Experiment 6: EDTA Titration of the Hardness of Water
Introduction:
This experiment was designed to familiarize the student with experimental design by allowing the student to develop his/her own
procedure to determine the hardness of tap water. The student was then instructed to extend his/her procedure to determine the
calcium content of a commercially-available antacid.
Procedure:
A pH meter was calibrated, then utilized to measure the pH of a premade ammonia buffer solution. Hydrochloric acid was added to
the buffer solution until its pH was approximately 10. Then, 8mL of the buffer solution was combined with 1.8612g EDTA in a 500mL
volumetric flask, which was then diluted to the mark with distilled water. A solution of calcium carbonate was prepared by dissolving
approximately 0.50g CaCO3 in 0.1M HCl in a 100mL volumetric flask. The flask was then diluted to the mark with 0.1M HCl. An
antacid solution was prepared similarly, using approximately 1.2743g of antacid and diluting to 100mL with 0.1M HCl. Three
different solutions were titrated using the EDTA solution. The first was 3mL of the CaCO3 solution combined with 5mL of buffer and
calmagite indicator. The second was 25mL of tap water combined with 5mL of buffer and calmagite indicator. The third was 3mL of
the antacid solution combined with 5mL of buffer and calmagite indicator. All titrations were repeated until 3 good trials were
obtained.
Data:
CaCO3 solution
EDTA solution
Mass (g)
0.5005
Mass (g)
1.8618
Volume (mL)
100
Volume (mL)
500
Standardization
3mL CaCO3 solution + 5mL buffer + 3 drops indicator
Trial
Initial Volume (mL)
Final Volume (mL)
1
0
16.3
2
0
14.2
3
0
16.7
Tap Water Titration
25mL tap water +5mL buffer + 3 drops indicator
Trial
Initial Volume (mL)
Final Volume (mL)
1
0
3.9
2
0
2.8
3
0
2.8
Tums Titration
3mL Tums solution + 5mL buffer + 3 drops indicator
Trial
Initial Volume (mL)
Final Volume (mL)
1
0
15.00
2
0
14.80
3
0
14.60
Formulas for Calculations:
Change in Volume Equation
Change in Volume Example
Δ𝑉 = 𝑉𝑓 − 𝑉𝑖
Molarity of CaCO3 Equation
𝑀𝐶𝑎𝐶𝑂3 = 𝑔𝐶𝑎𝐶𝑂3 ×
Molarity of CaCO3 Example
1𝑚𝑜𝑙
÷ 0.100𝐿𝑠𝑜𝑙𝑛
100.0869𝑔
Molarity of EDTA Equation
𝐿𝐶𝑎𝐶𝑂3 ∗ 𝑀𝐶𝑎𝐶𝑂3
𝐿𝐸𝐷𝑇𝐴
(because moles of EDTA = moles of Ca2+ = moles of CaCO3)
Molarity of Ca2+ in Tap Water Equation
̅𝐸𝐷𝑇𝐴
𝐿𝐸𝐷𝑇𝐴 ∗ 𝑀
𝐿𝑡𝑎𝑝 𝑤𝑎𝑡𝑒𝑟
(because moles of EDTA = moles of Ca2+)
𝑀𝐶𝑎2+ =
Hardness of Tap Water (ppm) Equation
40.078𝑔 1000𝑚𝑔
×
𝑚𝑜𝑙
1𝑔
(because 1 ppm = 1 mg/L)
𝑝𝑝𝑚𝐶𝑎2+ = 𝑀𝐶𝑎2+ ×
Claimed % Ca2+ in Tums Equation
0.500𝑔𝐶𝑎𝐶𝑂3
1𝑚𝑜𝑙𝐶𝑎𝐶𝑂3
1𝑚𝑜𝑙𝐶𝑎2+
×
×
1𝑡𝑎𝑏𝑙𝑒𝑡
100.0869𝑔𝐶𝑎𝐶𝑂3 1𝑚𝑜𝑙𝐶𝑎𝐶𝑂3
40.078𝑔𝐶𝑎2+
×
÷ 𝑔𝑡𝑎𝑏𝑙𝑒𝑡 × 100%
1𝑚𝑜𝑙𝐶𝑎2+
Grams Ca2+ in Tums Equation
1𝑚𝑜𝑙
÷ 0.100𝐿𝑠𝑜𝑙𝑛
