The p Ka of an Unknown Acid-Base Indicator Lab Report PDF

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Abstract In this experiment the purpose was to determine the pKa of an unknown acid-base indicator. In this experiment, the indicator dye was labelled as “Dye of Speed”. The pKa of the indicator was determined through using both qualitative and quantitative measures. These measures included the determination of the pH range qualitatively and quantitatively through the determination of the absorbance values and spectra of the undissociated and dissociated forms of the dye. To prove that the experimental procedure worked, first bromocresol green analyzed. The experimentally observed pKa of the “Dye of Speed” was determined to be approximately 10.18. The undissociated form of the dye was measured at a wavelength of 524 nm while the dissociated form of the dye was measured at a wavelength of 636 nm.

I.

Introduction The use of an indicator is one of the most common ways to determine the pH of a

solution. In titrations indicators help in the determination of pH through their color change process. When the indicator reaches the point of color change, its pK a is approximately equal to the solution’s pH. The intermittent color of the solution is used as the stopping indicator point as an indicator varies between two colors. The solution becomes a neutral color when the dissociated and undissociated forms of the indicator are equal. It is useful to know the pK a of an indicator as it helps to choose the indicator that is applicable to the equivalence point of the titration. The purpose of this experiment was to qualitatively and then quantitatively measure the pKa of an unknown amount of an unknown acid-base indicator, labeled as “Dye of Speed”. An important quality of an indicator is the ability to dissociate and its color change through dissociation from the acidic-basic composition of the solution. The dissociation is shown in equation 3 from the lab guide:

So then, the pKa of the indicator can be calculated by the derivation of the dissociation of the acid in this equation:

Through taking the logarithm, the equation can be written as:

This equation is called the Henderson-Hasselbalch equation and will be used to determine the pKa of the indicators in this experiment. To calculate the concentrations of undissociated and dissociated forms of the indicators in this experiment, values were found using a spectrophotometer and pH meter. The pH meter was used to qualitatively measure the pH the known (bromocresol green) and the unknown indicator (“Dye of Speed”). The spectrophotometer was used to determine the quantitative measure of the peak wavelengths and absorbances at those wavelengths of the dissociated and undissociated forms of the indicators. To find the wavelengths and absorbances of the dissociated and undissociated forms of indicator, solutions with 100% dissociated and 100% undissociated forms of the indicator were created.

II.

Experimental The program “Calvin” was used to conduct this experiment and code was submitted to

run the steps of the experiment. In the first part of this experiment the known indicator, bromocresol green, was used in order to determine the effectiveness of the procedure before continuing onto the unknown indicator. In the step 1 of the experiment, the pH was determined through qualitative measures. 100 ml of 0.1083 M standardized NaOH was added to a clean 100 ml beaker was named “stock_base” and 100 ml of 0.1090 M standardized HCl was added to a clean 100 ml beaker named “stock_acid”. Then a clean 100 ml beaker was named “titration_beaker” and 20 ml of solution from the stock_acid beaker was pipetted into the titration beaker. One drop of bromocresol green was added to the titration beaker. The NaOH from the stock_base beaker was added to a 50 mL buret until the buret read 0.45 mL (volume of 49.55 mL) and titration was performed until the buret read 20.60 mL (volume of 29.4 mL). The blue-green color of the titrated solution was recorded and using a pH meter the pH was measured. In step 2 of the experiment, 100 ml of 0.1083 M standardized NaOH was added to a clean 100 ml beaker was named “stock_base”. Then a 25 mL graduated cylinder was used to transfer a total of 50 mL of solution from the stock_base beaker to a beaker named optimum_concentration and one drop of bromocresol green was also added. The color of solution and pH were measured and recorded. Then using a spectrometer was used to find the spectrum of the solution in optimum_concentration beaker for the dissociated (In-) form of the indicator. This step 2 process was repeated with 0.0871 M standardized HCl to measure the pH and spectrum of the undissociated (HIn) form of the indicator.

In step 3 of the experiment, a titration was performed until a color change was reached. Two clean 100 mL beakers were filled with 100 mL of the stock solutions of 0.1123 M standardized NaOH and 0.1092 M standardized HCl. A 25 mL graduated cylinder was used to transfer 25 mL of solution from the stock_acid beaker to a 100 mL beaker named Beaker1, and one drop of bromocresol green was also added. Then a 25 mL graduated cylinder was used to transfer a total of 50 mL of solution from the stock_base beaker to Beaker2, and one drop of bromocresol green was also added. Then 49.35 mL of the solution in Beaker2 was added to a 50 mL buret and the titration was conducted. The color change from yellow to green in Beaker1 was recorded, the pH measured, and a spectrum using a spectrometer was obtained. The experimental pKa was calculated and the theoretical pKa of bromocresol green was used to determine the percent error. This 3 step process was repeated using the unknown indicator referred to as “Dye of Speed”. However, the stock concentrations used differed. In step 1 the stock concentrations were 0.0895 M standardized NaOH and 0.1125 M standardized HCl. In step 2, 0.1053 M standardized NaOH and 0.0999 M standardized HCl was used. In step 3, 0.1003 M standardized NaOH and 0.0941 M standardized HCl was used. After conducting the experiments, the pKa of the unknown indicator was calculated.

III.

