106 Journal of Agriculture, Victoria. [10 Feb., 1919. 



of copper soda, carbonate of soda is the alkaline substance employed. 

 It might at first sight appear that this substitution of soda for lime 

 would be of little consequence, provided a sufficiency be used to achieve 

 the right degree of neutrality. As a matter of fact, this change of 

 alkali has very far-reaching effects on the composition of the resulting 

 fungicide mixture. 



The complexity of the reactions which occur in the making of Bor- 

 deaux mixture has already been pointed out;* the chemistry of copper 

 soda is, however, even more complicated, the range of different com- 

 pounds which may make their appearance being greater. These vary, 

 not only according to the relative quantities of soda and copper sulphate 

 which interact, but also, and to a very considerable extent, according to 

 the order in which they are mixed, the concentration of the solution, 

 and the speed with which the actual mixing is effected. 



For a good many years after its first introduction, copper soda was 

 mixed in a rather haphazard manner, little being known concerning its 

 chemistry. At the Viticultural Congress held in 1914 at Lyons 

 (France), Dr. H. Fonzes-Diacon, Professor at the University of Mont- 

 pellier, read a paper on Copper Soda, the following extracts from which 

 will give some idea of the complexity of the question, and the reasons for 

 certain alterations recommended in the preparation of the mixture. 



• The following formula is often used to explain the action of soda 

 carbonate on copper sulphate when the quantities of each are those 

 necessary for complete neutralization : — 



CuSO,, + .:f^a, COs = Na^SO^ + Cu CO3 

 Copper Soda Soda Copper 



Sulphate. Carbonate. Sulphate. Carbonate. 



In reality the light-blue gelatinous precipitate formed in the above reaction 

 is not neutral copper carbonate, CuCOg, but a hydrocarbonate, resulting from 

 the partial dissociation of the former with liberation of carbonic acid, and answer- 

 ing to the formula CuCOg, CuO, 2H2O. 



2CUCO3 + 2H,0 = CuCOg, CuO, 2HoO + COj. 

 The carbonic acid gas thus literated partly remains in solution, dissolving a 

 little copper carbonate forming a bicarbonate which subsequently decomposes 

 slowly on exposure to air. 



So far as the quantities of the two salts which interact in the case of a 

 neutral mixture, the equation enables us to calculate that 1 kilog. of crystallized 

 copper sulphate will be entirely precipitated by 425.7 grms. of pure anhydrous 

 soda carbonate. 



In reality, though this is what happens when sulphate of copper, dissolved in 

 water, is poured into carbonate of soda solution, the reaction is no longer at all 

 the same when the operation is effected in the reverse order ( soda into copper ) . 



In the first case (copper into soda) the liquid retains its alkaline 

 reaction until the whole of the copper sulphate has been added, when it 

 suddenly changes to acid. In the case of soda into copper, instead of 

 a sudden change from an acid to an alkaline condition, we have, as also 

 occurs with Bordeaux mixture, a neutral or intermediate stage. There 

 is, in other words, a considerable difference between the quantity of soda 

 needed to bring about cessation of acidity and that required to cause the 

 first signs of alkalinity. 



Let us take, for example, 10 lbs. of copper sulphate (the quantity 

 necessary to make 50 gallons of 2 per cent, spray mixture) and dissolve 

 this in, say, 40 gallons of water. If we progressively add to this, soda 

 carbonate in solution, we shall find that when the quantity of this salt 



• See Journal for October, 1918, p. 594. 



