Test for Thiosulfate ion (S2O32-) - Qualitative Analysis

In qualitative analysis, there are several tests to identify thiosulfate ion (S2O32-). According to the metal cation, thiosulfate compounds can be aqueous solutions or precipitates. Also, some thiosulfate compounds have specific colours which is an important fact in identify thiosulfate from other anions. Thiosulfate ion contains two sulfur atoms and three oxygen atoms, but two sulfur atoms have different oxidation numbers, 0 and +4.

In this tutorial, we will study followings.

Oxidation numbers of Thiosulfate ion and redox reactions

According to the lewis structure of S2O32- ion, there are two double bonds. One double bond is located between the sulfur atoms other double bond is located between middle sulfur atom and one oxygen atom. (S=O). Electronegativity of oxygen is higher than sulfur. Therefore oxygen atoms should have negative oxidation numbers. Oxidation number of all oxygen atoms are -2. However, one sulfur atom has +4 oxidation number and other has 0.

oxidation number and molecular shape of thiosulfate S2O32- ion

Sulfur's maximum oxidation number is +6. Therefore thiosulfate ion can be oxidized or reduced to another oxidation state. Therefore thiosulfate can be converted into sulfur or sulfate ion in reactions which is possible to apply in identify thiosulfate ion in qualitative analysis.

Tests for thiosulfate ion

  • Thiosulfate and iodine with farina
  • AgNO3 + S2O32- reaction
  • Potassium permanganate and thiosulfate ion reaction
  • S2O32- with ferric chloride or lead acetate

Test for thiosulfate ion: iodine with farina

Add little bit of farina to iodine solution. The solution will take blue colour.

farina and iodine

Then add Sodium sulphite (Na2S2O3) to that blue colour solution. You can see blue color is disappearing from the solution due to loss of I2 because I2 is reduced to Iodide ions (I-) and thiosulfate ion is oxidized to tetrathionate (S4O62-). These physical observations confirm that there may be thiosulfate ions in the aqueous solution.

iodine and thiosulfate reaction

Test for thiosulfate ion with Silver nitrate

  • Add Silver nitrate (AgNO3) solution in small quantities. You may see a white precipitate of Silver thiosulfate (Ag2S2O3) is formed in the solution.
silver and thiosulfate

  • Add more thiosulfate ion (S2O32-) solution to the solution where Ag2S2O3 precipitate is. Now, you can see precipitate is started to dissolve and give a colourless solution.
silver thiosulfate and excess thiosulfate

  • But Ag2S2O3 is an unstable compound. Therefore it turns to black silver sulphide (Ag2S).
silver thiosulfate and silver sulfide

Test for thiosulfate ion with Potassium permanganate

Potassium permanganate (KMnO4) is a strong oxidizing agent and a purple colour aqueous solution when it dissolves in water. Potassium permanganate reacts with S2O32- ion in two ways to give different products.

Thiosulfate ion with Potassium permanganate in acidic medium

Potassium permanganate is reduced to Manganese +2 cation (Mn2+) cations by S2O32- in acidic solutions. You can see purple colour of aqueous potassium permanganate solution is changed to pink or colourless due to the Mn2+ cation.

As the oxidation half reaction, Sulfates and tetrathionates are formed in these reactions.

Thiosulfate ion with Potassium permanganate in alkaline medium

Potassium permanganate is reduced to Manganese dioxide (MnO2) by S2O32- in an alkali medium. Manganese dioxide is MnO2 is a black precipitate and not soluble in water.

Test for thiosulfate ion with Ferric chloride

Ferric chloride (FeCl3) gives a deep violet colour with thiosulfate ion solutions. The colour quickly disappears. It is probably due to the formation of ferric thiosulfates (or a corresponding complex ). The colour fades owing to the reduction of ferric ion (Fe3+) to ferrous ion (Fe2+).

FeCl3 and S2O32- | Ferric chloride and thiosulfate ion reaction

Test for thiosulfate ion with an acid

In the presence of a dilute acid, thiosulfate ion releases sulfur dioxide (SO2) gas and sulfur. Due to those formation, you can see a release of colourless gas (SO2) and a white / yellow precipitate (Sulfur). In both products, sulfur atom in sulfur is at 0 state and sulfur atom in sulfur dioxide gas is at +4 oxidation state. So, this reaction is not a redox reaction because oxidation numbers of respective sulfur atoms are not changed during the reaction.

thiosulfate and acids

Test for thiosulfate ion with Lead acetate

A white precipitate, Lead thiosulfate (PbS2O3) forms. But this PbS2O3 is an unstable compound. When it boils, lead sulfide (PbS) black precipitate is formed.

lead acitate and thiosulfate

Precipitates of thiosulfate ions and colours

  • Ag2S2O3 : white precipitate, but this unstable and changes to black colour Silver sulfide.
  • PbS2O3 : white precipitate. This is also an unstable compound and readily converts to Lead sulfide (PbS) which is a black precipitate.

Summary of tests of thiosulfate ion and reactions

identify thiosulfate ion reactions

Molecular shape of thiosulfate ion

One sulfur aton is the central atom in the thiosulfate ion. There are four sigma bonds and zero lone pairs around the central atom. Therefore molecular shape of thiosulfate ion is tetrahedral.

Preparation of thiosulfate ion

Reaction of sulfur and sulfites ion

Reaction of rhombic form of sulfur and sulfite ion will give thiosulfate ion.
SO32- + S → S2O32-

Oxidation number of S2O32- ion

In this section, we are going to find oxidation number of sulfur atoms in S2O32- ion using the equation. We assume oxidation numbers of sulfur atoms are same. Oxidation number of oxygen atoms are taken as -2.

Oxidation number of s is taken as x.

2*x + (-2)*3 = -6

x = +3

But, by calculating oxidation number from drawing the molecular shape, we got oxidation numbers of sulfur atoms are +4 and 0. Therefore we see, the oxidation number found by the formula is not much accurate.


Write a balanced chemical equation and the expected observations when dilute nitric acid is added to an aqueous solution of sodium thiosulfate

There can be two reactions depending on the concentration and temperature of nitric acid. You know nitric acid is an oxidizing acid. So it can oxidize other compounds.

with dilute cold nitric acid

NaNO3, SO2, S and H2O are given as products.

2HNO3 + Na2S2O3 = SO2 + S + 2NaNO3 + H2O

yellow/white colour precipitation (sulfur) can be observed. Bad smell of SO2 is felt.

Lewis structures

P2O5 lewis structure OH- lewis structure Ammonium ion (NH4+) lewis structure H2CO3 lewis structure