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 self-indicators.

End point ≠ Equivalence Point Indicator error 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 Example I have 5 mmol oxalic acid reacts with 2 moles of permanganate

How does one design a titration experiment? The reaction of the titrant and analyte must proceed according to a definite chemical equation The reaction should proceed to completion at the equivalence point Some method must be available to determine equivalence point Indicators Electrochemical monitoring Must be rapid Example I have 5 mmol oxalic acid reacts with 2 moles of permanganate

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: High purity High air stability Non-hygroscopic Readily available at modest cost Highly soluble 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: High purity High air stability Non-hygroscopic Readily available at modest cost Highly soluble 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 Temperature Solvent Common-ion effect Activity/Ionic strength pH 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 Add an excess of standard AgNO3 Solid AgCl is removed Analyte (ex. Cl-) + Excess Ag+ 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 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. In 2- Ag+ + Cl-  AgCl (s) + + + - - + - + - + - - + - + - + - + - + - + - + - + - + - - + - + - + - + - + - + - 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-, Fe(CN)64- Ag+ 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 Hg22+ NaCl to produce Hg2Cl2 Bromophenol blue PO43-, C2O42- Pb(Ac)2 to give Pb3(PO4)2 and PbC2O4. Dibromofluorescein or fluorescein

EXAMPLES 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? 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

change occurs over ~2 pH units Acid-base indicators are usually weak acids (HIn) which have different color than its conjugate base (In-). basic change occurs over ~2 pH units acidic

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