APPENDICES 



651 



It lias also been found tliat in pure water at this temperature there is 

 an equal number of H and OH ions, i.e. [H] = [OH]. Therefore the 

 concentration of hvdrooren ions 



orCH= Fio-i-' = 10-7 

 and Coh = Ch =10~' 



For short, the H ion concentration is often denoted by the logarithmic 

 exponent, i.e. 



Ch of 10-" = pH of 7. 



It has been found that all substances which make a solution acid do 

 so by increasing the number of H ions, e.g. 



NaH.PO^ ^ 



H..0 



/ 



+ Na+ + HPOi- 

 + OH" 



and as [H] x [OHJ is a constant, [OH] -.nnst be correspondingly decreased. 



Similarly, alkalies and alkaline salts cause an increase in the number of 

 free OH and a decrease in the free H ions. 



Any increase in tlie concentration of the H ions will be denoted by a 

 decrease in the denominator of the fraction expressing concentration. 



At the neutral point 



Therefore, if the pH is denoted by a figure less than 7 the solution is 

 acid, if by a figure greater than 7 the solution is alkaline. 



A value lying between two whole numbers may be written in either of 

 two ways. For example, water just slightly alkaline may have a concen- 



0'5 



tration of hydrogen ions of 1 in 20 million= q^q^^q - This may be 



written as Ch = 0-5x10-7 or the fraction may all be put as a power of 

 10 = 10-7-3 or pH = 7-3. 



