Thiosulfate ion testing, oxidation number, colours

Thiosulfate ion contains two sulfur atoms, but they have different oxidation numbers, 0 and +4. Sulfur's maximum oxidation number is +6. Therefore thiosulfate ion can be oxidized or reduced. Therefore thiosulfate can be converted into sulfur or sulfate ion in reactions.

Thiosulfate ion oxidation numbers

There is a double bond between two sulfur atoms(S=S) and middle sulfur atom and one oxygen atom. (S=O). Electronegativity of oxygen is higher than sulfur. Therefore oxygen has negative oxidation number. Oxidation number of all oxygen atoms are -2. One sulfur atom has +4 oxidation number and other has 0.

oxidation number and molecular shape of thiosulfate S2O32- ion

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-

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

Thiosulfate and 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.

iodine and thiosulfate reaction

Silver nitrate and sodium thiosulfate reaction.

A white precipitate (Ag2S2O3) is formed.

silver and thiosulfate

Add more S2O32- to Ag2S2O3. Precipitate is started to dissolve and give colourless solution.

silver thiosulfate and excess thiosulfate

But Ag2S2O3 is unstable. Therefore it turns black silver sulphide (Ag2S).

silver thiosulfate and silver sulfide

Potassium permanganate and thiosulfate ion reaction

Potassium permanganate (KMnO4) is purple colour solution. KMnO4 reacts with S2O32- ion in two ways.

  1. KMnO4 is reduced to Mn2+ cations by S2O32- in acidic solutions. Mn2+ aqueous solution is pink or colourless.
  2. KMnO4 is reduced to MnO2 by S2O32- in alkali solutions. MnO2 is a black precipitate.
  3. Sulfates and tetrathionates are formed in these reactions.

Ferric chloride and thiosulfate ion reaction

Ferric chloride (FeCl3) gives a deep violet colour with thiosulfate ion solutions. The colour quickly disappears. it is probaly due to the formation of ferric thiosulfates (or a corresponding complex ). The colour fades owing to reduction of ferric ion to ferrouss ion.

FeCl3 and S2O32- | Ferric chloride and thiosulfate ion reaction

thiosulfate and acid reaction

This reaction give SO2 and S(white precipitate) as products.

thiosulfate and acids

Lead acetate and thiosulfate ion solution reaction

White precipitate, lead thiosulfate (PbS2O3) forms. But this PbS2O3 is unstable. When it boils, lead sulfide (PbS) black precipitate is formed.

lead acitate and thiosulfate

Precipitates of thiosulfate ions and colours

  • Ag2S2O3 : white precipitate
  • PbS2O3 : white precipitate

Summary of tests of thiosulfate ion and reactions

identify thiosulfate ion reactions

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.