MICRO-NUTRIENT PROBLEMS 37 



0-5 ml. of a 5 per cent solution of lead acetate, the mixture 

 shaken, allowed to stand for 10 min. and then treated with 1 ml. 

 of a 20 per cent solution of sodium sulphate to remove excess 

 lead. After 30 min. the precipitate is removed by filtration or 

 centrifuging and the clear solution neutralized with 40 per cent 

 sodium hydroxide. Three or four drops of formaldoxime reagent 

 are added to the liquid and then more 40 per cent sodium hydro- 

 xide until a wine-red colour develops, the intensity of which is 

 said to be directly proportional to the concentration of man- 

 ganese. The liquid is made up to a standard volume and the 

 intensity of the colour determined. According to Sideris it would 

 appear that quantities of manganese of the order of 5/xg. can be 

 determined in the presence of 10-lOO^tg. of phosphate with an 

 error not exceeding 4 per cent. Under more favourable con- 

 ditions quantities of manganese down to 0-25/xg. would appear 

 to be determinable. 



Yet another method for the determination of small amounts 

 of manganese has been described by Wiese and Johnson (1939). 

 The nitric acid solution of the ash containing from 1 to 10/xg. of 

 manganese is first rendered free from chlorides by three times 

 evaporating it to dryness and redissolving in nitric acid +10 ml. 

 of distilled water. About 0-2 g. of sodium bismuthate is added 

 and the mixture boiled for 2-3 min. On cooling to below 30° C. 

 0-2-0-3 g. of sodium bismuthate is added, and after mixing the 

 sample thoroughly and allowing it to stand for a few minutes 

 the excess of sodium bismuthate is filtered off through a Gooch 

 crucible. The solution filters directly into the absorptiometer cell 

 containing two drops of a solution of benzidine (i per cent in 

 5 per cent acetic acid) in 3 ml. of distilled water. The solution is 

 made up to standard volume and the intensity of the yellow- 

 green colour which develops is estimated after 5 minutes. 



Zinc. For the determination of zinc in plant material by 

 means of the spectrograph, the flame method is not sufficiently 

 sensitive. Several workers, however, have described procedures 

 for the spectrographic determination of zinc in such material 

 by using the arc. Thus Rogers (1935) advocated using the zinc 

 line 2 138- 5 A. but found it was necessary to sensitize the 

 photographic plate by spreading mineral oil over the emulsion, 

 or, alternatively, using a special plate (Eastman spectroscopic 



