o-Phenylenediamine (OPD) is widely used as a reagent in biochemical assays, particularly in ELISA (Enzyme-Linked Immunosorbent Assay) tests, where it serves as a chromogenic substrate. When oxidized by horseradish peroxidase (HRP), OPD produces a colored product, allowing for the detection and quantification of target proteins or antibodies.

In the dye industry, OPD is a key intermediate for the synthesis of various dyes and pigments, including hair dyes, textile dyes, and fur dyes. Its ability to form complex structures makes it valuable for creating vibrant and stable colors.

Additionally, OPD is utilized in the production of polymers and plastics, as well as synthetic rubber, where it acts as a monomer, cross-linking agent, or antioxidant, enhancing material properties such as durability, thermal stability, and flexibility.

In the pharmaceutical industry, OPD serves as an intermediate in the synthesis of certain drugs, including antibiotics and fungicides.

In analytical chemistry, it is employed as a reagent for detecting substances like glucose and hydrogen peroxide, and as a chelating agent for metal ion detection, aiding in the identification and measurement of specific metals in samples.

In the electronics industry, OPD is used in the production of semiconductor materials and protective coatings.

However, due to its potential toxicity, carcinogenicity, and allergenic properties, handling OPD requires strict safety precautions to minimize health risks.