About us | Feedback

Standardization of iodine and thiosulfate solutions for use in potentiometric titrations

Potentiometric titrations

Permanganate titration

Iodometry

Need more info?

Measurement Uncertainty in Chemical Analysis Amazon

Measurement Uncertainty in Chemical Analysis

by Paul De Bievre (Editor)

Potentiometric titration » Iodometry - standardization

Balancer and stoichiometry calculator

operating systems:
XP, Vista, 7, 8, 10

single user license price:
€24.95 - approximately $33

Buy Now!

download 30-day free trial!
30-day money back guarantee!

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:

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 → 2AsO43- + 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:

2S2O32- + I2 → S4O62- + 2I-

Procedure to follow:

For calculations we will use rather strangely looking reaction equation:

KIO3 + 6Na2S2O3 + 6H+ → 3S4O62- + 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:

Cr2O72- + 6I- + 14H+ → 2Cr3+ + 3I2 + 7H2O

Iodine solution is then titrated with thiosulfate:

2S2O32- + I2 → S4O62- + 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:

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+ + 3S4O62- + 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:

2S2O32- + I2 → S4O62- + 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:

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.

2S2O32- + I2 → S4O62- + 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:

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.


Page was last modified on October 28 2010, 14:09:50.

titration at www.titrations.info © 2009 ChemBuddy