Introduction: Acid/Base Test Using Red Cabbage and a Spectrometer

By

In this project we look quantitatively at the transmission spectrum of red cabbage juice using a low cost educational spectrometer. It is well known that red cabbage leaves contain a substance called anthocyanin that changes color depending on whether mixed with an acid or a base. Here, by looking at the transmission spectrum of a vial that contains cabbage juice, the shift of the absorption peak of anthocyanin is seen to shift towards blue of red wavelengths.


Supplies

Shredded red cabbage to extract anthocyanin

A glass of hot water is used for this extraction

An educational fiber optic spectrometer (e.g. Spectryx Blue)

A white light source (e.g. a Maglite Xenon lightbulb)

A 3 V voltage source (e.g. from 3.3V output of an Arduino Uno board)

Helper Hands to construct a simple transmission measurement setup

Disposable plastic cuvettes

White Vinegar

Bleach

Q-tips

Step 1: Extract Anthocyanin From Red Cabbage

Fill a glass with shredded red cabbage. Fill with hot water. Wait 30 minutes. Transfer the purple juice to a clean glass.

Step 2: Make a Simple Transmission Measurement Setup

Fix the xenon lamp onto one of the crocodiles of the helper hands. Connect the lamp to an Arduino Uno 3.3V output (GND and 3.3V using two separate wires). Hold the fiber of the spectrometer with the other crocodile of the helping hands. Align the fiber so it looks at the light source, about 1 cm above the surface of the table.

Observe the light signal on the spectrometer software. Collect reference spectrum, and switch to transmission mode on GUI.


Step 3: Measure Red Cabbage Juice Transmission

First, pour red cabbage juice into the cuvette. Place the cuvette close to the fiber optic cable light input. Observe the transmission in the spectrometer software. There is a broad absorption line around green (~ 565 nm).

Step 4: Test Effect of Vinegar

Add a few drops of white vinegar into the cuvette. Observe transmission spectrum. It is seen that the absorption (the dip in the transmission curve) shifts towards UV wavelengths. The cuvette now turns orange/red because blue is absorbed due to acidic pH.

Step 5: Test Effect of Bleach

CAUTION: Bleach is corrosive and basic. Take care to not get on your hands, face or eyes.

Wash the cuvette and refill with red cabbage juice. Observe the purple color. Place again in front of the fiber optic cable.

Use a Q-tip to transfer a small drop of bleach. Mix with a toothpick or a clean Q-tip. The color shifts towards greenish hue. Observe the transmission using the spectrometer software. The absorption has shifted red towards 600 nm.



Step 6: Final Comparison

When we compare the transmission spectra, it is seen that vinegar (acidic) shifts the absorption peak towards Blue/UV, and Bleach (basic) shifts the peak towards Red. The peak position can be correlated with pH level.