48 METHODS OF INVESTIGATING 



mination of boron in tissues where this element is present in 

 quantity less than 50 p. p.m. 



Molybdenum. Molybdenum in plant material is generally 

 determined with the colorimeter or absorptiometer as molyb- 

 denum thiocyanate by the method described by Marmoy (1939). 

 A 50 ml. sample of the hydrochloric acid solution of the ash 

 containing not more than 20jug. of molybdenum and having an 

 acid concentration of 1 4 per cent by volume is treated first with 

 3 ml. of potassium thiocyanate and then with 3 ml. of stannous 

 chloride. The molybdenum thiocyanate produced is then ex- 

 tracted with ether, the extractions being repeated until the 

 ether layer is colourless, and the depth of colour of the com- 

 bined ether extract is compared with that of a standard solution 

 in ether of molybdenum thiocyanate prepared from ammonium 

 molybdate in the same way as that from the ash. 



Aluminium. Aluminium is present in quantity in many soils 

 and is generally present in plants, but so far its essentiality has 

 been indicated for only a few species. Perhaps for this reason 

 not so much attention has been given to the determination of 

 aluminium in plant material during recent years as to the better 

 established micro -nutrients. 



For the measurement of small quantities of aluminium, such 

 as might be expected in samples of plant material, direct measure- 

 ment of the intensity of the aluminium line in the air-acetylene 

 flame spectrum is not suitable, for Lundegardh (1929) found that 

 no line was obtained with even a 0- 1 M solution of aluminium 

 chloride. Mitchell and Robertson (1936) have, however, 

 described a means by which aluminium in concentrations 

 ranging from about 2 to 10 mg. per litre can be determined by 

 the Lundegardh method. It depends on the fact that the 

 presence of aluminium brings about a lessening of the intensity 

 of the calcium and strontium lines of the flame spectrum, the 

 decrease in intensity varying with the amount of aluminium 

 present and also with the relative, amounts of calcium and 

 strontium present. Hence with careful control of the conditions 

 the depression in the intensity of these lines can be used to 

 determine the aluminium content of solutions of the concentra- 

 tions indicated above. Using 15 ml. of solution for a determina 

 tion, this means that quantities down to 30/xg. of aluminium 

 can be measured in this way. 



