Instrumental and Analytical Chemistry: Gravimetric analysis

Gravimetric analysis/methods:

Gravimetric analysis:Based on mass measurements

Gravimetric methods: based on producing and measuring the mass of solids form from analytes. The mass of solid can be linked to the amount of analyte.

Precipitation Gravimetry:

The analyte is separated from a solution of the sample as a precipitate which is converted into a compound of known composition that can be weighed. The amount of analyte is determined from the weight of the compound.

Volatilisation Gravimetry:

The analyte is separated from the constituents of a sample by converting it to a gas of known chemical composition. The amount of analyte is determined from the weight of the gas.

Thermogravimetry:

The analyte is separated from the constituents of a sample by volatilisation whiclst the sample mass is continuously monitored as the applied temperature is increased.

Electrogravimetry:

The analyte is separated from the constituents of a sample by deposition of an electrode. The mass of deposited material is a measure of the analyte concentration.

Key to precipitation gravimetry:

Convert analytes into precipitates made of pure solids of known composition by the use of a precipitating agent.

Precipitating agent:

• should react specifically or at least selectively with the analyte

• Should deliver a precipitate that is:

  • highly insoluble

  • Easy to filtrate and wash to remove contaminates

  • Of known composition after it is dried/ignited

  • Stable when exposed to ambient air

Specific agent:

reacts only with the a single chemical species, the analyte of interest

Selective agent:

Reacts with a limited number of chemical species, including the analyte of interest

Ksp ( the solubility product constant)

AaBb(s) ←> aA(as) + bB(aq)

Ksp = [A]^a[B]^b

AaBb (s) : insoluble solid portion

Aa (aq) + bB (aq) : soluble dissolved portion of the compound

Soluble/ insoluble portions are in chemical equilibrium

Steps in precipitation gravimetry:

1. Weigh sample

2. Dissolve sample

3. Precipitate target analyte

4. Filter precipitate

5. Wash precipitate

6. Dry/ignite precipitate

7. Weigh dry/ignited precipitate

Types of precipitates:

• Colloidal precipitates

• Crystalline precipitates

Colloidal precipitates:

• obtained from colloidal suspensions (whose tiny particles are naked to the human eye)

• Tend not to settle and are difficult to filter

• Can be made to coagulate/agglomerate into larger particles that precipitate and can be filtered

Crystalline precipitates:

• obtained from crystalline suspensions

• Large particles are visible to the naked eye

• Tend to settle spontaneously and are easily filtered

Precipitation mechanisms:

1. Supersaturation

2. Nucleation

3. Growth

Precipitation mechanisms: Superstation

• occurs when a solution contains more dissolved salt than at saturation equilibrium

Precipitation mechanisms: Nucleation

• a few ions, atoms or molecules (as few as 4/5) join to form a stable solid cluster

Precipitation mechanisms: Growth

• ions/atoms/molecules are added to a cluster to form larger particles

Nucleation: colloidal suspension

When rate of nucleation predominates, many small particles are formed

Nucleation: crystalline suspension

When the rate of nucleation < rate of particles grown, big particles are formed

Particle growth predominates, smaller number of large particles precipitate, easy to filter, less impurities

Relative Supersaturation:

• key property determining the particle size of precipitates

  LARGE RSS VALUES → Q.S AND S IS SMALL

• Solution is highly supersaturated and very unstable

• Conditions that lead to colloidal suspensions and small particle precipitates

  SMALL RSS VALUES -. Q = S AND S LARGE

• Solution is slightly supersaturated and a little unstable

• Conditions that lead to crystalline suspensions and large particles precipitates

Homogenous precipitation:

Consists of generating a precipitating agent via a slow chemical reaction

• local precipitating reagent access does not occur since the reagent is gradually generated at every point of the solution

• Relative supersaturation is kept low during the entire precipitation facilitating the formation of larger particle purer precipitate.

Co-precipitation:

A process where an otherwise soluble impurity is precipitated along with the desired analyte

1. Surface adsorption

2. Inclusion

3. Occlusion

4. Mechanical entrapment

Co-precipitation: surface adsorption

Impurities are adsorbed on the precipitate surface

• high surface area colloidal particles can adsorb large amounts of impurities

• Low surface area crystalline particles are less prone to impurity adsorption

Digestion in hot precipitation solution allows the dissolution of adsorb impurities whilst particles grow denser and larger, improving precipitate purity

Co-precipitation: Inclusion

Impurities are included in the precipitate crystal lattice

• both colloidal and crystalline precipitates can suffer from inclusion

• Ion impurity must have the same charge an similar size to the displaces ion

Mixed crystals are formed little can be done to improve purity

Co-precipitation: Occlusion

Ions of the counter-ion layer are occluded (trapped) in the precipitate as impurities

• low RSS values can reduce the occlusion of impurities

Co-precipitation : Mechanical entrapment

A portion of precipitation solution is trapped between growing precipitate crystals

• low RSS values can reduce the trapping of precipitation solution impurities

Thermogravimetric analysis:

an analytical chemistry technique in which the mass of a sample is measured over time as the temp is changed in a controlled atmosphere

Thermogravimetric analysis: factors of mass changes

• thermal stability

• Composition

• Purity

• Decomposition Reactions

• Decomposition temperatures

• Absorbed moisture content of the sample

Electro gravimetric analysis

• signal is the mass of an electro deposit on the cathode/anode in an electrochemical cell

• The analyte is deposited as a solid film on an electrode either through oxidation or reduction

• The electrode is weighed before and after the exhaustive deposition of the analyte. The difference in electrode weight gives the analyte mass, then the concentration is calculated.

What does a large/small values of Ksp mean?

Large Ksp = soluble

Small Ksp = insoluble