pH Paper

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Two types of pH paper are commonly used. Litmus paper is indicator paper coated with an organic dye which changes color in the presence of acids and bases. Red litmus paper is used to check for bases, blue litmus paper is used to check for acids. Alkacid paper, a universal indicator paper, can check for both acids and bases and indicate the approximate pH of a solution.

First, choose the appropriate type of paper (red litmus if testing for a base, blue litmus if testing for an acid, or alkacid paper if the approximate pH is to be determined). Then test the solution with the paper.

Video: Using pH Paper ( 1.12 M ) Text description

Do not dip the pH paper into the solution to be tested! Why?

You added calcium to water, a reaction that produces calcium hydroxide. You want to check to see if the reaction has taken place. What type of paper should you use? Answer

Blue litmus paper will turn red in the presence of an acid, while red litmus paper will turn blue in the presence of a base.

Red litmus paper (above); blue litmus paper (below)

Alkacid paper indicates the approximate pH according to the following scale:

Be cautious in interpreting results; a neutral solution will simply wet the paper, which will cause it to look darker. This result may be misinterpreted as a weakly acidic solution. As an additional experiment, test the solution with blue litmus paper. If the blue litmus turns red, the solution is acidic, but if the paper does not change color the solution may be neutral.

What is the approximate pH of the solution tested in the picture? Is the solution an acid or a base? Is it strong or weak? Answer

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