phthaleiinien

Phthaleins, or phthalein indicators, are a family of organic compounds used as acid–base indicators in analytical chemistry. They are typically colorless in their acidic form and develop vivid colors in their basic form, allowing precise detection of endpoint pH during titrations. The color change results from structural changes that extend the conjugation of the molecule when the lactone ring opens under basic conditions.

Chemically, phthaleins are derived from phthalic anhydride linked to phenolic components. In acidic solutions, the molecules exist predominantly in a closed, colorless spirolactone form. When deprotonated in basic media, the ring opens and a quinonoid, highly conjugated system is formed, producing a distinct color. The exact hue and transition range depend on the specific substitutions on the aromatic rings.

Common members of the phthalein family include phenolphthalein and thymolphthalein, and there are several related derivatives. Phenolphthalein is widely used in educational and clinical laboratories as a traditional indicator, typically changing from colorless in acidic solutions to pink in basic conditions. Thymolphthalein, with different substituents, offers a higher pH transition and can appear blue to purple in its basic form. The various derivatives provide a range of transition pH values, enabling titrations across different alkaline strengths.

Synthesis generally involves condensation of a phthalic anhydride with chosen phenolic partners under acidic conditions to form the phthalein chromophore. Phthaleins are valued for sharp, distinct color changes, albeit with replacement in some applications due to safety or disposal considerations of certain derivatives.