COPPER. 
§ 836 .] 
vegetables, such as peas, beans, cucumbers, and so forth, preserve their 
green colour if boiled in copper vessels, has long been known. In this 
“ coppering ” the French have been more active than the English 
traders ; the French operate in two different ways. One method is, to 
dip from 60 to 70 litres of the green vegetables in 100 litres of 0-3 to 
0-7 per cent, of copper sulphate, to leave them there for from five to 
fifteen minutes, then to remove them, wash and sterilise in an autoclave. 
A second method is to put the vegetables into a copper vessel, the wall 
of which is connected with the negative terminal of an electric current ; 
the positive pole dips in a solution of salt in the same vessel, the current 
is allowed to pass for three minutes, and the vegetables are afterwards 
sterilised. Fruits are simply allowed to stand with water in copper 
vessels, the natural acidity of the juice dissolving sufficient copper. 
The amount of copper taken up in this way is appreciable, but yet 
not so much as might be expected. The prosecutions for selling 
“ coppered ” peas in England have been based upon quantities varying 
from 1 to 3 grains per lb. ; the highest published amount of copper 
found in peas artificially coloured is 0-27 grm. per kilo., or 18*9 grains 
per lb. 
The reason why vegetables preserve their green colour longer when 
treated with a copper salt has been proved by Tschirch 1 to be owing to 
the formation of a phyllocyanate of copper. 
Phyllocyanic acid is a derivative of chlorophyll, and allied to it in 
composition ; the formula of C 24 H 28 N 2 0 4 has been ascribed to it. Under 
the action of acids generally, mineral or organic, chlorophyll splits 
up into this acid and other compounds. Copper phyllocyanate, 
(C 24 H 27 N 2 0 4 ) 2 Cu, contains 8-55 per cent, of copper ; it forms black 
lamellae, dissolving easily in strong alcohol and chloroform, but in¬ 
soluble in water ; it is a little soluble in ether, insoluble in petroleum 
ether, and dissolved neither by dilute acetic acid nor by dilute nor 
concentrated hydrochloric acid. The compound dissolves in caustic 
alkali on warming. In alcohol it forms a beautiful non-fluorescent 
solution. A solution of 1 : 100,000 is still coloured strongly green. 
This solution, in a stratum 25 mm. thick, gives four absorption 
bands when submitted to spectroscopic observation, and Tschirch has 
worked out a process of estimation of the amount of copper phyllo¬ 
cyanate based upon the disappearance of these bands on dilution. 
Green substances, so carefully treated that they only contain 
phyllocyanate of copper, would yield but small quantities of copper, 
and probably they would not be injurious to health ; but the coppering 
is usually more extensive, and copper leguminate and other compounds 
are formed—for the vegetables, when exhausted by alcohol, give a 
residue which, successively exhausted by water, by soda-lye, and lastly 
1 Das Kupfer, Stuttgart, 1983. 
42 
