Experiment 6: EDTA Titration of the Hardness of Water
Purpose:
The purpose of this experiment is to titrate EDTA by your own design.
Procedure:
1) A pH meter was calibrated with buffer solution until the pH of 6M ammonia buffer was verified.
2) HCl concentration was added to the ammonia buffer until it reached a pH of approximately 10.
3) A .01M EDTA solution was prepared by massing approx. 0.9306g of EDTA adding 4mL of
ammonia buffer and placed it in a 250mL volumetric flask, diluting it to the mark.
4) A calcium chloride solution was prepared by reacting approx. 0.500g of calcium carbonate with
100mL of 0.1M HCl.
5) 3mL of calcium carbonate solution was then placed in a 250mL Erlenmeyer flask.
6) 5mL of ammonia buffer was added and a small amount of calmagite indicator
7) The solution was titrated with EDTA solution until the violet turned a blue color. Need 3 good
trials.
8) 50mL of tap water was pipeted into a 250mL Erlenmeyer flask along with 3mL ammonia buffer
and a small amount of calmagite indicator.
9) The solution was titrated with EDTA and repeated until 3 good trials were obtained.
10) 1 crushed Tums tablet was massed and placed in a 100mL volumetric flask.
11) 5mL of ammonia buffer was added to the solution and then the solution was diluted with 0.1M
HCl to the mark.
12) 5mL of this solution was placed into a 250mL Erlenmeyer flask with approx. 2mL ammonia
buffer and a small amount of indicator.
13) The pink solution was then titrated until a blue color appeared.
14) Was repeated until 3 good trials were obtained.
Data:
Mass of EDTA: .9313g
Mass of CaCO3: .5000g
Mass of tablet: 1.3214g
Trial
1
2
3
Average
Molarity of EDTA
EDTA added (mL)
13.7
13.4
13.2
13.43
Molarity of EDTA
0.0109
0.0112
0.0114
0.0112
Tap Water Titration
EDTA added (mL)
Ca2+ concentration (M)
5.1
2.28 x 10-4
6.2
2.78 x 10-4
6.6
2.96 x 10-4
5.97
2.67 x 10-4
Trial
1
2
3
Average
Tums Titration
EDTA added (mL)
5.5
5.1
4.1
4.9
Trial
1
2
3
Average
ppm
9.14
11.14
11.86
10.71
Ca2+ in sample (%)
3.71%
3.48%
2.80%
3.33%
Calculations:
Grams of EDTA needed
molar mass of EDTA disodium ×
. 01mol
× 0.25L = mass of EDTA needed
L
372.24g
x 0.01M x 0.25L = 0.9306g EDTA
mol
Molarity of EDTA
grams CaCO3 × molar mass CaCO3 ×
0.5000gCaCO3 ×
0.003L CaCO3
1
×
= Molarity of EDTA
0.1L
L EDTA added
1mol
0.003LCaCO3
1
×
×
= 0.0109M EDTA
100.09g CaCO3
0.1L
0.0137L
Ca2+ Concentration in Tap Water
L EDTA added × M EDTA ×
0.0051L × 0.0112M ×
1mol Ca2+
1
×
= Molarity of Ca2+
1mol EDTA . 25L
1mol Ca2+
1
×
= 2.28 × 10−4 M Ca2+
1mol EDTA . 25L
Hardness of Water
M Ca2+ × molar mass Ca2+ ×
2.28 × 10−4 M x
1000mg
= 𝑝𝑝𝑚
1g
40.078g Ca2+ 1000mg
x
= 9.14𝑝𝑝𝑚
mol
1g
Tums Tablet (Claimed)
1
1mol Ca2+
0.5000g CaCO3 ×
×
× molar mass Ca2+ = grams of Ca2+
molar mass CaCO3 1mol CaCO3
0.5000g CaCO3 x
1mol CaCO3
1mol Ca2+
40.078g Ca2+
x
x
= 0.2002g Ca2+
100.09g CaCO3 1mol CaCO3
1mol
0.2002g
x100 = 15.15%
1.3214g
Tums Tablet (Experiment)
1mol Ca2+
20 parts
L EDTA added × Mavg EDTA ×
× molar mass Ca2+ ×
= grams of Ca2+
1mol EDTA
solution
0.0055L × 0.0112M ×
1mol Ca2+ 40.078g 20 parts
×
×
= 0.049g
1mol EDTA
1mol
solution
0.049g
x 100 = 3.71%
1.3214g
Difference: 15.15-3.33 = 11.82%
Conclusion:
The purpose of this experiment was to determine the hardness of water. The hardness of water for this
experiment was determined to be 10.71ppm which is considered to be soft water. The claimed amount
of Ca2+ in the Tums tablet was calculated to be 15.15%, which was a large difference compared to the
experimental value of an average of 3.33%. There are several errors that could have been present when
conducting this experiment which were titrating past the endpoint, or not having clean glassware to
collect the tap water.