Ionode IJ pH V5.1 - Manual (Page 6)

Ionode IJ Instruction Manual 6
4. pH
4.1. Theory
pH is a scale of active hydrogen ion
concentration in aqueous solution. In pure
water, the concentration of hydrogen ions is
10
–7
M. The pH scale converts this to a
convenient positive number:
 
] [ log

 
H
a pH
pH indicates acidity (pH<7), alkalinity (pH >7) or
neutrality (pH = 7).
Pure water undergoes autoprotolysis to yield
equal numbers of hydrogen and hydroxide ions:
    
OH H O H
2
The equilibrium constant at 25
o
C is:
14
10 ] [ ] [
     
OH H K
W
pOH is the negative logarithm of the active
hydroxide
concentration. The equilibrium
expression can be rewritten as:
14
  
pOH pH pK
W
This equation applies for all aqueous systems.
Very strong acids and bases can have pH’s <0
and >14 respectively.
Although pH is defined for aqueous solutions,
measurements can be made in non-aqueous
solutions provided the solvent is sufficiently
conductive, or a conductive solvent is added to
a non-conductive sample.