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.
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.
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.
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 (S4O62-). These physical observations confirm that there may be thiosulfate ions in the aqueous solution.
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.
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.
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.
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+).
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.
A white precipitate, Lead thiosulfate (PbS2O3) forms. But this PbS2O3 is an unstable compound. When it boils, lead sulfide (PbS) black precipitate is formed.
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.
Reaction of rhombic form of sulfur and sulfite ion will give thiosulfate ion.
SO32- + S → S2O32-
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.
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.
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.