4 o 



BIOLOGICAL CHEMISTRY 



Platinum 

 saturated 

 with hydro- 

 gen 



Solution 



under 

 investigation 



Standard chloride 



solution saturated 



with mercurous 



chloride 



Mercurous chlor- 

 ide and 

 mercury 



Instruments required for measuring the electrical potential. 



FIG. 7. Diagram to illustrate the method for measuring electrical 



potentials. 



A known potential from the galvanic cell B is present at the two ends of the wire XY, the 

 moving contact Z can give a potential proportional to the distances. The potential to be 

 measured is led through the electrometer E in the opposite direction to that of the known 

 potential. When the electrometer shows no current the two potentials are iin the ratio 

 of XZ:XY. 



In the diagram a calomel electrode C, consisting of mercury covered by mercurous chloride 

 in contact with a standard chloride solution is connected through a concentrated potassium 

 chloride solution in vessel V with a hydrogen electrode H. The hydrogen electrode shows 

 the platinum plate P dipping^into a solution. The hydrogen enters through the lower tube 

 marked with arrows and escapes through the small mercury seal S. 



Instead of the combination represented here other combinations may be used and^the 

 electrical potentials in living tissues can be measured by two calomel electrodes connected 

 by means of the tissue instead of a vessel as represented by V in the diagram. 



There are electrical potentials at the contact of platinum 

 with solution under investigation, at the contact of the solution 

 under investigation with the standard chloride solution and 

 at the contact of the standard chloride solution with mercurous 

 chloride and mercury. 



The chloride solution is of constant concentration and it is 

 saturated with mercurous chloride ; thus the potential between 

 this solution and mercury does not vary. This potential can 

 be measured and deducted from that found for the whole 

 system. 



