Edta
Edta
Edta
HO C CH2 CH2 C OH
N CH2 CH2 N
HO C CH2 CH2 C OH
O O
This
salt
is
readily
available
from
many
commercial
sources,
and
often
in
such
a
high
purity
that
solutions
need
not
be
standardized
for
routine
work.
Primary
standard
calcium
carbonate
can
be
used
to
standardize
EDTA
solutions.
Of
the
various
EDTA
species,
only
the
Y4-
ion
(the
completely
deprotonated
anion
of
EDTA)
forms
a
1:1
complex
with
metal
ions.
To
increase
the
fraction
of
Y4-,
the
pH
needs
to
be
increased
to
10
in
this
experiment.
The
endpoint
of
an
EDTA
titration
is
determined
with
a
metallochromic
indicator.
These
indicators
are
complexing
agents
that
change
color
when
combined
with
metal
ions.
A
variety
of
indicators
can
be
used
for
EDTA
titrations.
In
this
experiment,
we
will
use
Eriochrome
black
T
(EBT)
indicator,
having
the
structure
shown
below.
OH OH
-
O3 S N N
NO2
(H2In-)
This
indicator
(shown
as
H2In-
in
the
equations
below)
changes
from
blue
to
red
when
combined
with
a
metal
ion,
forming
a
complex
ion:
1
M2+
+
H2In-
+
2H2O
<-->
MIn-
+
2H3O+
blue
red
EDTA
is
a
stronger
complexing
agent
than
the
indicator,
and
displaces
the
indicator
from
the
metal
ion
allowing
the
indicator
to
return
(through
shades
of
violet)
to
a
pure
blue
color,
indicating
the
end
of
the
reaction.
MIn-
+
Y4-
<-->
MY2-
+
H2In-
red
blue
Calcium
ion
(Ca+2)
does
not
form
a
stable
red
complex
with
the
EBT
indicator;
therefore
the
direct
titration
of
Ca2+
by
EDTA
may
not
cause
a
sharp
color
change
of
EBT
indicator
at
the
end
point.
The
magnesium
complex
with
EBT
is
stable
and
the
Kf
of
Mg2+
with
EDTA
is
lower
than
the
Kf
of
Ca2+
with
EDTA.
Thus,
a
displacement
titration
of
Ca2+
by
the
mixture
of
Mg2+
and
EDTA
will
help
to
determine
the
end
point
with
the
following
mechanism:
CaIn-
+
MgY2-
<-->
CaY2-
+
MgIn-
To
accomplish
this
displacement
titration,
a
small
amount
of
Mg2+
will
be
mixed
with
the
EDTA
solution.
The
EDTA-Mg
mixture
will
titrate
the
unknown
Ca2+
solution.
At
the
end
point,
Mg2+
will
be
released
from
the
EBT
indicator
and
complexed
with
EDTA,
causing
the
color
change
from
red
to
blue.
Solutions
needed
for
this
experiment:
Solutions
prepared
by
the
student
Solutions
provided
by
the
instructor
0.01M
disodium
EDTA
with
MgCl2
12M
Hydrochloric
acid
(standardized
by
student)
Standard
Ca2+
solution
8.5M
NH3-NH4Cl
Buffer
Solution
of
egg
shells
Eriochrome
black
T
indicator
2
Experimental
Procedure
Preparation
and
standardization
of
0.01
M
EDTA
solution.
1. Using
the
top
loading
balance,
weigh
between
3.6
and
3.7
grams
of
disodium
EDTA
dihydrate
into
a
clean
1
L
plastic
bottle.
EDTA
will
leach
metal
ions
from
soft
glass
containers,
and
should
never
be
stored
in
glass
containers.
Add
1
L
of
deionized
water.
EDTA
dissolves
SLOWLY.
Shaking
or
stirring
the
solution
vigorously
speeds
the
dissolution
process.
Nevertheless,
even
under
these
conditions
EDTA
dissolves
SLOWLY.
It
is
strongly
recommended
that
the
EDTA
solution
be
prepared
several
hours
or
even
the
day
before
you
plan
on
using
it.
Before
use,
check
the
solution
to
make
sure
all
of
the
solid
has
dissolved.
2. Using
the
analytical
balance,
weigh
out
~0.1
grams
of
MgCl2
and
add
the
MgCl2
to
the
EDTA
solution.
You
dont
need
to
wait
for
the
EDTA
to
dissolve
before
adding
the
magnesium
chloride.
3. Dry
about
1
gram
of
calcium
carbonate
(CaCO3)
in
the
oven
for
2
hours.
