The pH scale is used as a measure of acidity in the environment. In this lab you will learn to measure pH with a pH meter, determine the pH of some solutions, and investigate the properties of buffers.

The symbol pH is derived from the French for "hydrogen power." The numbers in the pH scale stand for negative logarithms (10^-n) of the concentration of hydronium ions (H₃O⁺), here verbally simplified to hydrogen ions (H⁺) expressed in moles per liter of water. To obtain a mole of any substance, you simply measure out its molecular weight in grams. One gram of hydrogen atoms is a mole; 44 grams of CO₂ is a mole; the atomic weight of C = 12, that of O = 16, and 12 + 2(16) = 44. A mole of any substance contains the same number of particles -- Avogadro's number, 6.02 x 10²³. The liter is a particular volume of water.

Thus a pH of 0 means a hydrogen ion concentration of 10⁰ molar or moles per liter, which is one mole.

Since normal dissociation of water molecules into H⁺ and OH^- yields 10^-7 moles of H⁺, pure water has a pH of 7, the neutral point in the scale. Since the pH scale is logarithmic, a difference in 1 unit is a 10-fold increase or decrease in concentration of hydrogen ions.

I. Measuring pH

Your instructor will demonstrate the use of the pH meter and of litmus paper.

Dilute each solution by half (add an equal volume of water). Measure and record the pH of each of the following aqueous solutions. Be sure to rinse the electrode with distilled water between each measurement and dry it carefully; use a twist of paper towel like dental floss to clean the groove in its tip.

Put a drop of each solution on pink and on blue litmus paper and record the color change. What is the effect of an acid and of a base on each color?

* clean the tip of the electrode thoroughly; detergent and soapy water both leave a film.

What would be the resulting pH if you mix equal amounts of an acid and a base that are the same numerical distance from neutrality (e.g., pH 5 + pH 9)?

II. Investigating the Properties of Buffers

Constant pH is maintained with buffers, which are solutions of weak acids or bases that either donate or accept hydrogen ions. Thus excess H+ is removed, and, when the addition of OH- uses up the H+ to form water, the H+ is replaced. In this exercise you will compare several solutions. Be sure to use equal volumes of liquid (water or buffer) in each beaker.

• Add acid (lemon juice) one drop at a time to an aqueous solution.
• Swirl the solution well to mix it after each drop is added.
• Record the change in pH of the solution.

When you add acid, you expect the pH to decrease. For the first 20 drops of lemon juice you have added, using graph paper, plot the pH (on the y-axis) as a function of the number of drops of added lemon juice (on the x-axis). Compare the rate of the decrease in pH in the different solutions (water or buffer). Did the pH change at a uniform rate with each drop by drop addition of lemon juice?

Buffers act to resist changes in pH (either additions of acid or base). Buffers, however, have a limited capacity, and when you exceed that capacity, the pH changes rapidly. Organisms depend on the buffering capacity of their environment and on buffered solutions within their own bodies in order to tolerate fluctuations in acidity. Changes that exceed the capacity of the buffers will often kill the organisms. Almost all cellular chemistry takes place at pH's between 6 and 8.

Rev.1/12/99 Wahlert & Holland

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