pH Lab Activity

Background Information:  Molecules that are dissolved in water may separate (dissociate) into charged fragments or ions.  Often one of these fragments is a hydrogen ion (H+).  The pH of a solution is a measure of the concentration of hydrogen ions.  Because enormous variations in ion concentrations are possible, pH is calculated in powers of 10, using the mathematical device of logarithms (base 10).  The alkalinity or acidity of a solution is determined by its concentration of H+ ions, that is, by its pH.

A water molecule ionizes when one of its two hydrogen atoms leaves its electron behind and, as a hydrogen ion (H+), joins a different water molecule.  Two ions are produced by this reaction, a hydroxide ion (OH-) and a hydronium ion (H3O+).  We can express this reaction as follows:

2 H2O <---> H3O+  +  OH-

Convention, however, allows us to express the ionization of water more simply as:

H2O <---> H+  +  OH-

If there are more H+ ions than OH- ions per mole, the solution is acidic.  If there are more OH- ions than H+ ions, the solution is basic.  If the concentration of H+ ions equals the concentration of OH- ions, the solution is neutral.  pH scale:

0-----1-----2-----3-----4-----5-----6-----7-----8-----9-----10-----11-----12-----13-----14

                                                >H+                     ....1000x    100x      10x  neutral  10x    100x    1000x .....                    >OH-

 

Directions:  Make a data table for each exercise using the following information/instructions.

Exercise A:  Testing some beverages using pH paper.  In your data table include the solution, your prediction of the pH (you can simple predict if the substance will be a strong acid, mild acid, neutral, mild base or strong base), and the actual pH.  You will test the following eight substances:  H2O, non-fat milk, orange juice, sparkling water, apple juice, coffee, carbonated lemon-lime soda, and sports drink.

Procedure:

1.  Use the trays in your boxes to put each of your samples in a well (don't forget which is which).

2.  Using forceps, dap a piece of pH paper into the first sample. 

3.  While the paper is still wet, compare its color with the standard pH color scale in your box.  You may to repeat step 2 if you waited too long.  Record the pH value in your data table.

4.  Be sure to wipe off the forceps after each substance...why?

5.  Repeat steps 2-4 for the rest of the beverages.

Exercise B:  Testing the pH of common medicine using cabbage juice (pink for acidic, green for base).  In your data table include the solution, your prediction of the color change of the cabbage juice, and the actual color change of the cabbage juice.  You will test the following four medicines: aspirin, Milk of Magnesia, Alka Seltzer, and Maalox.

Procedure:

1.  Place two or three drops of cabbage juice into your tray wells; record the color of the cabbage juice.

2.  Put a drop or two of each of the medicine samples into different wells (don't forget which is which).

3.  Record the color change of the cabbage juice (if there is a change).

4.  Repeat steps 1 - 3 if necessary.

Post Lab Questions:

1.  What is pH?

2.  Arrange the solutions from both exercises on a pH scale in order of increasing pH, noting the pH of each substance (you will have two scales)

                ex:   0-----------------------------------3.5---------------------------------7-----------------------------------10.5---------------------------------------14

3.  It is often recommended that aspirin be taken with a large glass of milk or water.  Based on your results in these lab exercises, explain why this is a good recommendation?

4.  Would apple or orange juice be a good accompaniment to aspirin?  Why or why not?

5.  Which of the medicines you tested are often used for the treatment of "acid indigestion" (too much acid in the stomach)?  Based on your results, how would you explain the action of these medicines?

6.   Enzymes function best at particular pH values.  In the normal human stomach, a pH of 2.0-3.0 provides the environment required for the proper functioning of the digestive enzymes found there.  What effect would the medicines that are basic have on these enzymes ability to digest (break down) food?

7.  In some flowers, soil pH affects the uptake of certain metals that can complex (join) with the pigment and prevent its normal color expression, pink.  For instance, in hydrangeas, high soil pH values prevent the uptake of aluminum and the flower appears blue.  Is the soil more acidic or basic in this situation?  What color would hydrangeas be if grown in a heavily industrialized region.   Explain.