Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration
Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration
Quantitative Determination of Total Hardness in Drinking Water by Complexometric EDTA Titration
ABSTRACT
The drinking water Hidden Spring, with NH3NH4+ solution as buffer and
EBT
(Eriochrome
Black
T)
as
indicator,
is
titrated
with
EDTA
(ethylenediaminetetraacetic acid) solution by complexometric titration to
determine its total hardness or total mineral content. The EDTA solution is
standardized by the primary standard CaCO 3 with 99.9% purity. The resulting
average molar concentration of EDTA after three (3) trials is 0.0059 M, while the
calculated titer is 0.593 mg CaCO3 per mL of EDTA. The endpoint when the water
sample was titrated was observed when the color changed from wine-red to
clear-blue. The theoretical total hardness of the drinking water is 206.559 ppm
CaCO3 which was obtained from its calcium and magnesium content of 25 ppm
and 35 ppm respectively, while 191.74 ppm is the average total hardness
obtained experimentally. Both data classify Hidden Spring under very hard
water according to the water hardness scale. The relative standard deviation
was 264.92 ppt and the confidence interval was 191.74 126.11 ppm CaCO 3.
The theoretical and experimental values of the total hardness of the drinking
water differ by 7.18%.
INTRODUCTION
Ethylenediaminetetraacetic
acid
(EDTA), the used titrant in this
experiment, is an aminocarboxylic
acid and a chelating agent, which
means it has two (2) or more groups
capable of complexing with a metal
ion. The completely protonated form
of EDTA, H6Y2+ (Figure 1), is a
hexaprotic weak acid.
In
this
experiment,
the
total
hardness of the drinking water
Hidden Spring was determined by
complexometric EDTA titration.
A complexation reaction involves
reactions
of
the
undissociated
neutral molecules and the complex
ions in the solution; the formed
complex must be very soluble. In a
complexometric titration, a solution
containing free metal ions is titrated
with a solution of a ligand an ion
capable of providing more than one
pair of electrons to a metal ion.
Ligands are also called chelating
agents which capture metal ions
present in water. When a chelating
agent is involved, the complex is
called a chelate. A titration with a
chelating
agent
is
called
a
chelometric titration a specific type
of complexometric titration.
METHODOLOGY
Table 1. Values of 4 for EDTA at
different pH
pH
pH
2.0
2.5
3.0
4.0
5.0
6.0
3.7 x 10-14
1.4 x 10-12
2.5 x 10-11
3.6 x 10-9
3.5 x 10-7
2.2 x 10-5
7.0
8.0
9.0
10.0
11.0
12.0
4
4.8
5.4
5.2
3.5
8.5
9.8
x
x
x
x
x
x
10-4
10-3
10-2
10-1
10-1
10-1
Tria
l
VCa(II) std.
Net
VEDTA
10 mL
7.9 mL
10 mL
8.8 mL
10 mL
8.2 mL
[EDTA]
0.0062
M
0.0056
M
0.0060
M
complex
Kf
Ca-EDTA
5.0 x 1010
Mg-EDTA
4.9 x 108
Mg-EBT
1.0 x 107
Ca-EBT
2.5 x 105
Ca-EDTA > Mg-EDTA > Mg-EBT > CaEBT
Likewise, the Mg-EDTA complex has a
greater Kf than the Mg-EBT complex
Trial
Vsample
Net VEDTA
Total
Hardness, ppm
Average
50 mL 50 mL 50 mL
14.0
21.1
13.4
mL
mL
mL
166.0
250.2
158.9
4
5
2
191.74 ppm
Water
Hardness
ppm CaCO3
Soft
Moderately soft
Moderately hard
Hard
Very hard
0 20
20 60
61 120
121 180
greater than 180
Since
the
theoretical
and
experimental total hardness of the
drinking water Hidden Spring are
both greater than 180, the drinking
water can be classified as very hard
water according to the water
hardness scale (Table 5). This
means that Hidden Spring is suitable
to be a drinking water since it
contains enough amount of minerals
(particularly Ca2+ and Mg2+ ions).
Some errors may be coming from
overtitration of the solution, a late
detection of the color change
because of its late transition,
REFERENCES
[3]
Petrucci,
Ralph.
