Reaction of sodium thiosulphate with acid

This is a well-known reaction found in most chemistry books and often carried out at GCSE level.

Sodium thiosulphate reacts with acid to produce sulphur which is deposited in the solution causing it to become opaque.

    Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O

The colorimeter can be used to measure the rate of this reaction since the increasing opacity scatters the light passing through.


There are no hazards associated with the basic investigations described here.
At 0.1M the concentration of hydrochloric acid used in the examples given is well below that requiring a hazard warning label.

Good laboratory practices should be observed.

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    thiosulphate graph

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This investigation can be carried out with small volumes and low concentrations of the reactants.

  • put 1.5cm3 of dilute hydrochloric acid into a cuvette
  • add 1.5cm3 of sodium thiosulphate
  • read absorbance (best with the blue LED).

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