College of Science

Determination of Calcium Ion Concentration

Safety Equipment Needed

Lab coats, safety glasses and enclosed footwear must 10 and 20 mL pipettes
be worn at all times in the laboratory. 250 mL conical flasks
Note that the concentrated (8mol L-1) sodium 100, 250 and 500 mL volumetric flasks
hydroxide solution used is highly corrosive and
should be handled with extra care: ideally wear pH indicator paper
rubber gloves when preparing and handling it. 10 mL and 100 mL measuring cylinders
burette and stand
Introduction
This method, called a complexometric titration, is used Solutions Needed
to find the calcium content of milk, the ‘hardness’ of EDTA: ethylenediaminetetraacetic acid 0.025 mol L−1
water and the amount of calcium carbonate in various solution. If possible, dry 5 g of the disodium salt of EDTA
solid materials. for several hours or overnight at 80°C, allow to cool.
The method uses a very large molecule called EDTA Weigh 4.65 g of the dried EDTA salt and dissolve it in
which forms a complex with calcium ions. EDTA stands 500 mL of distilled water in a volumetric flask.
for ethylenediaminetetraacetic acid. A blue dye called Patton-Reeder indicator triturate: a small amount
Patton and Reeder’s indicator (PR) is used as the may be available from Outreach at the University of
indicator. This blue dye also forms a complex with the Canterbury, see contact details on back page.
calcium ions changing colour from blue to pink/red Sodium hydroxide solution: (8 molL-1). (See safety
in the process, but the dye–metal ion complex is less notes) Weigh 32 g of solid sodium hydroxide into a 250
stable than the EDTA–metal ion complex. As a result, mL conical flask and carefully dissolve in 100 mL of
when the calcium ion–PR complex is titrated with EDTA distilled water. The solution will get very warm as the
the Ca2+ ions react to form a stronger complex with NaOH dissolves; the temperature may be controlled by
the EDTA. sitting the bottom of the flask in a small basin of cold
For the titration, the indicator is added to the sample tap water.
solution containing the calcium ions and forms the Dilute hydrochloric acid solution: (1-2 mol L−1)
pink/red calcium ion-indicator complex (Ca-PR). This
solution is then titrated with EDTA. The endpoint occurs Dilute sodium hydroxide solution: (1-2 mol L−1)
when the solution turns blue, indicating that the Ca-PR
complex has been completely replaced by the calcium
ion-EDTA complex and the PR indicator reverts to its
blue colour.

The reaction is:

