Bromothymol blue is a commonly used pH indicator. It is a large compound with a molecular weight of 625 g/mol also determines the rough pH of a substance . It is soluble in alcohol and aqueous solutions of alkali; sparingly soluble in water. Soluble in ethanol, ether and alkalis. Slightly soluble in water. It is a good indicator of dissolved carbon dioxide (CO2) and other weakly acidic solutions. Bromothymol blue solution ‘s colour depends on the pH of the stock water used to prepare this pH indicator solution may sometimes appear yellow or reddish. Low levels of carbon dioxide or acid in solution with bromothymol blue indicator will appear blue. It changes to yellow when level of carbon dioxide or acid increases. It is applied in biology experiments to indicate photosynthetic activity (solution turns blue as plants use up carbon dioxide) or respiratory activity (solution turns yellow as carbon dioxide content increases). It can also be used for testing photosynthesis and respiration, using the same technique as used for pH testing. As photosynthesis occurs, carbon dioxide is used up, and as respiration occurs, carbon dioxide is produced. The changing level of carbon dioxide also changes the pH of the solution because carbon dioxide reacts with water to form carbonic acid, and carbonic acid decreases the pH of the solution.