Exp. 15 – video
(time: 21:39 minutes)
Exp. 15: Volumetric Analysis: Total Hardness of Water by
EDTA
Hardness – is defined in terms of the capacity of cations in
the water to replace the sodium or potassium ions in soaps
and form sparingly soluble products (insoluble).
• Most multiple charged cations (>+1)
• Natural waters – most abundant Ca2+ and Mg2+
Hardness for most part is expressed in terms of
the concentration of calcium and magnesium in
the sample.
Why is hardness important?
Measure of quality of water for household and industrial
use (hard waters can precipitate calcium carbonate which
can clog filters, pipes, etc.)
Water Hardness is typically determined by an EDTA
titration buffered at a pH of 10 (condition for maximum
complexing).
EDTA – ethylenediaminetetraacetic acid
(1:1 mol ratio between divalent metal (Ca2+, Mg2+) and EDTA)
There is an order of stability of metal complexes formed
with EDTA. Calcium tends to form a more stable complex
with EDTA than magnesium.
This means if Ca2+ is present in a water sample, the
calcium EDTA complex will form before any Mg2+
forms a complex with EDTA
How will we determine eq pt?
Ca2+ - eriochrome black T complex – dark red (maroon)
uncomplexed black T – blue color
one drop red  blue (lack of red tint)
EDTA is standardized with primary standard CaCO3
To simplify calculations with hardness we use
CaCO3 titer = mg CaCO3
mL EDTA
After EDTA is standardized, we can use the ratio to
determine the hardness of a water sample by titrating with
EDTA and using the CaCO3 titer.
Total Hardness is typically reported in ppm CaCO3
ppm -parts per million
ppm CaCO3 = mg CaCO3
L water sample
aq: 1ppm = 1mg
1L
Standardization
Start experiment on page 104 - 2nd paragraph of
Standardization (skip blank run)
Changes: Use 10.00 mL of 0.01000 M CaCO3 (Standard
solution) instead of 25 mL.
2 mL pH 10 buffer instead of 5 mL
10 mL deionized water instead of 25 mL
4-6 drops black T indicator (note: add indicator
from start of titration)
Do a minimum of 3 runs – want good precision
avg titer ± s mg CaCO3
mL EDTA
end pt approx 8-12 mL
Standardization Calculation
Goal: mg CaCO3
mL EDTA
Note: molarity of EDTA not required
Unknown Water Titration
Do experiment as written except add indicator from start
and try to make first run count (don’t do rapidly).
Do a minimum of 3 runs – want good precision
Unknown end pt approx 5-12 mL
avg ppm ± s ppm CaCO3 of unknown water sample
Also must report based on the water hardness scale the
classification of your unknown water sample.
soft, slightly hard, moderately hard, hard, very hard
(must find scale on own and submit in report discussion)
Unknown Water Sample Calculation
Goal: ppm CaCO3 = mg CaCO3
L H2O sample
Amount of chemicals to obtain in small beaker per group:
EDTA – 150 mL
pH buffer – 40 mL (keep under hood until you need it)
0.01000 M CaCO3 standard solution – 50 mL
Unknown water – 200 mL
Additional question: Hard water is a major issue in which
science is used solve the problem. After your conclusion in
exp 15’s lab report, you will discuss another major issue in
which science is involved: Climate change is thought to be
accelerated by carbon dioxide from coal and natural gas
fueled power plants. Due to the increasing need for
electricity, other power sources will be required to lessen the
emissions from fossil fuels. List three possible alternative
sources for the production of electricity. For two of your
responses, describe an advantage and disadvantage
between it and current coal and natural gas power plants.