College of Science
Determination of Total Calcium and Magnesium Ion Concentration
Safety
then start to complex with ErioT indicator, immediately
changing its colour from blue to pink.
Lab coats, safety glasses and enclosed footwear must
be worn at all times in the laboratory.
Concentrated ammonia solution used in preparing
the buffer and indicator solutions is highly corrosive
− wear rubber gloves and take care when handling.
Both the buffer and indicator (and thus also the
titration solution) will liberate ammonia gas to some
extent. This gas may be harmful if inhaled in large
quantities. Work in a fumehood or well ventilated
area.
The main reaction is
Introduction
Burette
This method, called a complexometric titration, is used
to find the total calcium and magnesium content of
milk, sea water and various solid materials. It can also
be used to determine the total hardness of fresh water
provided the solutions used are diluted. The combined
concentration of calcium and magnesium ions is
considered to be the measure of water hardness.
20 mL pipette
The method uses a very large molecule called EDTA
which forms a complex with calcium and magnesium
ions. EDTA is short for ethylenediaminetetraacetic acid.
A blue dye called Eriochrome Black T (ErioT) is used
as the indicator. This blue dye also forms a complex
with the calcium and magnesium ions, changing
colour from blue to pink in the process. The dye–metal
ion complex is less stable than the EDTA–metal
ion complex. For the titration, the sample solution
containing the calcium and magnesium ions is reacted
with an excess of EDTA. The indicator is added and
remains blue as all the Ca2+ and Mg2+ ions present are
complexed with the EDTA.
A back titration is carried out using a solution of
magnesium chloride. This forms a complex with the
excess EDTA molecules until the end-point, when all
the excess EDTA has been complexed. The remaining
magnesium ions of the magnesium chloride solution
Ca2+ + EDTA4− → [Ca-EDTA]2−
Back titration
EDTA4− + Mg2+ → [Mg-EDTA]2−
Indicator reaction: note, ErioT is blue and ErioT-Mg is pink
ErioT + Mg2+ → ErioT-Mg
Equipment Needed
250 mL conical flasks
100 mL volumetric cylinder
Solutions Needed
EDTA: (ethylenediaminetetraaceticacid) 500 mL of a
0.05 molL-1 solution. Weigh 9.31 g of the EDTA salt and
dissolve it in 500 mL of distilled water in a volumetric
flask.
Buffer: Dissolve 7.0 g of ammonium chloride in 57 mL
concentrated ammonia (see safety notes). Dilute to
100mL with distilled water in a volumetric flask. The pH
should be 10.5.
MgCl2.6H2O: 0.025 molL-1 solution. Weigh 2.54 g of
magnesium chloride hexahydrate and dilute to 500 mL
with distilled water in a volumetric flask.
ErioT indicator: Dissolve 0.2 g of Eriochrome Black T
indicator in 15 mL of concentrated ammonia solution
(or 15 mL of triethanolamine) (see safety notes) and
5mL absolute ethanol. Do not store more than one to
two days before use. You may be able to get the ErioT
indicator from the University of Canterbury - see the
contact details at the end.
Titration Method for Seawater, Milk and Solid
Samples
1. Pipette 10 mL of the sample solution into a conical
flask.
2. Add 20 mL of 0.05 mol L−1 EDTA solution.
3. Add 10 mL of ammonia buffer, 50 mL of distilled
water and 1 mL of Eriochrome Black T indicator
solution.
4.Titrate the sample with the standard 0.025 molL−1
magnesium chloride solution until a permanent pink
colour appears.
Method
Sample Preparation
For samples that are already in solution, such as
freshwater, seawater and milk, no further preparation
is needed.
For solid samples such as eggshells and limestone,
the samples must first be dissolved in acid. Accurately
weigh about 0.5 g of the solid into a small beaker or
conical flask, add about 20 mL dilute hydrochloric acid
and allow the solid to completely dissolve (this may
take several minutes). Neutralise the unreacted acid
with dilute sodium hydroxide solution until the pH
Titration
Method
for Fresh
or Tap Water Samples
Eriochrome Black T indicator solution.
chloride
Preparechloride
a 0.0025
mol L magnesium
Eriochrome Black T indicator solution.
magnesium
4. Prepare a 0.0025 mol L 4.
of3. the
solution
is
almost
7
(according
to
pH
indicator
magnesium
solution
by
diluting
the
0.025
mol
L
magnesium
solution
by
diluting
the
0.025
mol
L
Titrate the
EDTA with the magnesium chloride
Titrate the EDTA with the3.magnesium
chloride
1.1/10.Add
a 100
ofofthe
solution
by amL
factor
1/10. sample solution into a 250 mL
chloride
by a factor ofchloride
paper).
