MICRO-NUTRIENT PROBLEMS 39 



On polarographing this solution well-defined and well-separated 

 waves of lead, cadmium, nickel and zinc are obtained. 



Reed and Cummings experienced some trouble with the method 

 of Stout, Levy and Williams, particularly when large quantities 

 of zinc or of cadmium, lead, copper, nickel or cobalt were 

 present, and they found that to effect a quantitative separation 

 of the zinc from aluminium, iron and alkali metals five or six 

 extractions, instead of only one, were necessary. They therefore 

 devised a different procedure in which the ash solution in hydro- 

 chloric acid was brought to a pH of between 4 and 5 by addition 

 of dilute ammonium hydroxide. This precipitates practically all 

 the aluminium and ferric iron which are filtered off. The filtrate 

 is evaporated to dryness on a steam bath and the residue dis- 

 solved in a solution of 0-1 N ammonium acetate of pH 4-6 and 

 containing also potassium thiocyanate of concentration 0-025 N, 

 and the resulting solution polarographed. It is stated that no 

 interference with the height of the zinc wave then results from 

 the presence of chloride, sulphate, phosphate, carbonate, sodium, 

 potassium, calcium, magnesium or manganese, and no inter- 

 ference results from lead, cadmium or nickel up to concentra- 

 tions ten times that of the zinc. Copper interferes if it is present 

 in concentrations ten times or more that of zinc, while the 

 cobalt and zinc waves tend to coalesce if the former is present 

 in relatively high concentration. Actually Reed and Cummings 

 found that if cobalt occurs in concentrations greater than 

 1 x 10 -5 g. per ml. it only interferes with determinations of zinc 

 when the ratio of cobalt to zinc exceeds 2. It would, as a matter 

 of fact, be a very exceptional plant ash in which any of the ions 

 noted occurred in sufficient amount to interfere with the polaro- 

 graphic determination of zinc in this way. 



The lowest measurable concentration of zinc is stated by Reed 

 and Cummings to be 0-2/xg. of Zn per ml., that is, about 

 Jf/300,000, so that, using 5 ml. of solution, 1/xg. of zinc should 

 be determinable. 



In Walkley's procedure the dried plant material is first sub- 

 jected to wet ashing, about a gram of the material being digested 

 with a mixture of 10 ml. of nitric acid, 1 ml. of sulphuric acid 

 and 1 ml. of perchloric acid. Frothing is prevented by addition 

 of a drop of kerosene. The cooled digest is taken up in 15 ml. of 