Ca-PR + EDTA4− → PR + [Ca-EDTA]2−
Note: Ca-PR is pink/red and PR is blue.
Method 3. Add 0.1 g of Patton-Reeder indicator and swirl the
solution to dissolve the indicator.
Sample Preparation 4. Titrate the sample with the diluted EDTA standard
Calcium samples that are already in solution, such as solution to the blue endpoint. Repeat until
tapwater and milk, do not need any further preparation. concordant results are obtained.
Seawater may need to be filtered to remove solid
material such as sand and seaweed. Result Calculations
Solid samples, such as limestone and eggshell, must
first be dissolved in acid. 1. Calculate the average volume of EDTA solution used
from your concordant titres.
1. Accurately weigh about 0.5 g of the solid into a
small beaker or conical flask, add about 20 mL dilute 2. Calculate the moles of EDTA required to complex the
hydrochloric acid and allow the solid to completely Ca2+ ions in the sample.
dissolve (this may take several minutes). 3. Using the method ratio Ca2+: EDTA = 1 : 1, calculate
2. Neutralise the unreacted acid with dilute sodium the concentration in mol L−1 of Ca2+ in your sample
hydroxide solution until the pH of the solution is solution.
almost 7 (according to pH indicator paper). With 4. Calculate the concentration, in mg/L (parts per
eggshells, the inner membrane will not dissolve and million or ppm), of Ca2+ in your sample solution.
should be carefully removed from the solution. 5. In the case of a solid sample which has been
3. Transfer the solution to a 100 mL volumetric flask and dissolved in acid, the concentration of Ca2+ in your
make up to the mark with distilled water. sample solution may be used to calculate the
percentage, by weight, of CaCO3 in the solid sample.
Titration This assumes that all the Ca2+ found has come from
For undiluted seawater, undiluted milk, eggshell and CaCO3.
limestone samples.
1. Pipette a 10 mL aliquot of the sample solution into a
conical flask.
2. Add 40 mL of distilled water and 4 mL of 8 mol L−1
sodium hydroxide solution (see safety notes), and
allow solution to stand for about 5 minutes with
occasional swirling. A small of magnesium hydroxide
may precipitate during this time. Do not add the
indicator until you have given this precipitate a Figure 1 Colour changes for calcium-EDTA titration in clear
chance to form. sample solution using Patton-Reeder indicator. Left flask: pink/
3. Add 0.1 g of Patton-Reeder indicator and swirl the red colour well before endpoint (excess Ca2+ ions present to
solution to dissolve the indicator. complex with indicator). Centre flask: last trace of purple colour
just before endpoint (Ca2+ ions almost all complexed by EDTA).
4. Titrate the sample with the EDTA solution. The Right flask: blue colour at endpoint (all Ca2+ ions complexed by
endpoint is a colour change from pink/red to blue. EDTA, indicator completely uncomplexed).
Repeat the titration with further samples until
concordant results (titres agreeing within 0.1 mL) are
obtained.
For tapwater the method is modified due to the much
lower Ca2+ concentration.
1. Dilute the ETDA standard solution by a factor of 1/50
by pipetting 10 mL into a 500 mL volumetric flask
and making up to the mark with distilled water. This
will give a 0.0005 mol L−1 solution.
Figure 2 Same colour changes for calcium-EDTA titration as in
2. Pipette a 50 mL aliquot of tapwater into a conical Figure 1, but for cloudy (opaque) sample solution, eg milk. Left
flask. Add 4 mL of 8 mol L−1 sodium hydroxide flask: pink/red colour well before endpoint. Centre flask: last
solution, and allow solution to stand for 5 minutes trace of purple colour just before endpoint. Right flask: blue
with occasional swirling. colour at endpoint
 used here is highly corrosive and should be handled with 4. The presence of some metal ions, such as
care: wear safety glasses and ideally also rubber gloves copper, iron, cobalt, nickel, zinc or manganese in high
when preparing and handling it. concentrations may cause errors using this method.
Additional Notes However, this is unlikely for the solutions and solids
4. The presence of some metal ions, such as copper,
suggested.
Additional Notes
1. Ethylenediaminetetraacetic acid, EDTA, is a large iron, cobalt, nickel, zinc or manganese in high
5. concentrations
This method formay determining
cause errorsthe using
concentration
this method.
1. molecule which creates a complex
Ethylenediaminetetraacetic with aismetal
acid, EDTA, a largeion, of CaHowever,
2+
in a sample
this may
is be used
unlikely forwith
the the method
solutions forsolids
and
bonding
molecule through
which sixacoordination
creates complex withsites.
a metal ion, measuring total Ca and Mg concentration using
suggested. 2+ 2+
bonding through six coordination sites. Eriochrome Black Tforindicator to determine, by difference,
5. This method determining the concentration
2- the concentration of Mgmay
of Ca2+ in a sample
2+
ions
beinused
the sample.
with theNote that
method
O
this method requires
for measuring a
total 0.0252+ mol L EDTA
−1
solution
Ca and Mg concentration−1
2+ while
the total Ca 2+
and Mg 2+
method
using Eriochrome Black T indicator requires ato0.05 mol L
determine,
C EDTA solution.
by difference, the concentration of Mg ions in the
2+

