12 



BACTERIOLOGICAL CHEMLSTRY 



of the Salt solution and the particular metal in t^uestion. 

 If two such electrodes, for instance, copper in copper 

 sulphate and zinc in zinc sulphate, are joined together in 

 an electrical circuit by connecting the two metals with a 

 wire and the two solutions either with another wire or 

 with a ri-tube containing a solution (Fig. 1), then a 



COO D ocTo ^ D" 



4- rH 



Cu 



sv 



Ca S + 



In 



lYx SO4 



Fig. 1 



current will flow in the direction of the higher to the 

 lower potential. An electrode of this type which is very 

 frequently employed for purposes of measurement is the 

 standard calomel electrode in which mercury is in contact 

 with saturated mercurous chloride and N-potassium 

 chloride, and which has a potential of +0-56 volt at 18^ C. 

 Another standard electrode is the hydrogen electrode 

 in which an electrode of platinum black saturated with 

 hydrogen gas is immersed in an acid solution normal in 

 respect to the hydrogen ions. For reasons of convenience 

 this potential is arbitrarily taken as being zero. If the 

 concentration of hydrogen ions is different from normal 

 the potential will also be different from the standard, and 

 will be a measure of the hydrogen ion concentration. 



The potential differences between a standard half -cell, 

 such as the calomel electrode, and an electrode immersed 

 in the unknown solution are measured by balancing them 



