Methylene blue is a heterocyclic aromatic chemical compound with the chemical formula C16H18N3SCl. It has many uses in biology and chemistry; for example, it can be used as a stain and as a pharmaceutical drug. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in water. The hydrated form has 3 molecules of water per unit of methylene blue. Methylene blue should not be confused with methyl blue, another histology stain, new methylene blue, nor with the methyl violets often used as pH indicators.

Function:
1. Mainly used for dyeing cotton, acrylic, hemp, silk, paper, also used for the coloration of bamboo and wood, used to make ink, lake, and can also be used for dyeing biological bacteria.
2. Used as redox indicator, adsorption indicator and biological dye
3. Bacterial stains. The serum protein biochemical test is accompanied by a mixed indicator. Prepare exfoliated cells to test staining solution. Preparation of alkaline methylene blue staining solution. Redox indicator.
4. Determination of mercury and tin. A titrant is reduced to precipitate perchlorate and citrate. Photometric determination of selenium and molybdenum with others.