The 40% caustic soda (sodium hydroxide, NaOH) is a caustic alkaline solution used in various scientific and industrial applications. In chemistry and biology, 40% caustic soda is often used to adjust the pH of solutions and media because it is a strong base and thus can raise the pH.

In histology and cell biology, caustic soda can be used to remove proteins and lipids from tissue samples. In microbiology, caustic soda can be used to break down bacterial cell walls and provide access to intracellular structures and substances, which is important for the extraction of nucleic acids and proteins.

In addition, the 40 % caustic soda solution is used in industry, for example in the production of soaps and detergents, the neutralization of acids, the treatment of waste water, and in the paper and textile industries.
In metallography, the 40 % caustic soda solution can be used as an etchant for aluminum welds (aluminum alloy/ pure aluminum/ hard aluminum alloys), as well as for macroscopic weld assessment. The 40 % sodium hydroxide solution provides a high-contrast image of the individual weld layers and the heat-affected zones.