Standardization of iodine and thiosulfate solutions for use in potentiometric titrations

0.05M iodine standardization against arsenic trioxide

Chemical characteristics of the arsenic trioxide As₂O₃ make it a good candidate for a standard substance in many potentiometric methods, however, because of its toxicity it is used less and less frequently.

Arsenic oxide is dissolved in sodium hydroxide, producing sodium arsenite, which is a good reducing agent. In iodometry it is quantitatively oxidized by iodine to arsenate:

Na₃AsO₃ + I₂ + H₂O → Na₃AsO₄ + 2I^- + 2H⁺

Direction of this reaction depends on pH - in acidic solutions As(V) is able to oxidize iodides to iodine. To guarantee correct pH of the solution we will add solution of sodium bicarbonate NaHCO₃.

Interestingly, when using As₂O₃ as a standard substance in other types of redox titrations, we often add small amount of iodide or iodate to speed up the reaction. For obvious reasons in the case of iodometric titration we don't have to.

Procedure to follow:

Weight exactly about 0.15-0.20g of dry arsenic trioxide and transfer it to Erlenmayer flask.
Add 10 mL of 1M sodium hydroxide solution and dissolve solid.
Add a drop of phenolphthalein solution.
Neutralize with 0.5M sulfuric acid, adding several drops of excess acid after solution loses its color.
Add slowly (to not cause the solution to foam up) 50 mL of 2% NaHCO₃ solution.
Add 5 mL of the starch solution.
Titrate swirling the flask, until a blue color persists for 20 seconds.

To calculate iodine solution concentration use EBAS - stoichiometry calculator. Download iodine standardization against arsenic trioxide reaction file, open it with the free trial version of the stoichiometry calculator.

Note, that to be consistent with the use of arsenic trioxide and its molar mass, reaction equation is not the one shown above, but

As₂O₃ + 2I₂ + 5H₂O → 2AsO₄^3- + 4I^- + 10H⁺

These are equivalent. Enter arsenic troxide mass in the upper (input) frame in the mass edit field above As₂O₃ formula. Click n=CV button below iodine in the output frame, enter volume of the solution used, read solution concentration.

0.1M thiosulfate standardization against potassium iodate

Potassium iodate is in fact not titrated directly, but after it is mixed with iodate in acidic solution, it is a source of iodine:

IO₃^- + 5I^- + 6H⁺ → 3I₂ + 3H₂O

This reaction needs presence of acid. As it was already signalled on the iodometric titration overview page, low pH both helps air oxygen oxidize iodides to iodine and speeds up thiosulfate decomposition. Both reactions are detrimental for the standardization, but they can be ignored if the water is oxygen free and titration doesn't take too long.

Iodine solution is then titrated with thiosulfate:

2S₂O₃^2- + I₂ → S₄O₆^2- + 2I^-

Procedure to follow:

Weight exactly about 0.10-0.15g of dry potassium iodate and transfer it to Erlenmayer flask.
Add 40 mL of freshly boiled distilled water
Add 2 g of (iodate free) potassium iodide.
Add 10 mL of 1M hydrochloric acid solution and swirl the soltion.
Titrate swirling the flask, until a pale yellow.
Add 5 ml of the starch solution.
Titrate swirling the flask, until blue color disappears.

For calculations we will use rather strangely looking reaction equation:

KIO₃ + 6Na₂S₂O₃ + 6H⁺ → 3S₄O₆^2- + I^- + K⁺ + 12Na⁺ + 3H₂O

Strangely as it looks, it correctly describes stoichiometry of the whole process.

To calculate thiosulfate solution concentration use EBAS - stoichiometry calculator. Download thiosulfate standardization against potassium iodate reaction file, open it with the free trial version of the stoichiometry calculator.

Enter potassium iodate mass in the upper (input) frame in the mass edit field above KIO₃ formula. Click n=CV button below thiosulfate in the output frame, enter volume of the solution used, read solution concentration.

