18 PHYSICAL CHEMISTRY 



nitrate in alcohol is purple, but on adding water it turns pink, 

 because in the water-alcohol mixture electrolytic dissociation 

 takes place (Noyes and Blanehard, 1900). 



Electric conductivity of solutions: Solutions of electrolytes 

 conduct the electric current and are called conductors of the 

 second class to distinguish them from the metals which are con- 

 ductors of the first class. 



If an electric current is passed through a solution from two 

 metallic conductors (electrodes) the positive ions (cations) are 

 attracted to the negative electrode (cathode) and the negative 

 ions (anions) are attracted to the positive electrode (anode). 



The more ions there are in a solution the greater the electric 

 conductivity, and hence the electric conductivity may be used to 

 determine the ionization. In doing this the electrical resistance, 

 in ohms, is measured, and the conductivity is the reciprocal of 

 the resistance. 



The electrical resistance of solutions is measured by means 

 of the Kohlrausch method. The solution to be examined is placed 

 in a conductivity cell which consists of a glass vessel containing 

 two platinum electrodes. The electrodes are made of platinum 

 foil and should stand vertically so that bubbles will not collect 

 on them. Every time the distance between the electrodes is 

 changed to the slightest degree the cell has to be standardized 

 again. To avoid this, the platinum is often made thick for stiff- 

 ness, or the four corners of one electrode are connected to the 

 corners of the other by means of glass rods fused to the platinum 

 by heat. To the upper edge of the platinum foil a stout platinum 

 wire is welded. This is done by heating foil and wire to a white 

 heat and welding the wire to the foil by means of a quick tap 

 with a hammer. The wire is fused into the end of a small glass 

 tube and a drop of mercury dropped into the tube so as to touch 

 the platinum wire. When connecting up the apparatus, a copper 

 wire is inserted down the tube into the mercury. The electrodes 

 may be fastened into the cell by passing the glass tubes through 

 a stopper that fits into the cell. 



For solutions used in pure chemistry the pipette forms of con- 

 ductivity cells that are on the market are very convenient. For 

 biological fluids when only a small quantity may be obtained', the 

 test tube form of cell shown in Fig. 2 is convenient. For tissues, 

 the apparatus shown in Fig. 3 is useful. 



