Complexometric Determination of Water Hardness

Topics: Water, Calcium, Hard water Pages: 5 (1301 words) Published: September 18, 2013
Complexometric Determination of Water Hardness

By Nick Williams

CHM 152
Lab dates: Aug 28th 2013

Dr. Weide

Abstract:

When a polyatomic ligand with multiple lone pairs of electrons available for bonding to a central metal ion forms a complex with a metal ion, a process known as chelation takes place. Metal ion impurities can be found by using disodium salt of EDTA to determine the concentration of M2+ by complexometric or chelometric titration. Erichrome Black T makes it easy to see when the EDTA has completely chelated the metal impurities. When the metal cation is still present in the water, the solution will be a complex with a pink tint. Once enough EDTA has been added to the M2+ solution the metal ions complex to the EDTA leaving the indicator solvated causing a color change from pink to blue. Once the color has changed to blue, the titration has reached an end point and the M2+ metal ion impurities in hard water can be calculated. Procedure:

Prepare 500 mL of 0.004 M disodium EDTA solution by dissolving 0.7-0.8g of Na2EDTA in 500 mL deionized water. Standardize the solution by using a 10 mL transfer pipet to add 10 mL of standardized calcium ion stock solution to a 250 mL Erlenmeyer flask and mixing it with 30 mL of deionized water. Add 3 mL of ammonia/ammonium chloride buffer (pH 10) to ensure that the calcium ions remain in the solution for accurate results. Add four drops of Eriochrome Black T indicator solution so that a color change can be visible during titration. Once the solution is prepared, titrate it with the disodium EDTA solution. Add the solution quickly at first and once the color changes from pink to violet slow down the titration until the color changes from violet to blue. Then record the results.

Part 2 Unknown

After receiving an unknown prepared water sample record the unknown in your lab notebook (unknown #76). After the unknown has been recorded, transfer 25 mL of the sample into a 250 mL Erlenmeyer flask with 20 mL of DI water. As in part one, add 3 mL of ammonia/ammonium chloride buffer (pH10) to the solution along with four drops of Erichrome Black T indicator. Once the solution has been made, titrate the solution with the standardized disodium EDTA solution and calculate the hardness of the prepared water sample from the titration. Compare the results to the expected range for municipal water hardness on your city’s water quality lab website.

Data:
|
Test 1: Standardization of EDTA

Mass of CaCO3: 0.01 g
Moles of CaCO3: 9.9x10^-5
Molarity of the 250 mL standard Ca2+: 3.96x10^-4 M

Column1| Trial 1| Trial 2| Trial 3|
Volume of Ca2 Titrated (mL)| 10| 10| 10|
Moles Ca2+ Titrated | 9.9x10^-5| 9.9x10^-5| 9.9x10^-5|
Moles EDTA| 9.9x10^-5| 9.9x10^-5| 9.9x10^-5|
Initial Buret reading (mL)| 1| 1| 1|
Final buret reading (mL)| 26.3| 26.5| 27|
Volume of EDTA (mL)| 25.3| 25.5| 26|
Molarity of EDTA (M)| 0.0039| 0.0039| 0.0038|
Average Molarity of EDTA| | 0.0039 M| |

Test 2: Unknown Determination Sample #76
Column1| Trial 1| Trial 2| Trial 3|
Volume of Water Sample Used (mL)| 25| 25| 25|
Initial buret reading (mL)| 1| 1| 1|
Final buret reading (mL)| 14.8| 15| 14.9|
Volume of EDTA used (mL)| 13.8| 14| 13.9|
Moles EDTA| 5.38x10^-5| 5.46x10^-5| 5.42x10^-5|
Moles Ca2+ in sample| 5.38x10^-5| 5.38x10^-5| 5.38x10^-5| Moles Ca2+ per liter| 5.38x10^-4| 5.38x10^-4| 5.38x10^-4| Grams CaCO3 per liter| 0.054| 0.055| 0.055|
Water hardness| 29.73| 30.61| 30.17|
Average water hardness (ppm CaCO3)| | 30.17| |

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Calculations:
Test 1:
Calculate Mass of CaCO3 used in the reaction:
10 mL Ca2x 1L/1000mLx 1g/1L= 0.01 g CaCO3
Calculate Moles of CaCO3
0.01 g CaCO3x 1mole CaCO3/100.1 g CaCO3= 9.9x10^-5 moles
CaCO3
Molarity of 250 mL of standard Ca2+ solution
9.9x10^-5 moles CaCO3/0.250 L= 3.96x10^-4 M Ca2+
Moles...
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