1.23: Experiment_624_Measuring pH_1_2

Student Name		Laboratory Date: Date Report Submitted:	___________________________
Student ID		Experiment Number and Title	Experiment 624: Measuring pH

Experiment 624: Measuring pH

Section 1: Purpose and Summary

• Determine the pH of some solutions using ‘red cabbage’ indicator.
• Determine the pH of solutions using a pH meter.
• Understand the effect of adding an acid or a base to buffer solutions.

In this experiment, the instructor will prepare a pH indicator by extracting colored substances from red cabbage. This pH indicator solution will be added to buffer solutions of pH range 1 – 13. Students will record the resultant color of each buffer solution. Students will then use the indicator to determine the pH of common solutions by comparing the color to the buffers. To measure the exact pH values of solutions, a laboratory pH meter will be used. Students will determine the hydrogen ion concentration, [H+] of the solutions from the pH value recorded. Students will determine the effect of adding HCl or NaOH to an acetic acid/acetate buffer solution.

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Use of eye protection is recommended for all experimental procedures.

Waste Disposal:

All reagents used in this experiment are generally non-toxic and may be disposed of in sinks with tap water rinses.

Section 3: Procedure

Part 1: Preparing the pH indicator and pH standards – INSTRUCTOR DEMO

NOTE: Part 1 may be replaced by the use of universal indicating solution at the instructor’s option.

Place several cut-up red cabbage leaves into a 600-mL beaker and cover the leaves with laboratory water.

Boil the cabbage leaves on a hot plate until the pigment has been extracted. The leaves will look whitish in color.

While the indicator cools, prepare an array of buffer solutions with pH 1 – 13. Label thirteen (13) 150-mm test tubes. Fill each test tube with about a half-full of the appropriate buffer solution.

Using a dropper pipet, add several drops of the cabbage indicator to each test tube. An array of colors should be observed. Students will record the color of each pH standard.

Buffer pH	Color with Cabbage Indicator
1
2
3
4
5
6
7
8
9
10
11
12
13

Part 2: pH determination of household products or everyday chemicals

Possible household items: white vinegar, colorless soda like Sprite^TM, dish soap solution (colorless), sparkling water, alkaline water, isopropyl alcohol

Everyday chemicals: 0.1 M HCl, 0.1 M acetic acid, 0.1 M ammonia, 0.1 M NaOH

Take a 150-mL beaker and obtain about 50 mL of cabbage indicator solution prepared in Part 1.

Label ten (10) clean medium test tubes with the solutions to be tested.

Transfer about 3 mL of each solution into the corresponding test tube.

Using a dropper pipet, add an equal volume of cabbage indicator solution. Mix well.

Compare the resulting color of the solution with the buffer array to determine the approximate pH of the solution. Record your observations on the table below.

Set aside these solutions for use in the following section.

Solutions tested	Approximate pH

Part 3: Quantitative determination of pH

This procedure describes how to use a portable Flinn pH meter (model AP8673). If the pH meter is not measuring properly (unstable, or inaccurate), refer to the manual for troubleshooting.

Check out a portable pH meter from the stockroom.

Remove the protective cap on the electrode. Clean any salt build-up off by rinsing with laboratory water.

Press the ON/OFF button once.

Rinse the electrode with laboratory water and blot dry with filter paper.

Take one of the solutions from Part 2 of this experiment.

Immerse the electrode in the test solution. Once the display stabilizes (approx. 1 min.), record the exact pH.

Remove the pH meter from the solution.

Repeat steps 4 – 7 using the rest of the test solutions.

When finished, rinse the electrode with laboratory water and blot dry with filter paper. Replace cap and return the pH meter to the stockroom.

Solutions tested	pH

Part 4: Effect of adding an acid or a base to the pH of a buffer

Obtain four (4) medium test tubes and label them A – D.

To test tubes A and C, add 10 mL of laboratory water.

To test tubes B and D, add 5 mL of 0.1 M acetic acid (HC2H3O2) and 5 mL of 0.1 M sodium acetate (NaC2H3O2). This mixture is your acetic acid/acetate buffer solution. Stir well to mix.

Using Hydrion^TM pH paper strips, determine the pH of the contents of test tubes A – D. Put four (small) strips of pH paper on a large watch glass. Using a stirring rod, dab a small drop of the test solution onto the pH paper. Compare the color obtained to the pH scale on the label of the pH paper casing. Record the pH values to the tenth place.

To test tubes A and B, add five (5) drops of 0.1 M hydrochloric acid (HCl). Determine the pH using pH (similar to step 4 above).

To test tubes C and D, add five (5) drops of 0.1 M sodium hydroxide (NaOH). Determine the pH using pH (similar to step 4 above).

Solutions tested	pH	pH after addition of 0.1 M HCl
A (laboratory water)
B (buffer solution)
Solutions tested	pH	pH after addition of 0.1 M NaOH
C (laboratory water)
D (buffer solution)

Post Lab Questions:

1. Based on your results, classify each test solution as acidic, basic, or neutral.

2. Write the mathematical equation that shows the relationship between pH value and the hydrogen ion concentration, [H⁺]. What is the unit of [H⁺]?

3. Using your results in Part 3 of this experiment, calculate [H⁺] for each test solution. Include units.

4. Compare a solution with pH 10.5 and a solution with pH 4.5. Which solution contains more H⁺ ions? Explain.

5. Two methods were used in determining the pH of the solutions – pH paper and pH meter. Do they show similar or different pH values? What was different?

6. Compare the pH of the buffer solutions in Part 4 of this experiment before and after the addition of 0.1 M HCl and 0.1 M NaOH. Does this confirm the expected behavior of a buffer solution against an added acid or base? Explain.