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.

Now and then you need to measure the pH of a solution, and this can be done by either using a digital pH-meter, or you can use pH papers (blotter paper soaked in an indicator solution, such as litmus or BTB). Now, both those methods are more or less expensive, the cheapest pH-meter will cost $50 and most will go for over $100. pH paper is cheaper, but many people have trouble finding them.

You really don't have to use either of them, as nature provides us with pH indicators in the form of plant pigments known as antocyanines, which change color over different pH ranges, depending on source. On this page, I will present the indicators derived from red cabbage and red beet. Red cabbage juice will function over a wide pH range, from as low as pH 1 up to pH 12. Red beet juice will change from red to yellow somewhere between pH 11 and 12.

The indicator solutions are prepared by chopping up the red beets or red cabbage into small pieces, preferably by running them through a blender. Then the juice is strained off from the resulting mush, and the mush can also be extracted with water to yield even more of the pigment. The solutions are filtered to remove any remaining plant matter, and can then be used as is, or diluted if the color is too intense. I have no idea for how long such solutions will keep, but stored in a refrigerator they should last for at least several weeks. If your solutions are too dilute, you may be able to concentrate them with gentle heating, I don't know at what temp the pigments will be destroyed, so be careful.

I made solutions of NaOH or HCl in water, to yield solutions with pH's of 0 through 13, and tested the juices, and the results can be viewed below. Outside the shown ranges, the indicators won't change color anymore.