Alkalinity means acid neutralization capacity of water. When we add acid in water (adding H+ ions) water absorbs H+ ions without showing significant change in pH mainly it is due to carbonate, bicarbonate & hydroxide ion present in water or the mixture of two ions present in water. The possibility of OH– and HCO3– ions together is not possible since they combine together to form CO3-2 ions.
OH- + HCO3- ⇒ CO3-2 + H2O
Two types of Alkalinity present in water,
- P-Alkalinity also called Phenolphthalein Alkalinity because Phenolphthalein indicator used for analysis
- M-Alkalinity also called Methyl orange Alkalinity because Methyl orange indicator used for analysis
In alkalinity analysis different ions can be estimated separately by titration against standard acid solution, using selective indicators like phenolphthalein and methyl orange.
Below reaction occurs during analysis:
- OH- + H+ ⇒ H2O
- CO3-2 + H+ ⇒ HCO3-
- HCO3- + H+ ⇒ H2O + CO2
The neutralization reaction up to phenolphthalein end point shows the completion of reactions (1) and (2) only.
The amount of acid used thus corresponds to complete neutralization of OH– plus half neutralization of CO32–.
The titration of water sample using methyl orange indicator marks the completion of the reactions (1), (2) and (3).
Based on above three reaction,
|Ion||P = 0||P = 1/2M||P > 1/2M||P < 1/2M||P = M|
1.Phenolphthalein alkalinity (P) = 0; that means the volume of acid used till the completion of reaction (1) and (2) is 0. This can only happen when both OH– and CO32– ions are not present in water. Alkalinity is present due to HCO3– ion only which can be determined using methyl orange indicator and called methyl orange alkalinity (M).
2. P = ½ M; indicates that only CO32– ions are present. Using phenolphthalein indicator neutralization reaches upto HCO3– but using methyl orange indicator the complete neutralization of HCO3– takes place.
3.P > ½M; implies OH– ions are also present along with CO32– ions. Upto phenolphthalein alkalinity OH– ions will be neutralized completely whereas CO32– will be neutralized upto HCO3– ion. But using methyl orange indicator HCO3– will be completely neutralized along with OH– and CO32–.
4.P < ½ M; indicates that beside CO32– ions HCO3– ions are also present. The volume of acid required for the neutralization upto phenolphthalein end point correspond half neutralization of CO32– (equation 2). Neutralization using methyl orange indicator corresponds to HCO3– obtained from CO32– and HCO3– originally present in the water sample
5.P = M; indicates only OH– ions are present.
Alkalinity of water is very important parameter in boiler water treatment program. Based on alkalinity analysis we can predict the presence of free caustic as mentioned in above table. Free caustic is responsible for caustic corrosion in boiler system.