I was recently studying about the properties of Sodium Thiosulfate, where I came across the fact that $\ce{S2O3^2-}$ reacts with $\ce{CN-}$ ions to produce $\ce{SCN-}$ ions. This was quite useful in treating cyanide poisoning.

I have tried to find the complete reaction, but with no success. I believe this must be a redox reaction, since $\ce{S2O3^2-}$ has an average oxidation state of $+2$, while $\ce{SCN-}$ has a state of $-2$ (correct me if this is wrong). Can someone give me the complete reaction?

  • $\begingroup$ Redox reactions can be very complicated. In this case there's probably no complete scheme. $\endgroup$ – Mithoron May 10 '17 at 13:07
  1. Source 1(If you consider $\ce{Na+}$ as counterions)

$$\ce{Na2S2O3 + NaCN -> Na2SO3 + NaSCN}$$

Sodium thiosulfate reacts with sodium cyanide to produce sodium sulfite and sodium thiocyanate.

  1. Source 2(If you consider $\ce{Ag+}$ and $\ce{Na+}$ as counterions for cyanide and thiosulfate ion respectively)

$$\ce{AgCN + 2Na2S2O3 → Na3[Ag(S2O3)2] + NaCN}$$

Silver(I) cyanide react with sodium thiosulfate to produce sodium dithiosulfatoargentate(I) and sodium cyanide.

In this case, thiocyanate does not form, rather a complex forms.

Explanation of this reaction can be found by observing the related reactions given in the Wikipedia article of sodium thiosulfate.

Silver halides, e.g., AgBr are typical components of photographic emulsions, dissolve upon treatment with aqueous thiosulfate:

$$\ce{2S2O3^{2−} + AgBr → [Ag(S2O3)2]^{3−} + Br^{−}}$$

Since, $\ce{CN-}$ and $\ce{Br-}$ hold same charge, you can easily replace it with silver bromide with silver cyanide to get cyanide ions as products.


Sodium thiosulfate is a component of an alternative lixiviant to cyanide for extraction of gold to form a strong soluble complex $\ce{[Au(S2O3)2]^{3−}}$. The advantage of this approach is that thiosulfate is essentially not toxic and that ore types that are refractory to gold cyanidation (e.g. carbonaceous or Carlin-type ores) can be leached by thiosulfate. Some problems include the high consumption of thiosulfate, and the lack of a suitable recovery technique, since $\ce{[Au(S2O3)2]^{3−}}$ does not adsorb to activated carbon, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.

This process is also suitable for silver extraction using sodium thiosulfate and silver cyanide to form corresponding silver complex, $\ce{[Ag(S2O3)2]^{3-}}$. Just replace gold cyanide with silver cyanide and it will work pretty much well.

  • $\begingroup$ Thanks for your answer. Can you please guide me to sources that explain the reactions of thiosulfate, particularly silver thiosulfate? $\endgroup$ – Pritt Balagopal May 11 '17 at 14:00
  • $\begingroup$ @PrittBalagopal Thanks for the feedback. I edited the answer to make it clear $\endgroup$ – Nilay Ghosh May 11 '17 at 14:59

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