Estimation of Alkalinity in Water Sample Expt. No.: Date: Aim: To determine the phenolphthalein and methyl orange alkalinity in the given water samples A and B. A standard solution of.. N sulphuric acid is provided Principle: Pure water is neutral in nature with ph 7. Due to the presence of dissolved minerals in rain water, the ph increases and becomes alkaline. Alkalinity in water is due to the presence of hydroxide (OH ), carbonate (CO 3 2 ) and bicarbonate (HCO3 ) ions. The various alkalinities are estimated by titrating the water sample against a standard acid using phenolphthalein and methyl orange indicators successively. Hydroxide ions is completely neutralized to water using phenolphthalein indicator (single step neutralisation) OH + H + H 2 O Carbonate ions is neutralized to bicarbonate ions using phenolphthalein indicator in the first sep. in the second step, these bicarbonate ions is completely neutralized to water and carbon dioxide using methyl orange indicator (double step neutralisation). CO 3 2 + H + + H + H 2 O + CO 2 Bicarbonate ions is completely neutralized to water and carbon dioxide using methyl orange indicator (single step neutralisation). + H 2 O + H + CO 2 Procedure: The burette is filled with standard sulphuric acid to the zero level, following usual precautions. Page 1 of 5
20 ml of the given water samplea is pipetted out into a conical flask. Two drops of phenolphthalein indicator is added and titrated against the standard sulphuric acid. The end point is the disappearance of pink colour. The titre value is noted. A drop of methyl orange indicator is added to the same solution after the phenolphthalein end point and the titration is continued until the solution becomes orange. The total titre value is noted. The titration is repeated to get concordant value and the titre values are noted in the Table 1. The above procedure is repeated for the water sampleb and the titre values are noted in the Table 2. Note P = M OH only P = 0 P = ½M 2 CO 3 P > ½M both OH 2 and CO 3 are present P < ½M 2 both CO 3 and HCO3 are present Result: 1. The given water samplea contains Phenolphthalein alkalinity = ppm Methyl orange alkalinity Since P ½M, and.. ions are present in the given water samplea. 2. The given water sampleb contains Phenolphthalein alkalinity = ppm Methyl orange alkalinity Since P ½M, and.. ions are present in the given water sampleb. Page 2 of 5
Volume of water samplea Titration1 Standard Sulphuric acid vs Water samplea Burette Reading Volume of Sulphuric acid consumed Initial Final P M P M Volume of water sampleb Titration2 Standard Sulphuric acid vs Water sampleb Burette Reading Volume of Sulphuric acid consumed Initial Final P M P M Page 3 of 5
Water samplea Phenolphthalein alkalinity Calculation Volume of sulphuric acid (phenolphthalein end point) =.. ml = N Volume of water samplea =. ml (Concordant Value) Phenolphthalein alkalinity = V acid X N acid X 50 X 1000 Methyl Orange or Total Alaklinity Volume of sulphuric acid (methyl orange end point) =.. ml = N Volume of water samplea = ml (Concordant Value) Methyl Orange alkalinity = V acid X N acid X 50 X 1000 Water sampleb Phenolphthalein alkalinity Volume of sulphuric acid (phenolphthalein end point) =.. ml = N Volume of water sampleb =. ml (Concordant Value) Phenolphthalein alkalinity = V acid X N acid X 50 X 1000 Methyl Orange or Total Alaklinity Volume of sulphuric acid (methyl orange end point) =.. ml = N Volume of water sampleb = ml (Concordant Value) Methyl Orange alkalinity = V acid X N acid X 50 X 1000 Page 4 of 5
Ions present in the given water samples Water SampleA Water SampleB P M ½M Condition (P<½M or P>½M) Species present Page 5 of 5