In the Classroom
Re-Blue-ing Blue Litmus Paper Margaret J. Steffel Department of Chemistry, Ohio State University Marion Campus, 1465 Mt. Vernon Ave., Marion, OH 43302-5695 Surely every lab instructor has experienced students’ confusion about “blue” litmus paper that isn’t really blue. Blue litmus paper gradually becomes less intensely blue, often becomes lavender, and might become as pink as slightly faded red litmus paper. Students who use this socalled blue paper see that their red paper turns blue with a particular solution and their “blue” paper also becomes much bluer with the same solution. Or their “blue” paper does not change color noticeably with a solution that they know is supposed to be acidic. The students’ learning, and also their grades on laboratory reports, might be adversely affected. Therefore, an instructor should make the effort to check the blue litmus paper that is used by each student in a laboratory class, since papers of different ages, and therefore of different shades of blue, might be in use.
Paper that is not distinctly blue can be easily and quickly “re-blued” in the following manner. Litmus paper is purchased in small plastic vials. Inside the vial there is a label that is not glued to the vial and is wrapped around the paper, presumably to protect it from light. Use a medicine dropper to put one drop of 15 M NH3 onto the inside wall of the vial (between the vial wall and this paper label) and restopper the vial. Within seconds the ammonia permeates the paper, changing it to a proper blue. The paper label becomes only slightly damp; the litmus paper remains dry and is ready for immediate use. This simple procedure can also be used to change red litmus paper to blue in case of a shortage of blue paper. I cannot guarantee that this blue paper conforms to ACS specifications, but it certainly works satisfactorily in a typical student laboratory setting.
JChemEd.chem.wisc.edu • Vol. 75 No. 2 February 1998 • Journal of Chemical Education
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