Solutions used in iodometric titrations
Two most important solutions used in iodometric titrations are solution of iodine and solution of sodium thiosulfate. Both substances can be easily obtained in a pure form, but their other characteristics (volatility, hard to control amount of water of crystallization) make them difficult to use as a primary standards.
It is also worth of mentioning that both solutions are not quite stable and they can not be stored for a prolonged period of time. Iodine can be lost from the solution due to its volatility, while thiosulfate slowly decomposes giving off elemental sulfur. The latter process is easily visible, as thiosulfate solutions become slightly cloudy with time.
Iodine solution
It is not difficult to prepare high purity iodine through sublimation, but - due to its volatility - iodine is difficult to weight accurately, as it tends to run away. To minimize losses it should be weight in closed weighing bottle. Iodine should be kept in a closed bottles also because it is highly corrosive and it vapor can damage delicate mechanism of analytical balance.
Commonly used solutions are 0.05M (0.1 normal).
To find out amounts of substances required to prepare the solution for a needed volume use ChemBuddy concentration calculator. Download the iodine solution preparation file. Open it with the free trial version of the concentration calculator. After opening the file enter solution volume and click on the button. Read amounts of the substances, but don't follow the general directions. It is better to use as small initial volume of the solution as possible, that is, dissolve potassium iodide in about 1/100th of the final volume of water, before adding iodine.
To minimalize losses it is important to transfer iodine to the solution as fast as possible, or even to weight a 1% excess. Solution should be kept in dark glass bottle with grinded glass stopper and standardized every few weeks or before use.
Sodium thiosulfate solution
Sodium thiosulfate (Na2S2O3·5H2O) can be realtively easily obtained in a pure form, but it is quite difficult to obtain samples with known amount of water of crystallization, as the exact composition of the nominal pentahydrate is highly temperature and humidity dependent. Thus solution has to be standardized against potassium iodate KIO3 or potassium dichromate K2Cr2O7.
Commonly used solutions are 0.1M (0.1 normal).
To prepare the recipe for a needed volume of the solution use ChemBuddy concentration calculator. Download the sodium thiosulfate solution preparation file. Open it with the free trial version of the concentration calculator. After opening the file enter solution volume and click on the button.
Small amount of carbonate added helps keep solution pH above 7, which slows down thiosulfate decomposition. Some sources also call for addition of 0.5 mL chloform per liter of the solution, to stop possible growth of bacteria that can speed up decomposition process.
Starch solution
Starch solution is used for end point detection in iodometric titration.
To prepare starch indicator solution, add 1 gram of starch (either corn or potato) into 10 mL of distilled water, shake well, and pour into 100 mL of boiling, distilled water. Stir thoroughly and boil for a 1 minute. Leave to cool down. If the precipitate forms, decant the supernatant and use as the indicator solution. To make solution long lasting add a pinch of mercury iodide or salicylic acid, otherwise it can spoil after a few days.
2% sodium bicarbonate
This solution is used for neutralization of sodium arsenite, before it is titrated with iodine solution during iodine solution standardization.
To prepare the recipe for a needed volume of the solution use ChemBuddy concentration calculator. Download the sodium bicarbonate solution preparation file. Open it with the free trial version of the concentration calculator. After opening the file enter solution volume and click on the button.
