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COMPLEXIMETRIC DETERMINATION OF CACIUM IN WATER SAMPLESDAVID, Frances Erika H.

1, DEVILLES, Jallyssa Jannel J.

2, EBON, Alyssa Mae P.

3, ERODIAS, Claro Ven T.

4

1Department of Biology, College of Science, University of the Philippines Baguio

August 14, 2013

ABSTRACT

The experiment aimed for the students to get acquainted with chelating agents; strengthen technical

skills involved in titrimetric analyses; learn and apply the concept of titer; and to study the physic-

chemical characteristics of water. The experiment was divided into two parts wherein in the first part,

the EDTA solution was prepared and standardized by the titration of a prepared Calcium chloride

solution using the EDTA solution; the second part was the analysis of the water sample win which

Calcium content was analyzed by obtaining a water sample where a buffer solution and an indicator

was added and titrated with the standardized EDTA solution. The water sample used was a

commercial mineral bottled water. Its mineral contents were determined by titrating it in the EDTA

solution. And computed for the hardness of the water. 

Introduction

In a complexometric titration, a

solution containing the free metal ion of

interest is titrated with a solution of chelating

agent until all of the metal ions are completely

complexed. The endpoint is usually measured

with an indicator ligand that forms a colored

complex with the free metal ion.

The most important chelating agent in

analytical chemistry isethelyenediaminetetraacetic acid (EDTA). The

tetrabasic form of this acid forms complexes

with virtually all metal ions. EDTA is a

hexadentate ligand; each of the acid oxygens

and each of the amine nitrogens can donate

one electron pair. The metal ion is usually held

in a one-to-one complex with EDTA. The

complexes have four or five 5-membered

rings, contributing significantly to their

stability. Unfortunately, EDTA cannot be easily

used as a primary standard. It is available in

several days to obtain the precise composition

of the dihydrate. In any case, standardization

of EDTA titrant against a solution of the metal

ion to be determined helps to eliminate any

errors in endpoint selection.

This experiment shows the way how to getacquainted with chelating agents and also to

strengthen technical skills involved in

titrimetric analyses, to learn and apply the

concept of titer and to study one physico-

chemical characteristic of water.

Results

A. 

Preparation and Standardiztion of

EDTA solution

It took 25 drops of 1:1 HCl to clear sol’n. Table 1 Calcium Carbonate Titrations

TrialInitial

volume

Final

volume

Molarity

of EDTA

solution

1 50 24.9 0.00843

2 50 23.7 0.00861

3 50 24.1 0.00871

N   N

COOH

COOH

HOOC

HOOC

Ethylenediaminetetraacetic acid (EDTA)

pKa-2 = 0.0

pKa-1 = 1.5

pKa1 = 2.0

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  Average 0.00858

B. 

Analysis of Water Samples

Table 2

Tria

l

Initial

volum

e (mL)

Final

volum

e (mL)

Ca 2+ 

Concentrati

on

Total

Hardne

ss of

Water

(ppm)

1 50 25.5 0.00219 M  219

2 50 27.2 0.00233 M  233

3 50 24 0.00206 M  206

Discussions

Three tests were executed in order

to determine how much Calcium

Carbonate solution was spent in every

trial. In Trial 1, with the initial volume of

50 ml, 25.1 ml Calcium Carbonate solution

was put to use. Accordingly, its final

volume was 24.9 ml making it the lowest

volume of Calcium Carbonate used with

regards to the three trials performed.

Trial 2 with its initial volume of 50 ml

yielded to a 23.7 ml final volume. So, only26.3 ml Calcium Carbonate solution was

depleted. As for the terminal trial, again

50 ml as the initial volume, which resulted

to 24.1 ml final volume. In the latter, 25.9

ml Calcium Carbonate was used. Alongside

its high volume amount utilization, the

pure blue color “indicator” was acquired

here for which the process aborts. All of

which resulted to the said blue color. The

Molarity of the EDTA solution wascalculated. For Trial 1, a Molarity of

0.00843 M was obtained, the lowest of the

three. For Trial 2 and Trial 3, 0.00861 M

and 0.00871 M was reckoned respectively.

In analyzing the water sample

obtained from drinking water, the concept

of titer was operated. Three trial runs

were executed in this section of the

experiment. From the collected drinking

water, given their initial volume of 50 mlfor Trial 1, only 24.5 ml usage of EDTA

solution was computed, with a final

volume of 25.5 ml. Performing Trial 2,

22.8 ml was consumed, a final volume of

27.2 ml was calculated. Finally, with a 50

ml initial volume, 26 ml was used up

resulting to a final volume of 24 ml.

Likewise, a pure blue “indicator” was

obtained for which the process aborts. All

of the trial runs completed resulted to theabove said final solution. None of the three

runs terminated as “overtitrated”. In

addition, the concentration total hardness

of water as ppm of Ca2+  was known. A

0.00219 M Ca2+ concentration, with a 219

CaCO3  ppm was obtained in Trial 1. For

Trial 2, a Ca2+ concentration of 0.00233 M

and a 233 CaCO3  ppm. Lastly, the Ca2+ 

concentration of Trial 3 was 0.00206 M

and a CaCO3 ppm of 206, the lowest of the

three.

Conclusion

The experiment tests the hardness of

the water sample. It is said that the water is

considered hard if it contains Ca+2, Mg+2,

and/or Fe+3  ions. And the mineral water used

contains Ca+2 and Mg+2  ,Viva Mineral water

which is commercially sold bottled water. This

experiment shows how much minerals are

contained in the water sample. Because

certain minerals have effect on humannutrition especially those who have

intolerance with such minerals.