O O CH2 6. sample. Noteconcentration

The precise that this methodof Carequires
2+ a 0.025
in the sample
C CH2 solution may well vary considerably dependingand
mol L−1
EDTA solution while the total Ca 2+
on Mg2+
O N method
the nature andrequires
sourceaof0.05the mol L−1 EDTA
sample. solution.
To obtain good
CH2 results,
Ca2+ 6. Thethe average
precise titre volumeofshould
concentration Ca2+ inideally be
the sample
O N CH2 between 10 and 30 mL. It may be necessary
solution may well vary considerably depending to vary theon
C CH2
concentrations
the nature ofandthe EDTA of
source solution used in
the sample. Toorder to good
obtain
O
CH2 obtain appropriate titre volumes.
results, the average titre volume should ideally be
O
C between 10 and 30 mL. It may be necessary to vary
Contact Us
the concentrations of the EDTA solution used in order
O to obtain appropriate titre volumes.
If you have any questions or comments relating to this
Complex
Complex formedbybyEDTA
formed EDTAand
and calcium
calcium ions
ions experiment, please contact us:
2. The Patton-Reeder indicator is used here in the Contact Us
2. The Outreach
form of aPatton-Reeder indicatorisisthe
“triturate”. Trituration used here inofthe
dilution a very If you of
have any questions or comments relating to this
form of acompound
“triturate”.with Trituration College Science
strong solid an inertispowder
the dilution
(calledofa a experiment, please contact us. Please note that this
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compoundbywith an inert University of Canterbury
diluent) a definite weight. Thispowder
practice service is 4800
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(called a diluent) Private Bag
is used extensively in in a definite proportion
pharmaceutical chemistry. by weight.
This practice is used extensively in pharmaceutical Christchurch only. We regret we are unable to respond
New Zealand
Because the undiluted compound is so strong, only to queries
chemistry. Because the undiluted New Zealandfrom overseas.
a very small portion is required and thiscompound
is difficult is
toso
weigh accurately. The dilution makes it possible to this
strong, only a very small portion is required and Outreach
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is difficult
accurately weighto weigh
a portion accurately. The dilution
of the mixture makes
containing theit College
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possible to accurately weigh a portion
correct amount of the compound. This triturate consists of the mixture University of Canterbury
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of 0.5containing
g of the purethe Patton-Reeder
correct amountindicator,of the compound.
2-hydroxy- Private Bag 4800
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Christchurch
This triturate consists of 0.5
1-(2-hydroxy-4-sulfo-1-naphthylazo)-3-naphthoic g of the pure Patton-
acid,
andReeder
50 g of indicator,
sodium sulfate2-hydroxy-1-(2-hydroxy-4-sulfo-1-
ground together to a fine New Zealand
naphthylazo)-3-naphthoic
powder. Thus addition of 0.1 g of acid,
theand 50 g actually
triturate of sodium Phone: +64 3 364 2178
sulfate ground
corresponds togetherof
to the addition tojust
a fine powder.
0.001 Thus
g of the Fax: +64 3 364 2490
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Patton-Reeder indicator compound. the triturate actually corresponds Email: outreach@canterbury.ac.nz
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3. This method for determining Ca2+ concentration
indicator compound.
in the presence of Mg2+ relies on the fact that the
pH3. of
This
themethod
solutionfor is determining
sufficiently high Ca2+toconcentration
ensure in
the presence of Mg 2+
relies on
that all magnesium ions precipitate as magnesium the fact that the pH
of the before
hydroxide solution theisindicator
sufficiently high to(The
is added. ensure thatbe
pH will
all magnesium ions precipitate as
approximately 12.5 due to the addition of concentrated magnesium
NaOH hydroxide
solution). before the indicator is
The Patton-Reeder added. (The
indicator is a pH
suitable indicator in this case as it produces a clear of
will be approximately 12.5 due to the addition
concentrated NaOH solution). The Patton-Reeder
indicator is a suitable indicator in this case as it
3 produces a clear colour change from pink/red to blue
in the pH range of 12 – 14.