Note 1).

[Note 1:  The blue color which results when free iodine and starch are brought together is probably not due to the formation of a true chemical compound.  It is regarded as a “solid solution” of iodine in starch.  Although it is unstable, and easily destroyed by heat, it serves as an indicator for the presence of free iodine of remarkable sensitiveness, and makes the iodometric processes the most satisfactory of any in the field of volumetric analysis.]

COMPARISON OF IODINE AND THIOSULPHATE SOLUTIONS

Procedure.—­Place the solutions in burettes (the iodine in a glass-stoppered burette), observing the usual precautions.  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.  Repeat until the ratio of the two solutions is established, remembering all necessary corrections for burettes and for temperature changes.