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References[1]Giron, Ofelia.  ANALYTICAL

CHEMISTRY (chem26.1) Laboratory Manual .

Philippines: University of the Philippines

Baguio[2]

Skoog, West, Holler & Crouch.

(2010). Fundamentals of Analytical

Chemistry . USA, Cengage Learning[3] The Complexometric Titration.

Retrieved August 13, 2013, from

http://www.nmt.edu/~fletcher/labs/lab03/La

b03_EDTA_Complex_Titration.html

Retrieved August 13, 2013, fromhttp://homepages.ius.edu/DSPURLOC/c121/w

eek13.htm

Retrieved August 13, 2013, from

http://answers.yahoo.com/question/index?qi

d=20080303075816AAAGr5n

[4]Brown, LeMay, Bursten. 2002.

CHEMISTRY, The Central Science, 8th ed.

Philippines: Pearson Education Asia Pte. Ltd.

[5]

Harvey, David. 2000. ModernAnalytical Chemistry. United States of

America: The McGraw-Hill Companies, Inc.

[5]http://www.csudh.edu/oliver/che23

0/labmanual/calcium.htm

[6]http://www.lasalle.edu/~prushan/W

ater-lab7.pdf

Appendix

Calculations for Molarity:

CaCO3 Titer:

M CaCO3= g CaCO3 

 

M CaCO3= 0.21g CaCO3 

 

M CaCO3= 0.008393 M

EDTA Solution

Since 25 mL aliquot was taken,

25 mL = moles CaCO3 in

aliquot

25 mL = 0.00021

moles CaCO3 in aliquot

MEDTA= moles CaCO3 in aliquot * 1000ml

Final Volume of EDTA

Trial 1:

MEDTA=  

MEDTA= 0.00843 M

Trial 2:

MEDTA=  

MEDTA= 0.00861M

Trial 3:

MEDTA=  

MEDTA= 0.00871M

Calculations for the Total Hardness of Water:

Find the concentration of Ca2+:

M Ca2+= ( )( )  

Trial 1:

M Ca2+= ()( )  

M Ca2+= 0.00219 M

Trial 2:

M Ca2+= ()()  

M Ca2+= 0.00233 M

Trial 3:

M Ca2+= ()( )  

M Ca2+= 0.00206 M 

To Compute for Total Hardness:

ppm CaCO3=

( ) ( ) ( )( )( )( )( )  

Trial 1:

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ppm CaCO3=

( ) ( ) ( )( )( )( )( )  

= 219

Trial 2:

ppm CaCO3=

( ) ( ) ( )( )( )( )( )  

= 233

Tria 3:

ppm CaCO3=

( ) ( ) ( )( )( )( )( )  

= 206

Analysis of Water Sample

Red wine colored Solution (Initial)

Pure Blue Solution (Final)

Answers to Questions

1. 

Determine the precision of your

analyses using standard deviation and

relative range in parts per thousand.

MEAN :

25.5 ml

27.2 ml

24 ml

76.7

76.7

3 = 25.6

AVERAGE DEVIATION:

│25.6 – 25.5 │ = 0.1 

│25.6 – 27.2│ = 1.6 

│25.6 - 24│ = 1.6

3.3

3.3

3 = 1.1

STANDARD DEVIATION

S2 = (0.1)2 + (1.6)2 + (1.6)2

3-1

= 0.01 + 2.56 + 2.56

2

= 2.57

2. 

Explain the advantage / disadvantage

of using Eriochrome Black T over

Calgamite as indicator.

The color of the Eriochrome

Black T complexes with Mg2+ in the

solution and it forms red color. Then,

the metal is displaced from the

solution and complexes with EDTA

which causes a color change from red

to blue. And by titration, the color

change from red to blue. And it cannot

be cannot be used to indicate the

titration of Ca2+

alone because it forms

too weak that thus not gives a distinct

end point.

3. 

A 20.0 ml volume of EDTA titrant is

required to titrate 25.0 ml of standard

0.0100 M CaCO3. A 75.0 ml sample

solution of chalk requires 30.0 ml of

this EDTA titrant in the analysis of

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calcium content. Calculate both the

ppm Ca and ppm CaCO3 in the sample

solution of chalk. If the 75.0 ml sample

solution is withdrawn from a stock

solution of 500.0 ml which contains

2.1345 g of chalk, what is the % Ca in

the solid sample?

EDTA = CaCO3

20ml x M = 25ml x 0.0100

M = 0.0125 M EDTA

ppm of Ca

= M EDTAxV EDTAxMW Ca/volsample

=0.0125Mx30.0mlx(40mg/mmol)/ 75.0ml

= 0.2 ppm Ca

ppm CaCO3 

=M EDTAxV EDTAxMW Ca/volsample

=0.0125Mx30.0mlx(100mg/mmol)/75.0ml

= 0.5 ppm CaCO3

mass of Ca

=mass of CaCO3 x (MW Ca/ MW CaCO3)

= 2.1345g x (40 g/mol)/(100g/mol)

= 0.8538 g

% Ca = (mass of Ca/ mass of CaCO3) x 100

=(0.8538 g/ 2.1345g) x 100=40% Ca in the sample

4. 

The titration of a 50.0 ml water sample

for total hardness required 4.08 ml of

0.0100 M EDTA. Calculate the hardness

of the water as ppm CaCO3.

ppm CaCO3=M EDTAxV EDTAxMW CaCO3/vol sample

= 0.0100M x 4.08ml x (100mg/mmol)/ 50.0ml

= 0.0816 ppm CaCO3

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