Experiment six: EDTA titration of the Hardness of Water
Andrew Mullenax
Purpose:
The purpose of this experiment is to prepare a procedure to determine the hardness of water.
Procedure:
1. Calibrate a pH meter with buffer solutions until the pH of 6M ammonia buffer can be
determined.
2. Add concentrated HCl to the ammonia buffer until a pH of 10 is reached.
3. Prepare a 0.01M EDTA solution by weighing approximately 0.9360g EDTA and add
4mL of the 6M ammonia buffer in a 250mL volumetric flask and fill to the mark.
4. Prepare a calcium chloride solution by mixing approximately 0.5000g CaCO3 with
100mL of 0.1M HCl.
5. Place 3mL of your CaCl2 solution in a 250mL Erlenmeyer flask with 5mL ammonia
buffer as well as a small amount of calmagite indicator.
6. Titrate the solution with your EDTA solution until the violet turns to a blue. Repeat for
three good trials.
7. Pipet 50mL of tap water into a 250mL Erlenmeyer flask. Add 3mL of ammonia buffer
and a small amount of calmagite indicator.
8. Titrate this solution with your EDTA solution. Repeat for three good trials.
9. Crush and weigh a tums tablet. Add it to a 100mL volumetric flask. Add 5mL ammonia
buffer then dilute to the mark with 0.1M HCl.
10. Place 5mL of this solution in a 250mL Erlenmeyer flask. Add 2mL of ammonia buffer
and a small amount of calmagite indicator.
11. Titrate until the solution turns from a pink to a blue color.
Data:
Standardization of EDTA
Mass CaCO3: 0.5000g
Mass EDTA: 0.9375g
pH of Ammonia Buffer: 9.65
Volume EDTA
Added(mL)
Concentration of
EDTA
Trial 1
Trial 2
Trial 3
Average
16.12
14.88
15.10
15.37
.00930
.01007
.00992
0.00976
Titration of Tap Water
Volume EDTA
added(mL)
Concentration of
Ca2+
Hardness (ppm)
Trial 1
Trial 2
Trial 3
Average
5.71
5.01
5.50
5.41
.00101
.00089
.00098
0.00096
40.51
35.69
39.30
38.50
Trial 1
Trial 2
Trial 3
Average
15.9
14.5
14.9
15.1
7.21%
6.58%
6.76%
6.85%
Titration of Antacid Tablet
Mass of tablet: 1.7258g
Volume EDTA
added(mL)
% Ca2+ in
Sample
Calculations:
Mass EDTA needed:
372.24g/mol ×
372.24g/mol ×
0.010 mol
× 0.25 L
1L
0.010 mol
× 0.25 L = 0.9306 g EDTA
1L
Mass Calcium Carbonate needed:
Molarity of CaCl2 × L CaCl2 used ×
0.1 M CaCl2 × 0.100L CaCl2 ×
1 mol CaCO3 molar mass CaCO3
×
1 mol CaCl2
1 mol CaCO3
1 mol CaCO3 100.09 CaCO3
×
= 1.0 g
1 mol CaCl2
1 mol CaCO3
Molarity EDTA:
mass CaCO3 ×
0.500 g CaCO3 ×
1 mol CaCO3 1 mol
1
×
× 0.003L CaCl2 ×
100.1g CaCO3 0.1 L
L of EDTA
1 mol CaCO3
1 mol
1
×
× 0.003L CaCl2 ×
= 0.0093M EDTA
100.09 g CaCO3 0.1 L
0.01612mL of EDTA
Ca concentration in tap water:
L of EDTA added × Molarity EDTA ×
0.00571L × 0.0098M ×
1 mol Ca
1
×
1 mol EDTA 0.055 L
1 mol Ca
1
×
= 0.00102 M
1 mol EDTA 0.055 L
Calcium Hardness of Tapwater:
Ca Molarity ×
0.00102 M ×
mass Ca 1000 mg
×
= ppm
1 mol
1g
40.078 gCa 1000 mg
×
= 40.88 ppm
1 mol
1g
Claimed wt% of Calcium in tablet:
mass CaCO3 ×
0.500 g CaCO3 ×
1 mol CaCO3
1 mol Ca
40.1g Ca
×
×
100.1g CaCO3 1 mol CaCO3 1 mol Ca
1 mol CaCO3
1 mol Ca
40.078 of Ca
×
×
= 0.2002g
100.09 CaCO3 1 mol CaCO3
1 mol Ca
grams Ca2+
× 100% = wt% claimed
total mass of tablet
0.2002 g Ca2+
× 100% = 11.6%
1.7258 g
Mass of calcium and wt% of calcium:
L EDTA × Molarity of EDTA ×
1 mol Ca
molar mass Ca 20 parts
×
×
= mass Calcium
1 mol EDTA
1 mol Ca
solution
0.0159 L × 0.0098 M ×
1 mol Ca
40.078 Ca 20 parts
×
×
= 0.1249g
1 mol EDTA 1 mol Ca solution
𝑤𝑡% =
𝑚𝑎𝑠𝑠 𝑐𝑎𝑙𝑐𝑖𝑢𝑚
× 100
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡𝑎𝑏𝑙𝑒𝑡
. 1249𝑔
× 100 = 7.24%
1.7258𝑔
Conclusion:
The purpose of this lab was to determine the calcium hardness of tap water as well as the content
of calcium in a commercial antacid through several EDTA titrations. We determined the calcium
hardness of the tap water to be about 38.50ppm. We determined the content of calcium in the
antacid tablet to be about 7.6% which was somewhat lower than the manufacturer claim of
11.6%. We attributed this error to our difficulty determining the endpoint of our titrations.