Titrimetry

1. Titrimetry

2. 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.

3. 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.

4. 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.

5. 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)

6. The goal of titration is be able to deliver a stoichiometrically equivalent amount of titrant to an analyte solution

7. 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

8. 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: VolhardMethods and Fajans Method RedOxTitrations – 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

9. 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

10. 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

11. 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

12. Precipitation Titration

13. Precipitation titration involves the formation of insoluble salts. RECALL: Factors affecting solubility Temperature Solvent Common-ion effect Activity/Ionic strength pH Complexation

14. 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+ Ag+ + SCN-AgSCN (s) After all Ag+ is exhausted… Fe3+ + SCN- FeSCN2+ (aq) (RED SOLUTION) Titrate with KSCN with Fe3+ as indicator +

15. Common methods involve silver ion (Ag+) and are called ARGENTOMETRIC TITRATIONS

16. 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- - - - + - + - + +

17. Common methods involve silver ion (Ag+) and are called ARGENTOMETRIC TITRATIONS

18. Common methods involve silver ion (Ag+) and are called ARGENTOMETRIC TITRATIONS

19. 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

20. Acid-Base Titrations

21. 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

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