42 METHODS OF INVESTIGATING 



It has already been mentioned (p. 33) that with the use of the 

 arc Melvin and O'Connor (1941) have achieved the simultaneous 

 determination of boron, manganese and copper. The copper line 

 used for the determinations was 3248 A., and the beryllium line 

 3 130 A. was used to provide the internal standard. From the 

 analyses published by Melvin and O'Connor it would appear that 

 the copper forming 0-001 per cent of a 10 mg. sample of material 

 can be measured, from which it would seem that quantities 

 of copper of the order of 0-1 fxg. can be estimated. 



For the spectrograph^ determination of copper in grasses 

 Rusoff, Rogers and Gaddum (1937), employing the arc, used 

 cadmium as internal standard. 



A procedure for the estimation of copper in plant materials 

 by means of the polarograph has been described by Reed and 

 Cummings (1941). The solution obtained by wet ashing of from 

 0-5 to 2 g. of plant material (see p. 32) is diluted to 15-20 ml., 

 heated to boiling and then rendered alkaline by the addition of 

 a slight excess of concentrated ammonium hydroxide. The 

 solution is then boiled for a minute and filtered, the residue being 

 washed with slightly ammoniacal water and the washings added 

 to the nitrate. The latter is evaporated to dryness and the 

 resulting residue dissolved in 10 ml. of a ground liquid consisting 

 of 4-5 ml. of 0-5 M sodium hydroxide +4-5 ml. of 0-5 M citric 

 acid + 1 ml. of 0-05 per cent of acid fuchsin. This liquid, when 

 polarographed in absence of oxygen, gives a well-defined wave 

 for copper. In this way, according to Reed and Cummings, con- 

 centrations of copper down to about 3 x 10~ 6 M can be deter- 

 mined, corresponding to about 2 p. p.m. of copper in the plant. 

 Published data indicate that in normal plants the amount of 

 copper is usually well above this value, but in plants showing 

 symptoms of copper deficiency it may be lower than this. In 

 such cases it would be necessary either to use larger quantities 

 of plant material for the determination or to use a method other 

 than a polarographic one. 



The absorptiometric method generally used for the deter- 

 mination of copper in plant material depends on the intense 

 colour produced by copper salts with diethyldithiocarbamate 

 (Callan and Henderson, 1929), the coloured complex being 

 extracted with amyl alcohol. Procedures particularly applicable 



