MICRO-NUTRIENT PROBLEMS 35 



to render the end-point of the wave for the latter rather indeter- 

 minate, even if the two waves do not coalesce. The large quantity 

 of potassium, calcium and magnesium chlorides present in the 

 solution acts as ground substance and the filtered solution can 

 be polarographed directly. Where the quantities of sulphate and 

 phosphate in the ash are relatively small, results obtained by this 

 treatment have been in reasonable agreement with results ob- 

 tained by other methods, but where much sulphate and phosphate 

 are present results are not reliable, owing probably to adsorp- 

 tion of manganese by barium sulphate or phosphate. For each 

 determination two solutions are taken, one consisting of ash 

 solution, the other consisting of ash solution containing the same 

 concentration of ash but with a known amount of added man- 

 ganese. The difference in the heights of wave given by the two 

 solutions is then attributable to the added manganese, to which 

 the height of the wave given by the pure ash solution is referred. 



Since a number of metals are reduced at more positive 

 potentials than manganese there is the possibility that they might 

 interfere with the wave for this ion. The principal elements 

 concerned are copper, cadmium, lead, chromium, molybdenum, 

 cobalt, nickel, iron and zinc. It is extremely unlikely that any of 

 these, with the exception of the last two, are likely to occur in 

 sufficient quantity in plant material to disturb the polaro- 

 graphic determination of manganese. As regards iron this is 

 practically all removed by the treatment of the ash solution 

 outlined above, and, as a matter of fact, no wave for iron appears 

 in the polarograms of solutions so treated. The possible influence 

 of zinc on the manganese wave was examined by the writer, who 

 found that no effect was produced on the wave of Mj 10,000 

 manganese by zinc in concentrations up to five times that 

 amount. No ash examined by the writer has been found to 

 contain a proportion of zinc to manganese approaching that 

 value. 



A number of methods for determining manganese with the 

 use of the absorptiometer are available. Most of these depend 

 on the oxidation of the manganese salt with the production of 

 permanganate, the intensity of the colour of which is determined 

 with the absorptiometer. The methods are known as the per- 

 iodate, persulphate and bismuthate methods, according to the 



