40 METHODS OF INVESTIGATING 



water and boiled, and on cooling 25 ml. of an ammonium citrate 

 buffer are added. This buffer is prepared by dissolving 5 g. of 

 citric acid in 50 ml. of water and 200 ml. of 4 jV ammonium 

 hydroxide, and then extracting impurities by three successive 

 shakings with 10 ml. of a solution of dithizone in chloroform 

 (1 g. of dithizone in 100 ml. of chloroform), and running off the 

 chloroform layers. The final purified buffer solution contains 

 dithizone in solution. 



The zinc is separated from the digest after treatment with the 

 ammonium citrate buffer by three extractions each with 5 ml. 

 of chloroform. The chloroform extracts are evaporated to dry- 

 ness, then treated with a mixture of 2-5 ml. of nitric acid, 0-5 ml. 

 of perchloric acid, and two drops of sulphuric acid and again 

 evaporated to dryness, boiling being avoided. The residue is 

 then dissolved in 1 ml. of a ground liquid of 0-1 N ammonium 

 chloride -I- 0-02 N potassium thiocyanate containing 0-0002 per 

 cent of methyl red and polarographed in a small electrolysis 

 vessel. Practical details for carrying out Walkley's method will 

 be found in Piper's book on Soil and Plant Analysis (1942). 



For the absorptiometric determination of zinc the most 

 promising method appears to be that based on the coloration 

 given by zinc salts with dithizone (diphenylthiocarbazone). 

 Reference has already been made in this chapter to the fact that 

 this reagent gives a red or purple colour when added to solu- 

 tions of compounds of a number of metals, including zinc (see 

 p. 25). The resulting coloured compound is quantitatively ex- 

 tracted with chloroform or carbon tetrachloride, and according 

 to R. H. Caughley (see Holland and Ritchie, 1939) sodium 

 diethyldithiocarbamate in 0-02 N ammonium hydroxide solu- 

 tion inhibits the reaction of dithizone with all metals except zinc. 

 Cowling and Miller (1941) have made use of this very useful fact 

 to work out an absorptiometric method for the quantitative 

 estimation of zinc in plant materials. Unfortunately, the 

 addition of the sodium diethyldithiocarbamate (generally 

 denoted by 'carbamate' for the sake of brevity) renders the 

 extraction of zinc by dithizone incomplete. By carefully stan- 

 dardizing the technique, however, Cowling and Miller claim that 

 this drawback can be overcome and the quantity of zinc in 

 plant ash determined with reasonable accuracy. 



