By Laura Smart
In recognition of the “Chemistry Colors our World” theme of Chemistry Week today’s experiment is a colorful one – we are conducting the red cabbage pH indicator experiment.
In order to understand the experiment we must first consider a few questions. What is pH? Why is it important to measure pH? How can we measure pH? What is an indicator?
What is pH?
pH is defined as the negative logarithm of the concentration of H+ ions in solution and is a measure of the acidity of a solution.
pH normally ranges from 0 to 14. A value of 7 indicates that the solution has neutral pH. A pH measurement which is lower than 7 indicates that a solution is acidic. A pH measurement which is higher than 7 indicates that a solution is basic.
Environmental and biological systems often have distinct pH ranges and a deviation from an expected range is likely to indicate a problem.
Monitoring of pH is particularly important in medicine, food science, agriculture and environmental science. For example, our blood must have a pH between 7.35 and 7.45 because exceeding this range by even just 0.1 of a pH unit could be fatal.
In chemistry pH is often monitored and modified in order to control reactivity and sometimes increase stability of chemicals.
How can we measure pH?
There are various methods to measure pH. In laboratories, the most common uses a pH‐sensitive glass electrode (which is considered the measuring electrode), a reference electrode and a pH meter with high sensitivity.
However, very often an indication of the pH is sufficient for monitoring purposes.
What is a pH indicator?
A pH indicator is a halochromic chemical compound (a substance which changes color when pH is altered) which can be added in small amounts to a solution so that the pH of the solution can be determined visually.
There are many indicators available and an indicator is selected according to the expected pH of a solution. For example, methyl orange changes from red to yellow between pH 3 and pH 4.5, whereas phenolphthalein is colourless below pH 8 and changes to fushcia between 10 and 13.
Where pH is unknown a more general indicator can used. Many of us will be familiar with the little colored tapes distributed in Chemistry class which are known as universal indicator paper. This paper is coated with several indicators so that color changes over a pH value range from 1-14 can be observed.
Today we are using the halochromic properties of red cabbage to assess the acidity of different drinks and household products. In order to produce the red cabbage indicator we blended red cabbage leaves with water and then filtered the solid cabbage to make the maroon indicator.
We added portions of the resulting solution to six glass jars. We then added a different household product to each jar.
The indicator turns red when acidic (pH1-3), blue/purple when neutral (pH7-8) and green/yellow when basic (pH10-12).
Take a look at our pictures and see whether you can work out which jar we added lemon juice to!