Copper Fungicides

Abstract

Investigations on the various products of the action of lime on copper sulphate, including ordinary Bordeaux mixture, led to the conclusion that the efficacy of such substances as fungicides depended on the proportion of copper in them which is rendered soluble by the carbon dioxide of the air, and, that if a deficiency of lime is used to start with, one of the lower basic sulphates of copper is obtained, from which carbon dioxide liberates a much larger proportion of copper than it does from the more highly basic sulphates present in ordinary Bordeaux mixture. The lowest basic sulphate is, owing to its physical condition, unsuitable for spraying purposes; but this is not so with the compound 10 CuO, SO3, obtained by adding lime-water to copper sulphate just short of alkalinity, and this substance has been put on the market under the name of Woburn Bordeaux Paste. In the absence of secondary reactions, carbon dioxide should convert one-tenth of the copper present in it into soluble copper sulphate, 2 (10 CuO, SO3) + 9 CO2 = 9 (2 CuO, CO2) + 2 CuSO4, whereas with ordinary Bordeaux mixture, any copper sulphate formed in this way, would be decomposed by the excess of lime present, and that ultimately remaining undecomposed would be very small in amount.