Download Intro to Titrimetry

Titrimetry
Titration is an analytical method that uses accurate and precise
volume delivery of one solution of known concentration to
determine the concentration of another solution through a
monitored reaction.
Titration is an analytical method that uses accurate and precise
volume delivery of one solution of known concentration to
determine the concentration of another solution through a
monitored reaction.
There are three essential parts of a titration experiment: (1)
Titrant, (2) Analyte, (3) Indicator
Titrant – solution of known
concentration which is accurately
delivered using a buret
Analyte – solution of unknown
concentration which reacts with
the titrant
Indicators – compounds which do not
participate in the general reaction between
analyte and titrant, but are indirectly
affected by the reaction producing notable
changes. They are used to monitor the
reaction.
Some reactions also produce selfindicators.
The goal of titration is be able to deliver a stoichiometrically
equivalent amount of titrant to an analyte solution
Equivalence point – is reached
when the quantity of titrant added
is the exact amount necessary for
reaction with analyte
End point – is marked by a
sudden change in a physical
property of the solution
End point ≠ Equivalence Point
Indicator error
End point ≈ Equivalence Point
(use of blank titration)
The goal of titration is be able to deliver a stoichiometrically
equivalent amount of titrant to an analyte solution
How does one design a titration experiment?
1. The reaction of the titrant and analyte must
proceed according to a definite chemical equation
2. The reaction should proceed to completion at the
equivalence point
3. Some method must be available to determine
equivalence point
a. Indicators
b. Electrochemical monitoring
4. Must be rapid
There are several types of titration according to the reactions of
the analyte
Acid-Base Neutralization reaction – utilizes the reactions of
acids with bases. Reaction is monitored using pH changes.
Ex: CH3COOH + NaOH  CH3COO- + H2O + Na+
Precipitation Titration – forms insoluble salts which also
serves as a monitor of reaction completion.
Ex: Volhard Methods and Fajans Method
RedOx Titrations – involves species which undergo redox
reactions. The reaction is monitored by RedOx indicators or
thru Electrochemical methods.
Ex: 2 MnO4- + 5 C2O42- + 16 H+  2 Mn2+ + 10 CO2 + 8 H2O
Complexometric Titrations – reactions involving
chelating/complexing agents.
Ex. EDTA titrations and Leibig Method
The concentration of the titrant must be known to a high
accuracy in order for the results to be useful
Standardization – is the process by which the concentration
of titrant is determined to a high degree of accuracy.
Primary standards are highly purified compounds that
serve as REFERENCE MATERIAL for all titrimetric
methods
Must satisfy most, if not all, of the criteria listed:
o High purity
o High air stability
o Non-hygroscopic
o Readily available at modest cost
o Highly soluble
o Large molar mass
The concentration of the titrant must be known to a high
accuracy in order for the results to be useful
Standardization – is the process by which the concentration
of titrant is determined to a high degree of accuracy.
Secondary standards are compounds whose purity is
established by chemical analysis using primary standards.
Must satisfy most, if not all, of the criteria listed:
o High purity
o High air stability
o Non-hygroscopic
o Readily available at modest cost
o Highly soluble
o Large molar mass
There are two ways of performing a titration experiment
Analyte
+
Titrant
DIRECT TITRATION
Until equivalence is reached
Analyte
+
Titrant 1
Excess
Titrant 1
+
Titrant 2
Until equivalence is reached
BACK TITRATION
Precipitation Titration
Precipitation titration involves the formation of insoluble salts.
RECALL: Factors affecting solubility
1. Temperature
2. Solvent
3. Common-ion effect
4. Activity/Ionic strength
5. pH
6. Complexation
Common methods involve silver ion (Ag+) and are called
ARGENTOMETRIC TITRATIONS
Volhard titration – involves the formation of a soluble,
colored complex at the end point
Analyte
(ex. Cl-)
+
Excess
Ag+
Add an
excess of
standard
AgNO3
+
Titrate with
KSCN with Fe3+
as indicator
Solid AgCl
is removed
Ag+ + SCN-  AgSCN (s)
After all Ag+ is exhausted…
Fe3+ + SCN-  FeSCN2+ (aq)
(RED SOLUTION)
Common methods involve silver ion (Ag+) and are called
ARGENTOMETRIC TITRATIONS
Species Analyzed
Notes
VOLHARD METHOD
Br-, I-, SCN-, CNO-, AsO43-
Precipitate removal is
unnecessary
Cl-, PO43-, CN-, C2O42-, CO32-,
S2-, CrO42-
Precipitate removal is
required
Common methods involve silver ion (Ag+) and are called
ARGENTOMETRIC TITRATIONS
Fajans titration – involves the adsorption of a colored
indicator on the precipitate at the end point.
Ag+ + Cl-  AgCl (s)
-
In
2-
+
+
-
In 2-
-
+
-
+
-
-
+
-
+
+
-
+
-
-
+
-
+
-
-
+
-
-
+
+
-
+
-
+
-
-
+
-
+
-
+
+
-
+
-
+
-
-
+
-
+
-
+
+
-
+
-
+
-
-
+
-
+
-
+
+
+
Common methods involve silver ion (Ag+) and are called
ARGENTOMETRIC TITRATIONS
Common methods involve silver ion (Ag+) and are called
ARGENTOMETRIC TITRATIONS
Species
Titrant
Indicator
FAJANS
Cl-, Br-, I-, SCN-, Ag+
Fe(CN)64-
Fluoresceine,
Dichlorofluorescein, eosin,
bromophenol blue
F-
Th(NO3)4 to produce ThF4
Alizarin Red S
Zn2+
K4Fe(CN)6 to produce
K2Zn3[Fe(CN)6]2
Diphenylamine
SO42-
Ba(OH)2 in 50% v/v MeOH
Alizarin Red S
Hg22+
NaCl to produce Hg2Cl2
Bromophenol blue
PO43-, C2O42-
Pb(Ac)2 to give Pb3(PO4)2 and Dibromofluorescein or
PbC2O4.
fluorescein
EXAMPLES
1. A Fajans titration of a 0.7908 g sample required 45.32
mL of 0.1046 M AgNO3. What is the %Cl of the
sample?
2. The bismuth in 0.7405 g of an alloy was precipitated
as BiOCl and separated from the solution by
filtration. The washed precipitate was dissolved in
nitric acid to convert all chlorine to Cl-. This was
then treated with 10.00 mL of 0.1498 M AgNO3,
causing the precipitation of AgCl. The excess AgNO3
required 12.92 mL of 0.1008 M KSCN for titration.
Calculate % Bi in the sample
Acid-Base Titrations
Acid-base indicators are usually weak acids (HIn) which have
different color than its conjugate base (In-).
basic
acidic
change
occurs over
~2 pH units
Acid-base indicators are usually weak acids (HIn) which have
different color than its conjugate base (In-).