STANDARDIZATION OF SOLUTIONS

Commercial iodine is usually not sufficiently pure to permit of its use as a standard for thiosulphate solutions or the direct preparation of a standard solution of iodine.  It is likely to contain, beside moisture, some iodine chloride, if chlorine was used to liberate the iodine when it was prepared.  It may be purified by sublimation after mixing it with a little potassium iodide, which reacts with the iodine chloride, forming potassium chloride and setting free the iodine.  The sublimed iodine is then dried by placing it in a closed container over concentrated sulphuric acid.  It may then be weighed in a stoppered weighing-tube and dissolved in a solution of potassium iodide in a stoppered flask to prevent loss of iodine by volatilization.  About 18 grams of the iodide and twelve grams of iodine per liter are required for an approximately tenth-normal solution.

An iodine solution made from commercial iodine may also be standardized against arsenious oxide (As_{4}O_{6}).  This substance also usually requires purification by sublimation before use.

The substances usually employed for the standardization of a thiosulphate solution are potassium bromate and metallic copper.  The former is obtainable in pure condition or may be easily purified by re-crystallization.  Copper wire of high grade is sufficiently pure to serve as a standard.  Both potassium bromate and cupric salts in solution will liberate iodine from an iodide, which is then titrated with the thiosulphate solution.

The reactions involved are the following: 

(a) KBrO_{3} + 6KI + 3H_{2}so_{4} —­> KBr + 3I_{2} + 3K_{2}so_{4} + 3H_{2}O,

(b) 3Cu + 8HNO_{3} —­> 3Cu(no_{3})_{2} + 2no + 4H_{2}O,
    2Cu(no_{3})_{2} + 4KI —­> 2CuI + 4KNO_{3} + I_{2}.

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!

Procedure.—­Weigh out into 500 cc. beakers two portions of about 0.150-0.175 gram of potassium bromate.  Dissolve each of these in 50 cc. of water, and add 10 cc. of a potassium iodide solution containing 3 grams of the salt in that volume (Note 1).  Add to the mixture 10 cc. of dilute sulphuric acid (1 volume of sulphuric acid with 5 volumes of water), allow the solution to stand for three minutes, and dilute to 150 cc. (Note 2).  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.  Make a blank test for the amount of thiosulphate solution required to react with the iodine liberated by the iodate which is generally present in the potassium iodide solution, and deduct this from the total volume used in the titration.