Notes for Teacher

MICROSCALE ANALYTICAL EXPERIMENT

Determine the Order of Reaction of Phenolphthalein in Alkaline Solution

A. Materials

Chemical

D.I. water

Ethanol

Phenolphthalein

Sodium chloride

Sodium hydroxide

Glassware

10-mL Beaker

Dropper & dropper teat

5-mL graduated pipette (optional)

10-mL graduated pipette

Pipette filler

10-mL volumetric flask

The following chemicals and glassware are for standardization of sodium hydroxide solution

Chemical:

D.I. water

Potassium hydrogen phthalate

Sodium hydroxide

Phenolphthalein

Glassware:

250-mL Beaker

Burette

Burette clamp

250-mL conical flask

Glass rod

Hot plate stirrer

25-mL pipette

Pipette filler

Stand

250-mL Volumetric flask

White tile

B. Preparation of materials

0.1% phenolphthalein solution is prepared by dissolving 0.01g of phenolphthalein in 10 mL of absolute ethanol.

The UV spectrum is recorded by Shimazhu UV-1700 spectrometer.

The data in this experiment are recorded by PASCO USB link with PASPORT Colorimeter Sensor and Replacement Cuvettes/Cap.

The Software for data recording is DataStudio.

C. Suggested time for the experiment

Part A – C: 15 minutes

Part D: 15 minutes

Part E: 55 minutes

Total: 85 minutes

D. Instruction for students

Experiment focus:

-  Chemical kinetics (rate law, rate constant)

-  Reactions of phenolphthalein in alkaline solution

-  Determination of reaction order by plotting appropriate graphs

- Advantages of using data loggers for repeating measurements

E. Student's common errors and possible mistakes and Suggestion to avoid the errors and mistakes

1. Error: The 0.3M NaCl solution was added exceeding the mark of the 10-mL volumetric flask during dilutions of NaOH solution.

Suggestion: Add the last 0.5 mL of 0.3M NaCl solution with a dropper slowly.

2. Error: More than 3 drops of 0.1% phenolphthalein solution were added to the sample solutions, caused the pink coloration became too intense for measurement.

Suggestion: Add the phenolphthalein solution carefully and slowly, repeat the measurement if necessary.

3. Error: After addition of phenolphthalein solution, the solution mixture was not shaken well before taking the measurement. This caused inaccurate measurement of the absorbance throughout the whole measuring period.

Suggestion: Be sure the solution mixture is shaken well before putting into the compartment of the colorimeter for data taking.

4. Error: The potassium hydrogen phthalate was not completely dissolved while titrating with NaOH solution. This cause inaccurate results and thus wrong concentration of the NaOH solution.

Suggestion: Make sure all the potassium hydrogen phthalate has been dissolved before pouring into the volumetric flask, heat the solution for longer time if necessary.

F. Further suggestions to the experimental procedures

1. The UV spectrum of phenolphthalein is shown below for your reference.

Based on this UV spectrum, the filter “Green (565nm) Absorbance” should be selected in step 19 for data collection.

2. According to Beer Lambert Law, A = ebc, the absorbance of the solution is directly proportional to the concentration of dianion of phenolphthalein, so

A = const [Ph2-],

Rearrange the equation [Ph2-] = const x A

then ln [Ph2-] = ln (const) + ln A

Therefore, the ln [Ph2-] in the plot ln [Ph2-] versus time can be replaced by ln A, and the graph should give a straight line with the slope of –k1.

3. The structures and reaction of phenolphthalein in alkaline solution are shown below

At pH = 0 – 8.2, phenolphthalein exists in the form of H2Ph, it is a colourless compound. Adding H2Ph to an alkaline solution higher than pH 8.2 causes rapid and successive deprotonation of H2Ph into mono and then diphenylate ion. The diphenylate ion then rearranges to give a pink colored quinoid species Ph2-. This is what happened when phenolphthalein is used as the indicator in an acid-base titration. Further addition of base to the solution results in the addition of a hydroxyl group to the central carbon and the structure rearranges to a colourless species POH3-. The conversion of Ph2- to POH3- is sufficiently slow to be measured.

4. The procedures for standardization of sodium hydroxide solution used in this experiment are as follows:

Dissolve about 20 g of potassium hydrogen phthalate in 100 mL of water in a 250-mL conical flask. Heat the solution to help dissolution. After all the solids dissolved, wait until the solution is slightly warm (the solids precipitate out if the solution is cooled to room temperature) and pour the solution into a 250-mL volumetric flask by aid of a glass rod. Rinse the conical flask several times with D. I. water, add this rinse to the volumetric flask as well. Add D. I. water up to the mark of the volumetric flask to make a total of 250 mL solution and shake well. This becomes the primary standard solution of potassium hydrogen phthalate. With a pipette, withdraw 25 mL of this potassium hydrogen phthalate solution into another 250-mL conical flask. Titrate the solution with ~0.3M (dissolving ~3 g of NaOH in 250 mL of water) sodium hydroxide solution, add 4 drops of phenolphthalein as indicator. Repeat the titration for 2 more times.

5. The exact concentration of phenolphthalein is not critical since the OH- concentration is in largely excess in any mixture.

6. Temperature changes can affect the rate of this reaction, it is better to perform all the measurements at constant temperature. One can also remove the cuvette from the compartment of the colorimeter except when actually measuring the absorbance since the light beam may cause increase in temperature.

G. Suggested answer to “Specific Chemical Hazards”

Hazard
Chemical / Flammable
/ Corrosive
/ Toxic
/ Irritant
/ Harmful
/ Oxidizing
/ Explosive
/ Others
Phenolphthalein / Ö / Ö / Ö / Carcinogenic
Reproductive hazard
Sodium chloride / Ö / Ö
Sodium hydroxide / Ö / Ö

H. Reference

1. Lois Nicholson, Journal of Chemical Education, 1989, 66, 725.

2. “General Chemistry: Principles and Modern Applications”, Edited by Petrucci, Ralph H., 9th Edition, New Jersey: Pearson Education Inc. 2007.

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