34 



BIOLOGICAL CHEMISTRY 



ELECTRICAL CONDUCTIVITY 



The electrical conductivity of a solution is the reciprocal 

 of the resistance of a column of liquid I square centimetre 

 in area and I centimetre long. The conductivity decreases 



FIG. 6. Diagram illustrating the method for measuring electrical 

 conductivity of solutions. 



A galvanic cell B actuates coil C so that an alternating current is delivered to the two ends 

 of the uniform resistance wire XY, i metre long with millimetre scale below it. 



A known resistance R is connected to one end of the wire XY and the solution whose resist- 

 ance is to be measured is put in the vessel U. 



A telephone T is connected to the wire joining R and U and to the sliding contact Z on 

 the wire XY. 



By this arrangement the vessel U forms one limb of a Wheatstone Bridge. 



The sliding contact Z is moved along the wire XY until the buzzing of the coil can no longer 

 be heard in the telephone : that is, until the alternating current at each end of the telephone 

 wires is equal. 



Under these circumstances the distances XZ : YZ as U : R or U = 



XZ 

 YZ 



R. 



The vessel U is standardised by means of a solution of known conductivity (such as potas- 

 sium chloride) and a constant C determined. The conductivity of the -solution is - 



The positions of the coil and the telephone are interchangeable. The resistance of the 

 solution is that due to the solution between the two horizontal platinised platinum electrodes 

 of vessel U. The electrodes are connected to the bridge by wires dipping into the mercury 

 in the two glass tubes. 



as the solution is made more dilute, due to the diminution in 

 the total number of ions in the solution, but the relative or 

 molecular conductivity increases owing to the greater dis- 

 sociation of those molecules which are present in the solution. 



