32 METHODS OF INVESTIGATING 



the kind of problem which faces the investigator of plant 

 nutritional and pathological problems. Broadly speaking it may 

 be said that the results of a single determination obtained by 

 the methods that have been outlined here are correct within 

 about 5-10 per cent. A higher degree of accuracy is no doubt 

 often possible, and with the absorptiometer, polarograph and 

 spectrograph it has been claimed in certain circumstances that 

 the error of a single determination does not exceed 2 per cent. 

 By making a number of replicate determinations and taking the 

 mean value a considerable increase in the accuracy of an estima- 

 tion may be achieved, but the limited amount of material 

 available may often render this procedure impossible. 



We may next turn to a consideration of the methods suit- 

 able for the estimation of the individual micro -nutrients in 

 plant material. They are usually determined in plant ash. This 

 should be prepared by first drying the material in an oven at 

 70-100° C, grinding it to a powder and then incinerating it in a 

 furnace at a temperature of from 450 to 600° C. Burning in a 

 crucible over a Bunsen burner or blowpipe is not to be recom- 

 mended, as this leads to an intense local heat causing partial 

 volatilization of some of the mineral matter. The ash, after 

 cooling, is dissolved in a small quantity of mineral acid and the 

 resulting solution evaporated to dryness on a water-bath. The 

 residue is then dissolved in a definite volume of water or dilute 

 acid. Insoluble silica may be removed by filtering. 



The preparation of ash in this way is not always to be recom- 

 mended. For example, Reed and Cummings (1941) found that 

 there was a considerable loss of copper, amounting to 50 per 

 cent or more, involved in this method of ashing, even when the 

 temperature of ignition was as low as 450° C. For the estimation 

 of copper in plant material they therefore recommend a pro- 

 cedure in which from 0-5 to 2 g. of the plant material are heated 

 with 5 ml. of concentrated nitric acid until brown fumes are 

 evolved, when 1 ml. of concentrated sulphuric acid is added and 

 heating continued until charring begins and all nitric acid is given 

 off. After addition of 1-2 ml. of 60 per cent perchloric acid, heating 

 is resumed until the solution is colourless or pale yellow and ex- 

 cess of perchloric acid also given off. The resulting solution should 

 then contain all the copper originally present in the sample used. 



