Phthalein dyes are a class of dyes mainly used as

pH indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...

s, due to their ability to change colors depending on pH. They are formed by the reaction of

phthalic anhydride Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commer ...

with various

phenols In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (— O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds are ...

. They are a subclass of

triarylmethane dye Triarylmethane dyes are synthetic organic compounds containing triphenylmethane backbones. As dyes, these compounds are intensely colored. They are produced industrially as dyes. Families Triarylmethane dyes can be grouped into families accordin ...

s. Common phthalein dyes include: *

Bromothymol blue Bromothymol blue (also known as bromothymol sulfone phthalein and BTB) is a pH indicator. It is mostly used in applications that require measuring substances that would have a relatively neutral pH (near 7). A common use is for measuring the pre ...

Bromocresol green Bromocresol green (BCG) is a dye of the triphenylmethane family (triarylmethane dyes). It belongs to a class of dyes called sulfonephthaleins. It is used as a pH indicator in applications such as growth mediums for microorganisms and titrations. ...

Bromocresol purple Bromocresol purple (BCP) or 5′,5″-dibromo-''o''-cresolsulfophthalein, is a dye of the triphenylmethane family ( triarylmethane dyes) and a pH indicator. It is colored yellow below pH 5.2, and violet above pH 6.8. In its cyclic sulfonate ester ...

* Cresol Red * ''o''-Cresolphthalein *

Chlorophenol red Chlorophenol red is an indicator dye that changes color from yellow to violet in the pH range 4.8 to 6.7. The pH of a substance is determined by taking the negative logarithm of the Hydronium ion concentration and the indictor changes color due ...

* Dixylenolphthalein * Guaiacolphthalein *

α-Naphtholphthalein α-Naphtholphthalein (C28H18O4) is a phthalein dye used as a pH indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or ...

Phenolphthalein Phenolphthalein ( ) is a chemical compound with the formula C20 H14 O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this applica ...

Phenolsulfonphthalein Phenol red (also known as phenolsulfonphthalein or PSP) is a pH indicator frequently used in cell biology laboratories. Chemical structure and properties Phenol red exists as a red crystal that is stable in air. Its solubility is 0.77 grams per ...

* Tetrabromophenolphthalein *

Thymol blue Thymol blue (thymolsulfonephthalein) is a brownish-green or reddish-brown crystalline powder that is used as a pH indicator. It is insoluble in water but soluble in alcohol and dilute alkali solutions. It transitions from red to yellow at p ...

Thymolphthalein Thymolphthalein is a phthalein dye used as an acid– base ( pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue. The molar extinction coefficient for the blue thymolphthalein dianio ...

* Xylenolphthalein

References

Triarylmethane dyes Phthalides {{organic-compound-stub

See also

References