100.0869𝑔
0.01000𝑀𝐸𝐷𝑇𝐴 =
0.00300𝐿𝐶𝑎𝐶𝑂3 ∗ 0.05000𝑀𝐶𝑎𝐶𝑂3
0.01500𝐿𝐸𝐷𝑇𝐴
Molarity of Ca2+ in Tap Water Example
0.005210𝑀𝐶𝑎2+ =
̅𝐸𝐷𝑇𝐴
0.00258𝐿𝐸𝐷𝑇𝐴 ∗ 0.01072𝑀
0.02500𝐿𝑡𝑎𝑝 𝑤𝑎𝑡𝑒𝑟
Hardness of Tap Water (ppm) Example
208.8𝑝𝑝𝑚𝐶𝑎2+ = 0.005210𝑀𝐶𝑎2+ ×
40.078𝑔 1000𝑚𝑔
×
𝑚𝑜𝑙
1𝑔
Claimed % Ca2+ in Tums Example
15.74%𝐶𝑎2+ =
0.500𝑔𝐶𝑎𝐶𝑂3
1𝑚𝑜𝑙𝐶𝑎𝐶𝑂3
1𝑚𝑜𝑙𝐶𝑎2+
×
×
1𝑡𝑎𝑏𝑙𝑒𝑡
100.0869𝑔𝐶𝑎𝐶𝑂3 1𝑚𝑜𝑙𝐶𝑎𝐶𝑂3
40.078𝑔𝐶𝑎2+
×
÷ 1.272𝑔𝑡𝑎𝑏𝑙𝑒𝑡 × 100%
1𝑚𝑜𝑙𝐶𝑎2+
Grams Ca2+ in Tums Example
1𝑚𝑜𝑙𝐶𝑎2+ 40.078𝑔𝐶𝑎2+
×
1𝑚𝑜𝑙𝐸𝐷𝑇𝐴
1𝑚𝑜𝑙𝐶𝑎2+
100𝑚𝐿𝑠𝑜𝑙𝑛
×
3𝑚𝐿𝑖𝑛𝑐𝑟𝑒𝑚𝑒𝑛𝑡𝑠
(because 3mL were used for each titration)
̅𝐸𝐷𝑇𝐴 ×
𝑔𝐶𝑎2+ = 𝐿𝐸𝐷𝑇𝐴 × 𝑀
% Ca2+ in Tums Equation
̅𝐸𝐷𝑇𝐴 ×
0.2199𝑔𝐶𝑎2+ = 0.01535𝐿𝐸𝐷𝑇𝐴 × 0.01072𝑀
×
40.078𝑔𝐶𝑎2+
100𝑚𝐿𝑠𝑜𝑙𝑛
×
1𝑚𝑜𝑙𝐶𝑎2+
3𝑚𝐿𝑖𝑛𝑐𝑟𝑒𝑚𝑒𝑛𝑡𝑠
1𝑚𝑜𝑙𝐶𝑎2+
1𝑚𝑜𝑙𝐸𝐷𝑇𝐴
% Ca2+ in Tums Example
%𝐶𝑎2+ =
𝑔𝐶𝑎2+
× 100%
𝑔𝑡𝑎𝑏𝑙𝑒𝑡
% Difference Equation
% 𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 =
0.05000𝑀𝐶𝑎𝐶𝑂3 = 0.5005𝑔𝐾𝐼𝑂3 ×
Molarity of EDTA Example
𝑀𝐸𝐷𝑇𝐴 =
%𝐶𝑎2+ =
16.30mL = 16.91𝑚𝐿 − 0𝑚𝐿
17.29%𝐶𝑎2+ =
0.2199𝑔𝐶𝑎2+
× 100%
1.2720𝑔𝑡𝑎𝑏𝑙𝑒𝑡
% Difference Example
|𝑡𝑟𝑢𝑒 − 𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙|
× 100%
𝑡𝑟𝑢𝑒
9.829% 𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 =
|15.74% − 17.29%|
× 100%
15.74%
Summary of Calculations:
Standardization
Change in Volume
Trial
(mL)
1
16.30
2
14.21
3
16.70
Average
Trial
1
2
3
Tums Titration
Change in Volume
(mL)
15.00
14.80
14.60
Average
0.009203658
0.010564763
0.008983212
0.009583878
Tap Water Titration
Change in Volume
(mL)
3.90
2.80
2.80
Average
g Ca2+
% Ca2+
% Difference
0.192051324
0.18949064
0.186929955
0.18949064
15.09837452
14.89706286
14.6957512
14.89706286
5.887789967
7.142619434
8.397448901
7.142619434
Molarity of EDTA
Molarity Ca2+
Hardness (ppm)
0.007801021
0.005600733
0.005600733
0.006334162
312.6493161
224.4661757
224.4661757
253.8605558
Molarity of CaCO3 : 0.050486128 M
Claimed % Ca2+ in Tums : 15.74025254 %
Conclusions:
The average calculated hardness of tap water was 208.27ppm. According to accepted standards, this indicates that the tap
water was very hard. This can pose certain health risks, and it is important to conduct further experimentation to determine if these
results were accurate. The commercially-available antacid analyzed in this experiment was Tums. The manufacturer’s claim is that
there are 500mg of calcium carbonate per tablet. This translates to a claimed Ca 2+ percentage of 15.74%. The average experimental
Ca2+ percentage was 16.54%. This is very close to the advertised percentage, although slightly higher. This may be due to
overshooting the end point of the titration. Overall, the percent difference of the titration was 5.106%, which is acceptable.