Expt 011 -- pH Buffers

A series of solutions of known [H⁺] is prepared by mixing two stock solutions in different proportions. 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 in a similar way:
  • pOH = - log₁₀ [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).
• A pH buffer is a solution that tends to maintain a constant pH. The solution contains significant concentrations of an acid and its conjugate base (see Experiment 008). Consider the following reaction:
  • H₂PO₄^- + OH^- --> HPO₄^2- + H₂O
  • HPO₄^2- + H⁺ --> H₂PO₄^-
• A mixture containing significant concentrations of both H₂PO₄^- and HPO₄^2- tends to maintain constant pH because the ratio of the concentrations of these species does not change much when either acid or base is added.
• A pH buffer has a buffer capacity. This expresses the amount of added acid or base that can be absorbed by reaction before a significant pH change occurs. Very roughly speaking, the buffer capacity is something like 0.5 x {[acid] + [conjugate base]}

Safety

Some of the buffer chemicals (e.g., boric acid) are toxic. Wear safety goggles and aprons. Avoid ingesting the chemicals. Use caution while handling the acids, bases, and indicators. Wash spills with water. Wash hands immediately upon contact with the chemicals. Wash hands after the experiment.

Procedure

1. Note buffers are prepared in a 24-well plate and then drops are transferred to the 96-well plate.
2. Prepare the following mixtures of solutions in a 24-well plate using a standard size plastic transfer pipet:

   pH	# drops A	# drops B
   2	39	0
   3	35	4
   4	31	8
   5	27	12
   6	24	16
   7	20	19
   8	17	22
   9	14	25
   10	11	28
   12	3	36

3. Place the 96-well microplate with the numbered columns at the top and the lettered rows on the left side.
4. Using a clean pipet, transfer 3 drops of pH 2 buffer from its well in the 24-well plate to each well to be tested at pH 2 in the 96-well plate. Repeat for each of the other solutions.
5. Add one or two drops of methyl orange indicator to each well in the top row. Add phenolphthalein to each well in the second row. Continue adding a different indicator to each row.
6. Place the 96-well microplate on a white paper background. Observe the colors.

   !!!Click here to See Picture.

7. 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, make up dilutions to test other natural indicators.
8. Empty the plate into a disposal beaker (or jar) provided by your instructor.
9. 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.
3. Briefly describe what is meant by the terms buffer and buffer capacity.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 011 pH Buffers

Record data while watching the the movies and pictures.

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.
• This is a very different experiment from that of pH Indicators, Experiment 004. The difference is that this experiment uses "magic" solutions to achieve specific pH levels, while the other uses a serial dilution procedure. Many teachers believe that the serial dilution procedure helps gain a better understanding of the pH scale. In this experiment, however, pH 5-9 are more likely to be close to "correct" than in SSExperiment 004 because of the extreme dilution for those solutions.

Safety-

Some of the buffer chemicals (e.g., boric acid) are toxic. Wear safety goggles and aprons. 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: 30 minutes (Add 20 minutes is many of the indicators must be prepared.)

Class Time: 50 minutes (For shorter lab periods, have two groups share preparing the 12 pH buffers in the 24-well plate. The recipe is enough for two groups.)

Materials-

• 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.)
• The following prepared solutions should be made available in plastic transfer pipets:
  • 10 mL of Solution A:
    • 0.2 M anhydrous boric acid and 0.05 M citric acid monohydrate. (Place 1.24 g H₃BO₃ and 1.05 g citric acid monohydrate in 60 mL of water. Dissolve. Add enough water to bring the final volume to 100 mL. You may want to use a 100-mL volumetric flask.)
  • 10 mL of Solution B:
    • 0.1 M trisodium phosphate dodecahydrate (Place 3.80 g trisodium phosphate dodecahydrate, Na₃PO₄•12H₂O, in 60 mL of water. Dissolve. Add enough water to bring the final volume to 100 mL. You may want to use a 100-mL volumetric flask.)
• 96-well plate
• 24-well plate
• plastic transfer pipets
• (disposal jar, NaHCO₃, vinegar)
• (100-mL volumetric flask)

Disposal-

The materials used in this experiment may be discarded at the sink.

Lab Hints-

See Experiment 004 on pH indicators.

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.

Q3. Briefly describe what is meant by the terms buffer and buffer capacity.

A3. A pH buffer is a solution that tends to maintain a constant pH. Buffer capacity expresses the amount of added acid or base that can be absorbed by reaction before a significant pH change occurs.

CoopLearn-

• There are two group challenges that can be issued. Groups can be charge with the task of finding interesting (attractive, effective, etc.) natural indicators. The groups can engage in an indicator contest.
• Universal indicators are ones in which there are many color changes throughout the pH range. Groups can be challenged to see which can concoct the best universal indicator from among indicators provided in the laboratory.

Reference-

Silberman, R. G.J. Chem. Educ. 1992, 69, A42-43.

Carmody, W. R. J. Chem. Educ. 1961, 38, 559

Key Words 1-

pH, acid, base, pH indicator, color change, reaction, applied chemistry, buffer, buffer capacity

Elements-

H O C B P