Question

Write the chemical formula and uses of the following: Hypo solution.

Answer 1

Hint: Sodium thiosulfate solution is commonly known as hypo solution in chemistry. It is present in the hydrated form with five water crystallisations. Hypo solution is used in chemistry laboratories for iodometric titrations.

Complete step by step solution
Hypo solution chemically known as sodium thiosulphate. It has the chemical formula which can be written as- $ N{a_2}{S_2}{O_3}.5{H_2}O $ .
Sodium thiosulphate can be prepared by boiling alkaline or neutral sodium sulphite solution with sulphur. The reaction can be represented as-
 $ N{a_2}S{O_3} + S \to N{a_2}{S_2}{O_3} $
But by evaporating the obtained reaction mixture, the monoclinic crystals of sodium thiosulphate are obtained.
 $ N{a_2}{S_2}{O_3} \to N{a_2}{S_2}{O_3}.5{H_2}O $
The salt of sodium thiosulphate is stable and numerous.
The hydrated sodium thiosulphate is mainly known as “hypo”. It forms very large colourless hexagonal crystals with melting point $ {48^\circ }C $ . It is readily soluble in water.
The hypo solution is used in many ways-
a) It is used in the volumetric titration of iodine analysis. Iodine oxidises thiosulphate ions to tetrathionate ions and itself reduces to iodide ions.
 $ {S_2}{O_3}^{2 - } + {I_2} \to {S_4}{O_6}^{2 - } + {I^ - } $
b) It is also used in the bleaching industries to destroy any excess $ C{l_2} $ on fabrics after which they have been through a bleaching bath. Since chlorine is a stronger oxidising agent than iodine, it oxidises thiosulfate to hydrogen sulphate rather than tetrathionate. The reaction can be written as –
  $ N{a_2}{S_2}{O_3} + 4C{l_2} + 5{H_2}O \to 2NaHS{O_4} + 8HCl $
c) It is also used in photography for fixing films and prints. Photographic emulsions are made of silver nitrate, silver chloride and silver bromide. When parts of film are exposed to light they get decomposed to silver. Thus formation of negative image takes place. Then after developing, the film or print is put into thiosulphate solution as the thiosulphate ions forms a soluble complex with silver salts. Hence no extra silver salt will be present on the print and it can be easily carried in light without any decomposition. The reaction of formation of complex can be written as
 $ N{a_2}{S_2}{O_3} + AgBr \to A{g_2}{S_2}{O_3} + N{a_2}{S_2}{O_3} \to N{a_3}[Ag{({S_2}{O_3})_2}] $ .

Note:
Sodium sulphite can be prepared by passing sulphur dioxide through a solution containing sodium carbonate in excess. Also sodium thiosulphate can be prepared by oxidising polysulphides with air. The reaction can be represented as- $ 2N{a_2}{S_3} + 3{O_2} \to 2N{a_2}{S_2}{O_3} + 2S $