Hydrogen ion concentration - pH scale
Sorensen, a Danish biochemist developed a scale to measure the acidity in terms of concentrations of H+ in a solution. As defined by the scientist, pH of the solution is negative logarithm to the base 10 of concentration of H+ ions which it contains.
Just as pH indicates the hydrogen ion concentration, pOH represents the concentration of the hydroxyl ion, which is given by
Considering the relationship, 
Taking log on both sides, we have
|
|
[H+]
|
[OH-]
|
pH
|
pOH
|
|
Acidic solution
|
> 10-7
|
< 10-7
|
< 7
|
> 7
|
|
Neutral solution
|
10-7
|
10-7
|
7
|
7
|
|
Basic solution
|
< 10-7
|
> 10-7
|
> 7
|
< 7
|
pH of some materials
|
Material
|
pH
|
Material
|
pH
|
|
Gastric juice
|
1.4
|
Rain water
|
6.5
|
|
Lemon juice
|
2.1
|
Pure water
|
7.0
|
|
Vinegar
|
2.9
|
Human saliva
|
7.0
|
|
Soft drinks
|
3.0
|
Blood plasma
|
7.4
|
|
Beer
|
4.5
|
Tears
|
7.4
|
|
Black coffee
|
5.0
|
Egg
|
7.8
|
|
Cow's milk
|
6.5
|
Household ammonia
|
11.9
|
The limitations of pH scale
(i) pH values of solutions do not provide us immediate idea of the relative strengths of solutions. A solution of the pH =1 has the hydrogen ion concentration 100 times that of the solution pH = 3 (not the three times). A 4 * 10-5 N HCl is twice the concentrated of a 2 * 10-5 N HCl solution, but pH values of these solutions are 4.40 and 4.70 respectively.
(ii) pH value zero is obtained in 1N solution of strong acid. If the concentration is 2N, 3N, 10N, etc. the respective pH values will be negative.
(iii) A solution of the acid having quite low concentration, say 10-8N, can not have pH 8, as shown by pH formula but the actual pH value will be less than 7.
pK value : p stands for negative logarithm. Just as H+ and OH- ion concentrations range over many negative powers of 10, it is convenient to represent them as pH or pOH, dissociation constant (as K) values also range over many negative powers of ten and it is convenient to write them as pK. Therefore, pK is negative logarithm of dissociation constant.
Weak acids have higher pKa values. Similarly weak bases have higher pKb values
For any conjugate acid-base pair in aqueous solution, Ka * Kb = Kw
Calculation of the pH of 10-8 M HCl & 10-8 M NaOH
If we use the relation 
we get the pH equal to 8, but this is not accurate because an acidic solution cannot have pH greater than 7. In this condition H+concentration of water cannot be neglected.
Therefore, 
Since HCl is strong acid and completely ionised,
Similarly if NaOH concentration is 10-8 M
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