Interesting Projects for Home or School - a031

Red Cabbage and pH

By Harold Eddleman, Ph.D. President of Indiana Biolab. (indbio@disknet.com)

When you cook vegetables the water soluble chemicals leak into the cooking water as the heat breaks the plasma membranes of the cells. When you boil beets the water becomes very red. Cooking vegetables causes the loss of vitamins, sugars, and some proteins. These chemicals are very important to good nutrition.

Red Cabbage Color in Acidic and Alkaline Solutions.

Begin by boiling chopped red cabbage in tapwater. You only need to boil long enough to extract some of the red color into the water. Save the colored water for these experiments. The cabbage is suitable for food.

Now the real fun and beautiful color begins. When you run the following experiments, use just a small portion of your boiled red cabbage water. For each experiment, you will usually begin with an new sample of the cabbage water. Rinse you test tube out with tap water between expts. If you hold he testtube against a white background it may be easier to see the color. Or try holding it up to a window. If you don't have test tubes; use whatever you can think of. A white plastic spoon may be better than a test tube for seeing the colors.

Try adding one or a few drops of vinegar. Does the color change?

In another sample add a toothpick tip of baking soda. You should see a sharp difference. Does the color change or disappear? I have heard that when boiling red cabbage cooks often add a little vinegar or baking soda to determine the color of the final dish. Which color would you like to see on your plate? Would you add vinegar or baking soda to get the color you prefer?

Try adding a drop or more of household bleach (Chlorox or similar product). Does it have any effect on the color? Right away or later. [Warning, for safety, never mix bleach and ammonia, toxic fumes may result.]

Vinegar is acidic. Some fairly safe other acidic chemicals are soda pop, cheddar and similar cheeses, some homes have citric acid, boric acid, and oxalic acid. Chemistry labs use these acids: hydrochloric, sulfuric, phosphoric, nitric, sulfamic acid. Perhaps you can get a science teacher to help you test red cabbage color with these acids.

Baking soda (sodium bicarbonate)is alkaline. Some other alkaline materials are washing soda (sodium carbonate), slaked lime (calcium hydroxide). Some dangerous, strong alkalline chemicals are lye (sodium hydroxide), some drain cleaners, ammonia,

Do all the acids give the same color with red cabbage water? If so, what color?

Do all alkaline substances give the same color?

Using drops of baking soda and vinegar, can you get the exactly in between color?

Warning

Do not read beyond this point unless you love science.

The pH Scale

Doctors, biologists, and chemists found long ago that, unlike water, strongly acid or strongly alkaline solutions were harmful to skin, intestines, and the normal chemical reactions in the body. More about pH later. I just wanted you to know that scientists must be very careful to have the right balance of acid and alkalinity for biological work. Chemists found all acidic solutions have lots of free protons (hydrogen ions, H+). Basic (alkaline) solutions have very few free protons. Since a proton has a charge it can attach or be attracted to proteins and other charged molecules. When a proton attaches to or becomes very close to a protein molecule, it usually changes the local charges in the molecule and since charges attract or repell, the shape of the protein often changes and that change of shape often affects the properties of the protein. The change in shape can made an enzyme inactive. Small changes of H+ concentration have less affect on sugars and carbohydrates because they seldom have a natural charge. Chemists us [ ] to mean concentration. Thus [H+] means concentration of protons; usually expressed as moles per liter.

Chemists have developed The pH Scale as way of describing the acidity-alkalinity balance of a solution. Chemists also use p as an abbreviation to mean concentration; so pH means concentration of hydrogen, but we all know they really mean concentration of H+ (protons). Thus, pH = [H+] = concentration of H+. If you really want to get ahead of our story, tell your teacher "pH is the negative logarithm of hydronium ion concentration, expressed as moles per liter." My teachers in beginning college chem always wrote, "very good", "Excellent", and other nice words when they asked me to define pH on a test. Just memorize that and it will serve you well. I will explain it later. If you become a physician or scientist all the stuff in his paragraph will be very important to you.

Heck, I am going to tell you right now what hydronium ions are. Recall that Water is H2O. The three atoms are hooked together as follows: H-O-H. The atoms are not in a straight line. There is a 135 degree angle bend at the Oxygen. Also Oxygen loves electrons and attracts some of the electron clouds of the hydrogens to its self. Therefore, the water molecule is dipolar, having two poles, one end of the molecule slightly negative due to Oxygen, and the other end is slightly positive due to the slight loss of electron clouds to the oxygen. Therefore, water molecules have an attraction for each other and we call it hydrogen bonding. That is the reason we have water on earth!! Without this attraction, water would have a very low boiling point and would exist on earth as a gas and, perhaps, due to earth's low gravity, that vapor would have escaped long ago! Hydronium ions consist of one proton attracting one or more water molecules around it by hydrogen bonding. A hydronium ion is a proton solvated with water (coated with water).

pH Indicators

Long ago, it was found that the dye called litmus, found in a lichen, also changes color like red cabbage. The color of red cabbage or litmus or the petals on hydrangea indicates the pH. They are pH indicators. Litmus is red in acid and blue in alkaline solutions. A wonderful thing about litmus is that it can be dried on paper and kept dry for years until needed. Red and blue litmus paper are the same stuff. The red paper has more protons attached to the litmus compound. There are some faults with litmus.

1. The color is very dilute, sometimes it is very hard to see. It is useless for deterimining blood and other colored solutions.

2. It is a biological substance which can get eaten in biological reactions. Bacteriologists sometimes add litmus powder to milk, sterilize it and then inoculate the litmus milk with a pure culture of an unknown bacteria. Some bacteria produce acid and turn it blue. Others produce alkaline and turn the litmus blue. Others destroy the litmus and the milk becomes white. As an added bonsus, some bacteria curdle the milk or cause other striking changes.

3. The biggest problem with litmus, is that the point where the color changes may not be the acid/base balance point important in a given experiment. Therefore, we need many different pH indicators.

Red cabbage color is more intense than litmus, but it loses its color and will not last years in the laboratory.

More about pH Scale and pH indicators

It happens that in netural water, the concentration of H+ is 0.0000001 moles per liter (6 zeros). Another way to write this is 1 x 10<&sup-7> .

-----to be continued.

Also very important to biology and chemistry is the use of Buffers. A link to buffers will be placed here. We will also add a link to acids and bases to cover additional experiments.

Safety Precautions

Do not use the strong acids and bases mention on this page for experiments at home. There are too many risks such as a child getting access to them. Also there is a risk of spillage or dropped bottles.