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51
Experiment 7
EDTA DETERMINATION OF TOTAL WATER HARDNESS AND CALCIUM
3 lab periods
Reading: Chapter 11, Quantitative Chemical Analysis, 8
th
Edition, Daniel C. Harris (7
th
Edition: Chapter 12).
Objective
This lab will introduce you to the concept of complexometric titrations. You will
learn how to standardize a solution of ethylenediaminetetraacetic acid (EDTA) and
how to determine the calcium and magnesium content of water. You may bring a tap
water sample from home to analyze. You will also be given a performance evaluation
sample.
Suggested Schedule
Lab 1 Prepare and standardize your calcium carbonate and EDTA solutions.
Lab 2 Analyze your water samples for total hardness and for calcium.
Lab 3 Finish your analyses.
Water quality is evaluated using a number of parameters, including total ionic content,
pH, total dissolved solids, organic compounds, and water hardness. Water hardness is a
measure of the concentration of all the polyvalent cations dissolved in the water. The
most common such cations are calcium and magnesium, although iron, strontium, and
manganese may contribute to water hardness. Water hardness is often defined as the sum
of the concentrations of Ca
2+
and Mg
2+
in water. “Hard” water typically contains high
concentrations of Ca
2+
and Mg
2+
, which react with the fatty acids in soap, causing them to
precipitate. “Soft” water, such as rainwater or water that has passed through a water
softener, has very little Ca
2+
and Mg
2+
.
Most waters contain more calcium than magnesium. The calcium usually comes from the
dissolution of calcium carbonate. Thus, water hardness is usually reported as milligrams
of calcium carbonate per liter of solution. The U.S. Geological Survey (www.usgs.gov)
provides the following general guidelines for classification of waters:
Soft: 0 to 60 mg/L hardness as CaCO
3
Moderately hard: 61 to 120 mg/L hardness as CaCO
3
Hard: 121 to 180 mg/L hardness as CaCO
3
Very hard > 180 mg/L hardness as CaCO
3
Both Ca
2+
and Mg
2+
can be determined by titration with ethylenediaminetetraacetic acid
(EDTA) at pH 10.
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52
HOOC-CH
2
CH
2
-COOH
NCH
2
-CH
2
N
HOOC-CH
2
CH
2
-COOH
Ethylenediaminetetraacetic Acid (EDTA, or H
4
Y)
The EDTA molecule can be represented as H
4
Y, where the four acidic hydrogen atoms
are those at the “ends” of the molecule. Approximately half of the EDTA dissolved at pH
l0 is in the form of HY
3-
and half is in the form of Y
4-
. The complexation reaction of
EDTA with either Ca
2+
or Mg
2+
can therefore be represented in either of the following
ways, where M
2+
represents the metal ion:
M
2+
+ HY
3−
MY
2−
+ H
+
M
2+
+ Y
4−
MY
2−
Standard EDTA solutions can be prepared directly from either disodium EDTA
(Na
2
H
2
Y) or disodium EDTA dihydrate ((Na
2
H
2
Y•2H
2
0).
The endpoints of EDTA titrations of Ca
2+
and Mg
2+
can be located with the
metallochromic indicator, Calmagite. This indicator forms a red complex with either Ca
2+
or Mg
2+
. The uncomplexed indicator can exist in the ionic forms H
2
In
–
, HIn
2–
, and In
3–
(red, blue, and orange, respectively). At a pH in the range 8.1 - 12.4, the blue HIn
2–
form
predominates; this form is in equilibrium with the red MIn
–
form when the metal M
2+
is
present:
M
2+
+ HIn
2–
MIn
–
+ H
+
(blue) (red)
Before the endpoint of the titration, the solution is red because of the excess metal ion. As
the EDTA titrant complexes more and more metal, the above equilibrium shifts to the
left. At the endpoint, the solution turns blue. The formation constants for the Calmagite-
magnesium and Calmagite calcium complexes are:
Calmagite-magnesium complex log K
f
= 5.69
Calmagite-calcium complex log K
f
= 3.67
EDTA forms a more stable complex with calcium (log K
f
= 10.65) than with magnesium
(log K
f
= 8.79). Thus, in solutions (such as natural water samples) that contain both
metals, EDTA reacts first with Ca
2+
, and, when all the Ca
2+
ions have been complexed,
with Mg
2+
. When all the free Mg
2+
has been complexed by EDTA, the remaining free
(uncomplexed) EDTA displaces the Calmagite from the red MgIn
–
complex. At the
endpoint, just enough EDTA has been added to displace all the Calmagite and the
solution turns blue because of the presence of HIn
2–
in solution.
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53
In an EDTA titration of natural water samples, the two metals are determined together.
