What makes phenolphthalein change colour

The pH scale runs from 0 to 14, with a pH of 7 being neutral. A substance below pH 7 is considered acidic; above pH 7 is considered basic. Phenolphthalein is naturally colorless but turns pink in alkaline solutions.

The compound remains colorless throughout the range of acidic pH levels but begins to turn pink at a pH level of 8. In , the German chemist Adolf von Baeyer discovered phenolphthalein by fusing phenol and phthalic anhydride in the presence of sulfuric acid or zinc chloride, the manufacture process still used today.

In the chemistry laboratory, phenolphthalein is mostly used in acid-base titrations. A solution of known concentration is carefully added into one of unknown concentration. The phenolphthalein indicator is added into the unknown concentration. When the solution turns from colorless to pink or vice versa , the titration or neutralization point has been reached, and the unknown concentration may be calculated.

In the past, phenolphthalein has been used as a laxative. Fill the other large beaker with water for pouring into small beakers. Instructions: Pour water into each beaker so each is half full. Ask students to observe carefully and describe the colour as you pour 1 into 2, and 4 into 3.

Watch as they both turn purple-pink! Dump both solutions together into the large beaker laced with vinegar and watch as they become colourless again! Extensions Can you predict the colour phenolphthalein will turn when added to milk? How about orange juice? Try it out with some other common solutions. Try making your own cabbage juice acid-base indicator. Related Resources Deep Purple Magic In this activity, students see colours change in a solution, and learn how chemistry can explain this magical result.

In aqueous solution, phenolphthalein will present the following equilibrium. Clearly there must be some intermediate situation where half the phenolphthalein is in the acid form and half in the colored conjugate-base form. That is, at some pH. In other words the color of phenolphthalein changes perceptibly between about pH 8. Other indicators behave in essentially the same way, but for many of them both the acid and the conjugate base are colored.

A drop of indicator solution is added to the titration at the start; when the color changes the endpoint has been reached, this is an approximation of the equivalence point. Indicators can be described as three types—outcome, process or structure — as first proposed by Avedis Donabedian The national safety and quality indicators of safety and quality in health care recommended in this report include indicators of all three types.

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Ben Davis February 9, How does phenolphthalein turn pink? At what pH does phenolphthalein change color? Why does phenolphthalein show pink Colour? Do acids change the color of indicators?