Transfer
to
the
desiccator
and
cool
(~
1
hour).
When
cooled,
weigh
a
0.5-gram
portion
of
calcium
carbonate
on
the
analytical
balance
and
transfer
it
to
a
clean
250
mL
beaker.
4. Add
approximately
25
mL
of
distilled
H2O,
then
5
mL
of
conc.
HCl
carefully
to
the
250
mL
beaker.
Calcium
carbonate
reacts
vigorously
with
acid,
producing
carbon
dioxide
gas,
which
may
spatter
the
beaker
contents.
Cover
the
beaker
with
a
watch
glass.
Note:
If
CaCO3
does
not
dissolve
completely,
add
another
5
mL
of
conc.
HCl.
When
the
calcium
carbonate
has
completely
dissolved,
boil
the
solution
gently
for
2-5
minutes,
keeping
the
watch
glass
on
the
beaker,
to
expel
carbon
dioxide.
Analytically
transfer
the
solution
to
a
500.00
mL
volumetric
flask
and
QS
with
DI
water.
5. Pipette
25.00
mL
of
standard
Ca2+
solution
prepared
in
step
4
into
a
250-mL
Erlenmeyer
flask.
Check
the
pH
using
pH
paper:
if
acidic,
use
dilute
sodium
hydroxide
solution
to
adjust
the
pH
to
~7.
When
the
pH
is
~
7,
add
10
mL
of
8.5M
NH3-NH4Cl
buffer.
This
buffer
has
been
prepared
for
your
use.
CAUTION!
This
buffer
is
dangerous;
it
is
caustic
and
ammonia
can
cause
pulmonary
paralysis
(it
can
interfere
with
your
ability
to
breath).
Exercise
caution
in
handling
and
dispensing
this
buffer!!
6. Add
20
mL
of
deionized
water
and
2-3
drops
of
EBT
indicator.
Titrate
the
Ca2+
standard
solution
with
the
EDTA
solution
until
the
color
changes
from
wine
red,
through
purple,
to
a
pure
rich
blue
color.
At
the
end
point,
the
last
traces
of
purple
in
the
solution
will
have
just
disappeared.
If
the
reaction
3
seems
to
proceed
slowly
near
the
equivalence
point,
after
each
addition
of
EDTA
wait
a
few
seconds
before
adding
the
next
drop.
Some
students
(not
all
of
them
male)
have
difficulty
distinguishing
between
purple
and
blue.
You
may
want
to
make
a
comparison
sample,
representing
the
endpoint
of
the
titration.
The
comparison
sample
is
made
from
50
mL
of
DI
water,
10
mL
of
8.5M
NH3-NH4Cl
buffer,
and
2
3
drops
of
EBT
indicator
solution.
The
solution
should
have
a
rich
blue
color
and
can
be
used
for
comparison
with
the
end
point
of
your
calcium-EDTA
titrations.
7. Perform
the
titration
in
triplicate.
Normally,
three
titrations
will
agree
within
0.04
mL.
Do
not
use
all
of
the
EDTA
solution
in
performing
the
standardization,
since
you
must
have
enough
EDTA
to
titrate
the
unknown
Ca2+
and
eggshell
solutions.
From
the
known
mass
of
CaCO3,
and
the
1:1
stoichiometry
between
Ca2+
and
EDTA,
you
should
be
able
to
readily
calculate
the
molarity
of
the
EDTA
solution.
Calculate
the
average
molarity
and
label
your
EDTA
solution
appropriately.
Determination
of
the
unknown
Ca2+
concentration.
1. Pipette
5.00
mL
of
your
unknown
solution
into
250.00
mL
volumetric
flask
and
QS
with
DI
water.
Pipette
a
50.00
mL
aliquot
of
the
Ca2+
solution
from
the
volumetric
flask
into
a
250-mL
Erlenmeyer
flask.
Adjust
the
pH
to
~7
(if
needed).
Add
10
mL
of
8.5M
NH3-NH4Cl
buffer,
and
2-3
drops
of
EBT
indicator
to
the
Erlenmeyer
flask,
and
titrate
with
EDTA.
Perform
this
analysis
in
triplicate.
Determination
of
the
percentage
of
Ca2+
in
eggshell.
1. Peel
off
any
membrane
attached
to
your
eggshell
(the
entire
eggshell
not
just
a
portion).
You
may
need
to
carefully,
thoroughly
examine
the
eggshell
to
find
all
of
the
membrane.
When
the
membrane
has
been
removed,
dry
the
eggshell
in
the
oven,
overnight,
at
110
oC.
