In titrations of iodine solutions with thiosulphate in the presence of ammonia or primary and secondary amines in near neutral solution (pH 5-7) sulphenamides are formed: This reaction competes with the formation of tetrathionate. The consumption of iodine is consequently up to double that expected for tetrathionate formation. The reaction is reversed by acidification, which gives tetrathionate and free iodine. The titrated solutions, containing no free iodine, retain their capacity to oxidise over long periods. They also retain the capacity to give the iodine-azide reaction, which is not given by tetrathionate. The stability of the sulphenyl-stage in the presence of amines can also be demonstrated in solutions containing an excess of iodine. In absence of amines the sulphenyl-stage in such solutions is known to disappear within minutes. The NH-effect is defined and its dependence on the pH, concentrations of titrants, iodide and NH-compounds is investigated. Other SH-compounds also give the effect, even at a lower pH, but this is often followed by irreversible oxidation by the iodine solution. The NH-effect of thioglycollic acid is only exceptionally (with morpholine) reversible by acidification. That of sulphide is not reversible and that of thioacetic acid is partly reversible. The postulated compound formed between thiosulphate and ammonia, NaO3S-S-NH2, sodium thiohydroxylamine-S-sulphonate, has been described in another context. The ready formation of an S-N bond, where an S-S bond would have been expected, should be considered in protein chemistry. The S-N compounds formed from thioacids and amino-acids should be of high interest.
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