ACIDITY 319 



of the solution. A possible interpretation of the complete reac- 

 tion is: 



C6H4O2 + 2e ^ C6H4O2-- + 2H+ ;=± CeHeOz 



Quinone + electrons ;;=± anion of + hydrogen ^ hydroquinone 



hydroquinone 



The method proceeds as follows. An "unattackable" metal 

 such as gold or platinum serves for the electrodes. No reference 

 (calomel) cell is needed, as a potential is not being measured; 

 instead, two potentials, a known and an unknown one, are 

 balanced. A small quantity of quinhydrone is added to both 

 solutions, the unknown and the known (the buffer mixture), 

 each in small beakers connected by an agar-salt (potassium 

 chloride) bridge. One member of the buffer pair is placed in its 

 beaker, and to it the other buffer is added drop by drop from a 

 burette until a potential (H+ ion concentration) is obtained which 

 balances the potential of the unknown in the other beaker, as 

 indicated by a zero reading of the galvanometer. With the two 

 systems in balance, then the pH of the unknown is obviously 

 equal to that of the buffer mixture which is determined from 

 the proportions of its two constituents. 



The quinhydrone method may be combined with the potenti- 

 ometric setup, as described for the hydrogen electrode. All 

 connections remain the same (Fig. 143). No buffers are then 

 used, as the potential is measured directly. A reference (calomel) 

 cell is now needed. The unknown solution with quinhydrone 

 and a metal electrode take the place of the hydrogen electrode. 



The quinhydrone method has replaced others in certain 

 types of research, particularly commercial work, as in the 

 manufacture of food products. But it has its limitations. 

 It is not good for pH values above 8.5. It is inaccurate for 

 whole-blood determinations owing to a reaction between hemo- 

 globin and quinhydrone, though fairly accurate for blood serum 

 and plasma. 



The Electrometer. — Instead of a galvanometer, an electrometer 

 may be used with the hydrogen electrode or with quinhydrone. 

 Of the two chief types, the quadrant electrometer is the more 

 sensitive (it is a very sensitive instrument). The capillary 

 electrometer is less delicate but very satisfactory and has the 

 advantage of being simple enough to be made in the laboratory. 



