APPENDIX 509 



This concentration is more conveniently expressed as a 

 logarithm : logio 97 = -9868, wherefore 97 X 10"^ = jq-9868-2 

 = io~^°^ It has been agreed to express hydrogen ion con- 

 centration as the exponent to the base 10 of the concentration 

 with the negative sign omitted, and this is represented by the 

 symbol P^. Hence the hydrogen ion concentration of the 

 above N/io hydrochloric acid would be P„ = i-oi ; if com- 

 pletely ionized it would be P^ = i. 



On this principle the following are synonymous methods 

 of expression : — 



100,000 



It will be seen from the above that the greater the value of 

 Pjj, the lower is the actual hydrion concentration. Moreover, 

 it is an established fact that the product of the concentrations 

 of the hydrogen and hydroxyl ions in any given solution, is a 

 constant, namely — 



C„ X C,„ = io-»» 



and consequently at exact neutrality, when the concentrations 

 of the two are exactly equal, C^ = 10"^°^ and C^g = io~^°^, 

 whence it follows that for absolute neutrality, in which the 

 concentration of hydrogen ions is exactly equal to that of the 

 hydroxyl ions Pjj = 7-07. 



There is no need to determine the OH ion concentration 

 since it is easily found from the difference between 14-14 and 

 the hydrion concentration. 



Thus for Pjji the hydroxyl ion concentration would be 

 P^jj 13-14, and for Pj,io it would be P^^ 4-14. 



Since for Pjj 7-07 there is exact equality between H and 

 OH ions, it follows that on either side of this value one 



