Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium hydroxide is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills immediately. Phenolphthalein is moderately toxic by ingestion. Crystal violet solution is flammable. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The dye solutions may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The actual amount of lab time needed for this experiment is about 30–40 minutes. An additional 20–30 minutes of computer time will be required if the calculations and graphing are done using the data collection software that accompanies the technology interface system. Alternatively, the calculations and graphing may be assigned as homework. The difference between the ln(Abs) and 1/Abs graphs is very distinct even in less precise hand-drawn graphs. The ln(Abs) versus time graph gives an excellent straight line fit, while the 1/Abs versus time graph is curved.
• Although this experiment serves a useful role in helping to meet technology goals for the curriculum, it is very challenging conceptually. Graphical analysis of kinetic data to determine whether a reaction is first or second order is usually reserved for honors-level or even advanced placement chemistry courses. The Background section contains a summary of the use of graphical analysis in kinetic studies. A more detailed explanation is provided in the Supplementary Information in the Further Extensions section.
• A review of the principles of light absorption and transmission will be necessary if this is the first colorimetry experiment in your lab program. The phenolphthalein solution, for example, absorbs light in the 350–600 nm region. The wavelength of light used in this experiment is 565 nm, corresponding to green light. The beam of light is passed through the sample, and the intensity of the light that is transmitted is measured electronically. The greater the concentration of phenolphthalein in solution, the more green light the solution will absorb. See the “Color and Light Spectrum Demonstrations Kit” (Flinn Catalog No. AP6172) for a large-scale demonstration of the relationship between the color of absorbed and transmitted light.
• Absorbance data may also be collected continuously via a timed run, as opposed to manually at selected time intervals. The drawback is that the temperature of the solution may increase more if the light source is on continuously.
• The experiment may also be performed using a conventional spectrophotometer rather than colorimetry to measure absorbance as a function of time. Below is the list of indicators and their corresponding maximum absorbance wavelengths to set for a conventional spectrophotometer.

  Phenolphthalein 565 nm
  Crystal violet 590 nm
  Bromphenol blue 590 nm

Teacher Tips

• This experiment can be extended to determine the reaction order with respect to hydroxide ions. Have different student groups collect absorbance data at different hydroxide ion concentrations. For each hydroxide ion concentration, a graph of ln(Abs) versus time should be linear with a slope equal to –k' (see the Background section), where k′ incorporates the hydroxide concentration (k' = k[OH^–]^m). The classroom data collected by different student groups can be compared to determine the reaction order in hydroxide ion. A graph of k′ versus [OH^–] is linear and demonstrates that the reaction is first order with respect to hydroxide ions (m = 1). See the Supplementary Information in the Further Extensions section for results and graphical analysis of the effect of hydroxide ion concentration on the rate of fading of phenolphthalein.
• All the indicators in this lab have similar structures and undergo similar reactions. For phenolphthalein, the reaction is:
  {13545_Tips_Figure_6}
  For crystal violet, the reaction is:
  {13545_Tips_Figure_7}
  For bromphenol blue, the reaction is:
  {13545_Tips_Figure_8}

Further Extensions

Supplementary Information

Graphing Exercises
Graphical analysis of first and second order reactions is based on combining the equations for the disappearance of reactant (Equation 6) and the rate law (Equation 7) for a pseudo-first order reaction.

The resulting equation is Using calculus, it can be shown that if m = 1, the so-called integrated rate equation is ln[A] = –k't + ln[A]_o. A graph of ln[A] versus time will be linear with slope –k'.

If m = 2, the integrated rate equation has the form 1/[A] = k't + 1/[A]_o. A graph of 1/[A] versus time will be linear with slope k'.

Order of Reaction in Hydroxide Ion
For the phenolphthalein reaction, the following ln(Abs) versus time graphs were obtained for different hydroxide ion concentrations. The value of k′ was equal to 0.13 for 0.2 M NaOH, and 0.061 for 0.1 M NaOH. Substituting these values into the equation for the pseudo-rate constant (k′ = k[OH–]^m) gives the following results. The fading reaction of phenolphthalein is first order in sodium hydroxide.

