Hydrogen ion concentration pH
Every aqueous solution whether acidic, alkaline or neutral contains both H+
and OH- ions. The product of their concentrations is always constant equal to 1
� 10
-14
at 298 K.
In 1909, Sorensen introduced a convenient scale to find out H+
ion concentration which he called the pH scale.
In the symbol pH, �p� stands for protenz (meaning strength) and �H� for
hydrogen in concentration.
So pH is defined as the negative logarithm to the base 10 of molar
concentration of hydrogen ions.
pH = -
log10
[ H+
] = log10
For pure water and neutral solution [ H+
] = 1 � 10-
7
\ pH = log10
[H+
] = - log (1 � 10-
7) = 7
pH Scale
In most practical cases the concentrations of H+
ions varies from100
to 10 -14
mol dm3 i.e. from
10-14
pH which is the basis of the pH scale. The mid point of the scale i.e. pH = 7
represents a neutral solution. If the pH value is less than 7 the solution is
acidic and if it is more than 7, then the solution is alkaline.
Example
Calculate the pH value of 1) 0.1m HCl 2) solution obtained by mixing 5 gms of
conc. HCl to get 1 liter volume.
1) \
pH = - log10
[ H+
]
= - log10
[0.1]
= - log10
(1 � 10-
1 ) = +1
2) Molar concentration = 5
36.46 g/mole
= 0.137 molar
\ pH = - log10
[ H+
]
= - log10
(0.137)
= - log10
(1.37 � 10-
1 ) = 0.86
pH indicators
An indicator is a chemical which indicates the nature of the solution, by
means of a sharp change in color.
|
Universal indicator
|
Acid-base indicator
|
- Is a mixture of organic dyes
- Indicates strength of solution
- Change in color is sharp at any pH
|
- is a chemical such as Methyl orange, phenolphthalein
- Does not indicate strength of solution
- change in its color is abrupt at a particular pH
|
pH values of common substances
| pH value |
Acidic
solutions |
pH value |
Alkaline
solutions |
| 1 |
0.1 N HCl |
7.3 |
Blood (Human) |
| 2.3 |
Citric acid (Lemons) |
7.9 |
Albumin (eggs) |
| 3 |
Carbonic acid (soft
drinks) |
8.5 |
Sea water |
| 3.5 |
Tartaric acid (grapes) |
10.5 |
Milk of Magnesia |
| 5.3 |
Boric acid (eye wash) |
11.6 |
0.1 N Na2CO3 solution |
| 6.6 |
Lactic acid (milk) |
13 |
0.1 N NaOH solution |
Utility of pH values
1) To determine the nature of solution
if pH = 7, then solution is neutral
pH > 7, then solution is alkaline
pH < 7, then solution is acidic
2) To compare the acidity and Basicity of two solutions.
3) For formulating drugs as some drugs are not stable at acidic pH.
4) In agriculture, to determine soil condition.
5) In Medicine pH values of blood and urine for diagnosis of various
diseases.
6) In the chemical industry, as certain reactions take place at particular pH
only.
7) In bio chemical processes, as enzymes are stable at a particular pH value.
|
