Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
Statistical analysis of EDTA titration vs. ICP-AES in the
determination of water hardness
Annika Larson
Department of Chemistry, Concordia College, 901 8th St S, Moorhead, MN 56562
Abstract
The determination of water hardness (or the concentrations of Ca2+ and Mg2+ ions
present in drinking water) is not a new phenomenon. In this experiment two different
methods of water hardness determination were used to find the concentrations of Ca2+ and
Mg2+ ions in multiple drinking water samples. The two methods were EDTA titration and ICPAES analysis. The results from the two methods were statistically compared to determine if
the data yielded by the two methods is statistically the same.
Introduction
Water hardness, or the presence of Ca2+ and Mg2+ ions in tap water, is not a new
concept to those who use running water on a regular basis to cook food, wash dishes, take
showers or flush the toilet. It is the “hardness” of the water that causes a white build-up on
tea kettles, little white spots on dishware, and rings around the bathtub and toilet bowl.1
There are many ways to determine the concentrations of calcium and magnesium ions
in tap water. Methods include titrations as well as ion analysis using various analytical
instruments. Complexometric titrations using ethylenediaminetetraacetic acid or EDTA is one
way to determine the concentration of Ca2+ and Mg2+ ions. EDTA binds to metal ions like Ca2+
and Mg2+ in a 1:1 ratio making the concentration calculations quite simple. The color change
in complexometric titrations is achieved using an indicator (like Eriochrome Black T) that is
one color when it is bound to a metal ion and another when it is unbound. Also the indicator
used binds the metals less strongly than EDTA so as the EDTA is added it removes the bound
metal ions from the indicator causing the color change.1
Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is another useful
method for the simultaneous determination of metal ion concentrations. A small sample of a
prepared solution is vaporized and passed through an argon plasma. The plasma excites the
atoms passing through it. As the atoms come back down to their ground states they emit light
that is detected and then analyzed to determine the concentrations of the specified ions. The
use of an internal standard is the most common analysis method.2
While both of these methods are capable of determining the concentration values of
Ca2+ and Mg2+ ions in water samples, the use of the ICP-AES is much more expensive and
requires a certain amount of knowledge about the operating software. The machine itself
costs tens of thousands of dollars and the argon used to make the plasma is also very
expensive. Titrations and the necessary calculations that accompany them are quite simple.
Also the materials and chemicals necessary for the titrations are much less expensive. What
isn’t known is if the results determined by each method are statistically the same. In this
experiment these two methods, EDTA titration and ICP-AES, will be compared to determine if
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Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
the two methods give statistically similar values for the Ca2+ and Mg2+ ion concentrations in
tap water samples.
Experimental
EDTA Titration:
Solution Preparation:
pH 10 Buffer: To create a pH 10 buffer solution 142 mL of 28 wt% aqueous NH3 and 17.5 g of
NH4Cl were added to a 250-mL volumetric flask. The flask was then filled to volume with
deionized water.3
EDTA solution: Na2H2EDTA.2H2O was dried for 1 hour and allowed to cool in a desiccator.
