38 METHODS OF INVESTIGATING 



plate type III-O) with ultra-violet sensitization. The tellurium 

 line 2143-OA. was used as internal standard. Vanselow and 

 Laurance (1936), on the other hand, recommended the use of 

 the zinc line 3345-OA. with cadmium line 3252-5A. as internal 

 standard. To a hydrochloric-acid solution of plant ash a known 

 amount of cadmium sulphate was added and the zinc and 

 cadmium then precipitated as sulphides by a special technique ; 

 the sulphides were then spectrographed. Rogers and Gall (1937) 

 reported unfavourably on the procedure of Vanselow and Lau- 

 rance and suggested that zinc in plant ash is not completely 

 extracted by hydrochloric acid. O'Connor (1941), in developing 

 spectrochemical methods for the determination of trace elements 

 in fertilizers, like Rogers used the zinc line 2 138-5 A. for the 

 determination of this element, but employed the beryllium line 

 2348-6 A. as internal standard. The spectrograms were obtained 

 on photographic plates with ultra-violet sensitization (Eastman 

 1-0 spectroscopic ultra-violet sensitive plate). O'Connor claims 

 that in this way zinc can be determined within the limits of 

 2 p. p.m. to 1 per cent with an accuracy within + 5 per cent. As 

 a 20 mg. sample is used for a determination this means that a 

 quantity of zinc as small as 0-04/xg. can be determined. 



Lundegardh (1934) recommends the use of the line 3345-OA. 

 for the determination of zinc by the spark method. 



Methods for the polarographic determination of zinc in plant 

 materials have been elaborated by Stout, Levy and Williams 

 (1938), by Reed and Cummings (1940) and by Walkley (1942). 



In the method of Stout, Levy and Williams the hydrochloric- 

 acid ash solution (about 100 ml.) from 1 to 2 g. of dried plant 

 material is treated with 5 ml. of N ammonium citrate and then 

 rendered slightly alkaline by the addition of ammonium hydro- 

 xide. The resulting solution is then shaken with 10 ml. of a 

 solution of 1-3 mg. of dithizone in chloroform. The resulting 

 chloroform layer, which then contains the zinc, nickel, cadmium, 

 lead and copper, is separated from the aqueous layer containing 

 iron and manganese. The zinc, and accompanying metals, are 

 then removed from the chloroform by two extractions with 

 10 ml. of 0-5 N hydrochloric acid. The hydrochloric-acid extracts 

 are evaporated to dryness and the residue dissolved in a solution 

 of 0-1 N ammonium acetate + 0025 A T potassium thiocyanate. 



