Gravimetric and Combustion Analysis

1. Gravimetric and Combustion Analysis

2. Gravimetric Analysis • Gravimetric analysis: the mass of a product is used to calculate the quantity of the original analyte. • An example: Ag+ +Cl-→ AgCl (s) • The mass of AgCl tells how much Cl- was originally present.

3. Precipitation • The ideal product of a gravimetric analysis should be insoluble, easily filterable, and very pure, and should possess a known composition. • Crystal growth: occurs in two phases • Nucleation • Molecules come together randomly • Particle growth • The addition of more molecules to a nucleus

4. Measure taken to promote formation of easily filtered precipitates: • Raising the temperature during precipitation • Slowly adding and vigorously mixing reagents • Maintaining a large sample volume • Using homogenous precipitation

5. Precipitation in the presence of electrolyte • Ionic compounds are usually precipitated in electrolytes • Ex: a colloidal particle of AgCl • Colloidal particles must collide to coalesce • The negative charged ionic atmosphere of the particles repel each other • Must have enough kinetic energy to overcome the electrostatic repulsion in order to coalesce

6. Heat increases the kinetic energy of the particles. • Increasing the electrolyte concentration decreases the volume of the ionic atmosphere and allows the particles to come closer together before the electrostatic repulsion becomes significant.

7. Digestion • The period of standing that slowly promotes the recrystallization of the precipitate.

8. Purity • There are three types of impurities: • Adsorbed: bound to the surface of the crystal • Inclusion: occupies lattice sites in a crystal • Occlusion: it becomes trapped in a pocket within a growing crystal • To overcome impurities reprecipitate the product. • Or you can treat the solution with a masking agent to prevent the impurities from reacting with the precipitate.

9. Product Composition • The final product must have a known stable composition. • Hygroscopic substance: substance that picks up water from the air and is therefore difficult to weigh accurately. • Ignition: strong heating used to change the chemical form of some precipitates. • Thermogravimetric analysis: a substance3 is heated and its mass is measured as a function if temperature.

10. Combustion Analysis • A sample is burned in excess oxygen and the products are analyzed. • Instead of weighing products, modern instruments use thermal conductivity, infrared absorption, or coulometry.

11. Gravimetric Combustion Analysis • A partially combusted product is passed through a catalyst at an elevated temperature to complete the oxidation of carbon dioxide and water. • The product is then flushed through a chamber containing Phosphorus pentoxide. • Then a chamber with Ascarite which absorbs carbon dioxide. • A gurad tube prevents atmosphereic water and carbon dioxide from entering the chamber.

12. Combustion Analysis Today • Elemental analyzers use an oxidation catalyst to complete the oxidation of a sample and a reduction catalyst to carry out any required reduction and to remove excess O2. • Dynamic flash combustion: creates a short burst of gaseous products.