MICRO-NUTRIENT PROBLEMS 27 



instrument for this purpose, but it has also been used success- 

 fully in my laboratory to determine small quantities of mag- 

 nesium, iron, aluminium and manganese in the course of work 

 not yet published . The smallest quantities of each of these elements 

 which have so far been determined in this manner are noted later. 1 



The polarograph is an instrument in which a solution of an 

 electrolyte in presence of another electrolyte in much higher 

 concentration (known as the ground substance or supporting 

 electrolyte) is subjected to a gradually increasing difference of 

 potential between two electrodes, one of which consists of a series 

 of small drops of mercury delivered from the end of a capillary 

 tube, while the other consists of a still mass of mercury with a 

 comparatively large surface. In these circumstances, when cer- 

 tain experimental conditions are fulfilled a current (the so-called 

 "wave") flows through the solution when the potential difference 

 reaches a certain value determined by the nature of the cation, 

 or in certain circumstances by the anion, present, while the 

 magnitude of the current is determined by the concentration of 

 these cations (or anions). By means of this instrument it is 

 possible to measure quantities of a number of cations and anions 

 of the order of Ifxg. or less. Among the ions which have so far 

 been determined in this way in my laboratory in the course of 

 the last five years are potassium, copper, manganese, aluminium, 

 iron, zinc, barium, chloride, sulphate and nitrate, though in 

 general it should be noted that it is not possible with the polaro- 

 graph to determine one alkali metal in presence of another. 



The polarograph was developed by Jaroslav Heyrovsky and 

 appears to have been first described by Heyrovsky and Shikata 

 in 1925, but although much of the pioneer work with the instru- 

 ment was described in English in the Collection of Czechoslovak 

 Chemical Communications, it appears until recently to have been 

 little used in this country. There can, however, be no doubt 

 whatever that this instrument is a most valuable tool for the 

 student of plant nutrition, and the recent publication in English 

 of a book on polarographic analysis by Kolthoff and Lingane 

 (1941) may render an appreciation of the value of the polaro- 

 graph more widespread. 



1 Since this was written there has appeared an authoritative work 

 on the absorptiometer by F. W. Haywood and A. A. Wood (1944). 



