Key Definitions in Chemical Analysis

Selectivity in Chemical Analysis

A reaction used in chemical analysis is selective when the reaction is characteristic of few substances, showing a preference for some of these. When the reaction is carried out only on a particular substance, it is specific.

Limit of Detection (LOD)

Refers to the minimum amount of a substance that can be detected in a trial.

Sensitivity of Analytical Methods

The ability of a method to discriminate a change in concentration caused by a variation in the measurement of the analytical signal.

Qualitative vs. Quantitative Analysis

The science that studies the principles and techniques of chemical analysis. It is divided into:

• Qualitative analysis: Its purpose is the identification of the constituents of a sample (organic and inorganic).
• Quantitative analysis: Aims to determine the amount of a component (analyte) in a sample. The determination of the analyte depends on the matrix environment within the sample.

There are two main techniques for quantitative determination:

• Standard techniques: Performed in small labs, they are inexpensive and include gravimetry and volumetry. Their basic instrument is the balance.
• Instrumental techniques: These are more modern and require specific apparatus for the determination. They are used in large laboratories, and their most important advantage is speed of determinations.

Volumetric Analysis Terminology

In volumetric analysis, one measures the volume of a reagent of known concentration (standard) and relates it to the amount of substance being determined.

Titration is the process of adding a volume of a solution of known concentration to a substance to be determined.

Standard solution: The solution of known concentration.

The equivalence point in titrations is where the number of equivalents of titrant is equal to the number of equivalents of the substance titrated. The equivalent gram is the equivalent weight in grams and is determined by the reaction that takes place in the titration.

Standardization (or Factorization): Is the process of correcting the approximate normality of a solution in order to obtain its exact normality.

Normality: Is a unit of concentration, expressed in gram equivalents per liter.

The endpoint of the titration: Seen through an indicator that changes color when there is an abrupt change of some property in the titration reaction. The endpoint is marked with indicators, which can be:

• Physical nature: These are based on the measurement of any physical property (conductivity, pH, etc.). In these methods, the endpoint and the equivalence point match.
• Chemical nature: These are chemicals that are added to the solution being titrated and are external to it. They have the property of having a color change at the equivalence point. The endpoint does not necessarily match the equivalence point.