Fortheeggshells,
the inner
will
remain
until
themembrane
endpoint is reached
–solution
a permanent
solution until
endpoint issolution
reached
– a permanent
conical
flask.
5. with
Titrate
sample
colour change from blue to pink.
5. Titrate
sample solution
thisthe
0.0025
molLsolution with this 0.0025 molL
colour change from
blue may
to pink.
undissolved
and
be carefully removed
fromthe
the
magnesium
chloride
solution until a permanent
pink
magnesium
chloride
solution
until
a
permanent
pink
4. Having
determined
the average titre of the
4. Having determined the average
titre of
the
2. Prepare
asamples
0.005
mol with
L−1 EDTA
solution by diluting the
solution.
Transfer the
solution
to asolution,
100
mL
volumetric
colour
appears.
Repeat
the titration
further samples
appears.
Repeat the
with
further
chloride
determine
the number
of titration
magnesium chloride solution,magnesium
determine the
number
of colour
−1 (titres agreeing within 0.1 mL)
until
concordant
results
until
concordant
results
(titres
agreeing
within
0.1
mL)
0.05 mol L EDTA solution by a factor of 1/10. Add
moles used.
flask
and make up tomoles
theused.
mark with distilled
water.
are obtained.
are obtained.
20mL of this diluted EDTA to the sample solution.
: EDTA ratio of 1 : 1, calculate the
5. of Given
the Mg the
1 : 1, calculate
5. Given the Mg : EDTA ratio
Standardisation
ofsolution.
the EDTAofSolution
concentration
your EDTA solution.
concentration of your EDTA
Result Calculations Result
3. AddCalculations
10 mL of the ammonia buffer and 1 mL of
Titration
Method
for Seawater,
Milk
and
Solid
1.Titration
Pipette
a 10formL
sample
the
EDTA
solution
into
a
Method
Seawater,
Milkof
and
Solid
Calculate
moles ofTEDTA
added to the
1. Calculate the total moles1. of EDTA
addedthe
to total
the Black
Eriochrome
indicator
solution.
Samples
Samples
sample solution.
sample solution.
conical flask.
1. solution
Pipette 10
mLa of
the sample
intothe
a conical
1. Pipette 10 mL of the sample
into
conical
amoles
0.0025
L−1 magnesium
chloride
2.4.Prepare
Calculate the
of themol
magnesium
chloride
2. solution
Calculate
moles of the
magnesium
chloride
flask.
flask.
2.
Add 10 mL of ammonia
buffer solution
and
1
mL
of
solution
used
in
the
back
titration
from
your concordant −1
solution used in the back titrationsolution
from your concordant
by
diluting
the
0.025
mol
L
magnesium
Frombelow,
the equation
From the equation of results.
the titration
the of the titration below, the
EDTA solution.
Add
20 mL ofsolution.
0.05 mol L results.
solution.
2. Eriochrome
Add 20 mL of 0.05
mol L2.TEDTA
Black
indicator
a moles
factor
of 1/10.
will
be
equivalentby
to the
of excess
moles
of Mg
tochloride
the
moles
ofsolution
excess
moles of Mg will be equivalent
3. Add
10 mL
of ammonia buffer, 50 mL of distilled
3. Add 10 mL of ammonia buffer,
50 mL
of distilled
EDTA.
EDTA.
3.water
Titrate
the
EDTA
with
the
magnesium
chloride
1 mL ofsolution.
