Reaction of sodium thiosulphate with acid
This is a well-known reaction found in most
chemistry books and often carried out at GCSE level.
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.
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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