Acids, Bases and Indicators

Posted on November 20th, 2010

Students that participate in the Lab Ratz Science Club often cite the “Acids and Bases” activity as one of their favorites.  The experiment involves determining the pH of several sample chemicals using standard pH paper and then using a “Mystery Indicator” to test the chemicals.  pH is a way to quantify how strong or weak an acid or base is by measuring the concentration of protons in the solution – Acids loose a positively charged proton when dissolved in water while bases, being the opposite of acids, loose a negatively charged electron when in solution.  By using results of the pH paper test the students are able to deduce what pH values correspond the the colors produced by the “Mystery Indicator”.

The composition of the “Mystery Indicator” is not really a mystery – It is just the juice of red cabbage.  The real mystery is what color it will turn when mixed with a solution of a particular pH.  This experiment is easily reproduced in the home kitchen with the help of a responsible adult, a stove, some water, a head of red cabbage and a pot or pan.  Disclaimer:  There are many things that smell better than red cabbage being cooked – It’s not the worst smell in the world, but it isn’t the most pleasant either.  Don’t let this discourage you from trying this at home, after all, we have to cook this up several time a week and our office reeks of cabbage indicator juice year-round.  But for the at-home scientist who mixes this up every once-in-a-while, the smell shouldn’t linger for more than an hour or so.  Just don’t say we didn’t warn you. Simply chop or tear the red cabbage – Only red cabbage will work – into small pieces and toss it into a pot of water.  The less water you use in proportion to the cabbage, the more concentrated the solution and the brighter your colors will be.  Bring the water and cabbage to a boil and allow it to simmer for 20-30 minutes.  It helps to use a spoon or other utensil to smash up the cabbage and squeeze out as much juice as possible  while it is simmering.  Allow the mixture to cool and strain out the chunks of cabbage and you will be left with a purple liquid that will change color when added to acidic or basic solutions.  Portion out small amounts into some cups and search the house for things to test!  Virtually any liquid that is water-soluble will work (Oils will not mix or dissolve with water so they may not produce a color change) as well as solids that dissolved in water, such as baking soda or salt.  Observe what color the indicator changes to with each chemical.  You may then want to search the internet for information on the pH of each sample chemical that you use.  Eventually you will be able to produce a color key chart so that you will know what pH values correspond to each color.

Another interesting way to use an indicator is to perform a titration.  When you mix an acid with a base, the positively charged protons from the acid and the negatively charged electrons from the base will be attracted to each other and will cancel one another out.  When the two bond they form water (The exact mechanism is a little more complicated than that, but water is still formed as a result).  The molecules of acid and the molecules of the base will also attracted each other and bond, forming molecules of sald (Not necessarily table salt, which is sodium chloride.  There are many different types of salt).  If you start with a solution of acid and indicator you can slowly add a base (Or vice versa) until the color of the indicator becomes purple, indicating a neutral pH.  By measuring how much base it takes to reach a neutral pH you can tell how strong the initial acid solution was.

Then you can publish your finding to our online scientific journal on our Facebook page.

NOTE:  Use adult supervision for this experiment.  Do not use household chemicals without the permission and supervision of an adult.  Do not use the stove without supervision.  Use safety goggles always.