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Phenol Red Chemical Structure

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What is Phenol red?

Phenol red also referred to as Phenolsulfonphthalein (PSP), is a  3H-2,1-Benzoxathiole 1,1-dioxide analogue. It shows yellow color below pH of 6.8 and bright pink (fuchsia) color above pH of 8.2 and is thus useful as a pH indicator. It has a role as a two-colour indicator, an acid-base indicator and a diagnostic agent.

Macsen Laboratories is the leading manufacturer and supplier of Phenol red dye with ISO certification.
Name of Product Phenol Red
IUPAC Name 4-[3-(4-hydroxyphenyl)-1,1-dioxo-2,1λ6-benzoxathiol-3-yl]phenol
Synonyms Phenolsulfonphthalein; Sulfonphthal; Sulphental; Sulphonthal; Phenolred
CAS No 143-74-8
Molecular Formula C19H14O5S
Molecular Weight 354.4 g/mol
Pubchem CID 4766
Pubchem SID 480474275


SR. No Criteria Limit/Specification
1 Appearance (form) Solid Powder
2 Appearance (color) Dark Red to Dark Brown
3 Odor Odorless
4 Solubility 0.77 g/L (In water)
2.9 g/L (In ethanol)
5 Melting Point >300° C
6 Density ~1.5 g/cm3 (Predicted)
7 Dissociation Constant (pKa) 8.0
8 Loss on Drying (at 110°Cr) NMT 5.0 %


  • Phenol Red is commonly used as a pH indicator in cell biology laboratories and in home swimming pool test kits (labelled with a different name, such as “Guardex Solution #2”).
  • In the past, it was employed as a method for determining kidney function based on the amount of phenol red that was passed out of the body via a colorimetric test.
  • Recent medical studies have shown that phenol red can imitate the effects of estrogen, making it useful in in-vitro fertility treatment and stem cell research.


Q. What does Phenol red indicate?

Phenol red is used to identify pH changes from neutral (red) to acidic (yellow) values in culture media.

Q. Is Phenol red acidic or basic?

The phenol red dye indicator is a weak organic acid.

Q. Is Phenol red toxic?

No significant toxic effects have been reported from the use of Phenol red; however, sufficient research has not been conducted on this subject.

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