To determine the concentration of each metal separately, we need to do an additional
measurement that is selective for one of the two metals. This can be done by raising the
pH to 12, which precipitates the magnesium as its hydroxide:
Mg
2+
+ 2OH
-
Mg(OH)
2
(s) pK
sp
= 9.2
The solid Mg(OH)
2
is not titrated, but the Ca
2+
, which remains in solution, is. To perform
this titration, Calcon, aka Eriochrome Blue Black R, is used as the indicator. Calcon
retains color better at pH 12 than does Calmagite. The endpoint is less precise, however,
so your estimate of Ca
2+
in the water will not be as reliable as your estimate of the total
Mg
2+
and Ca
2+
present.
The results of the first titration give the total moles of Mg
2+
and Ca
2+
in the sample. The
second titration gives the moles of Ca
2+
present in the sample. Obviously, the difference
between the two results is the moles of Mg
2+
present in the sample.
Prelaboratory Assignment
A 100.00-mL water sample was adjusted to pH 10 and titrated to the calmagite endpoint
with 10.87 mL of 0.0125 M EDTA solution. Then the pH of another 100.00-mL water
sample was adjusted to pH 12 and titrated with EDTA to the calcon endpoint. In this
second titration, 2.63 mL of titrant was required. Calculate the concentration of Mg
2+
and Ca
2+
in the water. Express your answers in mg/L of CaCO
3
and MgCO
3
,
respectively.
Apparatus
250- and 500-mL volumetric flasks
50 mL buret
3 to 4 250-mL Erlenmeyer flasks
10-mL graduated cylinder
25- or 50-mL volumetric pipet
Weighing bottle
250-mL plastic bottle
1 L plastic bottle
One or more types of pH indicator paper: need indicator paper that can be used to
indicate pH 2, pH 7, pH 10, and pH 12
Beakers of various sizes.
Droppers
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Chemicals
A sample of water from your home, or another source (~ 1 L).
pH 10 ammonia/ammonium chloride buffer (may be prepared by Lab Services)
Dissolve 16.9 g NH
4
Cl in 143 mL of ammonium hydroxide solution containing at
least 28% wt/wt NH
3
. Add 1.25 g magnesium salt of EDTA. Dilute to 250 mL
with DI water. Prepare this solution in the hood.
The following indicator solutions will be prepared for you:
Calmagite solution (0.1 g dissolved in 100 mL of water). This indicator must be
prepared fresh, as the quality rapidly deteriorates (within one day).
Calcon (Eriochrome Blue Black R) solution (0.2 g dissolved in 50 mL of methanol).
This indicator must be prepared fresh for this lab.
Disodium salt of EDTA or disodium EDTA dihydrate
Calcium carbonate primary standard (CaCO
3
)
Sodium hydroxide solution, 3 M. Prepare from a 50% wt/wt NaOH solution.
Sodium sulfide solution (5% wt /vol) in water (freshly prepared by Lab Services).
This step helps alleviate Fe interference.
Concentrated HCl
HCl solution (3 M). Prepare from concentrated (12 M) HCl solution.
Ice is useful to help cool your solutions after boiling.
Procedure
A Preparing a standard EDTA solution
1. Preparing the calcium carbonate primary standard.
Obtain ~ 0.1 1.0 g of calcium carbonate standard and dry at 110°C for ~1 hr, or to
constant weight. While you are waiting for it to dry, prepare your EDTA solution, as
in step 2. Once the calcium carbonate standard has dried and cooled, weigh 0.3 to 0.4
g (to the nearest 0.1 mg) by difference into a clean, dry, short-stemmed funnel set in
the mouth of a 500 ml volumetric flask. Tap the funnel gently to force the CaCO
3
into
the flask. Wash any remaining CaCO
3
into the flask using DI water. With the funnel
in place, add a small amount (< 2 mL) of concentrated HCl to the flask, washing any
remaining CaCO
3
into the flask with the HCl (Do this in the hood). Rinse the funnel
thoroughly with DI water and remove it from the flask. Swirl the flask until all the
CaCO
3
has dissolved. Dilute to the mark with DI water. Note: you are making a lot of
solution. You may wish to share this with others in your class.
2. Preparing your EDTA solution.
You will be using either the disodium salt of EDTA or disodium EDTA dihydrate
(M.W. 372.24 g/mol). Weigh ~ 0.9 g of EDTA. Transfer this into a 250 mL
volumetric flask (or Erlenmeyer flask, as you are going to standardize this solution)
and fill the flask ~halfway with DI water. Add 3-4 mL of the 3 M NaOH solution and
swirl to dissolve. This process might take ~15 minutes. You may warm the solution
gently to help the process along. Once the EDTA has dissolved, dilute to the mark
with DI water and mix thoroughly. Store in a 250 mL plastic bottle. Keep solution
capped when not in use.
