Solution 12 drops of the indicator solution and add

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solution, 12 drops of the indicator solution, and add NH 3 /NH 4 + buffer until the pH of the solution is 10 (using pH paper to check the pH). Titrate in order to standardize the EDTA solution. Subtract the average blank titration volume determined in step 2 . Repeat the titration on a second standard solution and average the results. Try to be as consistent as possible with the color at the endpoint. The two volumes should agree within ± 0.20mL. Report the molar concentration of EDTA titrant you determined through this standardization step. 4. Titration of Laboratory unknown Obtain a 15-mL unknown from the instructor and place into a 100-mL volumetric flask. Dilute to the mark and mix well. Carefully pipet a 25-mL portion of this diluted unknown into an Erlenmeyer flask. Pipet exactly 2-mL of the 4mM Mg 2+ solution, 12 drops of the indicator solution, and add NH 3 /NH 4 + buffer until the pH of the solution is 10 (using pH paper to check the pH) and titrate. Repeat on a second and third aliquot of the diluted unknown. Subtract the blank volume determined in step 2 from each result. By proportion of volume used compared to the 600ppm standard, calculate the hardness as ppm CaCO 3 (3 times, for each titration data), determine the average, standard deviation, and calculate the 95% confidence interval. Report the water hardness in the 15- mL unknown, and determine if the unknown is “hard” or “soft” water. Rinse the buret thoroughly with distilled water when done. Note (FYI): <60ppm CaCO 3 is generally considered “soft” water. >270ppm is considered to be “hard”
Lab 7: Determination of Fe by UV-Vis Spectroscopy Background A complex of iron (II) is formed with 1,10-phenanthroline, Fe(C 12 H 8 N 2 ) 3 2+ , and the absorbance of this colored solution over a range of wavelength is measured with a spectrophotometer. The spectrum is plotted (absorbance against wavelength) to determine the absorption maximum. Hydroxylamine (as hydrochloride salt to increase solubility) is added to reduce Fe 3+ to Fe 2+ and to maintain it in that state. 4 Fe 3+ + 2NH 2 OH 4Fe 2+ + N 2 O + 4H + + H 2 O Beer’s Law: A = ε bc where ε is molar absorptivity (M -1 ·cm -1 ), b (cm) is path length of light through the sample solution, and c is the molar concentration (M) Reagents a) 1,10-phenanthroline solution b) standard iron (II) stock solution c) Hydroxylammonium chloride solution d) Sodium acetate solution e) Unknown: 12 mL for each group Procedure 1. I nto each of five 100-mL volumetric flasks, pipet 5, 10, 15, 20 and 25 mL of the standard iron (II) stock solution respectively. Use a combination of 5-, 10-, and 25-mL volumetric pipets. 2. P our 50 mL of DI-water into a 6 th flask to serve as the “blank” (i.e. zero iron concentration). 3. O btain 12 mL unknown sample as directed by instructor or TA, place in a 7 th flask and treat it in the same manner as the standards, as indicated below.

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