POLAROGRAPHIC TECHNIQUE 



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POLAROGRAPHIC TECHNIQUE 



ion Fe"*'++, or Fe"*"*" z^ Fe"^'" + e (elec- 

 tron). Conversely, when an electron 

 is added to the ferric ion, the latter 

 is reduced to the ferrous state. In the 

 polarographic method the electrolysis 

 is carried out with a small easily polariz- 

 able electrode, the dropping mercury 

 electrode, and one large non-polarizable 

 electrode. 



In the reduction of cadmium ion to 

 metallic cadmium, for e.xample, the 

 applied potential is gradually changed 

 from 0.0 volts and the current flowing 

 through the solution is measured by 

 means of a galvanometer. From 0.0 

 volt to about —0.5 volt very little 

 current flows through the solution be- 

 cause the potential at which the cad- 

 mium ion is reduced has not been 

 reached. At about —0.53 volt the 

 current begins to increase as the voltage 

 is decreased (negative) and at —0.7 

 volt the current becomes steady again. 

 Between —0.52 volt and —0.7 volt an 

 S shaped curve is produced, and a 

 measurement of the mid-point of this 

 curve gives the half -wave potential, 

 a characteristic constant of any sub- 

 stance under controlled conditions while 

 the current obtained gives the amount 

 of cadmium in solution. 



In other words, the half-wave poten- 

 tial indicates the nature of the reducible 

 substance while the current observed 

 is a function of its concentration. 

 When the current in microamperes is 

 plotted against the applied voltage, 

 the resulting figure is called a polaro- 

 gram. The small current flowing 

 through the cell prior to reduction is 

 called the residual current. This is 

 due either to the reduction of traces of 

 oxjrgen, impurities or to the solution 

 of minute amounts of mercurj^ from the 

 dropping mercury electrode. The cur- 

 rent, which is obtained at about —0.7 

 volt for the reduction of cadmium ions, 

 is termed the diffusion current and it 

 is a function of the concentration. The 

 diffusion current is so named since the 

 ions in solution are reduced as fast as 

 they reach the dropping mercury elec- 

 trode — that is, the reduction is diffusion 

 controlled. Now if the concentration 

 of the cadmium ions is increased, the 

 diffusion rate will increase in propor- 

 tion to the concentration and this will 

 be reflected bj^ an increase in the diffu- 

 sion current. A plot of the diffusion 

 current minus the residual current 

 against the concentration can be used 

 for analytical purposes. 



The half-wave potential can be em- 

 ploj'ed to determine the nature of un- 

 known substances. For example, when 

 the electrolysis of a mixture of Fe^ 



Pb++, Zn+*- and Co++ is carried out in 

 a solution of 0.1 N KCl, the supporting 

 electrolyte, the half-wave potentials 

 in volts v.s. the saturated calomel 

 electrode of these ions are respectively 

 -0.396, -0.995, -1.20 and -1.3. The 

 half-wave potentials are separated 

 sufTiciently to permit the determination 

 of each one in a mixture of the four 

 ions, and they enable one to determine 

 any of these ions in an unknown solu- 

 tion. 



The principal objectives, therefore, 

 of the polarographic method are two. 

 In the first place the determination of 

 the nature of a substance from the half- 

 wave potential. This aspect of the 

 procedure is very important since it 

 enables one to get at structural altera- 

 tions in reducible compounds from a 

 single measurement. An application 

 of this aspect of the method in biology 

 (carcinogenesis) is given by (Car- 

 ruthers, C, and Suntzeff, V. Science 

 1950, 114, 103-107). Secondly, minute 

 amounts of several reducible substances 

 can be quantitatively determined in 

 a mixture without previous separation 

 by the classical chemical methods. 



The equation for polarographic analy- 

 sis was developed by Ilkovik (Collec- 

 tion Czechoslov Chem. Commun, 1934, 

 6, 498-513). This equation is 'd = 605 

 nD'/^Cm^'H"* in which 'd is the mean 

 diffusion current (in microamperes), n 

 is the number of electrons involved in 

 the reduction of one molecule of re- 

 ducible substance, F is the Faraday 

 (96,000 coulombs), D is the diffusion 

 coefficient (in cm" 8ec~i), C is its con- 

 centration (millimoles per liter), m is 

 the weight of mercury flowing from the 

 capillary per second in (mg.) and t (in 

 sec.) is the time necessary for the forma- 

 tion of one drop of mercury. This 

 equation can be reduced to 'd = KC 

 when all the other factors are constant. 

 Hence the concentration of a reducible 

 substance is given by the equation 

 C = 'd/K. Since K can be determined 

 experimentally from the electrolysis of 

 a reducible substance at known con- 

 centrations, the amount in an unknown 

 can be measured from the diffusion 

 current. 



For a general and detailed description 

 of the polarographic method of analysis, 

 the following books are recommended: 

 Kolthoff, I. M., and Lingane, J. J. 

 Polarography, New York: Interscience 

 Publishers, Inc. 1941, 215-254; Muller, 

 O. H., The Polarographic Method of 

 Analysis, Easton, Pa., Chemical Educa- 

 tion Publishing Co., 1951, 32-80, 141- 

 193; ilohn, H., Chemische Analysen 