Cool
the
eggshell
in
the
desiccator,
and
weigh
out
a
0.2
g
portion
on
the
analytical
balance.
Dissolve
the
eggshell
in
20
mL
of
50%
hydrochloric
acid.
Filter
the
solution
to
remove
the
organic
solid
in
the
solution.
2. Analytically
transfer
the
filtered
eggshell
solution
into
a
250.00
mL
volumetric
flask
and
QS
with
DI
water.
Pipette
a
25.00
mL
aliquot
of
the
solution
from
the
volumetric
flask
into
a
250.00
mL
Erlenmeyer
flask.
Adjust
the
pH
to
7
using
dilute
sodium
hydroxide
solution
(if
needed).
Add
10
mL
of
8.5M
NH3-NH4Cl
buffer
to
adjust
the
solution
to
~pH
10.
Add
2-3
4
drops
of
EBT
indicator
to
the
Erlenmeyer
flask
and
titrate
with
EDTA.
Perform
this
analysis
in
triplicate.
Calculations
Since
EDTA
forms
a
1:1
complex
with
Ca2+
under
these
experimental
conditions,
the
millimoles
of
calcium
present
can
be
found
by
calculating
the
number
of
millimoles
of
EDTA
consumed
during
the
titration
using
the
equation
millimoles
EDTA
=
mL
EDTA
X
molarity
EDTA
Since
the
total
volume
of
the
250
ml
volumetric
flask
has
not
been
titrated,
the
millimoles
of
calcium
titrated
(calculated
from
the
above
equation)
are
the
millimoles
present
in
the
aliquot
titrated.
A
dilution
factor
must
be
used
to
calculate
the
total
number
of
millimoles
of
metal
present
in
the
250.00
mL
volumetric
flask:
Total
millimoles
calcium
=
millimoles
calcium
X
(250.00
mL/50.00
mL)
Once
the
total
number
of
millimoles
of
calcium
present
in
the
sample
is
calculated,
the
mass
of
calcium,
in
milligrams,
is
readily
determined.
Laboratory
report.
A
sample
report
is
shown
at
the
end
of
the
procedure.
WASTE
DISPOSAL:
All
solutions
used
in
this
experiment
can
be
poured
down
the
sink
drain.
5
SAMPLE
REPORT
Determination
of
Ca2+
Unknown
#3.14159265358979323846
Lothar
of
the
hill
people
Standardization
of
EDTA
solution:
Mass
of
calcium
carbonate
used:
0.5112
g
Standard
Ca2+
Solution:
Sample
1
Sample
2
Sample
3
Volume
Ca+2,
mL
25.00
25.00
25.00
Molarity
Ca+2
0.01022
0.01022
0.01022
Vol.
EDTA,
mL
25.13
25.16
25.09
Molarity,
EDTA*
0.010167
0.010155
0.010183
Mean
EDTA
molarity
0.01017
*M1V2
=
M2V2
Analysis
of
calcium
unknown:
Ca+2 Unknown: Sample 1 Sample 2 Sample 3
Vol. Ca+2 unknown, mL 50.00 50.00 50.00
Vol. EDTA, mL 37.96 37.99 37.91
Ca+2 mg, aliquot1 15.548 15.561 15.528
Ca+2 mg, unknown2 77.740 77.805 77.640
Mean Ca+2 mg, unknown 77.73
1mL
EDTA
x
Molarity
EDTA
=
mmoles
EDTA
=
mmoles
Ca+2
mmoles
Ca+2
x
40.078
g/mole
=
mmoles
Ca+2,
aliquot
2(Ca+2
mg,
aliquot)
x
(250.00
mL/50.00mL)
=
Ca+2
mg,
unknown
6
Analysis
of
eggshell:
Mass
of
eggshell
used:
0.1997
g
Eggshell solution: Sample 1 Sample 2 Sample 3
Vol. titrated, mL 25.00 25.00 25.00
Vol. EDTA, mL 18.47 19.55 19.58
Ca+2 mg, aliquot3 7.5653 7.5795 7.5880
Ca+2 mg, solution4 75.6523 75.795 75.880
% Ca+2, eggshell5 37.883 37.954 37.997
Avg % Ca+2, eggshell 37.94
3mL
EDTA
x
Molarity
EDTA
=
mmoles
EDTA
=
mmoles
Ca+2
mmoles
Ca+2
x
40.078
g/mole
=
mmoles
Ca+2,
aliquot
4(Ca+2
mg,
aliquot)
x
(250.00
mL/25.00mL)
=
Ca+2
mg,
unknown
5(mg
Ca+2/mg
eggshell)
*100