The Expanation Why I3 Was Used in Winklers Redox Titration

Chemistry 26.1Total Ha

-

mo

les =

(

)

(

)

Unreacted Ha

-

moles =

(

)

(

)

2)

[Ha

2

] =

(

)

(

)

*

Number

of electrons (n) and m

oles of electron

(x)

in

t

he equation will be determined by the

reduction half

-

reaction

.

Example: Cl

2

+ 2e

-

-

> 2Cl

-

has n = 2 and x = 2 for its number and moles of electrons respectively.

2

nd

Written Laboratory Examination Reviewer

A.Y. 2010

–

2011

Experiment # 8

QUANTITATIVE DETERMINATION OF TOTAL HARDNESS IN DRINKING WATER BY

COMPLEXOMETRIC EDTA TITRATION

INTRODUCTION

Primary Standard: Calcium Carbonate (CaCO

3

)

Titrant: EDTA

Titrant/Analyte: Mineral Water Sample

Indicator: Eriochrome Black T (EBT)

Total hardness is a measure of the concentration of

calcium and magnesium ions

in water. It is,

however, measured primarily as

ppm of

calcium carbonate (CaCO

3

)

due to

the negligible

presence of other ions.

Ideal Freshwater Ca

2+

Concentration: 10

–

250 ppm

Ideal Freshwater Mg

2+

Concentration: 5

–

125 ppm

Choice of Titrant: Ethylenediaminetetraacetic acid (EDTA)

EDTA is set as the titrant due to its

hexadentate

property as a ligand in the formed metal

-

EDTA

complex. This means that it can bind with metal ions in a

1:1 ratio

due to its numerous binding

sites

, simplifying calculations.

It is important to note that EDTA exists in different states depending on the p

H of the solution.

At pH 10, EDTA is represented as

Y

4

-

in equations

.

A 1:1 ratio also means a

sharper endpoint

for the titration.

Choice of Indicator: Eriochrome Black T (EBT)

Eriochrome

Black T (EBT) was used in the experiment to determine the endpoint of the EDTA

titration process. The color of the Metal

-

EBT complex in a pH 10 solution (due to ammonia

buffering) is

wine red

*

. Upon reaching the endpoint, the complex is broken up and the

solution

appears

blue

.

Note that the

charge of In is

-

3

.

H

2

Y

2

-

+ MgIn

-

-

> MgY

2

-

+ HIn

2...