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Quantitative Analytical Chemistry
Joseph Orlando
Lab #6: EDTA Titration Of The Hardness Of Water
Purpose:
The purpose of this lab was to design an experiment that may help determine the hardness of
tap water. We detected [Ca2+] to determine the hardness of water.
Procedure:
Standardization of EDTA
1. Calibrate pH meter using solutions with known pH.
2. Prepare a 0.001 M EDTA solution
3. Prepare a Ca2CO3 solution by reaction 0.01g of Ca2CO3 with 100mL of 0.1 M HCl. Add 3mL of
this solution, 5mL of ammonia buffer and a few drops of calmagite indicator into a 250mL
Erlenmeyer flask.
4. Titrate the solution with EDTA until a blue endpoint is reached. Repeat titration three times
until three good results are obtained.
Tap Water Titration
1. Add 50mL of tap water, 5mL of ammonia buffer and a few drops of calmagite indicator into
a 250mL Erlenmeyer flask. Titrate this solution with standardized EDTA. Repeat titration
three times until three good results are obtained.
Titration of Tums
1. Crush one tums tablet, place into a 100mL volumetric flask and add 5mL of ammonia buffer.
Dilute to the appropriate mark with 0.1M HCl.
2. Add 5mL of the tums solution prepared in the previous step into a 250mL Erlenmeyer flask
with 2mL of ammonia buffer and a few drops of calmagite indicator. Titrate this solution
with standardized EDTA until endpoint. Repeat titration three times until three good results
are obtained.
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Data:
Standardization of EDTA
Amount (mL) of EDTA titrated
Molarity of EDTA
Trial #1
9.55
0.0099
Trial #2
9.30
0.0102
Trial #3
9.50
0.0099
Average
9.45
0.100
Tap Water Titration
Ca+ Concentration (M)
ppm
Trial #1
Amount(mL) of EDTA
titrated
7.90
0.00158
63.323
Trial #2
6.90
0.00138
55.307
Trial #3
6.00
0.00120
48.094
Average
6.93
0.00139
55.575
TUMS Titration
Amount(mL) of EDTA Titrated
Ca2+ in Sample (%)
Trial #1
0
0
Trial #2
16.20
0.42%
Trial #3
14.20
0.37%
Average
15.20
0.395%
Standard Deviation
±0.035
Relative Standard Deviation
8.86%
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Equations:
Mass EDTA Needed
0.01 mol EDTA
372.24 g EDTA
× 0.500 L ×
= 0.7306 g EDTA
1L
mol
M EDTA
0.3161 g CaCO3 ×
1 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3
1
0.003 𝐿
1
×
×
×
= 0.0099 𝑀
100.09 𝑔 𝐶𝑎𝐶𝑂3
0.100 𝐿 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
1
. 00955 𝐿 𝐸𝐷𝑇𝐴
Ca+ Concentration in Tap Water
0.0100 M EDTA ×
0.0079 𝐿 𝐸𝐷𝑇𝐴 𝑎𝑑𝑑𝑒𝑑 1 𝑚𝑜𝑙 𝐶𝑎 +
1
×
×
= 0.00158𝑀 𝐶𝑎 +
1
1 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 0.05 𝐿
Hardness of Water
0.00158 𝑀 𝐶𝑎 + ×
40.078 𝑔 𝐶𝑎 +
1 𝑚𝑔
×
= 211.61 𝑝𝑝𝑚
1 𝑚𝑜𝑙 𝐶𝑎 +
0.001 𝑔
Claimed Amount of Ca2+ in Tablet
0.3161 𝑔 𝐶𝑎𝐶𝑂3 ×
1 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3
1 𝑚𝑜𝑙 𝐶𝑎 2 + 40.078 𝑔 𝐶𝑎 2 +
×
×
= 0.1266 𝑔 𝐶𝑎 2 +
100.09 𝑔 𝐶𝑎𝐶𝑂3 1 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3
1 𝑚𝑜𝑙
0.1266 𝑔 𝐶𝑎 2 +
× 100% = 8.14%
1.5561 𝑔 𝑡𝑎𝑏𝑙𝑒𝑡
Amount Ca2+ in Sample
0.0100 𝑀 𝐸𝐷𝑇𝐴 ×
0.0162 𝐿 𝐸𝐷𝑇𝐴 1 𝑚𝑜𝑙 𝐶𝑎 2 + 40.078 𝑔 𝐶𝑎 2 +
×
×
= 0.0065 𝑔
1
1 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴
1 𝑚𝑜𝑙 𝐶𝑎 2 +
% Ca2+ in Sample
0.0065 𝑔 𝐶𝑎 2 +
× 100% = 0.42%
1.5561 𝑔 𝑡𝑎𝑏𝑙𝑒𝑡
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Conclusion:
The purpose of this experiment was to devise an experimental procedure that would allow us to
accurately determine the hardness of tap water. Hard water is defined as water that contains
many positively charged ions (cations). The procedure that I improvised measured the
concentration of Ca2+ in the tap water to measure hardness. A standard titrant, EDTA, was first
prepared to titrate against several samples of tap water. EDTA or Ethlenediaminetetraacetic acid
is a chelating agent that sequesters free metal ions (such as Ca2+). It was experimentally
determined that the average hardness of the tap water was 55.575 ppm. According to the
United States Geological Survey Water-Quality Information this tap water is considered to be
soft. TUMS was titrated to test the validity of the EDTA being used. The claimed amount of Ca2+
in a TUMS tablet is 8.14% whereas the experimentally determined value was 0.24%, a difference
of 8.11%. This leads me to believe that there was human error in the experiment somewhere
that lead to this large difference.