Determination of Total Water Hardness in an Unknown Sample Using EDTA
Lexus Hardeman
November 2018
Clark Atlanta University
Introduction
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Abstract
In this experiment the hardness of a hard water sample and unknown water sample was determined. The calcium in
the water was measured by performing a titration with EDTA and EBT as an indicator. After standardizing the
EDTA, the average molarity was found to be 0.0114. The calcium concentration was found to be 203.8ppm. Overall
the experiment-maintained precision and accuracy
Water hardness is a volume of the amount of calcium and magnesium salts in water.
Calcium and magnesium enter water through the withstanding of rocks. The higher amount of
calcium and magnesium in water, the harder the water. Water hardness is typically expressed in
milligrams per liter (mg/l) of dissolved calcium and magnesium carbonate.
Complexometric titration also known as chelatometry is a form of volumetric analysis in
which the formation of a colored complex is used to indicate the end point of a titration.
Complexometric titrations are predominantly useful for the determination of a mixture of
different metal ions in solution. An indicator capable of producing an unambiguous color change
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is typically used to detect the end-point of the titration. Ethylenediaminetetraacetic acid (EDTA)
is a colorless, crystalline solid, (C10H16N2O8), used as an industrial chelating agent, food
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preservative, etc. and in medicine to chelate lead, copper, etc. in metal poisoning, to prevent
coagulation of blood. EDTA is an indicator that can be used to determine calcium content in a
solution. At low pH's EDTA will be in an acidic form and will not be an effective complexing
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agent. The complex that is initially produced is red. After every individual calcium ions have
reacted, the solution then turns a bright blue. This indicates the endpoint of the titration.
In this experiment, a solution of EDTA was standardized by titration against a standard
solution made from Ca2+.The EDTA solution was then used to determine the hardness of one
different water samples. The buffer solution, 0.1M NH4Cl/0.5M NH3 buffer was used to maintain
the basic pH of 9-10. Eriochrome Black T (EBT) is used as the endpoint indicator.
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When EDTA is added into the hard water, the metal ions form stable complexes with EDTA by
releasing the indicator. When EBT is added to a hard water, EBT forms an unstable wine-red
color complex with Ca2+.When all the metal ions were taken by the EDTA from the indicator
metal ions complexes, the wine read color changes into steel blue, which indicate the end point.
The first experiment that was performed in this trial was Ca 2+ and EDTA to determine the
concentration of the EDTA solution after each replicate. The base was NH 4Cl/0.5M NH3 buffer
and the acid was Ca2+. Once all the solutions were mixed well in a flask, the titration of EDTA in
the solution begun until the endpoint was reached. It turned a bright blue with the assistance of
the EBT indicator. The precise molarity of EDTA solution was concluded through
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standardization.
Ca2+ + EDTA4- → CaEDTA2-
The second experiment was performed in this trial to determine the amount of calcium in
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a hard water sample. The base was NH 4Cl/0.5M NH3 buffer and the EBT was the indicator used
to determine to amount to calcium in the hard water sample. The precise molarity of Ca 2+, water
hardness and CaCO3 ppm were concluded.
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CaCO3 + EDTA4- → CaEDTA2- + CO32The third part of the experiment that was performed in this trial was used to determine the
calcium in an unknown sample of tap water. Hard water tap water and NH 4Cl/0.5M NH3buffer
were mixed well and titrated with EDTA till blue end point was met. The average molarity of
Ca2+ and water hardness were concluded.
Materials
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Devices/Supplies

50 ml buret

Buret clamp

Ring stand

125 ml flask

Beakers
Chemicals
EDTA Solution (0.01M)