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3. Standardizing your EDTA solution
a. Pipet 25.00 mL of your CaCO
3
standard solution into each of three 250 mL
Erlenmeyer flasks. Add ~1 ml of buffer solution using a 10 mL graduated
cylinder. The pH of your solution should be ~10 after the addition of buffer. Add
~2-3 drops of calmagite indicator solution. Titrate the solution until the last trace
of red color disappears upon addition of just a fraction of a drop of EDTA. The
final color change should be from a violet color to a pale blue. The change should
be fairly sharp. If it is not, it could mean that the indicator is old. Record each
endpoint volume to the nearest 0.01 mL.
b. Prepare a sample for a blank titration as follows:
Pipet a 25.00 mL sample of DI water into a clean 250 mL Erlenmeyer flask. Add
~ 1-2 mL of the buffer. Add a few drops of calmagite indicator solution. If the
solution turns blue, there is no measureable magnesium or calcium in the solution
and you will not have a blank correction. If the solution stays red or violet, titrate
with the EDTA solution until there is no trace of red or violet in your solution.
The color change on reaching the endpoint is very subtle. Ask the TA to help you
determine if you are near your endpoint. One thing that might help is to carry your
solution over to the window and look at it in natural light. The fluorescent lights may
make the solution have a pinkish tinge to your eye, even at the endpoint. Also, try to
reach the same color for each titration of your standards and unknowns. The
consistency of your technique will improve the precision of your measurements.
B Performance Evaluation
1. You will have a performance evaluation sample given to you by your TA or by Lab
Services. This sample will have a total volume of 300 mL and should be well shaken.
If you take more than 300 mL your lab grade will be decreased by 50%.
2. Use a volumetric pipet (either 2 x 50.00 mL or 4 x 25.00 mL) to place 100.00 mL of
your water sample in each of three 250-mL Erlenmeyer flasks. Add 5 - 10 drops of 3
M hydrochloric acid to each flask, or until the pH is ~2 (test using indicator paper),
and gently boil the solution for about 5 min. Dissolved carbon dioxide is removed
from the solutions during the boiling. The acid is added to convert dissolved
carbonate to carbon dioxide. Also prepare a solution for a blank titration.
3. Cool each solution to near room temperature. Use ice to speed things up. Add 3 M
sodium hydroxide solution dropwise to each flask until the pH is ~7 (use indicator
paper to test the pH). This may take anywhere from 5-15 drops of 3 M NaOH
solution. The hydrochloric acid which was added in step 2 is neutralized in this step.
4. Add 1 or 2 mL of pH 10 buffer and about 2-3 drops of calmagite indicator. The pH
should be ~10. Use indicator paper to check the pH. The solutions should themselves
be red in color.
5. Fill a 50-mL buret with the standardized EDTA solution and use the solution to titrate
each water sample to the endpoint. At the endpoint the titration solution changes from
red to blue. Record each endpoint volume to the nearest 0.01 mL.
6. Carry out a blank titration. Proceed as in steps 2 - 6, but using 100 mL of DI water as
a sample.
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Calculations (use report form provided)
1. Using your data from Part A, calculate the molarity of Ca
2+
for each replicate of the
performance evaluation sample. Find the average molarity and the standard deviation
for each sample.
Questions
Find the latest City of Moscow Annual Water Quality Report
(https://www.ci.moscow.id.us/pub_works/water/WaterQuality2010.pdf) to help you
answer the following questions:
1) Describe the source of your water sample. Use the USGS classification system to
describe the hardness of your water sample. Where does Moscow’s drinking
water come from? Based on the source of the water, explain the presence of the
magnesium and calcium ions in the water. If your sample is not Moscow tap
water, then explain the source of the hardness you determined in that water
sample.
2) What is a maximum contaminant level (MCL)? What is a maximum contaminant
level goal (MCLG)? What is an action level (AL)? Did any of the regulated
substances monitored by the City of Moscow exceed the MCL?
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Experiment 7 Name: _____________________________
EDTA Determination of Total Water Hardness Unknown #________
Purpose
Results
Concentration of EDTA solution:_______________M
PERFORMANCE EVALUATION SAMPLE, (ID number:____________)
CALCIUM
Replicate
1
2
3
EDTA Volume, mL
[Ca
2+
], M
Hardness, expressed
as mg/L of CaCO
3
Average Hardness: _______________ Standard Deviation: _________
Include sample calculations for all three experiments, and answer all questions. Check
your calculations carefully! Don’t forget to include copies of your lab notebook pages
for this experiment.
