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.

Titrant – solution of known concentration which is accurately delivered using a buretAnalyte – solution of unknown concentration which reacts with the titrantIndicators – 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 self-indicators.

There are three essential parts of a titration experiment: (1) Titrant, (2) Analyte, (3) Indicator

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 analyteEnd point – is marked by a sudden change in a physical property of the solution

End point ≠ Equivalence Point

Indicator errorEnd 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 pointa. Indicatorsb. 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 MethodRedOx 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

H2OComplexometric 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 purityo High air stabilityo Non-hygroscopico Readily available at modest costo Highly solubleo 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 purityo High air stabilityo Non-hygroscopico Readily available at modest costo Highly solubleo Large molar mass

There are two ways of performing a titration experiment

Analyte Titrant+

Until equivalence is reached

Analyte Titrant 2Titrant 1+ Excess Titrant 1 +

Until equivalence is reached

DIRECT TITRATION

BACK TITRATION

Precipitation Titration

Precipitation titration involves the formation of insoluble salts.

RECALL: Factors affecting solubility1. Temperature2. Solvent3. Common-ion effect4. Activity/Ionic strength5. pH6. 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-)

Add an excess of standard

AgNO3

Excess Ag+

Solid AgCl is

removed+

Titrate with KSCN with Fe3+

as indicator+

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 NotesVOLHARD METHODBr-, I-, SCN-, CNO-, AsO4

3- Precipitate removal is unnecessary

Cl-, PO43-, CN-, C2O4

2-, CO32-, S2-, CrO4

2-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 IndicatorFAJANSCl-, Br-, I-, SCN-, Fe(CN)6

4-Ag+ Fluoresceine,

Dichlorofluorescein, eosin, bromophenol blue

F- Th(NO3)4 to produce ThF4 Alizarin Red SZn2+ 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-, C2O4

2- Pb(Ac)2 to give Pb3(PO4)2 and PbC2O4.

Dibromofluorescein or fluorescein

EXAMPLES1. 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-).

acidic

basicchange occurs

over ~2 pH units

Acid-base indicators are usually weak acids (HIn) which have different color than its conjugate

base (In-).