Phenol red

Phenol red (phenolsulfonphthalein or PSP), a water-soluble dye that is used to measure pH in laboratories.

The pH scale ranges from 0 to 14. A pH of 7 means neutral. pH of less than 7 is acidic. pH of more than 7 is basic.

Phenol red is red at neutral pH.

Phenol red is purple at basic or alkaline pH.

Phenol red is orange at acidic pH.

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