Approximately 0.6 g of the dried Na2H2EDTA was weighed and placed in a 500-mL
Erlenmeyer flask with approximately 400 mL of deionized water. The solution was heated
until all of the solid had dissolved. Once the solution had cooled it was transferred into a 500mL volumetric flask and diluted to volume with deionized water.3
Sample Preparation:
Determination of Ca2+ and Mg2+ ions: Four 50.00-mL samples of drinking water were pipetted
into four 250-mL Erlenmeyer flasks. 3 mL of pH 10 buffer was added to each sample as well
as 6 drops of Eriochrome black T indicator.3
Determination of Ca2+ ions: Four 50.00-mL samples of drinking water were pipetted into
separate 250-mL Erlenmeyer flasks. 30 drops of 50 wt% NaOH was added to each flask. Each
flask was then allowed to mix for 2 min. After the solutions had been allowed to mix
approximately 0.1 g of solid hydroxynapthol blue was added to each flask. Also a 50-mL
sample of the deionized water was treated the same way as a control.3
Titration:
The EDTA solution prepared above was poured into a 50-mL buret. The first of the sample
unknowns was placed beneath the buret and the solution was titrated until there was a color
change from wine red to blue for the measurement of Ca2+ and Mg2+ ions, or from pink to blue
for the measurement of Ca2+ ions. The ending volume was recorded. This procedure was then
repeated for the other unknowns and the deionized water control. The volume used to titrate
the control (if any) was then subtracted from the volumes used to titrate the hydroxynapthol
blue solutions.3
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Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
ICP-AES:
Solution Preparation:
1% HNO3 Solution: Approximately 200 mL of deionized water was placed into a clean 1-L
plastic bottle. In the hood 5 mL of concentrated HNO3 was measured with a 10-mL graduated
cylinder and was then poured into the 1-L plastic bottle with the deionized water. The
solution was mixed and then the plastic bottle was filled to approximately 500 mL with
deionized water.2
Ca2+ Standard Solution: A 0.0917-g sample of CaCl2 was transferred into a 500-mL volumetric
flask. Approximately 300 mL of deionized water was added and mixed until the solid had
dissolved. The flask was then diluted to volume with deionized water.2
Mg2+ Standard Solution: A 0.1238-g sample of MgSO4 (Epsom Salt) was transferred into a 500mL volumetric flask. Approximately 300 mL of deionized water was added and mixed until
the solid had dissolved. The flask was then diluted to volume with deionized water.2
Standard Addition Preparation:
Five 50-mL volumetric flasks were obtained and labeled 1, 2, 3, 4, and S. Using a 10-mL glass
pipet 10.0 mL of the unknown was transferred into each of the five flasks. Using a 2-mL glass
pipet 2.00, 4.00, 6.00, and 8.00 mL of the Ca2+ standard was added to flasks 1-4 respectively.
The same was done for the Mg2+ standard. All of the flasks were then diluted to volume with
the 1% HNO3 solution prepared above.2
Instrument Preparation and Analysis:
The prepared solutions were analyzed using a Varian 715-ES ICP Optical Emission
Spectrometer. The argon source was set at 80 psi. The data was acquired using ICP Expert II
software with a standard addition method set to measure the concentrations of both calcium
and magnesium simultaneously. The tubing was set up and a blank was run. The four
solutions with added concentration were run and then the unknown sample was run twice. A
standard curve was generated to determine the concentration of the unknown sample. 2
Results and Discussion
Titrations:
The concentrations of Ca2+ and Mg2+ ions were calculated using the volume of EDTA added
and the molarity of the EDTA solution. In Table 1 the average concentration values in parts
per million (ppm) are listed with their standard deviations.
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Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
Table 1. Concentrations of Ca2+ and Mg2+ ions in various water samples as determined
by EDTA titration.
Source
Moorhead Tap
Fargo Tap
Detroit Lakes Tap (well)
Lab DI Tap
n
4
4
4
4
Concentration of Ca2+ (ppm)
23 ±1
46.3 ±0.6
17.6 ±0.4
0 ±0
Concentration of Mg2+ (ppm)
14.4 ±0.8
13.9 ±0.2
24.9 ±0.2
0 ±0
ICP-AES:
Data produced by the ICP-AES using standard additions was used to produce calibration
curves for both ions for each water sample. In Figure 1 the calibration curves that were
generated are shown.
