Litmus Paper Experiment - Acid/Base Indicator

One of the experiments that is commonly done in the classroom or laboratory is to use red and blue litmus paper strips to test a liquid as either acidic, basic, or neutral. This experiment is relatively inexpensive to do because the cost of litmus paper strips is low. You can purchase 100 strips for approximately $1.00 to $3.00 per bottle. The red strips look more pink than red and if they are dipped into a basic solution they will turn blue. The blue strips may look a bt more purple than blue, but they will turn pink (red) when dipped into an acidic solution. If the red and blue strips are dipped into a solution and they do not change in color then the solution is neutral. Water is usually a neutral solution, but remember that rainwater for example can pick up some acidic content as it falls to Earth.

One of the problems with conducting this experiment is to use solutions that are acidic or basic, but not dangerous. Acidic solutions can be produced with lemon juice, orange juice, but do not use moderate or strong acids which may be available in a laboratory if you are not familiar with their dangers and ways to properly and accurately dilute them. Basic solutions can be made with baking soda, antacid tablets, but again do not use moderate or strong bases unless you know how to properly use and dilute them. You may only need 5 or six different solutions. One should be pure water. Usually I would set up 6 test tubes in a rack and then 3/4 fill one of the central six test tubes with water. You might then add water to all the other test tubes to about 1/2 full and then add some lemon juice to one test tube until it is 3/4 full. You would add appropriate liquids to the other tubes as needed, but be careful to keep track of which fluid is added to each tube. Have enough fluid premade to make up more test tube racks if you are going to set up a number of lab stations.

You will want to do the experiment yourself once, to be certain it works the way you want. You would take a red litmus strip and dip it into the first test tube and see if it changes color. Then put this strip down, on to a paper towel would be best. Then use a new blue strip to test the same solution to see if it changes color. Then put it down on the paper towel right next to the first strip. Do not use these strips again. Label the section of the paper towel where these strips are now resting as "Test A" or "Experiment 1" etc. Remember just being wet is not a color change. If you use these same strips again you will contaminate the next test tubes. This means you will need to use two strips of litmus paper for each test tube test.

The test tube that contains only water should not change either strip to another color. One problem has been to develop a system of data collection that keeps students focused on the process. The graphic shows how this was done with only the "A" part of the experiment. The questions lead the student to the proper conclusion based on the data collected.

Be very careful, your selection of liquids to use must be limited to very weak acids and bases. In addition if you were to use stronger acids or bases and then plan to dilute them you must know what you are doing as this can be extremely dangerous. Adding water to a strong acid can even cause an explosion (As the water hits the acid it can disassociate into hydrogen and oxygen and the heat produced can explode the hydrogen gas.) Even dilute acids or bases can damage clothing. Remember students must know how to use safety equipment in a laboratory and must use goggles during most laboratory experiments. Other safety equipment may also be required.