MICRO-NUTRIENT PROBLEMS 



41 



After ashing a 5 g. sample of the dried plant material at 500- 

 550° C, the ash is dissolved in hydrochloric acid, insoluble matter 

 being removed by nitration. The metals which form complexes 

 with dithizone are then extracted from the solution by repeated 

 treatment with excess of a solution of dithizone in carbon 

 tetrachloride at a pH of 8-5-9 in presence of ammonium citrate ; 

 the latter prevents the precipitation of iron and aluminium. The 

 dithizone extract is then treated with 0-02 N hydrochloric acid; 

 copper and the excess of dithizone remain in the carbon tetra- 

 chloride phase while the zinc and other metals pass into the 

 aqueous phase. The pH of the latter is then adjusted to between 

 8-5 and 9 by the addition of ammonia-ammonium citrate buffer 

 containing carbamate and the zinc extracted with dithizone in 

 carbon tetrachloride. The resulting extract is used for the deter- 

 mination of the zinc, the reading obtained being compared with 

 those given by solutions of known zinc content which are plotted 

 to give a standard curve. It is essential that the same conditions 

 should be rigidly adhered to both in obtaining the standard 

 curve and in the analysis of samples of plant material; that is, 

 the same pK should be used in the extraction, the volumes of 

 phases, the amount of dithizone and the amount of carbamate 

 used should be the same. Tests made by the authors show that 

 if this is done the method is highly reliable. The presence of 

 other metals does not interfere significantly with the deter- 

 mination, a good degree of accuracy was obtained in the 

 reovery of zinc added to plant material, while good agreement 

 was obtained between determinations of zinc in duplicate samples 

 of the same material. The method would appear to be capable 

 of determining quantities of zinc as small as 2/xg. or even less. 

 Copper. The spectrograph^ determination of copper in plant 

 material can be carried out by both the flame and arc methods. 

 With the use of the Lundegardh flame method the sensitivity, 

 according to the recent experience of Griggs, Johnstin and 

 Elledge (1941), is about half that found for manganese, these 

 workers giving the minimum concentration of copper usable as 

 0-000025 M . As the minimum quantity of solution which can be 

 employed in their arrangement is 2 ml. this would give the lowest 

 measurable amount of copper to be 3-15/xg. The line used for the 

 measurements is 3247- 5 A. (Lundegardh, 1929). 



