Expt 004 -- pH Indicators

A series of solutions of known [H⁺] is prepared by successive dilutions from a stock solution. Dyes called pH indicators that have different colors depending upon the acidity are then added to these solutions.

Background

• pH indicators are organic acids that dissociate:
  • HIndicator --> H⁺ + Indicator^-
• When the HIndicator and Indicator- have different colors, a change is seen when the reaction takes place. In general, the color change is most dramatic when the ratio of [HIndicator]/[Indicator^-] changes from 10 to 0.1. This happens in a different pH range for each indicator. That is, each indicator has a characteristic range for change.
• Indicators are chosen on the basis of pH change range and intensity of color. When the indicator acid and/or salt are intensely colored, then a small amount imparts much color -- but does not interfere with the principal reaction being studied.
• The pH scale is a logarithmic scale used to describe acidity.
  • pH = - log₁₀ [H⁺]
• So, a solution that has [H⁺] = 0.01 has a pH of -log₁₀(10^-2)
  • = -(-2) = 2
• A pOH scale is defined ion a similar way:
  • pOH = - log₁º [OH^-]
• So, if [OH^-] = 0.01, pOH = 2.
• For any water solution, it turns out that pH + pOH = 14.
• The acids and bases used (HCl, NaOH) are strong. This means that they dissociate completely so that [NaOH] = [Na⁺] = [OH^-]. We do not expect to find any undissociated NaOH units in the solution. (A 0.1 M NaOH solution really has no NaOH; it has [OH^-] = [Na⁺] = 0.1 M).

Safety

Some of the indicators are toxic. Wear goggles and apron. Avoid ingesting the chemicals. Use caution in dealing with the acids, bases, and indicators. Wash spills with water. Wash hands immediately upon contact with the chemicals. Wash hands after the experiment.

Procedure

1. Place the 96-well microplate with the numbered columns at the top and the lettered rows on the left side.
2. In rows A, B, C, E, F, and G, place 9 drops of distilled water in wells #2 - #11. In the first well of each row place 10 drops of 0.1 M HCl and in well #12 place 10 drops of 0.01M NaOH.
3. Using a clean pipet, transfer one drop of acid from the first well to the second well in row A. Mix thoroughly by drawing up the entire contents of the second well into the pipet and then returning it to the well. Transfer one drop of the liquid from the second well to well 3, mixing as before and continue in this way up to well #6 which will be the last one of the acidic dilutions. Repeat this procedure for the other rows.
4. Repeat the dilution procedure using the 0.01 M NaOH in the same row as the acid dilution, but working backwards from #12 to #8, making #8 the last of the basic dilutions. Do not added anything to well # 7.
5. Each well has been diluted by a factor of 10. In well #1, the [ H⁺ ] = 1 x 10^-1 mol/liter, in well #2, the [ H⁺ ] = 1 x 10^-2 mol/liter. In well #12 the [ OH- ] = 1 x 10^-2 mol/liter, in well # 11 the [ OH- ] = 1 x 10^-3 mol/liter.
6. Add one or two drops of indicator to each well, using the figure as a guide. See below:

   

7. Place the 96-well microplate on a white paper background. Observe the colors.

   !!!Click here to See Picture.

8. Many indicator compounds are found as pigments in the flowers or fruits of plants. In row G, test the extract of the natural material you brought or one of the ones available in the lab. If time permits, you may make up dilutions to test other natural indicators. Red cabbage juice is shown here.
9. Empty the plate into a disposal beaker (or jar) provided by your instructor.
10. Write a summary of the observations from this experiment which includes the colors of the various indicators tested and the pH at which those colors were observed.

Questions

1. From among the indicators tested, which is the best choice for observing changes in pH around pH 7?
2. Predict the result of mixing bromthymol blue and phenolphthalein in the same line of wells.

Handout

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 004 pH Indicators

1. Note the colors in the picture on the CD-ROM
2. Write a summary of the observations from this experiment which includes the colors of the various indicators tested and the pH at which those colors were observed.

Curriculum-

Use this experiment with acids and bases, acidity, pH, indicators, titrations, applied chemistry (with many natural indicators), or color.

Safety-

Some of the indicators are toxic. Wear goggles and apron. Avoid ingesting the chemicals. Use caution in dealing with the acids, bases, and indicators. Wash spills with water. Wash hands immediately upon contact with the chemicals. Wash hands after the experiment.

Time-

Teacher Preparation: 20 minutes to prepare indicators

Class Time: 50 minutes

Materials-

• The following indicator solutions should be made available in plastic transfer pipets:

  (prepare 1.5 mL of each indicator per group of students.)

  • bromthymol blue (dissolve 0.1 g bromthymol blue in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
  • methyl orange (dissolve 0.1 g methyl orange in 100 mL of distilled water)
  • bromcresol green (dissolve 0.1 g bromcresol green in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
  • phenolphthalein (dissolve 0.5 g phenolphthalein in 60 mL 95% ethanol, and add enough distilled water to bring the final volume to 100 mL)
  • Alizarin yellow (dissolve 0.1 g alizarin yellow R in 100 mL of distilled water)
• natural indicator (red cabbage juice, blueberries, petals from flowers, etc.)
• NaHCO₃, and vinegar for disposal
• disposal jar
• 96-well plate
• clean, pulled, transfer pipet

Disposal-

The materials should be collected in a disposal jar. The resulting mixture should be neutralized (with NaHCO₃ or vinegar) and discarded at the sink.

Background-

The buffers can be approached from the perspective of equilibrium theory:

K_a = [H⁺][A^-] / [HA]

[H⁺] = K_a x [HA] / [A^-]

K_a is a constant. If the ratio [HA] / [A^-] does not change much, then [H⁺] does not change much either.

Data Table-

	acid	base	range
bromthymol blue	yellow	blue	6.0-7.6
methyl orange	red	orange/yellow	3.0-4.4
bromcresol green	yellow	blue	4.0-5.6
phenolphthalein	colorless	red	8.2-10.0
alizarin yellow	yellow	violet	10.0-12.0

Answers-

Q1. From among the indicators tested, which is the best choice for observing changes in pH around pH 7?

A1. Bromthymol blue is the best choice among those indicators used here.

Q2. Predict the result of mixing bromthymol blue and phenolphthalein in the same line of wells.

A3. The solution will turn from yellow to blue around pH 7, and then from blue to purple around pH 9. The blue of bromthymol blue and the pink of phenolphthalein will lead to a purple color.

Key Words 1-

pH, acid, base, pH indicator, color change, reaction, applied chemistry, conjugate acid, conjugate base

Elements-

H O C