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Ruth Pontoriero
CHM 337: Quantitative Analytical Chemistry
Lab #6: EDTA Titration of the Hardness of Water
Introduction:
This lab used a complexation titration, involving a metal-ligand complexation, to
determine overall hardness and calcium concentration of tap water. EDTA was used as a titrant.
EDTA is a weak acid with 6 binding sites: four carboxylate groups and two amino groups.
Methods:
A pH meter was calibrated. In this case, the pH meter was calibrated such that actual pH
10 yielded a reading of pH 9.15. 1M HCl was added to an ammonium buffer until the pH
reached the calibrated pH of 10. A 0.1 M EDTA solution was made by adding 1.4781g of EDTA
to 490mL distilled water and 10mL of pH adjusted ammonium buffer. A 0.1 M HCl was made
and 0.5089g of CaCO3 was added. A titration was performed using 3mL CaCO3 solution and
5mL buffer using Calmagite as the indicator and EDTA as the titrant. This was repeated for a
total of four trials. This entire process was repeated using 3mL of tapwater instead of 3mL
CaCO3 for a total of three trials.
A solution was made using a 1.9828g antacid tablet which, according to the
manufacturer, contained 0.500g CaCO3, and 10mL 1M HCl. A titration was performed using
3mL of the antacid solution and 5mL buffer with Calmagite as the indicator using EDTA as the
titrant. This was repeated for a total of three trials.
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Results and Calculations:
Titration CaCO3
EDTA (mL)
Molarity EDTA
Mean molarity
SD molarity
Trial #1
25.0
0.00610
0.00732
0.000814
Trial #2
19.8
0.00770
Titration Tapwater
EDTA (mL)
[Ca2+]
Ca2+ (ppm)
Average [Ca2+]
SD [Ca2+]
Average Ca2+ ppm
SD Ca2+ ppm
4.0
0.00976
391
0.00976
0.00024
391
9.50
Titration Antacid
EDTA (mL)
Average EDTA (mL)
SD EDTA (mL)
Mass Ca2+ (g)
Percent mass Ca2+ (%)
Mean % mass Ca2+
SD % mass Ca2+
20.1
19.9
0.208
0.120
6.05
5.97
0.0764
Trial #3
Trial #4
19.8
0.00770
Trial #1
19.6
0.00778
Trial #2
4.1
0.0100
401
Trial #1
Trial #3
3.9
0.00952
382
Trial #2
Trial #3
19.8
19.7
0.118
5.95
0.117
5.90
Mass of EDTA needed for solution:
0.500L ∗
0.01mol EDTA 292.14g EDTA
∗
= 1.4607g EDTA
1L
1mol EDTA
Molarity of CaCO3 solution:
0.5089g CaCO3 1000mL
1mol EDTA
∗
∗
= 0.0508M EDTA
100mL
1L
100.0869g EDTA
Molarity of EDTA:
0.0508M CaCO3 ∗ 0.003mL CaCO3 ∗
1
= 0.00610M EDTA
0.0250mL EDTA
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Concentration of Calcium in the tap water sample:
[Ca2+ ] = V EDTA ∗ M EDTA ∗
1 mol Ca2+
1 mol
∗
1 mol EDTA V water
= 0.004L EDTA ∗
0.00732mol EDTA 1 mol Ca2+
1
∗
∗
1L
1 mol EDTA 0.003L water
= 0.00976M
Hardness of water in ppm:
Hardness =
[Ca2+ ]
Molar mass 1000mg
40.087g Ca2+ 1000mg
∗
∗
= 0.00976M ∗
∗
1 mol
1g
1mol Ca2+
1g
= 391ppm
Expected mass of Ca2+ in antacid tablet:
Mass CaCO3 ∗
1mol CaCO3
1mol Ca2+ molar mass Ca2+
∗
∗
molar mass CaCO3 1mol CaCO3
1mol Ca2+
= 0.500g CaCO3 ∗
1mol CaCO3
1mol Ca2+ 40.087g Ca2+
∗
∗
100.0869g CaCO3 1mol CaCO3
1mol Ca2+
= 0.2003g Ca2+
Expected mass percent of Ca2+ in tablet:
mass Ca2+
0.2003g Ca2+
∗ 100% =
∗ 100% = 10.1% Ca2+
mass tablet
1.9828g tablet
Parts per solution:
0.2032g Ca2+
= 20.32 parts per solution
0.010L
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Mass of Ca2+ in tablet:
V EDTA ∗ M EDTA ∗
1mol Ca2+ molar mass Ca2+ 20.32 parts
∗
∗
1mol EDTA
1mol Ca2+
solution
= 0.0201L EDTA ∗ 0.00732M EDTA ∗
∗
1mol Ca2+ 40.087g Ca2+
∗
1mol EDTA
1mol Ca2+
20.32 parts
= 0.120g Ca2+
solution
Percent Ca2+ in sample:
mass Ca2+
0.120g Ca2+
∗ 100% =
∗ 100% = 6.05%
mass of tablet
1.9828g
Discussion:
As the pH lowers, the form of EDTA changes such that it has fewer binding sites. As the
pH is raised, more of the EDTA will be in the form which has six binding sites. The ammonia
buffer is used to maintain a constant pH during the course of the titration. Calcein is a
metallochromic indicator used to detect calcium. It is only useful at a high pH and therefore
could probably have been used in this experiment. The York Water Company lists the Calcium
content as being 23.6 ppm. The difference between this value and our calculated value may be
because of the place from which the water was collected or from the method of collection, or it
could be from experimental error. Soft water contains only sodium ions, therefore, according to
the values by the York Water Company, the water in York is somewhat hard. Our experimental
values show it to be considerably harder than reported, but those values may be inaccurate. The
calculated percentage of calcium in the Tums tablet was 10.1%. All three of our experimental
values (6.05%, 5.95%, and 5.90%) were less than this value.
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Conclusion:
Our experimental value of calcium concentration in the tapwater was considerably higher
than the reported value. Our experimental value of calcium concentration for the Tums tablet
was lower than the calculated value. These differences may be due to experimental error or they
may be accurate. One method to possibly improve accuracy would be the use of a different
indicator as the color change was a little ambiguous.