Preparation structure of Potassium dichromate and sodium thiosulphite.

Preperation.

$$Na_2Cr_2O_7+2KCl→K_2Cr_2O_7+2NaCl$$

KCl is added to hot concentrate solution of sodium dichromate.

It is an orange red in colour and dissolve in water.

1. Action with water.

$$4K_2Cr_2O_7=4K_2CrO_4+2Cr_2O_3+3O_2↑$$

2. Reaction with KOH.

$$K_2Cr_2O_7+2KOH=2K_2CrO_4+H_2O$$

Potassium dichromate is a powerful oxidizing agent in acid medium.

$$Cr_2O_7^{2-}+14H^++6e^-\rightleftharpoons 2Cr^{3+}+7H_2O;E=+1.36V$$

$$Cr_2O_7^{2-}+6Fe^{2+}+14H^+=2Cr^{3+}+6Fe^{3+}+7H_2O$$

$$Cr_2O_7^{2-}+6I^-+14H^+=2Cr^{3+}+3I_2+7H_2O$$

$$Cr_2O_7^{2-}+3H_2S+8H^+=2Cr^{3+}+3S+7H_2O$$

$$Cr_2O_7^{2-}+3SO_2+8H^+=2Cr^{3+}+3H_2SO_4+H_2O$$

Uses.

1. It is used as a primary standard in volumetric analysis.
2. Preparation of insoluble pigments and in the manufacture of inks example, chrome, alum, lead, chromate. The dichromate solution in concentrated sulphuric acid is used in degreasing glass wares.
3. It is also used as an oxidising agent in organic preparations.
4. Used as dichromate cells, leather tanning etc.

Potassium dichromate possesses advantage over potassium permanganate

1. Pot. dichromate is a primary solution while the permanganate is not.
2. Pot. dichromate has no action on dilute hydrochloric acid so that it can be used for titration in this acid medium (whereas potassium permanganate can be used.)

Structure.

Sodium Thiosulphate (Na₂S₂O₃).

Sodium thiosulphate is a salt of the thiosulphuric acid (H₂S₂O₃) The free acid is unstable at ordinary temperature.

$$H_2S_3O_3→SO_2+S+H_2O+H_2S+H_2SO_4$$

Structure.

Sodium Thio-Sulfate.

Preparation (Na₂S₂O₃).

1. When sodium sulphite or alkaline solution is boile with S, Na₂S₂O₃ is formed.

$$Na_2SO_3+S\xrightarrow{boil}Na_2S_2O_3$$

$$2Na_2S_3+3O_2\xrightarrow{heat}2Na_2S_2O_3+2S$$

2. Pure Na₂S₂O₃ is obtained from a solution of (NaHS+NaHSO₃). From the mixture Na₂S₂O₃ crystallize out.

$$2NaHS+4NaHSO_3→3Na_2S_2O_3+3H_2O$$

Sodium thiosulphate crystallize as Na₂S₂O₃.5H₂O.It is called Hypo.

Properties.

1. It is soluble in water.
2. Thio sulphated are decomposed by warming with dil HCl to give S and SO₂.

$$Na_2S_2O_3+2HCl→2NaCl+H_2O+SO_2↑+S$$

Precipitation of S and liberation of SO2 is an indication of S2O3-2 ion (this reaction is used in salt analysis.)

3. Reducing agent.

$$2S_2O_3→S_4O_6^{-2}+2e$$

$$(a);2Na_2S_2O_3+I_2→Na_2S_4O_6+2NaI$$

This reaction is basis of iodimetry in volumetric titration (estimation of Cu+2)

$$(b):\,Na_2S_2O_3+Cl_2+H_2O→Na_2sO_4+S+2HCl$$

$$(C);Fe^{+3}+2S_2O_3^{-2}→[Fe(S_2O_3)_2]^-$$

$$[Fe(S_2O_3)_2]^-+Fe^{+3}→2Fe^{2+}+S_4O_6^{-2}$$

Application in Photography.

Hypo is used for fixing film and prints. Uncharged AgBr in the photographic emulsion is removed by up. After developing the print is put in the solution of hypo. The reaction takes as follows and the AgBr is washed off from the photo.

$$Na_2S_2O_3+AgBr→Ag_2S_2O_3\xrightarrow{Na_2S_2O_3}Na_5[Ag(S_2O_3)_3]$$

When there is no photograph emulsion left, the print can be exposed to light.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.