Phenolphthalein

• 3,3-bis (4-hydroxyphenyl ) phthalide
• 3,3- bis (4- hydroxyphenyl) -1 ( 3H)- isobenzofuranone

A06AB04

White to pale yellow, crystalline, tasteless and odorless solid

Laxative

Fixed

1.30 g · cm -3

258-262 ° C

9.7

• Very poor in water ( 92 mg · l-1 at 20 ° C)
• Soluble in ethanol

Risk

> 1000 mg · kg -1 ( LD50, rat, oral)

Template: Infobox chemical / molecular formula search available

Phenolphthalein is one of the most pH indicators and was first presented in 1871 by Adolf von Baeyer. The name is composed of phenol and phthalic anhydride. Phenolphthalein is a triphenylmethane dye and is the base compound of the family of the phthaleins. The corresponding sulfonphthalein is the phenol red. At a pH value of less than 0, an aqueous solution of phenolphthalein is red-orange, colorless, with a pH value of 0 to about 8.2. At higher pH, the solution turned pink - violet, which becomes colorless again at a pH value of about 13 colors.

Representation

In a Friedel -Crafts acylation of two equivalents of phenol and one equivalent of phthalic anhydride are reacted small amounts of concentrated sulfuric acid or zinc chloride in the presence of:

Use

Until the discovery of a potentially carcinogenic effects of phenolphthalein was more than a hundred years, used as a laxative. The small amounts which are used when used as an indicator, however, are not dangerous. In construction of phenolphthalein solution for visualization of the carbonation of concrete and to determine the overcoating properties of newly plastered surfaces is used ( surface with a pH of less than 8.5 ).

Reduced Phenolphthalein is used to detect traces of blood in forensic science Kastle -Meyer test.

Properties

Phenolphthalein is a white crystalline powder in water and practically insoluble. It takes place mostly in a 1% alcoholic solution use. It is itself a weak acid.

Phenolphthalein has a pKa of 9.7. If one defines the transition range at one indicator acid / base ratio of 10:1 to 1:10, is obtained by the Henderson- Hasselbalch equation an envelope the range of pH = pKa ± 1 ( 8.7 to 10.7 ). At a pH value of less than 0 is an aqueous solution of phenolphthalein red-orange, colorless at a pH value of from 0 to about 8.2, at higher pH, the solution is colored pink - purple, in the strongly alkaline range, at a pH value close to 13, it becomes colorless again. It is therefore suitable as indicator for the titration, for example, alkaline solutions.

Structure and color change

Depending on the pH of the solution, the phenolphthalein change its structure and hence its color.

In the pH range 7.5 to about it is in its colorless, uncharged base form ( H2In, A in the drawing ). In more basic solution, the protons split off at the two hydroxy groups ( B1). In a resulting structure is a mesomeric quinoid system as a chromophore exists ( In2, B2). This is the colored structure of the indicator. In strongly basic environment accumulates at the central carbon atom of an OH group, thereby achieving the chromophore is impossible ( In (OH ) 3 -, C). In strongly acidic solution phenolphthalein color again. The lactone ring is cleaved by the H . Here, a positive charge at the central carbon atom that is so sp ² hybridized and thus is again stabilized by the mesomeric form of forms.

Considering the equation of the reaction ( A) to ( B), as is clear from the mass action law, why the color change is so fast:

It is (Ks constant, the quasi-constant concentration of the water is included in K )

The concentration of H3O ions is in a different order of magnitude. For acidic solutions it is high, the equilibrium lies on the side of ( A). However, once the concentration of H3O is very small or the concentration of OH - is large, the concentration of ( B) must massively increase due to the constancy of the term. Because (B ) arises from (A), the concentration of ( A) is extremely smaller - the color change is very rapid.