Experiment 6: EDTA Titrations and Hardness of Water
John Siller
Purpose: The purpose of this experiment was to use better understand EDTA titrations and use that
knowledge to determine the hardness of water. It was also important to design a lab that could be
repeated by our peers.
Procedure
1.) Calibrate a pH meter with buffer solution until the pH of 6M ammonia buffer was confirmed
2.) Add HCl until ammonia buffer reaches a pH of 10
3.) Weigh approximately 0.93g of EDTA, add 4mL of ammonia buffer and place into a 250mL
volumetric flask and diluting to get 0.01M EDTA
4.) Prepare a calcium chloride solution by reacting approximately 0.5g of calcium carbonate with
100mL 0.1M HCl
5.) Place 3mL of calcium carbonate in a 250mL Erlenmeyer flask then add 5mL of ammonia buffer
and some calmagite indicator
6.) Titrate with EDTA solution (3 times)
7.) Place 55mL of tap water into a 250mL Erlenmeyer flask then add 3mL of ammonia buffer and
calmagite indicator
8.) Titrate with EDTA solution (3 times)
9.) Crush and weigh 1 tablet then place in 100mL volumetric flask
10.) Add 5mL of ammonia buffer and then dilute with 0.1M HCl
11.) Place 5mL of new solution in a 250mL Erlenmeyer flask with 2mL ammonia buffer and indicator
12.) Titrate until blue (3 times)
Data
Mass of CaCO3 = 0.500g
Mass of EDTA = 0.9375g
Mass of Antacid = 1.7258g
Molarity of EDTA
Trial
EDTA added(mL)
Molarity of EDTA(M)
1
16.12
0.0093
2
14.88
0.0101
3
15.1
0.0099
Avg
15.37
0.0098
Tap Water Titration
Trial
EDTA added(mL)
Ca2+ Molarity(M)
ppm
1
5.71
0.00102
40.88
2
5.01
0.000893
35.79
3
5.5
0.00098
39.28
Avg
5.41
0.000964
38.64
Antacid Titration
Trial
EDTA added(mL)
% Ca2+ in sample
1
15.9
7.24
2
14.5
6.6
3
14.9
6.8
Avg
15.1
6.9
Calculations
Mass of EDTA needed for 0.01 M solution
molar mass EDTA disodium 0.010 mol
×
× 0.25 L = mass of EDTA needed
1
1L
372.24 g 0.010 mol
×
× 0.25 L = 0.9306 g EDTA needed
1
1L
Grams Calcium Carbonate needed for CaCl2 solution
Molarity of CaCl2 × mL CaCl2 used ×
1 mol CaCO3 molar mass CaCO3
×
= Grams Calcium Carbonate
1 mol CaCl2
1 mol CaCO3
0.1 M CaCl2 × 0.100L CaCl2 ×
1 mol CaCO3 100.09 CaCO3
×
= 1.0 g
1 mol CaCl2
1 mol CaCO3
Molarity of EDTA
grams CaCO3 ×
1 mol CaCO3
1 mol
1
×
× 0.003L CaCl2 ×
= Molarity EDTA
molar mass CaCO3 0.1 L
mL of EDTA
0.500 g CaCO3 ×
1 mol CaCO3
1 mol
1
×
× 0.003L CaCl2 ×
100.09 g CaCO3 0.1 L
0.01612mL of EDTA
= 0.0093M EDTA
Ca2+ concentration in tap water
L of EDTA added × Molarity EDTA ×
0.00571L × 0.0098M ×
1 mol Ca2+
1
×
= Ca2+ Molarity
1 mol EDTA 0.055 L
1 mol Ca2+
1
×
= 0.00102 M
1 mol EDTA 0.055 L
Hardness of Water
Ca2+ Molarity ×
grams Ca2+ 1000 mg
×
= ppm(hardness of water)
1 mol
1g
0.00102 M ×
40.078 g Ca2+ 1000 mg
×
= 40.88 ppm
1 mol
1g
Claimed Value of Ca2+ in tablet
grams CaCO3 ×
1 mol CaCO3
1 mol Ca2+
grams of Ca2+
×
×
= Ca2+ in tablet
molar mass CaCO3 1 mol CaCO3
1 mol Ca2+
1 mol CaCO3
1 mol Ca2+
40.078 of Ca2+
0.500 g CaCO3 ×
×
×
= 0.2002 g of Ca2+
100.09 CaCO3 1 mol CaCO3
1 mol Ca2+
grams Ca2+
× 100% = Amount of Ca2+ in tablet
total mass of tablet
0.2002 g Ca2+
× 100% = 11.6% Ca2+ in tablet
1.7258 g
Amount of 𝐂𝐚𝟐+ in sample
L of EDTA added × Molarity of EDTA ×
0.0159 L × 0.0098 M ×
1 mol Ca2+ molar mass Ca2+ 20 parts
×
×
= grams Ca2+
1 mol EDTA
1 mol Ca2+
solution
1 mol Ca2+ 40.078 Ca2+ 20 parts
×
×
= 0.1249 grams Ca2+
1 mol EDTA 1 mol Ca2+
solution
grams Ca2+
× 100% = Amount of Ca2+ in sample
total mass of tablet
0.1249 g Ca2+
× 100% = 7.237% Ca2+ in sample
1.7258 g
Conclusion:
This lab was designed to determine the hardness of water, in other words the Ca2+
concentration. This was done to a water sample and an unknown sample. For the unknown sample it
was found that it contained 7.237% Ca2+. The reported value was 11.6% so there must have been some
experimental error that resulted in loss of product. Overall however the values are not significantly
different enough to show a flaw in the procedure.