Test for Thiosulfate ion (S₂O₃^2-) - Qualitative Analysis

In qualitative analysis, there are several tests to identify thiosulfate ion (S₂O₃^2-). 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 S₂O₃^2- 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.

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 + S₂O₃^2- reaction
• Potassium permanganate and thiosulfate ion reaction
• S₂O₃^2- 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.

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 (S₄O₆^2-). These physical observations confirm that there may be thiosulfate ions in the aqueous solution.

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.

• Add more thiosulfate ion (S₂O₃^2-) solution to the solution where Ag₂S₂O₃ precipitate is. Now, you can see precipitate is started to dissolve and give a colourless solution.

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

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 S₂O₃^2- 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 S₂O₃^2- 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 S₂O₃^2- 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⁺).

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.

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.

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

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

Oxidation number of S₂O₃^2- ion

In this section, we are going to find oxidation number of sulfur atoms in S₂O₃^2- 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.