258 J. M. Lewis: 



lion, and has received the name " Sdrenson's negative hydrogen 

 ion exponent," and is indicated by the sign P H , or />H. 



In dealing with such a notation, one must bear in mind the 

 fact that a rise in the P H signifies a fall in the H* ion concen- 

 tration. (5) As Baylis remarks, while it is "easy to see that a 

 "hydrogen ion concentration of 4xlCh 6 is double that of 2xl0" 6 , 

 it is not at once obvious that a P H of 5.393 is double that of 

 5 699. One has to get accustomed to thinking in negative log- 

 arithms. " (1) 



To this end it may perhaps be helpful to the student if the 

 pH of familiar laboratory reagents such as decinormal hydro- 

 chloric and acetic acids is calculated. 



We may do this if we know the normality of the solution, and 

 its degree of dissociation. 



As Macleod points out, decinormal hydrochloric acid is dis- 

 sociated 91 per cent., it is therefore 0.091 N or 9.1x10--. The 

 P H is found by subtracting from the power of ten, the logarithm 

 of the reciprocal of the normality in hydrogen ions. Thus: — 

 Log. 9.1 is .96, now -2 minus .96 = -1.04, which is the p¥L 

 sought. 



Again, acetic acid in decinormal solution is dissociated 1.3 

 per cent; it is therefore 0.0013 N in hydrogen ions, or 1.3xl0 -3 . 

 Now, log. 1.3 is .11 ; subtracting .11 from -3, we get -2.89, and 

 the />H is therefore 2.89.* 



To find the normality in ordinary notation, we must subtract 

 the pYL from the next higher whole number, and take the anti- 

 logarithm of this number. 



Thus, if the P H be 7.45, subtracting this number from 8, 

 Ave get 0.55. Now, the antilogarithm of this is 3.55, and the 

 hydrogen ion concentration, or C H is therefore 3.55xlO _!S . (21) 



We would again lay stress on the fact previously mentioned, 

 i c, the formula of Nernst shows that for a tenfold difference 

 in the hydrogen ion concentration of the fluid under examina- 

 tion, the electrode potential changes by only 0.058- volt. 



In the case of the concentration cell previously mentioned, 

 i: is found that " when a univalent electrolyte is employed, an 

 e.m.f. of 0.058 volt is developed where one solution is ten times 

 the concentration of the other, and the electrode potential be- 

 tween monad metal and a normal solution of the correspond- 

 ing metallion is thus 0.058 volt smaller than the electrode poten- 

 tial between the same metal, and a decinormal solution of the 

 metallion." (4) 



* Algebraic euin would probably be tlie better expression to use in tbese cases. 



