ACIDITY 315 



(S, Fig. 145). Such a cell has been devised by Weston. It 

 is an H-formed vessel containing cadmium and mercury and has 

 been adopted as the international normal electrical element, or 

 standard cell, of voltage 1.0183. The voltage remains very 

 constant. Similar is the Eppley cell. The cell is not used 

 permanently but just long enough to calibrate the source of 

 electromotive force. In an experiment, the known electromotive 

 force E is first balanced against the standard cell S by adjusting 

 resistance R included in the circuit. To do this, the unknown 

 electromotive force E' is disconnected, and the standard cell S 

 inserted in its place. When the system M (with R) is balanced 

 with system N (with *S), then the galvanometer will show no 

 deflection, and the voltage drop from A to 5 is equal to the 

 voltage of the standard cell, or 1.0183 volts. All that has been 

 done is to make the voltage of system M, i.e., of E when in series 

 with R, equal to a definite and accurately known potential, 

 viz., that of the standard cell S. This done, that is to say, 

 with system M calibrated, S is disconnected, and E' reconnected 

 and balanced with the system M as before. The formula then 

 becomes 



CD 

 E' = ^X 1.0183 volts 

 AB 



If AB is of 1,000 units of length (1 m.), then 1 unit (1 mm.) is 

 equal to 0.0010183 volt (or 1 mv.), because the total drop from 

 A to 5 is 1.0183 volts, the voltage of the standard cell; conse- 

 quently, the potential drop from C to D can be read directly. 



Most of the above-mentioned parts of the Poggendorff setup 

 are, with others, put into the modern "type-K" potentiometer 

 of Leeds and Northrup, illustrated in Fig. 146; the inner connec- 

 tions are shown in Fig. 147. 



If we are concerned with the potential produced by hydrogen 

 ions, we must have a hydrogen electrode, which simply means an 

 electrode sensitive to hydrogen ions. This may be made by 

 coating a noble metal, such as platinum, with spongy (colloidal) 

 platinum or palladium and then saturating the electrode with 

 hydrogen. Platinum is known to have the property of holding 

 hydrogen; the minute pores of the spongy coating, greatly 

 increase the surface and therefore the adsorption of hydrogen 

 ions. The spongy platinum when saturated with hydrogen gas 



