The Role of Impurities in Chain Reactions

The inhibiting action of impurities (inhibitors) on chain reactions generally induces chain termination due to the decay of active centers. An example is the action of NCI3, one of the most active inhibitors of the chlorine-hydrogen reaction. Exceedingly small amounts of NCI3 decrease the quantum yield of HCl in the photochemical reaction H2 + CI2 from 0 = 10 to 2, i.e. convert the chain reaction to a non-chain reaction [170]. 

An extensive study of the inhibition effect shows that a non-chain reaction is often only observed when the inhibitor is added to the initial mixture. For instance, the addition of sulfur vapour (0.5—1 torr) to a mixture of hydrogen sulfide and oxygen has been found to completely inhibit the H2S oxidation whereas the same amount of sulfur added in the course of the reaction (after the reaction has reached its maximum rate) does not affect its progress [125]. 

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