Answers to Prelab Questions

Methylene blue (MB) is another indicator dye that undergoes a color fading reaction. In this reaction, the color fading results from the reduction of methylene blue by ascorbic acid (Asc) (see Equation 5). Methylene blue is added to a solution of 0.1 M ascorbic acid and the solution immediately turned blue. After 80 seconds, the color faded and the solution was almost colorless. The following absorbance measurements were recorded in Table 1.

Table 1

1. Calculate the values of ln(Abs) and 1/Abs for each absorbance measurement and enter the values in the table.

   See the table.

2. Use the following graphs to plot ln(Abs) versus time (Graph 1) and 1/Abs versus time (Graph 2).
   {13545_Answers_Figure_7}
3. Which graph more closely approximates a straight line? Is the reaction of methylene blue with ascorbic acid (Equation 5) first or second order in methylene blue?

   The graph of ln(Abs) versus time gives a straight line. The graph of 1/Abs versus time is curved. The reaction is first order in methylene blue.

Sample Data

Phenolphthalein

*Computer-generated tables and graphs may be substituted for the data table and Post-Lab Questions 1–3.

Crystal Violet

*Computer-generated tables and graphs may be substituted for the data table and Post-Lab Questions 1–3.

Bromphenol Blue

*Computer-generated tables and graphs may be substituted for the data table and Post-Lab Questions 1–3.

Answers to Questions

Phenolphthalein

1. Plot or obtain a graph of absorbance versus time. Does the “rate of fading” of phenolphthalein depend on the concentration of the dye? Explain.
   {13545_Answers_Figure_9}
   Yes, the “rate of fading” depends on the concentration of the dye. The graph of absorbance versus time is curved, suggesting that the rate decreases as the concentration of phenolphthalein decreases over the course of the reaction.

Crystal Violet

1. Plot or obtain a graph of absorbance versus time. Does the “rate of fading” of crystal violet depend on the concentration of the dye? Explain.
   {13545_Answers_Figure_10}
   Yes, the “rate of fading” depends on the concentration of the dye. The graph of absorbance versus time is curved, suggesting that the rate decreases as the concentration of crystal violet decreases over the course of the reaction.

Bromphenol Blue

1. Plot or obtain a graph of absorbance versus time. Does the “rate of fading” of bromphenol blue depend on the concentration of the dye? Explain.
   {13545_Answers_Figure_11}
   Yes, the “rate of fading” depends on the concentration of the dye. The graph of absorbance versus time is curved, suggesting that the rate decreases as the concentration of bromphenol blue decreases over the course of the reaction.
2. Calculate the values of ln(Abs) and 1/Abs for each absorbance measurement and enter the results in the table. Note: This may be done directly with the data saved in the technology program or separately using a calculator or spreadsheet program.

   See the data table.

3. Plot or obtain graphs of both ln(Abs) versus time and of 1/Abs versus time. (See the Background section and the Prelab Questions.)
   {13545_Answers_Figure_12}
4. Which graph more closely approximates a straight line?

   The points on the ln(Abs) versus time graph all fall on a straight line. The 1/Abs versus time graph is curved.

5. Is the reaction of the indicator with hydroxide ions (Equation 2) first or second order in the indicator?

   The reaction is first order in the indicator.

6. Did the temperature of the solution change over the course of the reaction? What effect, if any, would the temperature change have on the results of the experiment?

   The temperature of the solution increased from 21.5 to 22.8 °C over the course of the reaction. In general, the rate of a reaction increases as the temperature increases. However, the temperature increase in this case is fairly small and does not seem to affect the results. Note: The temperature effect is probably due to “heating” of the sample by the light source. To reduce the temperature effect, remove the cuvette from the colorimeter compartment between measurements. The temperature change does not seem to be large enough to warrant the increased probability that the solution might spill as it is repeatedly inserted and removed.

7. The concentration of sodium hydroxide is assumed to be constant throughout the reaction and is thus included in the “reduced” rate law expression (see Equation 4 in the Background section). Is this assumption valid? Prove it.

