litmus paper turns red under acidic conditions, and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5–8.3 at 25 °C (77 °F). Neutral litmus paper is purple.

 APPLICATION: Litmus paper is a filter paper that has been treated with a lichen-provided natural-soluble dye.

Blue litmus paper is prepared by impregnating white paper in an infusion of the litmus mixture mentioned above. The paper is then carefully dried in open air. Red litmus is similarly prepared but a small percentage of sulfuric or hydrochloric acid is added to cause it to turn red.

LITMUS BLUE INDICATOR PAPERS