WATER TECHNOLOGY Characteristics alkalinity types of alkalinity and determination hardness types and estimation by EDTA method (problems), Domestic water treatment disinfection methods (chlorination, ozonation, UV treatment boiler feed water requirements disadvantages of using hard water in boilers internal conditioning (phosphate, calgon and carbonate conditioning methods) external conditioning demineralization process desalination and reverse osmosis. Water is the essential commodity for any engineering industry. The sources for water are of stationary type such as pond, lake etc. or of flowing type such as river. The purest source of water is rain but rain also during its travel through atmosphere contains trace levels of dissolved carbon dioxide. The impurities in any water sample influence its characteristics. The physical characteristics of water are colour, odour, taste etc. The chemical characteristics of water evolve from the dissolved gases, salts etc. Microorganisms such as bacteria, viruses etc present in any water sample impart biological characteristics such as pathogenic nature. Depending on the source, water sample may consist of impurities in soluble or dispersed or suspended form. The impurities in water sample impart some undesirable properties to water and hence render the sample ineffective for the particular engineering application. The important characteristics of water, which influence its utility in engineering industry, are alkalinity and hardness. The alkalinity of water (pH>7) is attributed to the presence of the following species: hydroxides (OH-), carbonates (CO32-) and bicarbonates (HCO3-) of alkali or alkaline earth metals. The alkalinity is of types namely hydroxide alkalinity and carbonate alkalinity. The alkalinity caused by the alkali metal ions Na+, K+ is termed as caustic alkalinity. It is obvious that the strength of alkalinity for the above anions is in the order OH- > CO32- > HCO3Worked Example 1: A water sample has the following composition: Sodium bicarbonate = 2.5 mg / lit Sodium Carbonate = 2.6 mg / lit; Potassium carbonate = 2.9 mg / lit Potassium bicarbonate = 1.7 mg / lit and carbon dioxide = 1.5 mg / lit. Identify the types of alkalinity present in the sample. Calculate the total alkalinity of the water sample. Solution: Alkalinity due to any species in terms of CaCO3 equivalent = = Weight of the species in water (mg / l) X 50 Equivalent weight of the species Weight of the species in water (mg / l) X 100 Molecular weight of the species (OR)

Alkalinity due to carbonate = [(2.5 / 84) + (1.7 / 100)] 50 Alkalinity due to bicarbonate = [(2.6 / 106) + (2.9 / 138)] 50 Alkalinity due to carbon dioxide = [(1.5 / 44) 50 Total alkalinity = alkalinity due to (CO32- + HCO3- - CO2) = 2.338 + 2.277 1.7045 = 2.9105 ppm CaCO3 equivalent Here it is to be noted that dissolved carbon dioxide tends to reduce the alkalinity of the water sample, as it amounts to the formation of carbonic acid (which reduces pH). Worked Example 2: A water sample has the following composition: Sodium hydroxide = 4 mg / lit and potassium hydroxide = 5.6 mg / lit. Identify the type of alkalinity present in the sample. Calculate the total alkalinity of the water sample. Ans: The type of alkalinity in the sample is caustic alkalinity. Total alkalinity = [ (4 / 40) + (5.6 / 56) ] 50 = 10 ppm CaCO3 equivalent. Determination of alkalinity: Principle / Theory: The alkalinity of water sample can be determined by titration against a standard (or previously standardized) acid using the indicators phenolphthalein and methyl orange. The reactions for the neutralization of various anionic species are as follows: OH- + H+ H2O CO32- + H+ HCO3HCO3- + H+ H2O + CO2 (i) (ii) (iii)

Phenolphthalein indicator can be used with stronger alkalinity only whereas the indicator methyl orange works well with any type of alkalinity. Thus, the titration of the water sample against a standard acid with phenolphthalein indicator (end point is termed as phenolphthalein end point) represents the completion of reactions (i) and (ii) only. On the other hand, the titration of the water sample against a standard acid with methyl orange indicator (end point is termed as methyl orange end point) represents the completion of all the reactions (i), (ii) and (iii). Hence, the amount of acid used for the phenolphthalein end point corresponds to all bicarbonates plus one half of normal carbonate while the total amount of acid used corresponds to the total alkalinity (caused by hydroxides, bicarbonates and carbonates). For practical purposes the alkalinity of water sample is expressed as types such as hydroxide alkalinity, carbonate alkalinity and bicarbonate alkalinity. Amongst the combinations of various ions causing alkalinity, the possibility of

hydroxide and bicarbonate ions existing together is ruled out, as these ions combine instantaneously to form carbonate ions. e.g. NaOH + NaHCO3 Na2CO3 + H2O. Similarly, all these three anions cannot exist together. The alkalinity due to these ions is expressed commonly in terms of CaCO3 equivalent. Procedure: 100 ml of the water sample is pipetted out into a clean conical flask. 2-3 drops of phenolphthalein indicator are added and titrated against N/50 sulphuric acid taken in the burette. End point is the disappearance of pink colour. Then to the same solution, 2-3 drops of methyl orange indicator are added and the titration continued for the reappearance of pink colour, which is the end point. Observations & Calculations: Let Vp and Vm be the volumes of acid used with phenolphthalein and methyl orange indicators respectively. (Obviously total volume Vt = Vp + Vm) Phenolphthalein alkalinity = (Vp X 50 X 106) / (50 X 100 X 1000) ppm CaCO3 equiv. Methyl Orange alkalinity = (Vm X 50 X 106) / (50 X 100 X 1000) ppm CaCO3 equiv. Total alkalinity = (Vt X 50 X 106) / (50 X 100 X 1000) ppm CaCO3 equiv. Following table enables the calculation of alkalinity due to various ions: (Let P = Vp & M = Vm) Alkalinity Titre Value P=0 P=M P>M P