44 METHODS OF INVESTIGATING 



mixture vigorously shaken for 30 sec. The amyl alcohol extract, 

 which contains the coloured copper diethyldithiocarbamate, is 

 centrifuged or filtered through acid-extracted filter paper to 

 remove any water in suspension and the depth of colour deter- 

 mined with a colorimeter or absorptiometer by comparison with 

 a standard solution of copper diethyldithiocarbamate prepared 

 in the same way as that from the tissue digests. With adequate 

 precautions it would appear that quantities of copper as small 

 as 0-3 g. are determinable by the procedure of Eden and Green. 



In the method as used by Piper a wet ashing process is also 

 used, and after diluting the digest with water and treating it 

 with ammonium citrate solution to dissolve any hydrolysed 

 manganese and to prevent precipitation of phosphates, the 

 resulting solution is brought to pH 3 and the copper extracted 

 with dithizone in carbon tetrachloride. The carbon tetrachloride 

 is then removed by evaporation and the dithizone by heating 

 with a little sulphuric acid containing a drop or two of per- 

 chloric acid. After dilution with water and rendering the solu- 

 tion alkaline with ammonium hydroxide, the copper is pre- 

 cipitated by the addition of a few drops of a 3 per cent aqueous 

 solution of sodium diethyldithiocarbamate. The copper diethyl- 

 dithiocarbamate is then extracted with amyl alcohol and the 

 intensity of colour of the extract compared with that of a 

 standard. For full details of the experimental procedure refer- 

 ence should be made to Piper's book. 



Boron. As regards the determination of boron by means of 

 the spectrograph, the usual flame method is useless. Even with 

 a 0-1 M solution of boric acid no boron line was obtained by 

 Lundegardh in a flame spectrum. Boron in plant material has, 

 however, been determined by means of the spark method by 

 Foster and Horton (1937). The plant material was used fresh, 

 being neither dried nor ashed, but crushed and disintegrated 

 into a fine pulp in a small copper mortar. The samples used 

 consisted of 100 mg. of fresh material. Measurements were 

 made on the boron line 2497-7 A., and the gold line 2427-9 A. was 

 used as internal standard. Six replicate determinations of the 

 boron in turnip leaves gave values varying from 3-7 to 4-3/zg. of 

 boron per g. of fresh tissue, with an average value of 4-lyu,g. 

 Foster and Horton conclude that the error of a single deter- 