Eriochrome Black T indicator solution.
and 1 mL of Eriochromewater
Black and
T indicator
5. Titrate
the+ sample
solution with this 0.0025 molL−1
4−
2− 4−
EDTA
Mg2+ → [Mg-EDTA]2−
EDTA
+ Mg2+ → [Mg-EDTA]
4.endpoint
Titrate the
sample
with the –
standard
0.025 molL
untilwith
thethe
ismolL
reached
a permanent
4. solution
Titrate the sample
standard
0.025
magnesium
chloride solution until a permanent
chloride
solution until
permanent
pink
3. :Given
of Ca + Mg : EDTA = 1 : 1, calculate
magnesium chloride solutionmagnesium
until a permanent
pink
3. aGiven
the ratio
of Ca + Mg
EDTA the
= 1 : ratio
1, calculate
colour
change
from
blue
to
pink.
colour
appears.
that mustRepeat
have been the titration with further
themust
moles
ofcolour
Ca
pink
appears.
colour appears.
have
beenand Mg
the moles of Ca and Mg that
complexed
EDTA
by subtracting the excess EDTA
complexed with EDTA by subtracting
the with
excess
EDTA
4.Having
determined
the
average
titre
of
the
samples
until
concordant
results (titres agreeing
Method
for Fresh or
Tapthe
Water
Titration Method for FreshTitration
or Tap Water
Samples
moles of EDTA added to the sample.
from
total Samples
moles of EDTA from
addedthe
to total
the sample.
magnesium
chloride
solution,
determine
the
number
within
mL)
areandobtained.
1. solution
Add a 100
mLa of
250 mL
1. Add a 100 mL of the sample
into
250the
mLsample
Mg in the sample
is the0.1
moles
of Ca
andresult
Mg in
sample
Thissolution
result isinto
the amoles
of Ca This
conical
flask. used. conical flask.
solution.
solution.
of moles
2. solution
Prepare aby0.005
mol L EDTA solution by diluting
diluting
2. Prepare a 0.005 mol L EDTA
2+
EDTA
by a factorthe
of 1/10. Add 20
the
molofL 1/10.
5.theGiven
Mgsolution
: EDTA
ratio
of
1Add
:solution
1,20calculate
by 0.05
a factor
0.05 molthe
L EDTA
of thissolution.
diluted EDTA to the sample solution.
mL of this diluted EDTA to themL
sample
concentration of your EDTA solution.
−1
−1
−1
−1
−1
−1
2+
2+
−1
−1
2+
2+
-1
-1
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
−1
−1
−1
−1
3. buffer
Add 10
mL1 mL
of the
3. Add 10 mL of the ammonia
and
of ammonia buffer and 1 mL of
Eriochrome Black T indicator solution.
Eriochrome Black T indicator solution.
Figure 2chloride
Same colour
changes as
forinmagnesium chloride back-titration as in
1 Colour
changes in
forclear
magnesium chloride
in clear for magnesium
Figure 2 back-titration
Same colour changes
back-titration
Figure 1 Colour changes for magnesiumFigure
chloride
back-titration
1, but
cloudy
solution
Left1, flask:
colour
well sampleFigure
Figure
but forblue
cloudy
(opaque)
solution,
egfor
milk.
Left (opaque)
flask: bluesample solution, eg milk. Left flask: blue
solution using Eriochrome Black T indicator.
Leftusing
flask:Eriochrome
blue colour Black
well T indicator.
Figure
1 Colour
changes
for
chloride
back-titration
Figure
Same
colour
for magnesium
chloride backcolour
well before
endpoint.colour
Centre flask:changes
last trace of blue/purple
colour
before
endpoint
Ca all
/Mg
ions
complexed
excess
EDTA,
all indicator
colourby
well
before
endpoint.
Centre flask:
last trace
of2blue/purple
before endpoint
(all Ca /Mg
ions complexed
bymagnesium
excess(all
EDTA,
indicator
just
beforeatendpoint.
Right
flask: pink/red
colour at
endpoint.
Centre
flask:
last
of blue/purple
colour
justpink/red
justtrace
before
endpoint.
Right
flask:
colour
endpoint.
uncomplexed).
Centre
flask: molecules
last
trace ofuncomplexed).
blue/purple
colour
just
inmolecules
clear
solution
using
Eriochrome
Black
T
indicator.
Left
flask:
titration
as
in
Figure
1,
but
for
cloudy
(opaque)
sample solution,
beforecomplexed
endpoint (excess
EDTA
totally
before endpoint (excess EDTA almost totally
by added
Mgalmost
).
2+ complexed by added Mg ).
blue
colour
before
endpoint
(all
Caat2+endpoint
/Mg
ionscomplexed
complexed
eg milk. Left flask: blue colour well before endpoint. Centre
flask:complexed
pink/red
colour
by added Mg ,
Right flask:
pink/redwell
colour at
endpointRight
(all
EDTA
by added
Mg
, (all EDTA
indicator also complexed).
indicator also complexed).
by
excess EDTA, all indicator
molecules uncomplexed). Centre
flask: last trace of blue/purple. Right flask: pink/red colour at
2
2flask: last trace of blue/purple
colour just before endpoint
endpoint
(excess EDTA almost totally complexed by added Mg2+).