0.1 NH4Cl/ 0.5 NH3 buffer

0.0100 MZn(NO3)2, Zinc Nitrate

EBT Indicator

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
Hard Water Sample
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Procedure
Part 1: Standardization of EDTA Solution
1) A 0.01M EDTA solution was prepared and exact concentration was determined
2) A ring stand, buret clamp and 50 ml buret were set up to conduct titration
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3) Buret was rinsed and filled with EDTA
4) 10.00ml of standard Ca2+ was pipetted into a 125ml flask
5) 20.00ml of NH4Cl/0.5M NH3 buffer was added to the flask
6) 2-3 drops of EBT indicator was added to the flask
7) All solutions were mixed well
8) Titration begun using the EDTA titrant solution till a blue end point was reached
9) Initial and final volume of buret was recorded
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10) Repeated procedures for 3 more trials
Part 2: Determination of Calcium in a Hard Water Sample
1) A 100ml cylinder was used to measure 50ml of hard water sample and added into a 150ml
flask
2) 20.0ml of NH4Cl/0.5M NH3 buffer was added to the flask
3) Initial and final volume of buret was recorded
4) Repeated titration of the hard water sample twice.
5) Calculated the water hardness after each replicate.
6) Calculated the average water hardness for the hard water sample
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Part 3: Determination of Calcium in the Tap Water Sample in the Lab
1) A 100ml cylinder was used to measure 100ml of hard water sample
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2) 100ml of tap water was added into a 250ml flask
3) 20ml of NH4Cl/0.5M NH3 buffer was added to the flask
4) 2-3 drops of EBT indicator was added to the flask
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5) Titration begun using the EDTA titrant solution till a blue end point was reached
6) Calculated volume of EDTA used and mole of Ca2+ in water sample
7) Repeated titration of the hard water sample twice.
Results and Discussion
Results and Discussion
Part 1
About 0.01
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Estimated molarity of EDTA solution
Concentration of Ca^2+
0.0101 M
Trial 1
Trial 2
Trial 3
Final buret reading, EDTA
10.93 mL
11.23 mL
9.87 mL
Initial buret reading, EDTA
.000mL
.000mL
.000mL
Volume used, EDTA
10.93 mL
11.23 mL
9.87 mL
Molarity of EDTA solution
.0092 M
.0090 M
.0010 M
Average molarity of EDTA solution
.0064 M
.0064 M
.0064 M
Based on the stoichiometry of the equation, 1:1 ratio was used to assume that the molarity of the
Ca^2+ ion is equal to the molarity of the EDTA solution. Throughout the experiment, it was
concluded to determine the molarity of EDTA, we must multiply it by the volume in liters
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Calculations
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Molarity x Vol (L) = moles
(.0101M)(.0100L) = .000101 moles
See results in table using similar equation to solve
Once the moles were calculated, we used the solution to determine the molarity if all three
experiment.
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Molarity = moles/vol(L)
(.000101 mol/ .01093 L) = .00924 M
See results in table using similar equation to solve
Average molarity of EDTA = (.0092M + .0090M + .0010M) / 3 = .0064 M
Data Collected
Absolute Error = Absolute (True Value – Experimental Value)
% RE = (ABS Error/True Value) × 100
Abs Error = [(.01031mol/L- .0064M)] = 0.00391 M
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% Rel. Error (.00391M/.01031 mol/L) x 100 = 37.92
Accuracy= 100-37.92
= 62.08%
Precision
Standard Deviation = [.0064M-.0092M)^2 +(.0064-.0090M)^2]/3
= .000014587
Part 2
Concentration of EDTA
Trial 2
Final buret reading
5.73mL
4.73mL
Initial buret reading
0.00mL
0.00mL
Volume used
5.73mL
4.73mL
Molarity of Ca^2+ in the sample
.0365 M
.0301 M
Average molarity of Ca^2+ in the sample
.0333 M
.0333 M
Average water hardness in CaCO3 mmol/L
.0000333 mmol/L
.000333 mmol/L
Average water hardness in CaCO3 ppm
.00333 ppm
.00333 ppm
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Trial 1
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Average Molarity of Ca^2+ = (Molarity of Ca^2+ of trial 1 + Molarity of Ca^2+ of trial 2)/ 2
= (.0365M + .0301M)/2
=.0333M
Converted into mmol/L to calculate water hardness
Molarity/1000 - mmol/L
(.0333M)/1000 = .0000333 mmol/L
Converted into ppm to calculate water hardness
Molarity x molar mass of CaCO3 = g/L = ppm
(.0000333 mmol/L)(100.0869 g/mol) = .00333 ppm
Precision
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Standard Deviation = [(.0333M- .0365M)^2+ (.0333M - .0301M)^2] / 2
= .0000124
Concentration of EDTA
Trial 1
Trial 2
Final buret reading, EDTA
10.53 mL
13.33mL
Initial buret reading, EDTA
.000mL
.000ml
Volume used, EDTA
10.53ml
13.33ml
Molarity of Ca^2+ in the sample
.0674 M
.0853 M
Average molarity of Ca^2+ in the sample
.0764
.0764
Average water hardness in CaCO3 mmol/L
0000764 mmol/L
.0000764 mmol/L
Average water hardness in CaCO3 ppm
.00764 ppm
.00764 ppm
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Calculations
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Molarity of Ca^2+ in sample A
(.0064M x .01053 L) / .0010 L
= .0674 M
See results in table using similar equation to solve
Average Molarity of Ca^2+
(0.674 M + .0853 M)/ 2
= .0764 M
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Converted into mmol/L to calculate water hardness
Molarity/1000 - mmol/L
(.0764 M) /1000 = .0000764 mmol/L
Converted into ppm to calculate water hardness
Molarity x molar mass of CaCO3 = g/L = ppm
(.0000764 mmol/L)(100.0869 g/mol) = .00764
Precision
Standard Deviation = [(.0764M - .0674M)^2 + (.0764M-.0853M)^2] / 2
= .000080105
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Conclusion
The concentration of hardness of water was determined using the complexometric titration.
Complexometric titration is one of the best ways of measuring total water hardness. We found
the total water hardness in a water sample and an unknown sample of tap water. The total
hardness was discovered by the amount of calcium and magnesium dissolved in the water. The
determination of the water hardness provided us with the understanding on how water hardness
can differ. In addition, what is considered and not considered hard water. The overall accuracy of
the experiment was calculated as mid- accurate, stating that there were errors throughout the
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experiment.
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Reference
Complexometric Titration.
www.srmuniv.ac.in/sites/default/files/downloads/Complexometric_Titration.pdf.
Libretexts. “9.3: Complexation Titrations.” Chemistry LibreTexts, Libretexts, 17 Aug. 2018,
chem.libretexts.org/Textbook_Maps/Analytical_Chemistry/Book
%3A_Analytical_Chemistry_2.0_(Harvey)/09_Titrimetric_Methods/9.3%3A_Complexatio
n_Titrations.
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Libretexts. “Case Study: Hard Water.” Chemistry LibreTexts, Libretexts, 8 Sept. 2018,
chem.libretexts.org/Textbook_Maps/Inorganic_Chemistry/Supplemental_Modules_(Inorga
nic_Chemistry)/Descriptive_Chemistry/Main_Group_Reactions/Case_Study
%3A_Hard_Water.
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