Litmus Reddeners are a type of ACID used to test the pH of a solution. It is a mixture of litmus and a weak base, such as sodium hydroxide, that turns red in the presence of an ACID and blue in the presence of a base. The acidity or alkalinity of a solution can be determined by the color change of the litmus paper. Litmus Reddeners are commonly used in laboratories and other scientific settings to measure the acidity or alkalinity of a solution.