Experiment 6: EDTA Titration of the hardness of water
Purpose: The purpose of this lab was to have students design their own procedure to determine the
hardness of water using an EDTA titration.
Procedure:
1. Using buffer solutions of 4,7, and 10, calibrate a pH meter
2. Weigh out 0.09396g of EDTA and dissolve in a 250mL Erlenmeyer flask (fill to the line with
distilled water)
3. Prepare a solution of CaCl2 by weighing out 0.5g CaCO3 and dissolving it in 100mL 0.1M HCl
4. Combine 3.0mL CaCl2 solution with 5.0mL ammonia buffer and about 4 drops of calgamite
indicator
5. Titrate with EDTA until three good trials
6. Dissolve crushed antacid in 5.0mL buffer
7. Combine 5mL solution from step 6 with 2mL buffer and add calgamite indicator
8. Titrate with EDTA until three good trials (blue endpoint)
9. Determine the percent of Ca2+ in tablets
10. Add 25mL tap water to a 250mL Erlenmeyer flask
a. Add about 3mL buffer and several drops of calgamite indicator
b. Titrate with EDTA until three good results (blue endpoint)
Data:
Mass CaCO3= 0.5g
Mass EDTA= 0.9313g
Tums tablet= 1.3214g
Molarity of EDTA
Trial
1
2
3
Average
EDTA added
13.7mL
13.4mL
13.2mL
13.43mL
Molarity
0.0109M
0.0112M
0.0114M
0.0112M
Antacid (Tums tablet) titration
Trial
1
2
3
Average
EDTA added
5.5mL
5.1mL
4.1mL
4.9mL
Amount Ca2+
0.049g
0.046g
0.037g
0.044g
% Ca2+ in sample
3.71%
3.48%
2.78%
3.32%
Tap water titration
Trial
1
2
3
Average
Ca2+ concentration
2.28x10-4M
2.78x10-4M
2.96x10-4M
2.67x10-4M
EDTA added
5.1mL
6.2mL
6.6mL
5.97mL
ppm
9.16
11.1
11.8
10.7
Calculations:
Molarity of EDTA:
0.5𝑔 𝐶𝑎𝐶𝑂3 ∗
1 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3 1𝑚𝑜𝑙
1
∗
∗ 0.003𝐿 𝐶𝑎𝐶𝑂3 ∗
= 0.0109𝑀
100.09𝑔
0.1𝐿
0.0137𝐿
Ca2+ concentration in tap water:
0.0051𝐿 ∗ 0.0112𝑀 𝐸𝐷𝑇𝐴 ∗
1𝑚𝑜𝑙 𝐶𝑎2+
1
∗
= 2.28 ∗ 10−4
1 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 0.25𝐿
Hardness of water:
2.28𝑥10 −4 𝑀 𝐶𝑎2+ ∗
40.07𝑔 𝐶𝑎 1000𝑚𝑔
∗
= 9.16𝑝𝑝𝑚
1𝑚𝑜𝑙
1𝑔
Tums Tablet Claimed:
0.5𝑔 𝐶𝑎𝐶𝑂3 ∗
1𝑚𝑜𝑙
1𝑚𝑜𝑙 𝐶𝑎2+ 40.07𝑔 𝐶𝑎2+
∗
∗
= 2.002𝑔 𝐶𝑎2+
100.09𝑔 𝐶𝑎𝐶𝑂3 1𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3
1𝑚𝑜𝑙
2.002𝑔 𝐶𝑎2+
𝑥100 = 15.15%
1.3214𝑔 𝑡𝑢𝑚𝑠 𝑡𝑎𝑏𝑙𝑒𝑡
Tums tablet experimental:
0.0055𝐿 ∗ 0.112𝑀 𝐸𝐷𝑇𝐴 ∗
1𝑚𝑜𝑙 𝐶𝑎2+ 40.07𝑔 𝐶𝑎2+
∗
= 0.049𝑔 𝐶𝑎2+
1𝑚𝑜𝑙 𝐸𝐷𝑇𝐴
1𝑚𝑜𝑙
0.049𝑔 𝐶𝑎2+
𝑥100 = 3.71%
1.3214𝑔 𝑡𝑢𝑚𝑠 𝑡𝑎𝑏𝑙𝑒𝑡
Difference:
15.15% − 3.32% = 11.83%
0.2002𝑔 𝐶𝑎2+ − 0.044𝑔 𝐶𝑎2+ = 0.1562𝑔 𝐶𝑎2+
Conclusion:
The goal of this lab was to determine the hardness of tap water. According to our experiment results,
the average hardness of water was 10.7ppm. The amount of Ca2+ in a Tums tablet was also measured.
The claimed amount of calcium in the tablet was 15.15% but the average we calculated after gathering
results was only 3.32%. Possible sources of error could have occurred during titration. Our samples
containing the tap water and Tums tablets didn’t require a lot of EDTA before the endpoint.