Experiment 6: EDTA Titration of Water Hardness
Objective:
The purpose of this lab is to design and test and experiment for the hardness (hard=high
number of cations) of water.
Procedure:












Calibrate a pH meter with buffer to confirm the pH of 6M NH3 Buffer
Mix HCL and NH3 buffer to a pH of 10
Make a 0.01M EDTA solution using 0.9306g EDTA and 4mL of NH3 buffer in a 250mL
volumetric flask
Make a CaCl2 solution with 0.5g Ca2+ and 100mL of 0.1M HCl
Add 3mL of CaCl2 solution in a 250mL Erlenmeyer flask with 5mL of NH3 buffer and
calmagite indicator
Titrate with EDTA until Blue for 3 good trials
Add 55mL Tap water in a 250mL Erlenmeyer flask with 3mL NH3 buffere and calmagite
indicator
Titrate with EDTA until Blue for 3 good trials
Add 1 crushed TUMS tablet to a 100mL volumetric flask
Add 5mL of NH3 buffer and dilute with 0.1M HCl
Add 5mL of TUMS solutions to a 250mL Erlenmeyer flask with 2mL of NH3 buffer and
calmagite indicator
Titrate with EDTA until blue for 3 good trials
Data:
Standardization of EDTA with CaCO3
Mass EDTA: 0.9307g
Mass CaCO3: 0.5023g
Trial
1
2
3
Average
Titration of Tap Water
Trial
Water Volume
(mL)
1
55.0
2
55.0
3
55.0
Average
Volume EDTA (mL)
14.4
14.0
14.8
Volume EDTA (mL)
5.4
5.4
5.7
Molarity EDTA (M)
0.01046
0.01075
0.01017
0.01046
Ca2+ Concentration
(M)
0.001027
0.001027
0.001084
0.001046
Ca2+ ppm
41.16
41.16
43.44
41.92
Titration of TUMS Tablet
Mass TUMS Tablet: 1.3000g
Trial
Volume of TUMS
Solution (mL)
5.0
5.0
5.0
Average
1
2
3
Volume of EDTA
(mL)
4.0
3.7
3.8
Ca2+ in Tablet (g)
% Ca2+ in Tablet
0.0321
0.0297
0.0305
0.0308
2.47
2.28
2.35
2.37
Calculations:
Title
Mass of
EDTA
Needed
Mass of
CaCO3
Needed
Molarity
of EDTA
Concentra
tion of
Ca2+ in tap
Water
Ca2+ ppm
in tap
Water
Ca2+ in
TUMS
tablet
%Ca2+ in
TUMS
tablet
Formula
Calculation
𝐸𝐷𝑇𝐴 = 𝑉𝑤𝑎𝑛𝑡𝑒𝑑 (𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦)(𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠 𝐸𝐷𝑇𝐴)
𝐶𝑎𝐶𝑂3
1𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3
= (𝑀 𝐻𝐶𝑙)(𝑉 𝐻𝐶𝑙) (
) (𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠 𝐶𝑎𝐶𝑂3 )
2𝑚𝑜𝑙 𝐻𝐶𝑙
1
𝑀 𝐸𝐷𝑇𝐴 = 𝑀 𝐶𝑎𝐶𝑂3 (𝑉 𝐶𝑎𝐶𝑂3 ) (
)
𝑉 𝐸𝐷𝑇𝐴
[𝐶𝑎2+ ]
1𝑚𝑜𝑙 𝐶𝑎2+
1
= 𝑉 𝐸𝐷𝑇𝐴 (𝑀 𝐸𝐷𝑇𝐴) (
)(
)
1𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 0.055𝐿 𝑊𝑎𝑡𝑒𝑟
0.01𝑚𝑜𝑙
𝐸𝐷𝑇𝐴 = (0.250𝐿) (
)
𝐿
372.24𝑔 𝐸𝐷𝑇𝐴
(
) = 0.9306𝑔 𝐸𝐷𝑇𝐴
1𝑚𝑜𝑙 𝐸𝐷𝑇𝐴
1𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3
𝐶𝑎𝐶𝑂3 = (0.100𝑀 𝐻𝐶𝑙)(0.100𝐿 𝐻𝐶𝑙) (
)
2𝑚𝑜𝑙 𝐻𝐶𝑙
100.09𝑔
(
𝐶𝑎𝐶𝑂3 ) = 0.5005𝑔 𝐶𝑎2+
1𝑚𝑜𝑙
𝑀 𝐸𝐷𝑇𝐴 = 0.0502𝑀 𝐶𝑎𝐶𝑂3 (3.0𝑚𝐿 𝐶𝑎𝐶𝑂3 )
1
(
) = 0.1046𝑀
14.4𝑚𝐿 𝐸𝐷𝑇𝐴
1𝑚𝑜𝑙 𝐶𝑎2+
[𝐶𝑎2+ ] = 0.0054𝐿 𝐸𝐷𝑇𝐴 (0.01046𝑀 𝐸𝐷𝑇𝐴) (
)
1𝑚𝑜𝑙 𝐸𝐷𝑇𝐴
1
(
) = 0.001027 𝑀
0.055𝐿 𝑊𝑎𝑡𝑒𝑟
𝐶𝑎2+ 𝑝𝑝𝑚 = 𝑀 𝐶𝑎2+ (𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠 𝐶𝑎2+ )(𝐹𝑎𝑐𝑡𝑜𝑟)
40.078𝑔
𝐶𝑎2+ 𝑝𝑝𝑚 = 0.001046𝑀 𝐶𝑎2+ (
𝐶𝑎2+ )
1𝑚𝑜𝑙
1000𝑚𝑔
(
) = 41.92𝑝𝑝𝑚
𝑔
1𝑚𝑜𝑙 𝐶𝑎2+
𝐶𝑎2+ = 𝑉 𝐸𝐷𝑇𝐴(𝑀 𝐸𝐷𝑇𝐴) (
)
1𝑚𝑜𝑙 𝐸𝐷𝑇𝐴
2+
(𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠 𝐶𝑎 )(𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛)
𝑚𝑎𝑠𝑠 𝑜𝑓 𝐶𝑎2+
∗ 100 = % 𝐶𝑎2+
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡𝑎𝑏𝑙𝑒𝑡
1𝑚𝑜𝑙 𝐶𝑎2+
𝐶𝑎2+ = 0.0040𝐿 𝐸𝐷𝑇𝐴(0.01046𝑀 𝐸𝐷𝑇𝐴) (
)
1𝑚𝑜𝑙 𝐸𝐷𝑇𝐴
40.078𝑔
20 𝑝𝑎𝑟𝑡𝑠
(
𝐶𝑎2+ ) (
) = 0.0321𝑔
1𝑚𝑜𝑙
𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛
% 𝐶𝑎2+ =
0.0321𝑔 𝐶𝑎2+
∗ 100 = 2.47% 𝐶𝑎2+
1.3000𝑔 𝑡𝑎𝑏𝑙𝑒𝑡
Conclusion:
The hardness of the tap water as determined to be 41.92ppm. This falls into the soft range of 060ppm described by the United States Geological Service. The Ca2+ content in a TUMS tablet was found
to be 2.37%. The value on the container is 15.4%. Our value is significantly lower than expected. A major
source of error was determining the end point of the titration. The end color was blue and occasionally
the solution would change fully to blue but after a short rest it would revert back to the previous pink
color. This would have caused an increase in the amount of EDTA used.