Experiment #28: Hardness of Water
Student: Assem Abichshayeva
Lab. Partner: Marat Aitkhozha
The main purpose of this experiment is to standardize a solution of EDTA by titration against a standard solution made from calcium carbonate, CaCO3. Procedure:
A 50-mL buret, a 250-mL volumetric flask, and 25- and 50-mL pipets were obtained from stockroom. The 0.5 g of calcium carbonate was weighted in a small 50-mL beaker. Approximately 0.4 g of the carbonate was transferred to a 250-mL beaker. 25 mL of distilled water was poured to the large beaker and 40 drops of 6 M HCl were added. Then, solution was heated until it just begins to boil. After boiling 50 mL of distilled water was added to the beaker. The flask was stoppered and mixed thoroughly by inverting the flask. The buret was filled with the EDTA solution. The blank was prepared by adding 25 mL distilled water, 5 mL of the pH 10 buffer to a 250-mL Erlenmeyer flask. Small amount of solid Eriochrome Black T indicator was added. EDTA was added to the solution until the last tinge of purple just disappears. Three 25-mL portions of the Ca2 + solution were drawn separately into clean 250-mL Erlenmeyer flasks. To each flask add 5 mL of the pH 10 buffer, a small amount of indicator, and 15 drops of 0.03 M MgCl2 were added. The solution was titrated in one of the flasks until its color matches that of your reference solution. Data and Calculations:
Volume EDTA used to titrate blank (ml)| 16.3| 29.6| 20.1| Volume EDTA used to titrate Ca2+ (ml)| 20.5| 21.3| 19.6| Average volume of EDTA required to titrate Ca2+ (ml)| (20.5+21.3+19.6)/3=20.5| no. moles Ca2+ (M)| 0.56/100.1=0.005|
Molarity of EDTA= in aliquot/average volume EDTA required (ml)*1000 (M)| 0.005/20.5*1000=0.27|
In this experiment the hardness of water was checked due to the presence of calcium and magnesium ions. These ions do not pose any health...