pH indicator using Red cabagge

From left to right: White vinegar, Lemon Juice, Nothing added, Sodium bicarbonate, Sodium Carbonate

Abstract

The use of red cabbage (Brassica oleracea var. capitata F. rubra) extract as a pH indicator is a versatile and popular demonstration that is employed by educators in many schools and universities. Previous variations of this demonstration have used red cabbage extracts to prepare paper that changes color when solutions of acid or base are applied as “inks”. In this report, we highlight the ability of red cabbage extracts to function as invisible inks. Colorless solutions are prepared by bleaching anthocyanin extracts with aqueous sodium metabisulfite, and the resulting mixture can then be used to write hidden messages on paper. The invisible messages can be revealed by the addition of citric acid solution, or by using potassium aluminum sulfate (alum) as developers. The demonstration does not require specialist equipment or chemicals, and options to use additional plant sources to prepare the ink are described.

Abstract

Herein we describe The Purple Flask, a novel color-changing redox reaction that uses ascorbic acid as an electron donor, atmospheric oxygen as an electron acceptor, a copper–iron mixture as a redox catalyst, and the purple phenothiazine dye thionine as redox indicator. This formulation has several advantages over other versions of the classic “blue bottle” reaction for classroom demonstrations: (i) It uses noncaustic chemicals and proceeds at mildly acidic pH 4.2. (ii) The purple color generated by the oxidation reaction is intense, but fades rapidly. (iii) The reduction reaction rate is very temperature sensitive, which enables exploration of the impact of temperature on reaction kinetics. The Purple Flask is a vivid classroom demonstration that quickly captures student attention and can be used to illustrate the principles of redox chemistry in settings ranging from middle school through high school advanced placement chemistry and introductory college chemistry.