Titrations

1. Titrations

2. Homework Ch 5 Homework Ch 5Problems 3,4,6,9,12,13,14 Due Wed Oct 16

3. Principles of Volumetric analysis A titration is a process whereby a known standard reagent is delivered from a buret to the analyte until the reaction is complete. Titrations may be based on a variety of chemical reactions such as acid/base, redox, complexation, or precipitation.

4. Principles of Volumetric analysis Generally an indicator is required to signal when the end point of the reaction has occurred and tells the analyst to stop the addition of the titrant.

5. Principles of Volumetric analysis The exact stopping point for the titration is known as the equivalence point. Where the indicator changes is the end point. It is desirable that these two points be close to each other. The difference between the end point and the equivalence point is the titration error.

6. Principles of Volumetric analysis In a few titrations the end point is signaled by the change in the color of one of the rectants. More common is the addition of a dye that changes color to signal the end point. Various instrumental methods are also used to signal the end point, as the following slide shows with the pH meter.

9. Principles of Volumetric analysis The standard titrant solution may be prepared either by 1) accurately weighing out very pure reagents (primary standards) or

10. Principles of Volumetric analysis The standard titrant solution may be prepared either by 1) accurately weighing out very pure reagents (primary standards) or 2) by making an approximate solution and then standardizing it against some pure primary standard reagent.

11. Criteria of Primary Standard Reagents

12. Criteria of Primary Standard Reagents • Greater than 99.9% purity

13. Criteria of Primary Standard Reagents • Greater than 99.9% purity • Stable

14. Criteria of Primary Standard Reagents • Greater than 99.9% purity • Stable • Ability to be dried to remove adsorbed moisture.

15. Principles of Volumetric analysis There are two (2) different ways of using a volumetric determination, depending on the nature of the reaction between the analyte and the titrant. • Direct titration – titrant is added to the analyte until the end point is reached.

16. Principles of Volumetric analysis There are two (2) different ways of using a volumetric determination, depending on the nature of the reaction between the analyte and the titrant. 2. Back titration – a measured excess amount of a standard reagent is added to the analyte, followed by titration with a second standard reagent to determine the amount of unreacted first reagent.

17. Principles of Volumetric analysis Back titrations are useful whenever there is not a suitable indicator or the kinetics or equilibrium constant are not extremely favorable for the direct titration.

18. Calculations for Titrations The key steps in the calculations of a direct titration are 1. The moles of titrant = Volume x Molarity

19. Calculations for Titrations The key steps in the calculations of a direct titration are 1. The moles of titrant = Volume x Molarity 2. Using the stoichiometry of the balanced equation, relate the number of moles of the titrant to the number of moles of the analyte.

20. Calculations for Titrations In a back titration, the calculation consists of • Calculation of the total number of moles of the excess reactant. • From the titration with the second reactant, calculate the number of moles of analyte that has reacted with the first reactant.