Question

A 100-ml sample of water is titrated with 0.02 N H2SO4. The initial pH is 9.5, and 6.2 mL of acid is required to reach the pH 8.3 endpoint. An additional 9.8 mL is required to reach the 4.5 endpoint. Determine the species of alkalinity present and the concentration of each species.

Answer 1

Determination of total alkalanity

total volume of sulfuric acid = 6.2+9.8= 16ml of sulfuric acid

Normality of sulfuric acid = 0.02 N

volume of sample= 100ml =0.1L

Phenolphthalein Alkalinity=

Volume of sulfuric acid to reach pH 8.3 * N1( Normality of H2SO4)*100000 mg CaCo3/ eq/sample size = 0.02*6.2*100000/100 mg/L=124 mg/L

This is due to hydroxyl ions

Total Alkalinity

= Amount of acid used to reach pH 4.5 (ml) * Normality of acid (eq/L) * 50,000 (mg CaCO₃/eq) / sample volume (ml)

Volume of sulfuric acid to reach pH 4.5 is =6.2+9.8 =16ml

Total alkalinity= 16*0.02*50000/100 =160 mg/L

This is due to OH-, CO₃^-2 and HCO₃^-2

Alkalinity due to carbonates and bicaronated= 160-124= 36 mg/L

Concentration of OH- = 124/160= 0.775

Concentration of carbonates and bicarbonates= 36/160= 0.225