Results & Discussion Before conducting the experiment to determine the pKa of the unknown acid-base

indicator, first the pKa of bromocresol green was determined. The pKa of a known indicator was determined to verify that the procedure used to calculate pKa was valid and could be used to calculate the pKa of the unknown indicator,“Dye of Speed”. In experiment 1 of bromocresol green, using eyeballing, the stock acid and base solutions, and the pH meter the pH = pKa was estimated to be 4.99 for a solution of a blue-green color. In experiment 2, the best/peak wavelengths were found to be 442 nm and 616 nm. The optimal wavelengths of the dissociated and undissociated forms of bromocresol green was determined using highly acidic and basic solutions, and the addition of one drop of bromocresol green. This step allows for the determination of the peak wavelength for calculating pH of solutions with bromocresol green. Table 1. Determination of Peak Wavelengths of Both Dissociated and Undissociated Forms of Bromocresol Green

pH of Diluted Solution:

Peak Wavelength (nm)

Absorbance

Basic- Dissociated (In-) or Acidic- Undissociated (HIn) Form

1.06

442

0.190151

Acidic HIn

13.01

616

0.406651

Basic In-

In experiment 3 of bromocresol green, the titration of acid (HCl) and base (NaOH) was performed. The pH meter determined that the pH of the green titrated solution was pH = 4.55. Table 2. Absorbance of HIn (Undissociated) and In- (dissociated) in Bromocresol Green during Titration

pH of Solution

Absorbance of HIn (λ= 442 nm)

Absorbance of In(λ = 616 nm)

4.55

0.056502

0.590161

The pK a of bromocresol green can be determined by using two main equations: concentration-absorbance equation and the Henderson-Hasselbalch equation.  The proportion of HIn and In- ions was determined using equation 4 from the lab guide, shown

below:

This equation was used to determine the relative concentrations of each dissociated (In-) and undissociated (HIn) form:

 This is the relative concentration of In- in the solution. The same calculation was performed to

determine the relative concentration of HIn in the solution:

The relative concentrations of both HIn and In- can be assumed to be the same so the proportion of the ions can be determined and used in the Henderson-Hasselbalch equation to determine pKa:

The theoretical pKa of bromocresol green is known to be 4.70. The percent error was calculated to be approximately 11.46%, which was determined to be small enough to continue onto the unknown indicator part of the experiment.

The same procedure was then used for the “Dye of Speed”. In experiment 1 of “Dye of Speed”, using eyeballing, the stock acid and base solutions, and the pH meter the pH = pKa was estimated to be 9.58. In experiment 2, the best/peak wavelengths were found to be 442 nm and 616 nm. The optimal wavelengths of the dissociated and undissociated forms of “Dye of Speed” was determined using highly acidic and basic solutions, and the addition of one drop of “Dye of Speed”. This step allows for the determination of the peak wavelength for calculating pH of solutions with “Dye of Speed”.

Table 3. Determination of Peak Wavelengths of Both Dissociated and Undissociated Forms of “Dye of Speed”

pH of Diluted Solution:

Peak Wavelength (nm)

Absorbance

Dissociated (In-) or Undissociated (HIn) Form

1.00

524

0.136596

HIn

13.02

636

0.437173

In-

In experiment 3 of “Dye of Speed”, the titration of acid (HCl) and base (NaOH) was performed. The pH meter determined that the pH of the blue titrated solution was pH = 9.85.

Table 4. Absorbance of HIn (Undissociated) and In- (dissociated) in “Dye of Speed” during Titration

pH of Solution

Absorbance of HIn (λ= 524 nm)

Absorbance of In(λ = 636 nm)

9.85

0.091548

0.631574

To get the pKa of the “Dye of Speed”, the same equations and methods as bromocresol green can be used. The Henderson-Hasselbalch equation and equation 4 from the lab guide can be used again:

This equation was used to determine the relative concentrations of each dissociated (In-) and undissociated (HIn) form:

 This is the relative concentration of In- in the solution. The same calculation was performed to

determine the relative concentration of HIn in the solution:

The relative concentrations of both HIn and In- can be assumed to be the same so the proportion of the ions can be determined and used in the Henderson-Hasselbalch equation to determine pKa:

The experimentally determined pKa of the unknown indicator, “Dye of Speed”, was calculated to be approximately 10.18.

The analysis wavelengths were chosen because they were the peak wavelengths of the spectra of the dissociated and undissociated forms of each indicator. The peak wavelengths were used because they have the lowest signal-to-noise ratio. There are several possible sources of error in this experiment. One may be that in step 3 indicator was added to the base in the buret when it should’ve only been added to the beaker, which may have led to too much indicator in solution and may have caused a more rapid color change affecting the pH measurements. Another source of error is the subjectivity of qualitatively determining the pH range of the indicator through the color shift. It is hard to determine the exact middle point of the indicator’s color change. If the pH determined was off then it makes it difficult to calculate the the pK a quantitatively using relative concentrations, because in order to use the relative  concentrations the assumption is made that pH= pK a . The calculation of pKa could be corrected for by using a graphical method to calculate pH. Another possible source of error may be the pH meter. It depended on how the measurement through Calvin was obtained, but in normal experimental conditions the volatility of the pH meter would affect results. If the meter was not properly calibrated, the pH measured would not be accurate affecting the pKa calculations. An accurate pH value is important to obtain because it has a major influence on the determination of pK a of the indicators. With the  Henderson-Hasselbalch equation used, a shift in 0.1 pH units has quite an influence on the pK a

value and could change the experimental error by quite a lot. Ensuring the pH meter is properly calibrated is essential....