Reaction of Several Acid-Base Indicators

Problem Acids as well as base solutions are mostly colorless and cannot easily be identified. Color indicators are suitable for deciding whether a solution is acidic or alkaline. Special universal indicators (in solution or paper form) can do even more it shows the approximate pH value of a solution through various red and blue shades. The pH value can thus be introduced in a practical manner without the necessity of wondering about concentration of hydronium ions or even logarithmic calculations. 

Material Small and large beakers, test tubes, pipettes common laboratory acid and alkaline solutions, sodium chloride solution, concentrated sulfuric acid, universal indicator solution and indicator paper (with a color chart), phenolphthalein solution. 

Procedure (a) Dilute a little amount of universal indicator solution with tap water, place the green solution in three small beakers (can be projected with an overhead projector). Drop hydrochloric acid into one of the beakers, in the other sodium hydroxide solution, in the third common salt solution. Place several drops of the solutions on strips of universal indicator paper, observe and compare the colors with the color chart, determine the pH values, (b) Fill a large beaker with tap water and add a few ml of phenolphthalein solution while stirring. Prepare a second beaker of the same size with a few drops of sodium hydroxide solution, a third beaker of the same size with a few drops of concentrated sulfuric acid. Pour the contents of the first beaker into the second one, the second one into the third beaker. 

Observation (a) Hydrochloric add colors the green indicator red sodium hydroxide solution makes it blue. The indicator paper test shows colors, and comparing with the colors of the pH scale on the color chart, pH values can be approximated in each case, (b) Colorless liquid in the first beaker immediately turns wine red by pouring into the second beaker this solution becomes colorless when it is poured into the third beaker. 

Tip It is also possible to gain color extracts from red cabbage juice and cooked radishes, which demonstrate the change in colors by adding drops of acid and alkaline solutions. 

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