14-2Using Indicators to Determine pH

14-2 Using Indicators to Deter= mine pH

Many kinds of solutions must be tested to determ= ine whether they are acidic, basic, or neutral. The pH scale represents the hydronium ion concentration of a solution and is used to indicate how acidi= c or basic a solution is. The normal pH range is from the highly acidic pH 1 to = the neutral pH 7 to the highly basic pH 14. The most accurate method of pH determination is by use of an electronic instrument known as a pH meter. For simpler though less accurate pH determination, indicators= are commonly used. pH indicators are organic compounds that change color with changing pH. Some indicators are mixtures of compounds, each of which = changes color at a different pH. Indicators are chosen for specific applications according to the pH range in which they show a color change. In this experiment, you will observe the variation in color with pH of several indicators.<= /o:p>

OBJECTIVES

• Prepare solutions of varying pH.<= o:p>

• Observe the color changes of pH indicators in these solutions.

• Determine the pH range of each indicator.

HYPOTHESIS:<= /p>

MATERIALS

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apron

microtip pipets (3)

goggles

100-mL beaker

microplate

distilled water

PROCEDURE

1. Arrange your microplates so that there are six c= olumns and 12 rows on a blank bookcover. Label the columns A-F at the top. On the = left label the rows 1-12 starting with one at the top.

2. Fill a 100-mL beaker half-full with distilled wa= ter.

3. Place 9 drops of distilled water in each of well= s 2 through 11 in rows A - F.

4. Add 10 drops of 0.1M HCl to each of wells= A1, B1, C1, D1, E1, and F1.

5. Using a clean microtip pipet, add 10 drops of 0.= 01M NaOH to each of the wells A12, B12, C12, D12, E12, and F12.

6. Remove 1 drop of acid solution from well A1 and = add it to well A2 with your acid pipet. Mix the contents of the well using a tooth= pick.

7. Remove 1 drop of solution from well A2 and place= it in well A3. Mix well A3 in the same way that you did well A2.

8. Repeat the dilution and mixing process of steps = 6 and 7 through well A6, but do not transfer any solution to well A7.=

9. Repeat the acid dilution process used in row A f= or the wells in rows B, C, D, E, and F. Do not transfer any solution to well 7 in = any row.

10. Use a clean pipet to remove 1 drop of solution f= rom well A12 and place it in well A11. Mix as in step 6.

11. Repeat the dilution and mixing process of step 1= 0 from well A12 back through well A8, but do not add any solution to well A7.

12. Repeat the base dilution process used in row A f= or the wells in rows B, C, D. E, and F. The rows of solutions now have H+ concentrations that vary by a factor of 10. Each= row of wells approximates the pH scale from 1 to 12. Well 4 of each row contain= s a solution that is about pH 4, well 9 of each row contains a solution that is about pH 9, and so on.

13. Add 1 drop of universal indicator to each well i= n row A

14. Add 1 drop of bromothymol blue to each well in r= ow B.

15. Add 1 drop of phenolphthalein to each well in ro= w C.

16. Add 1 drop of thymol blue to each well in row D.=

17.<= /span> Pick an indicator not used and add 1 drop to ea= ch well in row E. Record name on Data Table.

= 18. Finally, pick an indicator not used and add 1 dr= op of to each well in row F. Record name on Data Table.

= 17. Observe the wells in each row. Record your observations of colors on your Data Table. Discard the solutions in your microplate according to your teacher’s instructions and rinse the microplate.

DATA AND OBSERVATIONS

indicator<= /p>

1

2

3

4

5

6

7

8

9

10

11

12

Row A: Universal indicator

Row B: Bromothymol blue

Row C: Phenolphthalein

Row D: Thymol blue

Row E:

Row F:

ANAYLSIS

1. For each indicator, list the color changes and t= he pH range or ranges at which they occur.

Universal indicator:

Bromothymol blue:

Phenolphthalein:

Thymol blue:

2. Which indicators changed color at more than one = pH?

CONCLUSIONS<= /p>
1. Which of the indicators tested is a good acid pH= range indicator? Why?

2. Which of the indicators tested is a good basic pH range indicator? Why?

3. Which of the indicators tested is the best choic= e when the acidity or basicity

of a solution is not known? Why?

4. For what reason do you think universal indicator= is called universal?

EXTENSION AND APPLICATION=

1. Suggest a reason for the common use of bromothym= ol blue to test the pH of water in swimming pools.

2. Phenolphthalein is not a good indicator to use in testing the pH of water in a fish tank. Suggest a reason why.

3. What characteristics are necessary for a univers= al indicator? Based on your observations in this experiment, suggest a composi= tion for a universal indicator.

indicator<= /p>	1	2	3	4	5	6	7	8	9	10	11	12
Row A: Universal indicator
Row B: Bromothymol blue
Row C: Phenolphthalein
Row D: Thymol blue
Row E:
Row F: