Alkalinity (exp:6) - assignment - Q-A

ASSIGNMENT:
(1) Q: Discuss the environmental significance of “alkalinity”.

Ans:
The alkalinity of waters is due principally to salts of weak acids and strong bases, and such substances act as buffer to resist a drop in pH resulting from acid additions. Alkalinity is thus a measure of the buffer capacity and in this sense is used to a great extent in wastewater treatment practice. Alkalinity is very important in iron removal from water by oxidation precipitation process. It has been found that rate of oxidation of dissolved ferrous iron (Fe2+) into insoluble ferric iron is vary slow for low alkaline waters (alkalinity less than 130 mg/L as CaCO3). Because the alkalinity of many waters is primarily a function of carbonate, bicarbonate and hydroxide content, it is taken as an indication of the concentration of these constituents. Alkalinity of water has little public health significance. Highly alkaline waters are usually unpalatable.
Although Bangladesh Environment Conservation Rule (1997) does not set any limit for its presence in drinking water, limits for carbon dioxide has been prescribed for many industrial uses.
(2) Q: Define total alkalinity, phenolphthalein alkalinity and methyl orange alkalinity.
Ans: phenolphthalein alkalinity: When alkalinity is measured to the phenolphthalein end point (pH 4.5), it is called phenolphthalein alkalinity. Which is due to the presence of either hydroxide or carbonate or both.
CO2 + CaCO3 + H2O = Ca2+ + 2HCO3-
Methyl orange alkalinity: When alkalinity is measured to the methyl orange (or bromocresol green) end point (pH), it is called Methyl orange alkalinity.
CO3 + H+ = HCO3-
Total alkalinity: Which alkalinity is due to the presence of hydroxide, carbonate and bicarbonate, it is called Total alkalinity.
HCO3- + H+ = CO32-
(3) Q: Calculate hydroxide, bicarbonate and carbonate alkalinities of a water sample with a total alkalinity of 210 mg/L as CaCO3 and a pH of 7.7. Which type of alkalinity dominates the total alkalinity? Also calculate the concentrations of carbonate (CO32-) and bicarbonate (HCO3-) ions.
Ans:
Carbonate alkalinity = [50,000{alkalinity/50,000 + (H+) - Kw/(H+)}]/ [1+(H+)/2KA2}]
mg/L as CaCO3 ----- (1)
Bicarbonate alkalinity = [50,000{alkalinity/50,000 + (H+) - Kw/(H+)}]/ [1+2KA2/(H+)}]
mg/L as CaCO3 ----- (2)
Kw =[H+][OH-] =10-14 (at 250C)
KA1 = [H+][HCO3-]/ [H2CO3] = 10-6.3 (at 250C)
KA2 = [H+][CO32-]/ [HCO3-] = 10-10.3 (at 250C)
Total alkalinity = 210 mg/L as CaCO3
pH = 7.7
As pH value is less than 8.3
Carbonate alkalinity = 0
Hydroxide alkalinity = 0
Bicarbonate alkalinity dominates Total alkalinity
From equation (2)
Bicarbonate alkalinity = [50,000{210/50,000 + 10-7.7 - 10-14 /10-7.7]/ [1+2*10-10.3 /10-7.7]
mg/L as CaCO3
= 208.93 mg/L as CaCO3
Bicarbonate [HCO3-] ions concentration = Bicarbonate alkalinity × 1.22
= 208.93 × 1.22 mg/L = 254.89 mg/L
Carbonate alkalinity = 0
Hydroxide alkalinity = 0
Bicarbonate alkalinity = 208.93 mg/L as CaCO3
Bicarbonate [HCO3-] ions concentration = 254.89 mg/L

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