Right flask: pink/red colour at endpoint (all EDTA complexed by
added Mg2+, indicator also complexed).
2+
2+
2+
2+
2+
2+
2+
2+
Result Calculations
1. Calculate the total moles of EDTA added to the
sample solution.
nature and source of the sample it may be necessary to
vary the concentration of the EDTA( if the titre volume is
too low) or to dilute your solutions (if the titre volume
is too high). The average titre volume should be in the
range of 10 – 30 mL.
2. Calculate the moles of the magnesium chloride
solution used in the back titration from your
concordant results. From the equation of the titration Contact Us
below, the moles of Mg2+ will be equivalent to the
If you have
Safety
Contact
Usany questions or comments relating to this
moles of excess EDTA.
experiment, please contact us. Please note that this
4−
2−
The concentrated
used in preparing
If you
haveisany
questions
or comments
service
for senior
school
chemistryrelating
studentstointhis
EDTAammonia
+ Mg2+solution
→ [Mg-EDTA]
the buffer and indicator solutions
is
highly
corrosive
and
experiment,
please
contact
us:
New Zealand only. We regret we are unable to respond
2+
2+
3. Given
ratio with
of Cacare.
+ Mg
: EDTA
=glasses
1 : 1, calculate
should
be the
handled
Wear
safety
and
to queries from overseas.
Outreach
the moles
of Ca2+ and Mg2+ that must have been
rubber
gloves.
Outreach
of Science
complexed with EDTA by subtracting the excess EDTA College
College
of
University
of Science
Canterbury
Both
the
buffer
and
indicator
(and
thus
also
the
titration
from the total moles of EDTA added to the sample.
University
of Canterbury
solution) will liberate ammonia2+gas to some
extent. This Private Bag 4800
This result is the moles of Ca and Mg2+ in the sample
Private
Bag
gas may be harmful if inhaled in large quantities. Work
Christchurch 4800
solution.
Christchurch
in a fumehood or well ventilated area.
New
Zealand
New Zealand
Phone:
2178
Phone:+64
+643 364
3 364
2178
AdditionalNotes
Notes
Fax:
+643 3364
3642490
2490
Fax: +64
Additional
Email:outreach@canterbury.ac.nz
outreach@canterbury.ac.nz
Email:
1. 1. Ethylenediaminetetraacetic
Ethylenediaminetetraaceticacid,
acid,EDTA
EDTAisisaalarge
large
www.outreach.canterbury.ac.nz
molecule
which
creates
a
complex
with
a
metal
ion,
www.outreach.canterbury.ac.nz
molecule which creates a complex with a metal ion,
bondingthrough
through
coordination
sites.
bonding
sixsix
coordination
sites.
2-
O
C
O
O
C
O
CH2
CH2
N
Ca2+
N
O
C
O
CH2
O
C
CH2
CH2
CH2
O
Complex formed by EDTA and calcium ions
Complex formed by EDTA and calcium ions
2. The ammonia buffer (pH ~ 10.5) used here is needed
as Eriochrome Black T only changes colour in the pH
2. The7 –ammonia
buffer (pH ~ 10.5) used here is needed
range
11.
as Eriochrome Black T only changes colour in the pH
3.range
The7 presence
of some other metal ions − eg
– 11.
copper, iron, cobalt, nickel, zinc, manganese − in
3. The presence of some other metal ions
high concentrations may introduce error to the
− eg copper, iron, cobalt, nickel, zinc, manganese −
determination of calcium and magnesium ions using
in high concentrations may introduce error to the
this method, although this is unlikely.
determination of calcium and magnesium ions using
2+
and Mg2+ in the sample
4.thisAs
the concentration
of Ca
method,
although this
is unlikely.
solution may vary considerably2+depending
on the nature
4. As the concentration of Ca and Mg2+ in the sample
and source of the sample it may be necessary vary the
solution may vary considerably depending on the
concentration of the EDTA( if the titre volume is too
low) or to dilute your solutions (if the titre volume is too
high). The average titre volume should be in the range of
10 – 30 mL.