a)
Calcium Calibration Curve
(Moorhead)
Magnesium Calibration Curve
(Moorhead)
2000000
y = 221124x + 1E+06
R² = 0.9998
1000000
0
0
2
4
6
8
Int. (c/s)
Int. (c/s)
3000000
600000
y = 36824x + 116958
400000
R² = 0.9998
200000
0
0
10
Added Concentration (ppm)
b)
Calcium Calibration Curve
(Fargo)
4000000
4
6
8
10
Magnesium Calibration Curve
(Fargo)
600000
y = 222980x + 2E+06
R² = 0.9986
Int. (c/s)
Int. (c/s)
6000000
2
Added Concetration (ppm)
2000000
0
y = 37814x + 112328
R² = 0.9996
400000
200000
0
0
2
4
6
8
10
0
Added Concetration (ppm)
2
4
6
8
Added Concentration (ppm)
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Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
c)
Calcium Calibration Curve
(Detroit Lakes)
Magnesium Calibration Curve
(Detroit Lakes)
600000
2000000
Int. (c/s)
Int. (c/s)
3000000
y = 219237x + 812443
R² = 0.9997
1000000
0
400000
y = 36052x + 226473
R² = 0.9997
200000
0
0
2
4
6
8
10
0
Added Concentration (ppm)
d)
2000000
400000
1500000
300000
y = 224792x + 25842
R² = 1
500000
0
0
2
4
6
4
6
8
10
Magnesium Calibration Curve
(DI Tap)
Int. (c/s)
Int. (c/s)
Calcium Calibration Curve
(DI Tap)
1000000
2
Added Concentration (ppm)
8
y = 36112x + 3981.1
R² = 0.9997
200000
100000
0
0
10
2
4
6
8
10
Added Concentraion (ppm)
Added Concentration (ppm)
Figure 1: Calibration curves determined by ICP-AES for the standard additions of calcium
and magnesium in a) Moorhead tap water, b) Fargo tap water, c) Detroit Lakes well
water, and d) DI water.
The concentrations of Ca2+ and Mg2+ were determined using calibration curves generated by
the ICP-AES. The values determined by the ICP-AES were averaged and listed in Table 2 along
with their standard deviations.
Table 2. Ca2+ and Mg2+ concentrations in various water samples as determined by ICP-AES.
Source
n Concentration of Ca2+ (ppm) Concentration of Mg2+ (ppm)
Moorhead Tap
Fargo Tap
Detroit Lakes Tap (well)
Lab DI Tap
2
2
2
2
24.247 ±0.005
44.72 ±0.03
18.06 ±0.05
-.084 ±0.002
15.6 ±0.3
14.64 ±0.04
31.189 ±0.002
-.090 ±0.002
Once the average values and the standard deviations for each water sample were calculated
they were compaired to see if the values were within the range designated by their standard
deviations. Figures 2 and 3 compare the values determined by each method for each of the
water samples with the standard deviation bars to show the ranges.
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Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
Water Sample
"DI water"
Detroit
Lakes
ICP-AES
EDTA
Moorhead
Fargo
-10
0
10
20
30
40
50
Calcium Ion Concentration (ppm)
Figure 2. Comparison of the concentrations of Ca2+ ions with standard deviation bars in
various water samples as determined by EDTA titration and ICP-AES.
Water Sample
"DI water"
Detroit
Lakes
ICP-AES
EDTA
Moorhead
Fargo
-5
0
5
10
15
20
25
30
35
Magnesium Ion Concentration (ppm)
Figure 3. A comparison of the concentrations of Mg2+ ions with standard deviation bars in
various water samples as determined by EDTA titration and ICP-AES.
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Concordia College Journal of Analytical Chemistry 3 (2012), 40-46
Conclusions
The results from the experiment appear to suggest that the values determined by both
methods are statistically the same. The results appear to be within the ranges set by the
standard deviations with the exception of the magnesium concentration in the Detroit Lakes
sample. More runs of the experiment as well as more sample would likely make it more clear
if the results obtained by both methods are statistically the same. Also water samples that
have been treated wth a water softener like the well water sample from Detroit lakes should
be omitted from further experiments because it appears that it may have an effect on the total
ion concentration EDTA titration.
References
1) DuPre, D.B. Complexometric Titrations: Competition of Complexing Agents in the
Determination of Water Hardness with EDTA. J. Chem. Educ., 1997, 74 (12), p 1422.
http://pubs.acs.org.cordproxy.mnpals.net/doi/pdfplus/10.1021/ed074p1422(Accesse
d (Accessed 4/5, 2012)
2) Jensen, M.B. Concordia College Analytical Chemistry Laboratory Manual 2011.
http://www.cord.edu/dept/chemistry/analyticallabmanual/
3) Harris, D.C. Quantitative Chemical Analysis, 8th Edition: EDTA Titration of Ca2+ and
Mg2+ in Natural Waters. 2010, 8, p 58.
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