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Using beads to teach pH.

The inverse relationship between pH and hydrogen ion concentration is difficult for students to visualize. More often than not, students cannot comprehend why pH increases when hydrogen ion concentration decreases. While they can understand the pH scale, interpreting its association with hydrogen ion concentration is confusing. I play" a simple game with my students to help them grasp this concept. We then do a real experiment to confirm it.

[FIGURE 1 OMITTED]

Each group of students is given five tubes with a varying number of small beads (Figure 1). They count the beads and assign a value of 1 for every, 10 beads (e.g., 100 beads equals -10:50 beads equals -5). We then tabulate and compare results. This demonstration clearly shows the students that the more beads in a tube, the lower the numerical value, and vice versa. I then explain that the same concept applies to pH. A solution with more hydrogen ions ("beads") has a lower pH value than a solution with fewer hydrogen ions.

Now we do a simple experiment to reinforce the association between pH and hydrogen ion concentration. Students obtain 10 ml of 1 M hydrochloric acid in a test tube and determine the pH using pH paper. 1 then have them assume that 10 ml of this acid contains 1000 hydrogen ions, or beads. (Note: It is very important to emphasize that the number 1000 is used for simplicity and that it is not real.) Next, students add 1 ml of the acid to 9 ml of water in a second tube. I tell them to reason that if 10 ml of the original acid contains 1000 hydrogen ions, then 1 ml contains 100 hydrogen ions. Hence. the diluted solution has 100 hydrogen ions in a total of 10 ml. That is, the acid was diluted tenfold. Students repeat the serial dilution four to five more times. I emphasize that for each subsequent dilution, they reduce the hydrogen ion concentration another tenfold. After all dilutions are completed, students predict whether the pH has increased, decreased, or remained the same in each tube. Finally'. they use pH paper to find the actual pH (Figure 2).

* Experiment. pH of Serially Diluted Acid
Materials

* 1MHCI

* water

* pH paper

* test tubes

* rack

Procedure

1. Label five tubes #1 through #5; place in the rack.

2. Fill Tube # 1 with 10 ml of 1 M HCl; find the pH using pH
paper.

3. Fill Tubes #2 through #5 with 9 ml of water.

4. Add 1 ml of 1 M HCl to 9 ml of water in Tube #2.

5. Continue to dilute the acid by, adding 1 ml from one tube
to 9 ml of water in the next.

6. Mix dilutions thoroughly; find the pH using pH paper.


[FIGURE 2 OMITTED]

* Questions

1. What happens to the hydrogen ion concentration when you dilute the acid?

--

2. Does the pH increase, decrease, or remain the same as you dilute the acid? Why?

--

* Results
 Hydrogen ion
Tube Dilution concentration pH (predicted) pH (observed)

1 1x
2 10x
3 100x
4 1000x
5 10000x


BIO

LALITHA JAYANT, Ph.D., is Assistant Professor, Borough of Manhattan Community College, The City University of New York, Science Department, New York, NY 10007; e-mail: ljayant@bmcc.cuny.edu.
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Title Annotation:QUICK FIX
Author:Jayant, Lalitha
Publication:The American Biology Teacher
Geographic Code:1USA
Date:Jan 1, 2009
Words:546
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