Acid-Base Indicators: Spectrophotometric Methods Lab Report

Emily Muller

Lab Partner: Tamara Afflerbach

7/22/12 Introduction:

Acids and bases can be transformed when their protons become dissociated in water. Many times this change would go unnoticed if it weren’t for indicators. Indicators are used to show a visible color change as the pH changes. The pH represents how acidic or how basic a solution is. The indicator demonstrates the change in pH and therefore the change from more acidic to more basic or vice versa.

The purpose of this experiment is to determine the equilibrium constant (K) for the dissociation of the weak organic acid bromcresol green. In order to complete this experiment,

+ [H3O ] will be adjusted by changing the concentration of [HOAc] and [OAc]. Then the value for the ration [B-]/[HB] can be found using the spectrophotometer. These two values can then be

+ - - combined to calculate Ka. This formula is Ka = [H3 O ][B ] where [B ] = (Aobserved − AHB )

[HB] [HB] (AB- - Aobserved)

+ −5 and where [H3O ] = Ka [HOAc] when Ka = 1.75 x 10 .

[ OAc−]

To begin, the following chemicals will be needed: 1.000 M acetic acid (HOAc), 3.0 x 10-4 M bromcresol green solution (BCG), 0.200 M sodium acetate solution (NaOAc), and 3 M Hydrochloric acid (HCl) containing 1.5 x 10−5 M BCG. Add 25.00 mL HOAc solution to 100-ml volumetric flask. Add 5.00 ml of BCG solution to this same flask. Dilute to the mark with distilled water. This is Solution A. Mix 5.00 ml BCG solution and 5.00 ml NaOAc. Dilute to the mark, this is Solution 1. Record the absorbance of Solution 1 versus a blank of distilled water.

Determine the wavelength of maximum absorbance and call it λB-. Pour sample back into the flask for 100 ml of Solution 1. Pour Solution A into a clean, dry buret. Add 2.00 ml of Solution

A. This is Solution 2. Measure its absorbance at λB−. Put sample back to have 102.00 ml of Solution 2. To Solution 2 add 2.00 ml of Solution A. This is Solution 3. Mix well and measure the absorbance at all fourteen wavelengths used for Solution 1. Put sample back to have 104.00 ml of Solution 3. To Solution 3, add 2.00 ml of Solution A. This is Solution 4. Mix it well and measure its absorbance at λB-. Repeat until 10 mL of Solution A has been added, taking absorbance measurements after every 2 mL added. Then to 110.00 ml of Solution 6, add 1.00 ml of 3 M HCl containing 1.5 x 10-5 M BCG. This is Solution 7. Measure the absorbance at all of the wavelengths used for Solutions 1 and 3. Data:

Solution 1 Solution 2 Solution 3 Wavelengt Wavelengt Absorbanc h Absorbance h e Wavelength Absorbance 400 0.118 400 0.134 400 0.155 425 0.07 425 0.15 425 0.214 450 0.023 450 0.139 450 0.252 475 0.03 475 0.118 475 0.187 500 0.062 500 0.09 500 0.109 525 0.121 525 0.081 525 0.044 550 0.208 550 0.102 550 0.013 575 0.325 575 0.158 575 0.001 600 0.487 600 0.223 600 -0.003 605 0.517 605 0.231 605 -0.002 610 0.541 610 0.242 610 -0.002 615 0.555 615 0.248 615 -0.003 620 0.549 620 0.234 620 -0.002 625 0.528 625 0.238 625 -0.003

Average ka value: 2.65e-5

Percent Error: 26.8%

Standard deviation: 1.08e-5

Results and Discussion: The purpose of this lab was to calculate a value for Ka. By following the procedure and calculations such value was calculated. The average calculated value was 2.65e-5. The accepted value found via the internet was 2.09e-5. This makes there a 26.8 % error in association with the values for K. The standard deviation for the given set of data was 1.80e-5. This is a very small number representing the numbers were very close. The relatively large percent error and the small standard deviation demonstrates the procedure was held constant throughout the experiment however there was a reason that caused the average calculated values of K to be off the accepted value.

One possible source of error in lies in the fact that each solution is contingent upon the previous reacting correctly. The reaction of protons can be different every time. This reaction causes a different output absorbance. If this difference is multiplied by the 7 solutions created, this can lead to a big difference in final values. Also, another possible source of error is the exact concentration of the chemicals used from the beginning being different than they are labeled. For example if [NaOAc] = 0.19 M instead of 0.20 M and [HoAc] = 1.01 M instead of 1.00 M then the Ka value for Solution 2 would change from 1.73e-5 to 1.81e-5. This would affect the average and the standard deviation value. However, assuming the numbers stay similar the new calculated Ka value would still fall within the normal distribution of the standard deviation which is from 6.25e-5 to 9.50e-6. Therefore, even slight differences in concentrations would not greatly affect the final conclusion and representation of data. This leads to the conclusion the 26.8% error may have come from the reaction of protons occurring in the chemicals. The reaction of protons for this specific reaction may be very specific. This means many different factors could affect the rate at which the protons move such as pressure, temperature, and orientation.

+ Using the values for [H3O ] the pH values were calculated. The range found was 3.76- 4.46. Based upon the second listed internet source, bromcresol green should distinguish between the pH of 3.8-5.4. The low end of the pH indication is very close. This is because the

+ concentration of [H3O ] in Solution 2 must be closer to the concentration in Solution 1 than the concentration in Solution 6 is to Solution 7. The range 3.76-4.46 disregards the pH of Solution 1 and 7. This would represent the outliers of the pH range. The pH were B- and HB would be equal is in between the values. This is where the solution is neither acidic nor basic. This value would have to be included in the range because it is where the solution changes over. It cannot change from acidic to basic or vice versa without passing this value. http://www.chemicalforums.com/index.php?topic=21230.0 http://antoine.frostburg.edu/chem/senese/101/acidbase/indicators.shtml

Post-Lab:

1. What two properties (one chemical and one physical) combine to make bromcresol green useful as an acidity indicator?

Bromcresol green is useful as an acidity indicator because it changes to yellow as the pH becomes more acidic. Also, the bromcresol green protons are easily dissociated making the color change visible and relatively quick.

2. What criteria are used to select a wavelength for measuring absorbances in these solutions? Why?

The wavelength with the greatest absorbance is used because at the highest absorbance the margin of error is the least. It is a constant which can be used throughout the whole experiment for comparison usage and accurate graphs.