Topic 6/7 - Gravimetric Analysis & Topic 8 - Activity and EQ

What are the three types of Gravimetric Analyses?

Precipitation, Volatilization, and Electroanalytic/Electrogravimetric

What is the general method behind Gravimetric Analysis?

The phase of the analyte is changed to separate the ion in its pure form from the original mixture

What is precipitation?

When a denser phase is produced by a less dense phase (e.g., solid from liquid)

What is volatilization?

Gas comes out of a liquid or solid medium

What method is generally used for volatilization?

Volatiles samples are separated or chemically decomposed by heating at a high temp

What is electroanalytical/electrogravimetric analysis?

Analysis based on electrochemical properties

What are the two types of precipitating agents?

Specific and Selective

What is a Specific precipitating agent?

Reacts only with a single chemical species

What is a Selective precipitating agent?

Reacts with a limited number of species

What four features make a good precipitate?

1. A substance that is easily filtered and washed

2. A substance with low solubility

3. A substance that is unreactive with the atmosphere

4. A substance of known composition

What makes something filterable?

Particle size

What is a colloidal suspension?

Particles 10^-7 to 10^-4 cm in diameter, can be hard to centrifuge and filter

What is a crystalline suspension?

Larger particles, often settle spontaneously, easier to centrifuge and filter

What four methods can be used to make as much crystalline precipitate as possible?

1. Elevate temperature to increase solubility and cool slowly

2. Slow addition of precipitating agent

3. Using dilute solutions

4. Cause nucleation and particle growth (introduce a seed crystal)

What is coprecipitation?

When a soluble compound is removed from the solution during precipitate formation

What are the 4 types of coprecipitation?

1. Surface adsorption

2. Mixed crystal formation

3. Occlusion

4. Mechanical Entrapment

What is surface adsorption?

Ions retained on the surface of a solid (solved by washing)

What is mixed crystal formation?

One of the ions in a crystal lattice being replaced by another (solved by recrystallization)

What is occlusion?

Trapping one substance within another, caused by crystals growing too fast (solved by recrystallization)

What is mechanical entrapment?

Crystals that are close together trapping pockets of solution as they grow together (solved by recrystallization)

What 3 methods can be used to minimize coprecipitation?

1. Recrystallization

2. Washing precipitates

3. Increasing analyte concentrations

What are the 7 main methods of Electrogravimetric Analysis

1. Potentiometry

2. Amperometry

3. Conductometry

4. Electrogravimetry

5. Voltammetry

6. Polargraphy

7. Coulommetry

What is Potentiometry?

Measuring electric potential (voltage) in a sample while keeping constant current between electrodes

What is Amperometry?

Measuring current (amperage) in a sample while keeping constant potential between electrodes

What is Conductometry?

Measuring conductance while AC current is constant between electrodes

What is Electrogravimetry?

Weighs the solids that are plated out onto an electrode

What is Voltammetry?

Measuring the changes in current while the potential is varied

What is Polargraphy?

A subtype of Voltammetry using a liquid metal (Mercury) electrode

What is Coulommetry?

Measuring the electricity consumed (coulombs) during an electrochemical reaction

What are the advantages of Gravimetric Analysis?

When methods are followed carefully, it is very precise

1. Often no required standards to calibrate for an unknown

2. There is little issue with instrument error

3. There is little expensive equipment required

What are 3 disadvantages of Gravimetric Analysis?

1. Can typically only analyze 1 element at a time

2. Modern methods of flash combustion w/ gas chromatography are superior to regular combustion analysis

3. Methods are often very convoluted or detailed

How do neutral species affect equilibrium?

Equilibrium independent of electrolyte concentration

How do ionic species affect equilibrium?

Electrolyte effect increases with charge

How does ionic strength affect solubility?

Extra electrolytes cause an overall decrease in attraction between ions, increasing the solubility