   The initial concentration of phenolphthalein, for example, in the cuvette is roughly 1 x 10^–4 M—one drop of 0.015 M phenolphthalein is diluted with about 3 mL (60 drops) of sodium hydroxide. The initial concentration of sodium hydroxide is 0.2 M, more than 1000 x greater than the phenolphthalein concentration. The amount of sodium hydroxide consumed during the reaction is insignificant—the assumption is valid.

References

This laboratory has been adapted from Flinn ChemTopic^™ Labs, Volume 14, Kinetics, Cesa, I., Ed., Flinn Scientific, Batavia, IL, 2003.

Introduction

The dyes phenolphthalein, crystal violet, and malachite green, along with bromphenol blue, are used as acid–base indicators. The indicators turn various bright colors as their solutions become basic. In strongly basic solutions these colors slowly fade and the solutions becomes colorless. The kinetics of these “fading” reactions can be analyzed by measuring the absorbance or color intensity of the dye solutions versus time and graphing the results.

Concepts

• Kinetics
• Reaction rate
• Order of reaction
• Colorimetry

Background

The indicators used in this experiment are all large organic molecules with similar structures, and they undergo the same general type of reaction in the presence of hydroxide ions. As an example, phenolphthalein has the colorless structure shown in Figure 1 when the solution pH < 8.

As the solution becomes basic and the pH increases (pH 8–10), the phenolphthalein molecule (abbreviated H₂P) loses two hydrogen ions to form the red-violet dianion (abbreviated P^2–) shown in Figure 2. The colorless-to-red transition of H₂P to P^2– (Equation 1) is very rapid and the red color develops instantly when the pH reaches its transition range (pH 8–10). If the concentration of hydroxide ions remains high, the red P^2– dianion will slowly combine with hydroxide ions to form a third species, POH₃^– (Equation 2), which is again colorless. The rate of this second reaction is much slower than the first and depends on the concentration of phenolphthalein and hydroxide ions. Thus, the color of the red P^2– species will gradually fade in a basic solution (pH >3). The kinetics of the “fading” reaction can be followed by measuring the concentration of P^2– versus time and graphing the results. Figure 3 illustrates how the concentration of a reactant decreases with time over the course of a reaction. Notice that the graph of concentration versus time is a curved line, not a straight line. The curve levels off as it approaches the x-axis. This means that the reaction slows down as the reactant concentration decreases.

Exactly how much the rate decreases as the reactant concentration decreases depends on the rate law for the reaction. In the case of the reaction of P^2– with OH^– ions, the rate law has the general form

The exponents n and m are defined as the order of reaction for each reactant and k is the rate constant for the reaction at a particular temperature. The values of the exponents n and m must be determined by experiment. If the reaction is carried out under conditions where the concentration of OH^– does not change—by using a large excess of hydroxide ions—then the rate law will reduce to the form where k' is a new “pseudo” rate constant incorporating both the “true” rate constant k and the experimentally constant [OH^–]^m term.

Mathematical treatment of the equations for the reaction rate and the rate law predicts the following outcomes:

• If the fading reaction is first order in [P^2–] (that is, n = 1), a graph of the natural log (ln) of [P^2–] versus time will give a straight line. The slope of the line is equal to –k′.
• If the fading reaction is second order in [P^2–] (that is, n = 2), a graph of 1/[P^2–] versus time will give a straight line. The slope of the line is equal to –k′.

Experiment Overview

The purpose of this technology-based experiment is to use colorimetry and graphical analysis to determine how the rate of a specific indicator fading reaction depends on the concentration of the dye. A colorimeter is a special instrument that measures the absorbance of light at a specific wavelength. A known amount of indicator will be added to a large excess of sodium hydroxide, and the absorbance (Abs) of the colored solution will be measured at regular time intervals. Graphing the absorbance data (ln Abs versus time and 1/Abs versus time) should reveal whether the fading reaction is first or second order with respect to the indicator dye. Three different indicators may be investigated: phenolphthalein, crystal violet, and bromthymol blue. Different groups will do the experiment using a different dye and the class results will be compared to see if there is any pattern in the kinetics of the three dye reactions.

Absorbance is directly proportional to concentration, and so a graph of absorbance versus time will have the same characteristics as a graph of concentration versus time (see Figure 3 in the Background section). The following table lists each dye, their colors in basic solutions, the concentration of NaOH needed for the fading reaction and the specific wavelength to use.