0.1M thiosulfate standardization against potassium dichromate

Dichromate - which can be easily obtained in a very pure form - oxidizes iodides to iodine:

Cr₂O₇^2- + 6I^- + 14H⁺ → 2Cr³⁺ + 3I₂ + 7H₂O

Iodine solution is then titrated with thiosulfate:

2S₂O₃^2- + I₂ → S₄O₆^2- + 2I^-

First reaction is not too fast, so after mixing reagents they should be left for 5 minutes. Also final color is different from what we are usually seeing during iodometric titrations, as solution contains trivalent, green chromium.

Procedure to follow:

Pour 80 mL of freshly boiled, distilled water into Erlenmayer flask (or better - flask with glass stopper).
Add 2 g of (iodate free) potassium iodide.
Add 25 mL of 1M hydrochloric acid solution and swirl the soltion.
Close the flask and left it in a dark place for a 5 minutes.
Titrate swirling the flask, until yellow iodine tint is barely visible.
Add 5 ml of the starch solution.
Titrate swirling the flask, until blue color disappears.

This is a two step process, but iodine is only intermediate, and the stoichiometry of the overall reaction that we are interested in is

K₂Cr₂O₇ + 6Na₂S₂O₃ + 14H⁺ → 2Cr³⁺ + 2K⁺ + 12Na⁺ + 3S₄O₆^2- + 7H₂O

This is not an exact reaction equation describing what is happening in the solution, but it has correct stoichiometric coefficients and allows easy calculation of amount of thiosulfate reacting with a given mass of potassium dichromate.

To calculate thiosulfate solution concentration use EBAS - stoichiometry calculator. Download thiosulfate standardization against potassium dichromate reaction file, open it with the free trial version of the stoichiometry calculator.

Enter potassium dichromate mass in the upper (input) frame in the mass edit field above K₂Cr₂O₇ formula. Click n=CV button below thiosulfate in the output frame, enter volume of the solution used, read solution concentration.

0.05M iodine standardization against thiosulfate

This procedure is in fact one of the two based on the reaction of thiosulfate with iodine:

2S₂O₃^2- + I₂ → S₄O₆^2- + 2I^-

If we have iodine solution of known concentration we can easily use it as a standard for thiosulfate solution standardization and vice versa. This is a common situation in the lab practice.

The only problem is selection of the volume of thiosulfate sample. If we use 50 mL burette, and both solutions are 0.1N (that means 0.05M solution of iodine and 0.1M solution of thiosulfate), we should use 45 mL of thiosulfate - to make sure we use as large volume of the iodine solution as possible to minimize effects of the volume reading error. However, there are no single volume volumetric pipettes of 45 mL volume :) The most logical approach is to use 20 mL pipette and 25 mL burette.

Procedure to follow:

Measure 20 mL of thiosulfate solution and transfer it to Erlenmayer flask.
Add 5 ml of the starch solution.
Titrate swirling the flask, until a blue color persists for 20 seconds.

To calculate iodine solution concentration use EBAS - stoichiometry calculator. Download iodine standardization against thiosulfate reaction file, open it with the free trial version of the stoichiometry calculator.

Click n=CV button over thiosulfate. Enter concentration and volume of the sample, click Use button. Click n=CV button below iodine in the output frame, enter volume of the solution used, read solution concentration.

0.1M thiosulfate standardization against iodine

This is almost exactly the same procedure we have described above, just titrant and titrated substance are switched.

2S₂O₃^2- + I₂ → S₄O₆^2- + 2I^-

Again, we have a problem with selection of the volume of titrated sample, and again the most logical approach is to use 20 mL pipette and 25 mL burette.

Procedure to follow is also very similar, just the moment of adding the indicator is different and we titrate not till color appears, but till it disappears:

Measure 20 mL of iodine solution and transfer it to Erlenmayer flask.
Titrate swirling the flask, until a pale yellow.
Add 5 ml of the starch solution.
Titrate swirling the flask, until blue color disappears.

To calculate thiosulfate solution concentration use EBAS - stoichiometry calculator. Download thiosulfate standardization against iodine reaction file, open it with the free trial version of the stoichiometry calculator.

Click n=CV button over iodine. Enter concentration and volume of the sample, click Use button. Click n=CV button below thiosulfate in the output frame, enter volume of the solution used, read solution concentration.