Learn about acids, bases, and pH indicators by measuring the pH of various substances.
Why do we have to define a pH scale?
The pH scale was defined because the enormous range of hydrogen ion concentrations found in aqueous solutions make using H+ molarity awkward. For example, in a typical acid-base titration, [H+] may vary from about 0.01 M to 0.0000000000001 M. It is easier to write "the pH varies from 2 to 13".
The chemistry behind pH, Acids, and Bases:
As mentioned above, an acidic solution contains an excess of protons or H+. pH is a measure of how 'acidic' a solution is. The lower the pH, the more acidic the solution. In chemical terms, pH means "the negative log of the concentration of protons" in solution. The theoretical definition is:
pH = -log[H+].
For example, if the concentration of H+ is .01M, the pH will be:
-log[.01] = -log[10^-2] = -(-2) = 2 (very acidic!).
"Neutral" solutions, such as water, have a pH of 7. This number coincides with the amount of H+ naturally formed in water from the equilibrium reaction: H2O <--> H+ + OH- (H+ experimentally known to be ~10^-7M; OH- is also the same concentration). "Basic" solutions have a pH greater than 7; meaning they have less free H+ than that of neutral water.
What did actually happen in this experiment?
By boiling the red cabbage leaves, you extracted a class of pigment molecules called anthocyanins into solution. Anthocyanin molecules will change their color depending upon the pH of their environment and can indicate the pH of a solution. This experiment can tell you whether a substance is an acid or base, but not the exact value of pH; the pH scale ranges from acid (0-6), through neutral (7) to base (8-14). If you want to calibrate your cabbage juice pH indicator, you will have to test your substances with another quantitative indicator (e.g. litmus paper) and compare those results to the colors of the cabbage juice pH indicator in those solutions. (Litmus paper can be obtained from several scientific suppliers or from your local swimming pool store.)
For more information on this experiment, check out: http://www.madsci.org/experiments/archive/859332497.Ch.html
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