A substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver. Synonyms:reducer, reductant.
... is an easy matter to obtain sodium nitrite (NaNO{2}), as the reaction given on the previous page indicates. Instead of merely heating the nitrate, it is better to heat it together with a mild reducing agent, such as lead, when the reaction takes place which is expressed ... — An Elementary Study of Chemistry • William McPherson
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