Environmental Engineering Practical Series - Alkalinity Test of Water
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Alkalinity of water is essential parameter to be found before its consumption. Following is a self explanatory presentation of why it is important and how to find the same in any sample of water.
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Environmental Engineering Practical Series - Alkalinity Test of Water
- 1. ALKALINITY TEST OF WATER ? It is the Acid Neutralizing property of water ?Caused due to presence of OH, CO3 & HCO3 ions ?Is analyzed by volumetric method by, ?Using Phenolphthalien & Methyl Orange indicators ?The principle behind this is mere chemical reaction, ?Between H+ and OH– ions ? It is to be noted that this is independent of pH Theory / Principle
- 2. ? The presence of OH ions occurs at pH 8.5 or above whereas CO3 ions occur at pH ranges between 6.5 to 8.5 and lastly bicarbonate ions ie. HCO3 occur when the pH falls down to 4.5 from 6.5. ? The reaction of acid and a base always result in neutralizing pH to a scale of 7 and thus in order to determine the Alkalinity of any water body an acid is used for neutralizing OH, CO3 & HCO3 ions to H2O and CO2 respectively
- 3. Procedure ? Take 100mL sample in a 250mL capacity conical flask ? In a burette take N/50 H2SO4 solution ? To the sample containing flask add 4-5 drops of Phenolphthalien indicator ? Pink color indicating presence of OH and CO3 ions will appear
- 4. ? Now titrate the sample against H2SO4 till the pink color disappears (colorless) (in this OH combines with H and gets converted to H2O and CO3 gets converted to HCO3) ? Note the value of N/50 H2SO4 consumed for the color change of the sample as “P” or the phenolphthalien end point. ? Now to the same sample add 1 drop of Methyl Orange indicator
- 5. ? Pale yellow color indicating the presence of HCO3 ions will appear ? Now without filling the burette again start the titration from the same end point “P” till the color changes to light orange (in this the HCO3 ions gets converted to H2O and CO2 by reacting with H+ ions of the acid) ? Note down the final value of N/50 H2SO4 consumed to complete the analysis of alkalinity as “T”
- 6. ? Repeat the whole experiment for finding the concordant value of the acid consumed for neutralizing the sample of water ? Using the equation – N1V1 = N2V2 We have to find out the amount of Alkalinity causing ions present in water either in mg/L or in p p m (parts per million) Calculations
- 7. 1. Phenolphthalien end point – Vol. of H2SO4 used is “P” and is said to be V2 where its normality is known as to be N2 ie. 1/50 By this the value of N1 or the concentration of alkalinity causing ions can be found by implying the formula – N1 = N2V2/V1 = 1 X P = OH + CO3 alkalinity 50 X 100 In simpler way it can be taken as – OH + CO3 Alkalinity = P X 1000 or P X 10 mg/L 100
- 8. 2. Methyl orange / Total end point – Vol. of H2SO4 used is “T” and is said to be V2 where its normality is known as to be N2 ie. 1/50 By this the value of N1 or the concentration of alkalinity causing ions can be found by implying the formula – N1 = N2V2/V1 = 1 X T = HCO3 alkalinity in mg/L 50 X 100 In simpler way it can be taken as – OH + CO3 Alkalinity = T X 1000 or T X 10 mg/L 100
- 9. ? Results are further interpreted by using the below table Values of P& T OH CO3 HCO3 P = T T 0 0 P = T / 2 0 2 P 0 P < T / 2 0 2 P T – 2 P P > T / 2 2 P - T 2 (T – P) 0 P = 0 0 0 T
- 10. VIVA QUESTIONS ? Q. (1) why a given water sample can not be alkaline due to the simultaneous presence of OH- and HCO3 -? ? Q. (2) What do you mean by the alkalinity of the water sample? ? Q. (3) What are the drawbacks of the high alkalinity of the water ? ? Q. (4) How you will conclude that the given water sample is alkaline due to simultaneous presence of OH- and HCO3 -? ? Q.(5) How the alkalinity data are useful in environmental engineering Practice? ? Q.(6) Why the pH of pond water is high? What type of alkalinity would be present?