318 PROTOPLASM 



its potential is 0.2848 volt at 25°C. The following equation 

 (based on the Nernst formula) gives the relationship between 

 the potential difference measured and the hydrogen-ion con- 

 centration of a solution: 



E^ - E (calomel) ^ 



P^ - 0:0591 ^^ ^^ ^• 



In practice, as the wire is so calibrated that a thousandth part 

 is equal to one millivolt, it is necessary only to read the wire 

 length {CD, Fig. 145), which gives directly, by its length, the 

 potential difference in millivolts between the calomel and hydro- 

 gen electrodes. A table converts the potential difference into a 

 pH value. A wire length of 567.6 mm. represents a voltage of 

 0.5676, which, at 15°C., indicates a pH value of 5.5; a voltage of 

 0.6819, at the same temperature, indicates a pH of 7.5. 



The Quinhydrone Method. — The quinhydrone method in its 

 simplest form involves balancing the potential produced by 

 quinhydrone in a solution of unknown pH with that produced 

 by quinhydrone in a buffer mixture of known pH. Quinhydrone, 

 through the addition of water, breaks down to hydroquinone 

 and quinone. (These last two substances could be used sepa- 

 rately in proper proportions; it is simply more convenient to 

 start with quinhydrone.) An equilibrium is established between 

 the hydroquinone and quinone, resulting in the giving off of two 

 ions of hydrogen and two electrons. The two electrons produce 

 the potential which is measured, or, rather, there is a difference 

 in the level of free energy which is utilized by means of platinum 

 electrodes; an electric current, with a difference in potential, 

 results. Zero deflection of the galvanometer in the circuit 

 indicates that the potentials, and therefore the hydrogen-ion 

 concentrations, are the same in both solutions. 



Quinhydrone yields equimolecular concentrations of hydro- 

 quinone which is a reductant and quinone which is an oxidant. 

 What is actually being measured in the quinhydrone method of 

 pH determination, is an electromotive force which has a definite 

 relation to the free energy produced in a chemical reaction, in 

 this case brought about by the reversible process of an oxidation- 

 reduction equilibrium. The rate of the reaction, and therefore 

 the potential, is determined by the hydrogen-ion concentration 



