Gravimetric Analysis Principles and Techniques

Gravimetric Analysis Fundamentals

Gravimetry is based on the law of mass action, essentially involving two experimental steps:

• Weight of the sample.
• Weight of the analyte or a known chemical substance of known composition containing the analyte.

Types of Gravimetric Analysis

Gravimetry by Volatilization

• The analyte is separated from other constituents of a sample by converting it into a known gas composition.
• The weight of this gas serves as a measure of the concentration of the analyte.

Gravimetry by Precipitation

• The analyte is separated from the components of a solution as a precipitate that is treated and converted into a stable compound of known composition.

Precipitators Agents

• Specific: They are rare; they react with a single chemical species.
• Selective: It is more common; reacts with a limited number of chemical species.

Gravimetry by Precipitation Requirements

For a precipitate to be useful in gravimetry, it is necessary:

• That it is insoluble in the medium in which it occurs.
• That it can be filtered easily.
• That it is pure and has a constant and known composition.

Classification of Precipitate Particles

The size of the precipitate particles is a function of the nature of the precipitate and the experimental conditions under which they occur. The precipitate size can be:

• Colloidal
• Crystalline

Types of Precipitates

Colloidal Precipitate

• The size is in the order of micrometers (μm).
• Does not sediment easily.
• Cannot be filtered using common filtration media.

Crystalline Precipitate

• Its size is in the order of millimeters (mm).
• Sediments with ease.
• Can be filtered using gravity means.

Titration Method

Titration is an analysis method that consists of measuring a precise volume of a reagent solution of known concentration to react stoichiometrically with the analyte contained in the sample.

Nomenclature in Titration

• Titrant: A substance in a solution of known concentration (standard).
• End-Point (EP): When the reaction point or the point of volumetric equivalence is displayed or sensed.
• Indicator: A substance or technique that visualizes and detects the equivalence point (PE).
• Volumetric Error: |EP - PE|.

Background Requirements for Titration

1. The volumetric reaction must be complete and stoichiometric.
2. The volume of the analyte solution (substance to value) must be easily manageable.
3. The titrant is added gradually so that its intake can be monitored easily.

Note: It is not possible to assess very small trace amounts of analyte using this method.