The Estimation of Acidity. 243 



Let A be a vessel containing acid of known concentration, 

 N/10 HC1 for example, whose H ion concentration is known, 

 while F represents a similar vessel containing acid of unknown 

 strength. Dipping into the acid are two " hydrogen electrodes," 

 B, B ; , and means are provided whereby hydrogen is bubbled 

 through the acid by means of the fine tubes CO ', creating an at- 

 mosphere of this gas about the exposed surfaces of the electrodes. 

 The hydrogen is at atmospheric pressure, and serves to keep the 

 electrodes in a state of saturation. Bent tubes from each electrode 

 vessel make contact with a saturated solution of potassium chlor- 

 ide in the vessel D (the reason for which will be explained later), 

 and wires from the electrodes complete a circuit which includes 

 .a delicate galvanometer. 



On closing the circuit a current will flow in the direction of 

 the vessel containing the weaker acid, since, in this case, the 

 electrolytic solution pressure of hydrogen is greater on the elec- 

 trode which is in contact with the acid of lesser concentration, 

 i.e. of lesser hydrogen ion concentration. Thus, the electrical 

 balance is upset, this electrode having a lower potential than that 

 in the vessel containing the stronger acid. Such an arrangement 

 is termed a " gas chain." 



Nernst conceived the idea that these electrical phenomena might 

 be treated quantitively, and he was able to construct a formula 

 by which the hydrogen ion concentration of the unknown solution 

 might be calculated, provided the e.m.f. developed, and the hydro- 

 gen ion concentration of the other solution were known. 



In developing this formula Nernst was able to utilise the work 

 of van't Hoff, who showed that if osmotic pressure be substituted 

 for gas pressure in the formula pv = RT, the " gas laws " are 

 applicable to substances in solution. Their assimilation may be 

 shown in the following way: Consider the case of a metal in 

 contact with a solution of one of its salts, and call the " electro- 

 lytic solution pressure " of the metal P, and the osmotic pressure 

 of the dissolved salt p; then the dissolved metal as it gives off 

 ions may be regarded as going from pressure P to pressure p. 

 Now, when a gram molecular weight of a gas expands isother- 

 mally from a pressure p l to another p 2 , the amount of work 

 •done A, is expressed by the formula A = RTl»P 1 // 2 where R is the 

 gas constant, and T the absolute temperature. Similarly, where 

 a formula weight in grams of metal in dissolving and dissociating 

 into ions, goes from solution pressure P to an osmotic pressure 

 p, the maximum work is shown by the formula, A =RTl™ P/ p 



