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Tuesday, March 20, 2012

Red Cabbage pH Indicator in Respiration Labs



A new twist on using cabbage juice?

Most every science teacher is aware that the juice extracted from red cabbage makes an excellent acid/base indicator.  But at our school, we recently used it for a different (but related) purpose.  Our biology classes do a cellular respiration lab in which the students observe the production of carbon dioxide during respiration.  Normally we use phenol red as an indicator.  Our supply of phenol red was running low and we looked for an alternative.  The answer was cabbage juice indicator!!

How was red cabbage used as an indicator of carbon dioxide?  I'll come back to that in a minute.  First, here is a little background about red cabbage......

Background:   Red cabbage contains a natural pH indicator in the form of the pigment, anthocyanin.  It is a water soluble pigment that is commonly found in nature in red cabbage, of course, as well as purple plums, grapes, and apples.  Red cabbage juice will turn a wide variety of colors in the presence of acids and bases.  In a neutral solution, the cabbage is a fairly dark purple color.  In the presence of acids, the cabbage juice will turn red/pink, and in the presence of bases, the cabbage juice turns a greenish-yellow color.

How to Make Cabbage Juice Indicator:  This process is so simple that I usually allow my students to make their own indicator.  Simply chop up a head of red cabbage.  Place a small handful of cabbage leaves into a beaker.  Cover the leaves with water and boil for about 15 minutes.  As the cabbage boils, the pigment will be extracted from the leaves and the water will turn a dark purple color.  Use a slotted spoon to remove the cabbage, or pore the solution through filter paper.  Your indicator is now ready to use!

Our Cellular Respiration Lab:  In order to complete the chemical reactions of cellular respiration, oxygen is required.  Carbon dioxide is released during the reactions and is given off as a waste product.    Although it is very difficult for us to observe the conversion of glucose to ATP, it is possible to observe the gas exchange that must take place in order for respiration to be carried out.   The lab that we do at our school has three objectives:
1.   To observe the release of carbon dioxide during cellular respiration in animals.
2.   To determine if plants carry out the same gas exchange as animals.
3.   To observe, record, and analyze the results of an acid-base indicator.


In the first part of our lab, the student will learn how to use the cabbage juice to indicate the presence of carbon dioxide.  (See above photo.)  Carbon dioxide and water will react to form carbonic acid.  Since our breath and carbonated beverages contain carbon dioxide, the color change from dark purple to pinkish/red indicates the formation of carbonic acid.


In the second part of the lab, we want the student to determine if plants also give off carbon dioxide during cellular respiration.  As you can see in the above photo, the first tube serves as a control, the second tube contains germinating seeds, which have a high rate of respiration, and the third tube contains dry seeds, which are alive, but dormant.  The photo clearly shows that the germinating peas are releasing carbon dioxide.  (Since the peas are not photosynthetic, they are not consuming carbon dioxide.)  In the tube containing the dry peas, there is a very slight color change that does not show clearly in this photo.  The dry seeds do carry out cellular respiration, but at a very slow rate.  The results seen in the above photo were obtained after allowing the tubes to sit for just 24 hours.

This lab is easy to set up, easy to clean up, and best of all, our students really enjoy doing it.

Happy Teaching!



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