6 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



the cathode, and the other is called the anode. It is customary 

 to speak of the current as flowing into an electrolytic cell at the 

 anode and out of the cell at the cathode, that is, the electric cur- 

 rent is considered to flow in the direction in which the metallic con- 

 stituent of the solution is carried in an electrolytic cell. 



Consider a solution of hydrobromic acid (HBr). When an 

 electric current is passed through this solution, hydrogen (H) 

 is liberated at the cathode and bromine (Br) is liberated at the 

 anode. In general, the molecule of any dissolved salt or acid is 

 separated into two parts by electrolysis ; one part is liberated at 

 the cathode and is called the cathion, and the other part is liber- 

 ated at the anode and is called the anion. Thus, hydrogen (H) 

 is the cathion and bromine (Br) is the anion of hydrobromic 

 acid. In all metallic salts the metal constitutes the cathion and 

 the acid radical or halogen constitutes the anion. In acids the 

 hydrogen constitutes the cathion and the acid radical or halogen 

 constitutes the anion. Thus, the cathion of copper sulphate 

 (CuSO 4 ) is copper (Cu), and the anion is the acid radical (SO 4 ). 



In many cases the cathion and anion are not actually liberated 

 at the electrodes because of what are called secondary reactions. 

 Thus, iathe electrolysis of an aqueous solution of sodium chloride 

 (NaCl), the cathion (Na), when it is liberated at the cathode, 

 immediately reacts upon the water, forming NaOH and free 

 hydrogen ; in the electrolysis of copper sulphate between copper 

 electrodes, the anion (SO 4 ) combines with the copper of the 

 anode forming fresh CuSO 4 which goes into solution or is 

 deposited as crystals on the anode if the solution is saturated ; in 

 the electrolysis of H 2 SO 4 between inert electrodes such as car- 

 bon or platinum, the hydrogen is liberated at the cathode as a 

 gas, and the anion (SO 4 ) reacts on the water according to the 

 formula SO 4 + H 2 O = H 2 SO 4 -j- O and the free oxygen escapes 

 as gas. The reason for taking the unfamiliar substance hydro- 

 bromic acid in the above example is that in the electrolysis of 

 hydrobromic acid there are no secondary reactions at the 

 electrodes. 



