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COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS LAB

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COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
DR. BUDRUK
CHM 152 LL
SEAN MARKIS
4 FEBRUARY 2015

Introduction
Using a Lewis base neutral molecule to donate electron pairs (ligands) to a Lewis acid metal ion center to form a single cluster (complex) ion. When the complex ions forms with a metal ion (chelation) the ligand used is called the (chelating agent). EDTA acts as a great chelating agent due to the Nitrogen and Oxygen donating an electron pair to the metal ion center to form an octahedral complex. The metal ions especially with a +2 charge or higher are the reason for water hardness to form on various objects known as “scum”. Calcium ions are typically the most common contributing factor
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Chapter 4 is also used for titration of redox reactions using a standard solution (Na2 EDTA) to add into another solution of unknown concentration (unknown sample + ammonia/ammonium chloride buffer + Eriochrome Black T) until the equivalence point is reached (has fully reacted) as visually identified by the indicators (Eriochrome Black T) from the color change of pink to violet to blue.
Procedure
1. “Prepare about 500mL of approximately 0.004M disodium EDTA solution. To prepare your solution, weigh out 0.7-0.8g of Na2EDTA and dissolve in about 500mL deionized water in your plastic bottle. Seal the bottle and shake vigorously for a few minutes to dissolve the salt.
2. Standardize the Na2EDTA solution using a stock calcium ion solution as the primary standard:
a. Use a 10-mL transfer pipet to add 10.00 mL of standardized calcium ion stock solution (1.000g CaCO3/L solution) to a 250-mL Erlenmeyer flask.
b. Add about 30 ml of deionized water to this titration flask.
c. Add a magnetic stir-bar, place on a magnetic stirrer and begin stirring. A piece of white paper under the flask gives good contrast for easier detection of the indicator color change.
d. Inside the fume hood, add about 3mL of ammonia/ammonium chloride buffer (pH 10). The buffer is an inhalation irritant. Stir for 30 seconds.
e. Just prior to titrating the flask, add four drops of Eriochrome Black T indicator solution. Continue stirring for another 30
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