EXPERIMENT 5 Determination of Total Water Hardness by Compleximetric Titration The total concentration of alkaline earth

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EXPERIMENT 5 Determination of Total Water Hardness by Compleximetric Titration The total concentration of alkaline earth metal ions, such as calcium and magnesium, in water determine the hardness of water. The term hard water comes from the fact that these metal ions precipitate soap molecules from water making it "hard" to get things clean. The calcium in hard water precipitates as calcium carbonate (lime scale). Water hardness is usually determined by measuring the total amount of calcium and magnesium present, since the concentrations of these ions far exceed those of other alkaline earth metals. The accepted practice for reporting hardness is as mg CaCO3/L, as if all of the hardness was from calcium carbonate. Table 1 gives a classification of the hardness of water. Table 1: Water Hardness Classification Hardness, mg CaCO., L Hardness <15 very soft 15-50 soft 50-100 medium hard 100-200 hard 200 very hard There are several methods used for measuring hardness. One of the most common titrants used for water hardness determination is ethylenediamine-N,N,N', N'-tetraacetic acid (EDTA). EDTA is a chelating agent that can donate electrons (Lewis base), thereby forming a complex with metal ions (Lewis acid). The EDTA will complex first with the Ca²+ and then with the Mg2+. As with any titration we will need an indicator to determine when all of the Ca²+ and Mg2+ have complexed with the EDTA (i.e. the endpoint). The indicator used in this experiment is Eriochrome Black T. At the endpoint, the EDTA will complex with the calcium and the indicator becomes unbound, which is indicated by the solution's color change from red to blue.
CHEMICALS/REAGENTS: ● . Distilled water • MgCl2.6H₂O NHẠCH ● Concentrated NH3 . Disodium EDTA dihydrate Eriochrome Black T indicator
Concentrated HCI solution MATERIALS/EQUIPMENT: ● • Volumetric flasks, 500-ml Amber reagent bottles, 50-mL • Beakers, 100-mL & 300-mL Watch glass ● ● Erlenmeyer flasks, 250-ml • Analytical balance
PROCEDURE: Note: All glasswares must be rinsed with 0.1 M HNO3 before using. A. Preparation of reagents • EDTA Titrant (0.005 M) Note: The solution is good for 1 student. The EDTA titrant is prepared by dissolving 1 g of disodium EDTA dihydrate (372.24 g/mol) in 500 mL volumetric flasks and dilute to mark with distilled water. Add a pinch of MgCl₂.6H₂O (approx 0.05 g). Good for 1 student. NH3-NH4Cl Buffer (pH 10.0) Note: Use the fume hood for preparation. One pair will prepare this for the whole class. Dissolve 32 g NH4Cl in 200 mL distilled water. Carefully add 285 mL concentrated NH3 with constant stirring. Add enough water to make a total of 500 mL. • Eriochrome Black T (EBT) Indicator Note: Indicator will be prepared by one student for the whole class. This solution is only good for one week only. Dissolve 0.25 g in 50-mL of methanol. Store in amber reagent bottle and mark date of preparation. Store away from sunlight, preferably refrigerated. • CaCO3 Standard Note: This solution is going to be prepared by one student for the whole class. Use the fume hood for solution preparation. Weigh 0.20 -0.25 g (± 0.1 mg) CaCO3 in a 250-ml beaker. Add concentrated HCI drop by drop until the CaCO3 dissolves. Cover the beaker with watch glass and evaporate the HCI to incipient dryness. Quantitatively transfer the residue to a 500.00 mL volumetric flask and dilute to volume. B. Standardization of EDTA titrant Pipet 10.00 mL aliquot of the standard CaCOs solution in 250-mL Erlenmeyer flasks. Add 5 mL of the buffer and 5 drops of EBT indicator. The solution should turn wine red in color. Titrate with EDTA until the solution becomes blue. Do triplicate determinations. Compute for the molarity of EDTA.
C. Analysis of unknown sample Submit a clean, dry, and properly labeled 50-mL reagent bottle for your unknown solution. Pipet 20.00 mL of the sample in 250-mL Erlenmeyer flask. Add 5 mL of buffer and 5 drops of indicator. Titrate the solution until it turns light blue. If the titration consumes more than 50 mL of the titrant, dilute the sample accordingly. Repeat in triplicates. Compute for the ppm Ca2+ in the sample. Report the mean, sd, and % rsd for the analysis. For the blank correction, perform a blank titration with 50-mL distilled deionized water and subtract the value of the blank from each test. GUIDE QUESTIONS 1. What is water hardness? Differentiate temporary from permanent water hardness. 2. Describe the underlying principles behind the complexometric determination of water hardness. Include pertinent equilibria. 3. Why does the glasswares need to be rinsed with 0.1 M HNO3 prior to use? 4. Why should deionized water be used throughout the analysis? 5. Why should the titration solution be buffered at pH 10.0? 6. Describe the mechanism of end point detection by EBT. 7. Why is CaCO3 standard prepared in the manner described in Part A? 8. What are the possible interferences for this analysis?