HYDROGEN-ION CONCENTRATION 



31 



will serve as a good conductor, with another electrode, the two elec- 

 trodes being also connected by wires with electrical apparatus for 

 measuring the total potential of the battery; then by adding +0.560 v. 

 to or subtracting this value from the total potential (depending on the 

 sign of the unknown electrode) we can tell the potential of the unknown 

 electrode. 



We have discussed these principles of electrochemistry because they 

 form the basis upon which depends the standard method for the deter- 

 mination of the H-ion concentration of fluids. Suppose, for example, 

 that in place of using a metal in the construction of one electrode, we 

 use an electrode consisting of a layer of pure hydrogen gas in contact 

 with a solution in which are free H ions; then the rate at which H ions 



CapVVf&vy .. _. I 



5Cv3e - 



, --(-Jh)-- / 



AMMwlqkpr 



Fig. 8. Diagram of apparatus for the measurement of the H-ion concentration. The cur- 

 rent generated in the battery (composed of calomel electrode, connecting vessel with KC1 solu- 

 tion, and the H electrode) or that from the normal element is transmitted through a reversing 

 key to the bridge wire, where the voltage is compared with a steady current flowing through the 

 bridge wire from an accumulator. The capillary electrometer is used to detect the flow of 

 current at various positions of the movable contact on the bridge wire. (Modified from 

 Sorensen.) 



become added to the solution from the H layer, or taken from it, will de- 

 pend on the concentration of H ions in solution. In order to secure a 

 hydrogen electrode fulfilling the above requirements, it is necessary to 

 employ some means by which a layer of hydrogen may be furnished, and 

 fortunately this can be done by taking advantage of the property which 

 spongy platinum possesses of absorbing large quantities of this gas. It 

 is also necessary to keep an atmosphere of pure H in contact with the 

 fluid. 



As is the case of the simpler cells described above, there are two 

 types which we might use for measuring the electromotive force gen- 

 erated in the unknown electrode: a concentration cell composed of two 