Materials

Prelab Questions

Methylene blue (MB) is another indicator dye that undergoes a color fading reaction. In this reaction, the color fading results from the reduction of methylene blue by ascorbic acid (Asc) (see Equation 5). Methylene blue was added to a solution of 0.1 M ascorbic acid and the solution immediately turned blue. After 80 seconds, the color faded and the solution was almost colorless. The following absorbance measurements were recorded in Table 1.

Table 1

1. Calculate the values of ln(Abs) and 1/Abs for each absorbance measurement and enter the values in the table.
2. Use the following graphs to plot ln(Abs) versus time (Graph 1) and 1/Abs versus time (Graph 2).
   {13545_PreLab_Figure_4}
3. Which graph more closely approximates a straight line? Is the reaction of methylene blue with ascorbic acid (Equation 5) first or second order in methylene blue?

Safety Precautions

Sodium hydroxide is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills immediately. Phenolphthalein is moderately toxic by ingestion. Crystal violet solution is flammable. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Read the entire Procedure before beginning the experiment. Record your assigned indicator in the data table, along with the correct concentration of sodium hydroxide to use for that indicator. (See table in Experiment Overview section.)

1. Handle the cuvet by its ribbed sides or its top to avoid getting fingerprints on the surface.
2. Connect the interface system to the computer or calculator and plug the colorimeter sensor into the interface.
3. Select Setup and Sensors from the main screen and choose “Colorimeter.”

   Note: Many newer sensors have an automatic calibration feature that automatically calibrates the colorimeter before use. If the sensor has the autocalibration feature, set the wavelength on the colorimeter to the corresponding proper wavelength for your assigned indicator. Press the autocalibration key, and proceed to step 9. If the sensor does not have the autocalibration feature, follow steps 4–8 to calibrate the colorimeter for 100% transmission (0 absorbance) with a “blank” cuvet containing only the sodium hydroxide solution.

4. Select Calibrate and Perform Now from the Experiment menu on the main screen.
5. Obtain a clean and dry cuvet and fill it about ⅔ full with the proper sodium hydroxide solution. Wipe the cuvette with a lint-free tissue, then place the cuvet in the colorimeter compartment.
6. Set the wavelength knob on the colorimeter to 0%T—the onscreen box should read zero. Press Keep when the voltage is steady.
7. Turn the wavelength knob on the colorimeter to the indicator wavelength—the onscreen box should read 100. Press Keep when the voltage is steady.
8. Return to the main screen and set up a live readout and data table that will record absorbance as a function of time.
9. Select Setup followed by Data Collection. Click on Selected Events to set the computer for manual sampling.
10. Remove the “blank” cuvet from the colorimeter compartment. Measure and record the initial temperature of the sodium hydroxide solution.
11. Add one drop of the indicator solution to the cuvet and immediately press Collect on the main screen to begin measuring time. This ensures that the absorbance versus time measurements will accurately reflect the time of reaction from the time of mixing.
12. Place the lid on the cuvet and carefully invert the cuvet several times to mix the solution.
13. Place the cuvet in the colorimeter compartment. When one or two minutes have elapsed from the time of mixing, press Keep on the main screen to automatically record the absorbance.
14. Continue making absorbance measurements at regular time intervals (at least every two minutes). Press Keep on the main screen to automatically record the absorbance at each desired time.
15. When 16 minutes have elapsed from the time of mixing, press Stop on the main screen to end the data collection process.
16. If possible, save the data on the computer or calculator and obtain a printout of the absorbance versus time data table and graph. Otherwise, record the absorbance and time measurements in the data table.
17. Remove the cuvet from the colorimeter compartment. Measure and record the final temperature of the dye solution.
18. Rinse the cuvet several times with distilled water and allow it to air dry.
19. (Optional) Calculations and graphical analysis may be done on the computer or calculator using either the data collection software that accompanies the technology interface system or a conventional spreadsheet program, such as Microsoft Excel^™. See the Post-Lab Questions section.

Student Worksheet PDF

13545_Student1.pdf