Reagent — chemical substances used for detection, transformation and analysis

Types and characteristics

Reagents are classified by function, specificity and form. Important categories are:

• Indicators: molecules that change color at particular pH or redox conditions (for example, litmus or phenolphthalein).
• Titrants: standardized solutions used to determine concentrations through titration.
• Precipitating and complexing reagents: form insoluble salts or colored complexes to separate or detect ions.
• Derivatizing reagents: chemically modify an analyte to improve detection by chromatography or spectrometry.
• Biological reagents: antibodies, enzymes and nucleic acid probes used in diagnostics and biochemical assays.

Common examples and analytical tests

Certain reagents are closely associated with classic tests. For instance, potassium dichromate oxidizes many primary and secondary alcohols and produces a visible color change as chromium(VI) is reduced; this property is utilized in some alcohol tests (alcohol). Tollens' reagent gives a reflective silver deposit with aldehydes. Concentrated nitric acid attacks many metals, but gold resists nitric acid alone and is dissolved only by mixtures such as aqua regia; this chemical behavior is exploited in metal analysis and refining (gold).

History and development

Use of reactive substances to reveal material properties goes back to ancient metallurgy and early chemistry. Over centuries, systematic analytical chemistry produced tailored reagents for identifying functional groups, metal ions and clinical markers. The 19th and 20th centuries brought standardized preparation methods and the concept of reagent-grade chemicals with specified purity for reliable measurements.

Practical considerations

Choosing a reagent involves trade-offs in selectivity, sensitivity and safety. Many effective reagents are hazardous—chromates are toxic, strong acids are corrosive, and some reagents are air- or moisture-sensitive—so proper storage, labeling, handling and disposal are essential. Reagent-grade versus technical-grade distinctions indicate suitability for analytical applications. Certified reference reagents and standards support accuracy and comparability across laboratories.

Distinctions and notable facts

In usage the term "reagent" emphasizes a chemical's role in testing or inducing a change, while "reactant" is a broader term for substances consumed in a reaction. Modern practice extends the term to biological and instrumental reagents used in assays and automated analyzers. Reliable analytical chemistry depends on well-characterized reagents, good laboratory technique and appropriate controls.