Determination of Ca2+ and Mg2+ with EDTA
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Procedure:
1-It is important to use good deionized water for this procedure-traces of heavy metal ions will form irreversible complexes with calmagite and interfere with end point.
2-Weigh 3.0 g NaH2Y. H2O , transfer to 1L plastic bottle and dissolve in about 500 mL water. Add 15 mL 1% Mg Cl2 solution and 2 mL 6M NH3, Mix thoroughly by shaking gently and warm if necessary.
3-Weigh accurately 0.5 g of previously dried primary standard CaCO3. Transfer to a 125-mL Erlenmeyer flask and add 20mL water and 5 mL conc. HCl drop-wise. Warm the flask gently till the CaCO3 is dissolved, then boil gently for a few minutes to remove all CO2. Transfer to a 100-mL volumetric flask and dilute to the mark.
4-Transfer, accurately , a 10-mL aliquot of the standard Ca solution from step 3 to a 250mL Erlenmeyer flask. Add 10 mL pH 10 buffer (32 g NH4Cl+285 mL Conc. NH3 per 500 mL of solution). Add 4 drops of fresh 10% Na2S solution and 5 drops of Calmagite solution.
5-Titrate immediately with EDTA solution from Step 2 until the color changes from red to blue. If the color appears to fade before the end point, add a few more drops of indicator.
6-Repeat steps 4-5 two more times. With the data from the three titrations, calculate the EDTA concentration in the titrant.
7-The calcium unknown will be issued as solution in a 100-ml volumetric flask which you will have turned in to your TA ahead of time, Add 8 mL conc. HCl. Dilute to the mark and mix well.
8-Transfer a 10mL aliquot of the unknown solution to a 250mL Erlenmeyer flask and repeat the titration procedure of steps 4-5.
9-Repeat step 8 two more times.
Determine the [Ca] in ppm in the original diluted 100 mL flask.
Data:
Flask# Added EDTA to Step 4 Added EDTA to the unknown
1 35.22 28.44
2 35.35 28.17
3 35.09 28.36.
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Solution Summary
A determination of Ca2+ and Mg2+ with EDTA is provided in the solution.
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Solution:
In the CaCO3 derived Ca2+, the molar concentration for Ca2+ is (0.5/100)/0.1=0.05 M (100 is the molecular weight for CaCO3). Suppose ...
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