Phenolphthalein Paper

A qualitative indicator paper that shows the crossing of pH 8.3, where phenolphthalein turns from colourless to pink. It gives a yes or a no, not a value.

Why it is measured

Phenolphthalein has one very sharp transition, and a great deal of routine chemistry is built around exactly that point: it is the endpoint of an alkalinity titration and the marker used to check whether concrete has carbonated.

Where it is used

• alkalinity endpoint checks
• concrete carbonation testing
• caustic residue detection
• teaching laboratories

What it answers

A qualitative paper does one thing and does it without ambiguity. There is no scale to interpolate and no lighting to argue about: the colour changes or it does not.

That makes it the right tool when the decision is binary — proceed or do not — and the wrong tool when anyone needs to know how far past the threshold the sample is. For a value, use an indicator paper with a scale or a meter.

Practical note

The pink fades in air as carbon dioxide is absorbed; read it at once and do not judge a strip left on the bench.

Supplied in 2 presentations covering different measuring ranges.