Phenolphthalein is a weak acid commonly used in titrations. In acidic or neutral pH (below pH 8), the solution remains colourless (H-phph), whereas it turns pink in more alkaline conditions (phph-). Adding hydrogen ions (H+), making the solution more acidic, moves the equilibrium to the left and the indicator turns colourless. In contrast, adding hydroxide ions (OH-) removes the H+ and tips the balance towards a pink colouration (phph-). This change occurs at around pH 8.2, but it’s difficult to measure accurately when the solution starts to change color.

Kastle-Meyer test:
This compound can be used in a test to determine if a substance contains blood. For the test, a few drops of alcohol, then a few drops of phenolphthalein and finally a few drops of hydrogen peroxide are added to the sample. If the sample contains blood (haemoglobin), the test will indicate pink. Although the test indicates the presence of blood, it cannot distinguish between human blood and animal blood, so further testing is necessary for positive readings.

This compound is also used in toys, in disappearing inks which are mixed with sodium hydroxide (NaOH). As the NaOH reacts with the carbon dioxide (CO2) in the air, H+ is released and the pH drops and the ink turns from pink to colorless.

Test carbonation reactions in concrete:
The same reaction is also useful to test signs of carbonation in concrete. Normal carbonation levels occur as the cement reacts with carbon dioxide in the atmosphere, reducing the pH to around 9. However, excessive carbonation can undermine the strength of the concrete. A 1% phenolphthalein solution added to a sample can detect the level of carbonation, with a pink response indicating normal levels, but a colorless reaction meaning the concrete has undergone significant carbonation.

Universal indicator:
Phenolphthalein is also an ingredient in a universal indicator, alongside methyl red, thymol blue and bromothymol blue. In this case, colours range from red and yellow at low pH to blue and violet at high pH.

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