10 Feb., 1919.] Copper Fungicides for Vine Diseases. 107 



is equal to 3.1927 lbs. of pure anhydrous soda carbonate, the mixture is 

 no longer acid — it ceases to redden blue litmus paper. It will, however, 

 be necessary to continue the addition of soda until 4.257 lbs. of pure 

 carbonate has been added before the first signs of alkalinity appear. 



. The explanation of this curious difference is to be found in the fact 

 that when copper sulphate is added to soda carbonate solution, the latter 

 salt being in excess until the close of the reaction, the whole of the 

 copper is precipitated as blue hydrocarbonate, the respective quantities 

 of copper sulphate and of pure soda carbonate necessary for complete 

 neutralization being 10 lbs. and 4.257 lbs., in accordance with the formula 

 first quoted above. 



In the second case (soda into copper), copper sulphate is in excess, 

 and the copper is no longer precipitated as hydrocarbonate, but in the 

 form of basic sulphates. In connexion with Bordeaux mixture,* a whole 

 series of basic sulphates was enumerated. Fonzes-Diacon points out, 

 as a result of his investigations, that in the case of copper soda made 

 by pouring the soda solution into the copper, " one of these basic sul- 

 phates is readily formed; it is the one answering to the formula 

 CuSO,, 3CuO, 4H,0, which may also be written 4CuO, SO., 4H2O, for 

 which reason it is known as tetracupric sulphate. The following formula 

 explains the reaction " — 



4CuS0,, .5H,0 + SNa^COa = .SNa.SO^ + CUSO4, 3CuO, 4H3O + SCOp. 



Copper Soda Soda Soda Tetracupric Carbonic 



Crystals. Carbonate. Sulphate. Sulphate. Acid. 



According to this formula, 10 lbs. of crystallized copper sulphate 

 would be completely precipitated in the form of blue tetracupric sulphate 

 when 3.1927 lbs. of pure anhydrous soda carbonate have been used, say, 

 for convenience' sake, 3.2 lbs. of soda carbonate. 



If the addition of soda carbonate be continued, this salt will react on the in- 

 soluble basic sulphate previously formed, decomposing it so as to give at the end 

 of the reaction blue hydrocarbonate as in the first case (copper into soda), as is 

 explained by the following equation: — 



CuSO , 3CuO, 4H 0, Na CO , CO , Na SO , 2(CuC0 , CuO, 2H 0). 



4' ' 2 ' 2 n' 2' 2 4' ^ 3' ' 2 ' 



It is only after the completion of this reaction that the excess of soda carbonate 

 will manifest its presence by reddening phenolphthalein paper. 



In reality, the total precipitation of the copper sulphate in the form of tetra- 

 cupric sulphate is a somewhat theoretical conception; in actual practice, the 

 precipitate formed at the commencement of the reaction quite answers this com- 

 position, but with the diminishing concentration of the copper sulphate solu- 

 tion, a little hydrocarbonate of copper makes its appearance, the proportion of 

 which increases towards the end of the reaction. 



To resume, in the case of copper into soda, the precipitate consists 

 entirely of copper hydrocarbonate, whereas, in the case of soda into 

 copper, we have two distinct precipitates, viz., tetracupric sulphate, if 

 the soda be in the proportion of 3.2 lbs. for 10 lbs. of copper sulphate, 

 and copper hydrocarbonate if the quantity of soda carbonate be increased 

 to 4.257 lbs. 



"With intermediate quantities of soda carbonate, the precipitate would 

 consist of a mixture of basic sulphates and copper hydrocarbonate, one 

 or other of which predominating according as the quantity of soda car- 

 bonate more nearly approaches the lower or the upper limit mentioned 

 above. Although the most important basic sulphate seems to be the tetra- 

 cupric, sulphates of higher basicity are no doubt also present in the 



♦ See Journal for October, 1918, page 595. 



