Effect of Strong Base on Phenolphthalein

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Summary

This is a demonstration of the effect of strong base on phenolphthalein. The 7 thumbnail images summarize the content of the video. Click an image to see the image gallery.

Narration

The three beakers shown each contain phenolphthalein in a weak base solution.

To the beaker on the left, 40 mL of water will be added. This will be the control.

To the beaker in the middle, 40 mL of a one molar sodium hydroxide solution will be added. To the beaker on the right, 40 mL of three molar sodium hydroxide solution is added.

The liquids are poured simultaneously.

The beaker on the left shows only the effect of dilution on the color.

As time passes, it can be seen that the color fades in the other two beakers. Notice that the stronger base causes the phenolphthalein to fade at a faster rate than the weaker base.

Discussion

This experiment is conducted to show the effect of strong base on phenolphthalein.

In freshmen chemistry lab, phenolphthalein is used as an indicator in acid-base titration experiments. The concentrations of acid and base used in the exercises are rather dilute, on the order of 0.1 molar. Phenolphthalein in a basic solution of this concentration has a purple-pink hue that is vivid and very stable. The color will remain for days if left undisturbed.

However, in the presence of a strong base, with hydroxide ion concentrations of 1.0 molar or more, the phenolphthalein will undergo a reaction. The quinoid form of the ion, which has a purple-pink color, is transformed to a carbinol form, which is colorless. The more concentrated base used in the reaction, the faster the color changes.

In this experiment three beakers are used. Initially each of the three contain 100 mL of a 0.05 molar sodium hydroxide solution to which 10 drops of phenolphthalein have been added. To the beaker on the left (in the video), 40 mL of distilled water is added. This shows the effect of dilution on the color, and serves as a control. Different concentrations of base are added to the other beakers. To the beaker in the middle, 40 mL of 1.0 molar sodium hydroxide solution is added, while 40 mL of a 3.0 molar sodium hydroxide solution is added to the beaker on the right.

It is readily seen that the color of the phenolphthalein begins to fade almost immediately in the right-most and middle beakers, with that of the right-most fading most quickly.

Keywords

Kinetics | Acids / Bases

These ChemEd DL Resource Groups Include This Video

Rates of Reactions
Reactions in Aqueous Solutions
Acid Base Reactions


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