CHAPTER 3: GRAVIMETRIC ANALYSIS

PROCESS

FEATURES

  1. Analyte is selectively coverted to an insoluble form.
  1. Quantitatively measure the mass of a pure compound to which the analyte is chemically related.

Uses the mass of a product to calculate the quantity of analyte.

Traditional method

Slow

Labour Intensive

Wide range of sample concentration

No calibration needed

Accurate to better than +0.01% (atomic weight)

TYPES

PRECIPITATION

VOLATILIZATION

ELECTROGRAVIMETRY

The analyte is separated by deposition on electrode by electrical current. Mass of product provides a measure of analyte concentration.

Analyte is separated from solution as precipitate and converted to compound of known composition. Can be weigh.

Analyte separated from sample by conversion of gas of known composition.

STEPS

  1. Sample dissolution
  1. Precipitation
  1. Digestion
  1. Filtration & washing precipitates
  1. Drying precipitates

Weighing

Data calculations

REQUIREMENTS FOR PRECIPITATES

Low solubility

Large size for easy filtration

Easily washed

Free of impurities (pure)

Unreactive to atmosphere

Known composition after drying

FACTORS THAT INFLUENCE THE PROPERTIES OF PRECIPITATES

pH

Solubility of precipitates

Precipitating agents

Temperature

If temperature increases, Ksp will also increases

Increasing the pH will decrease solubility

PROCESS OF PRECIPITATION

  1. Analyte + Precipitating agent
  1. Supersaturation
  1. Primary nucleation
  1. Crystals growth (secondary nucleation)

FACTORS DETERMINE PARTICLES SIZE OF PRECIPTATES

Relative Supersaturation = (Q - S) / S

Q= conc. of reactants before precipitation

S= solubility of precipitation at equilibrium

(Q - S) = degree of supersaturation

How to get small (Q - S)?

Precipitate from diluted solution

Add dilute precipitate slowly with stirring

Precipitates from hot solution

Chosen for their ability to be selective and to form highly insoluble precipitates.

Type

Inorganic

Organic

Not very selective

Form slightly soluble salts

useful for metals

IMPURITIES IN PRECIPITATES

Co-Precipitation

Post Precipitation

soluble compounds carried out of solution by precipitation

Foreign compound precipitate on top of desired precipitate because stand too long before being filtered

3 Types

Surface adsorption

Mixed-crystal formation

Occlusion & mechanical entrapment

Soluble compound carried out of solution on the surface of a precipitate

Interferences are incorporated into precipitating crystals

Two crystals grow together and trap a species in between them

Produce larger & purer particles

To remove adsorbed impurites & mother liquor

Remove water/absorbed electrolyte/volatile species

Repeat process of heating & weighing until constant weight obtained

GF= (mass of substance sought) / (mass of precipitate)