34 METHODS OF INVESTIGATING 



method for the simultaneous determination of manganese, 

 boron and copper in fertilizers, and their method would appear 

 to be suitable for the determination of these same trace elements 

 in plant material. The lines used were the manganese 2605-7 A., 

 boron 2497-7A. and copper 3247-5A., with the beryllium line 

 3130 A. as internal standard. An accuracy of about ± 5 per cent 

 is claimed for the estimations. Analyses published by the 

 advocates of the method indicate that quantities as small as, or 

 smaller than, 0-1 fig. can be determined by means of their pro- 

 cedure. 



Manganese is readily determined with the polarograph, a 

 good, well-defined wave being obtained when a chloride of an 

 alkali or alkaline earth is present in considerable excess. It is 

 usual to employ a few ml. of solution, and as concentrations of 

 from if/250,000 to if/300,000 are measurable, with the use of, 

 say, 5 ml. of solution l/u,g. of manganese is determinable. With 

 the use of special micro -cells taking smaller quantities of solu- 

 tion, very much smaller quantities of manganese can be mea- 

 sured. 



But although the polarograph would at first sight appear to 

 offer an ideal way for determining manganese in plants, actually 

 the polarographic determination of manganese in plant ash is 

 not straightforward, and in general is not to be recommended. 

 This is particularly so where ash or extract contains a con- 

 siderable quantity of phosphate. Plant ash consists chiefly of 

 oxides, phosphates and sulphates of potassium, calcium, mag- 

 nesium and other metals and is only soluble in acid. Since 

 hydrogen-ion gives a wave in solutions of alkali or alkaline earth 

 chlorides very near to that of manganese, an ash solution cannot 

 be polarographed for manganese directly because the waves for 

 manganese and hydrogen tend to coalesce. On neutralizing the 

 solution the manganese precipitates as phosphate, and in con- 

 sequence no wave for manganese is then given. To deal with this 

 situation the following procedure has been found by the writer 

 to give in some cases fairly reasonable results. After removal of 

 sulphate by barium chloride the phosphate is removed from the 

 ash solution by the addition of barium carbonate in excess and 

 filtering. This also removes ferric iron and aluminium, the wave 

 for the latter of which is sufficiently close to that of manganese 



