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To test for depth of carbonation into a concrete surface, a solution of phenolphthalein indicator can be applied. Uncarbonated concrete is indicated as a pink colouration i.e. the pH is greater than ~9. 

The  solution comprises 1g of phenolphthalein powder dissolved in a solution of 70ml ethanol and 30 ml of deionised water. Alternative indicators giving a colour change in the range pH 8 to 11 can be used to give a sufficiently clear colour change in concrete to differentiate the uncarbonated zone, e.g. thymolphthalein.

WARNING: Phenolphthalein is listed in REACH Appendix 2 and is listed as a substance of very high concern. Phenolphthalein powder should be handled with the greatest care using safety gloves in a fume cupboard fitted with an extractor. Always follow the COSHH guidelines when using chemicals.


Acknowledgement: Concrete Society

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