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The pH Scale & Standard Indicators

—by Anne Wallingford

The pH Scale, numbered 1 through 14, is a way of measuring the strength of an acid or alkali. A reading of pH = 1 indicates that a solution is a strong acid. A reading of pH = 14 indicates a strong alkali. A reading of pH = 7 means the solution is neither acidic nor alkaline, but neutral.

Click for pH Strip (will open in new window—enlarge window for larger graphic)

Standard pH indicators are chemical substances that tell whether a solution is an acid, an alkaline (base), or neutral. Color changes from red through deep purple occur as a solution changes its strength.

Litmus papers are general purpose acid/alkaline indicators. Use litmus strips when all you need to know is whether a solution is an acid or an alkaline (base). Unlike other indicator strips, litmus strips can be used several times before the litmus dye is leached from the paper. Red litmus turns blue when touched by an alkaline (base) and blue litmus turns red when touched by an acid. A trick for remembering this is to think of RBI's in baseball. An RBI means a run is batted in; the runner has to run the bases. When thinking of litmus paper, an RBI means red -> blue -> base.

Wide range pH test strips provide a distinct color for each pH unit from one through fourteen. Other types of test strips, such as Hydrion A and Hydrion B, show pH changes in incremental steps. Hydrion A has a range of 2-4-6-8-10, while Hydrion B has a range of 1-3-5-7-9-11.

All-purpose Hydrion Test Paper Kits usually have single roll dispensers of pH strips in narrow ranges. This includes a full spectrum 0-13 test paper as well as test papers covering 1.0-2.5, 3.0-5.5, 6.0-8.0, 8.0-9.5, 10.0-12.0, and 12.5-14.0.

To use an indicator strip, use a clean stirring rod to place a few drops of the solution on the indicator strip. Do not dip the strip into the solution because that would contaminate the solution.

Although it is theoretically possible to reconstitute test papers by soaking the test paper in a saturated indicator solution, it is much less expensive—and less messy—to simply buy replacement strips.

Besides litmus and Hydrion strips, other common indicators include cobalt chloride, Congo red, methyl orange, phenolphthalein, and turmeric.

Cobalt chloride changes color when exposed to high humidity. Dry cobalt chloride paper is blue; when exposed to water, the paper turns pink. Cobalt chloride is often used in weather instruments to show changes in humidity. In fact, making a humidity indicator can be a fun activity for students!

Congo red measures solutions with a pH range of 3.0-5.0. Test strips will change from red to blue.

Phenolphthalein measures solutions with a pH range of 8.0-10.0. Red phenolphthalein turns colorless when acid is added to a solution. Clear phenolphthalein turns red when a base is added to a solution. Phenolphthalein can add a bit of "magic" to an introduction demonstration on pH indicators.

Turmeric, sometimes called curcamin, measures solutions with a pH range of 7.5-8.5. It will change from yellow to red.

Methyl orange turns red when acid is added. It measures the 3.0-4.4 range.

Bromothymol blue turns yellow when acid is added. It measures the 6.0-7.6 range.

To demonstrate acids and bases to young students, remember it doesn't take a business administration degree to accomplish this, boil a few red cabbage leaves to get red cabbage water. (You can always eat the rest of the cabbage.) Pour a bit of the cabbage water into a clear container, or test tube, and then take turns adding small amounts of baking soda (a base) and vinegar (an acid) into the container. Have students guess whether the cabbage water is an acid or a base by observing the color changes.

To make your own litmus paper, cut some strips about ½" wide x 3" long from white construction paper. Next, remove the stems from a half cup of blueberries or blackberries. Crush the berries. Add a little water to make the pulp more liquid-y.

Next, dip a strip of construction paper into the berry juice. Cover the strips completely with the juice; make certain the strips are well-covered. After the strips are completely soaked with the berry juice, use your fingers to wipe any excess pulp from the strips. Place the strips on the edge of a plate, or on a paper towel, so they can dry. After the strips have dried, you have your own litmus paper!

Purple litmus strips made from blackberries turn pinkish-red in acids and deep purple in bases (alkalis.)

Purple litmus strips made from blueberries turn reddish-purple in acids and a light bluish-purple in bases.

© 2004 Anne Wallingford. All rights reserved.


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112009 Sunday, April 21, 2013