General
Chemistry: Principles and Modern
Applications. 10th ed. Ontario City:
Pearson Canada, 2011. 1426. Print.
[4]
Southway,
Dr.
C.
"Complexometric Titrations EDTA."
APCH231: Chemical Analysis. Dr. C.
Southway. Web. 6 Apr. 2015.
<http://cheminnerweb.ukzn.ac.za/lib
raries/apch231_h_govender_s_notes/
apch231_edta.sflb.ashx>.
[5] De La Camp, Ulrich, and Oliver
Seely.
"Complexometric
Ca
Determination." Complexometric Ca
Determination.
Web.
<http://www.csudh.edu/oliver/che23
0/labmanual/calcium.htm>.
[6]
Harvey,
David.
"Titrimetric
Methods
of
Analysis.
Modern
Analytical
Chemistry.
1st
ed.
McGraw-Hill Companies, 2000. 816.
Print.
APPENDIX A
DATA SHEET
EXPERIMENT 7
QUANTITATIVE DETERMINATION OF TOTAL HARDNESS IN DRINKING
WATER
BY COMPLEXOMETRIC EDTA TITRATION
Standardization of EDTA solution
Standard CaCO3 solution
Weight of pure CaCO3: 0.4917 g
Formula mass of 1 standard:
100.086
% Purity of pure CaCO3: 99.9%
Volume of final solution: 100 mL
Working standard CaCO3 solution
Volume of standard CaCO3
solution: 50 mL
Volume of final solution: 88.15 mL
Trial
10
10
10
10.6
11.5
19.7
2.7
2.7
11.5
7.9
8.8
8.2
M EDTA
0.0062
0.0056
0.0060
Average M EDTA
0.0059
Titer
0.593
Sample Analysis
Brand of mineral water used: Hidden Spring
Mineral Water content
ppm Ca: 25
ppm Mg: 35
Trial
Volume of water sample, mL
Final volume of EDTA, mL
Initial volume of EDTA, mL
Net volume of EDTA, mL
1
50
15.5
1.5
14.0
166.04
ppm
2
3
50
50
26.7
28.9
5.6
15.5
21.1
13.4
250.25
158.92 ppm
ppm
191.74 ppm CaCO3
264.92 ppt
(191.74 126.11) ppm CaCO3
Total Hardness
Average
RSD
Confidence Interval
APPENDIX B
SAMPLE CALCULATIONS AND WORKING EQUATIONS
100.086
g
1
mol
CaCO
1.
2+
1 L 10
100 mL stock Ca
M Ca2+=
0.00491 M Ca2+
2.
M1 EDTA =
7.9 mL EDTA
0.0062 M1 EDTA
3.
M2 EDTA =
8.8 mL EDTA
0.0056 M2 EDTA
4.
M3 EDTA =
8.2 mL EDTA
0.0060 M3 EDTA
5.
6.
M1 +M2 +M3
=5.924932626 x 10-3 0.0059 M EDTA
M EDTA =
g
100.086 g
mg CaCO3
g
0.593 0.593
L
mL EDTA
Sample Analysis
1. ppm CaCO3 from ppm Ca and ppm Mg
Ca
CaCO3
1 mol MgCO
3 1 mol CaCO3 100.086 g
1 mol
Mg
CaCO3 =144.1271343 ppm CaCO3
24.305
g
1
mol
Mg
1
mol
CO
1
mol
2.
Total hardness = ppm CaCO3 from ppm Ca + ppm CaCO3 from ppm Mg =206.5591419 206.559 ppm
3.
g
0.593002806 L 14 mL EDTA 1000 mg
ppm1 CaCO3
166.0407857 ppm1 CaCO3
50 mL sample
1g
4.
g
0.593002806 L 21.1 mL EDTA 1000 mg
ppm2 CaCO3
250.2471841 ppm2 CaCO3
50 mL sample
1g
5.
g
0.593002806 L 13.4 mL EDTA 1000 mg
ppm3 CaCO3
158.924752 ppm3 CaCO3
50 mL sample
1g
6.
ppm CaCO3
7.
8.
3 - 1
RSD
9.
Additional parameters
1.
206.5591419 - 191.7375739
100% 7.175459708% 7.18%
206.5591419
percent error
APPENDIX C
ATTACHED DATA SHEET