Part 10 (1/2)
[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 t.i.tration.
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 a.n.a.lysis.]
COMPARISON OF IODINE AND THIOSULPHATE SOLUTIONS
PROCEDURE.--Place the solutions in burettes (the iodine in a gla.s.s-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 t.i.trate 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.
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 pota.s.sium iodide, which reacts with the iodine chloride, forming pota.s.sium 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 pota.s.sium 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 a.r.s.enious 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 pota.s.sium 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 pota.s.sium bromate and cupric salts in solution will liberate iodine from an iodide, which is then t.i.trated 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 pota.s.sium bromate. Dissolve each of these in 50 cc. of water, and add 10 cc. of a pota.s.sium 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, t.i.trate 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 pota.s.sium iodide solution, and deduct this from the total volume used in the t.i.tration.
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.
[Note 1:--Pota.s.sium iodide usually contains small amounts of pota.s.sium iodate as impurity which, when the iodide is brought into an acid solution, liberates iodine, just as does the pota.s.sium bromate used as a standard. It is necessary to determine the amount of thiosulphate which reacts with the iodine thus liberated by making a ”blank test”