Bromothymol blue (alt: dibromothymolsulfonephthalein, bromthymol blue, BTB) is a chemical indicator for weak acids and bases. The bromothymol blue acts as a weak acid in solution and therefore can be in acid or base forms which appear yellow and blue respectively. It is green in neutral solution. Bromothymol blue is typically sold in solid form as the sodium salt of the acid indicator. It also finds occasional use in the laboratory as a biological slide stain. At this point it is already blue, and a drop or two is used on a water slide. The coverslip is placed ontop of the water droplet and the specimen in it, with the blue coloring mixed in. It is sometimes used to define cell walls or nuclei under the microscope. Bromothymol blue is mostly used in measuring substances that would have relatively low acidic or basic levels (near a neutral pH). It's often used in pools, fish tanks, or measuring the presence of carbonic acid in a liquid. Synonyms: 3',3''-Dibromothymolsulfonephthalein sod