Chemistry Lab Critique Essay

Category: Chemistry
Last Updated: 27 Aug 2020
Pages: 4 Views: 108

In this lab, the pKa of the unknown indicator of expression was determined both qualitatively and quantitatively. To verify our procedures, the experiment was tested using a known indicator, bromocresol green. Qualitatively, we used the color change of the solution with an indicator to obtain the pKa value solely using a pH meter. Quantitatively, we used a pH meter and the spectrophotometer with varying concentrations of the acid and base. The maximum wavelengths of absorbance used to quantify the dissociated and undissociated forms of the bromocresol green were 440 nm (yellow, undissociated) and 616 nm (blue dissociated). For the unknown indicator, the wavelengths were 505. 96 nm (red, undissociated) and 601. 66 nm (blue, dissociated). For bromocresol green, the experimentally obtained pKa value measured qualitatively was 4. 04. Quantitatively, the pKa came out to be 4. 16. The percentage error (10. 47%) obtained was within a reasonable range, allowing the same procedures to be used to determine the pKa value of the unknown indicator, Indicator of Freedom. The indicator was Freedom was found to have a pKa value of 5. 32 qualitatively essay writer typer. On the other hand, the quantitative pKa was a measure to be 4. 265.


This lab applies the concept of an indicator dye, which is principally a weak acid that changes colors when reacting with the amount of hydronium ion in a solution. Thus, the qualitative part of the lab assumes that at the point where the solution changes color into an intermediate shade of the two, the concentrations of [HIn] and [In-] are approximately equal.

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Using the Henderson-Hasselbalch equation for them: the pH equals pKa when the ratio between the indicator and its conjugate base is 1. Subsequently, the quantitative portion of the lab deals with the relationship between absorbance and concentration: A1(? )A2(? )= c1c2.

Thus, a certain absorbance of a solution can provide the concentration of the indicator and its conjugate base when taking into consideration the maximum wavelength of the two colors.

Experimental Section: To reduce the volumes of acid and base needed and the amount of time to complete the lab, the stock solutions of strong acid and base were diluted to at least one-fourth their starting amount. Thus, 60 mL of deionized water was added to both stock solutions of 20 mL HCl and NaOH to create 2 new 80 mL diluted solutions.

Part 1: Finding pKa Qualitatively 25 mL solution of the diluted NaOH with 3 drops of the indicator was titrated against the diluted HCl dropwise until a color change occurred. To determine the pKa of the indicators, the pH of the solutions was taken with a pH meter when there was a color change. Determining the color of bromocresol green to be yellow when acidic and blue when basic, the point of color change to measure pH was when the solution turned green. Likewise, the indicator of Freedom was red when acidic and blue when basic, which established the point of color change to be purple. These steps were performed three times for both the known and unknown indicators to find an average pKa value.

Part 2: Finding pKa Quantitatively For the quantitative determination of pKa, multiple solutions of varying acid/ base concentration were prepared with 3 drops of indicator. Despite the varying concentrations of each solution, the total volume stayed constant at 20 mL.

The qualitative part of the lab had room for a lot of errors. The unreliability of visual observation of the slight color change in solution could have dramatically affected the results. Because the experimentally determined pKa was less than the actual pKa for both cases, strong enough color change was probably not observed. Since the titration was performed roughly using drops, the drops could have added more acid than needed, resulting in a smaller pKa. Quantitatively, the pKa can be determined by plotting the data, with absorbance as a function of pH. There will be two separate lines, one for each wavelength of the two colors. The intersection of these two lines will indicate the point at which the pH should be equal to the pKa.

This works because at the intersection point: pH=pKa+logIn-HIn.

It is known that A1A2= c1c2.

Since A1= A2 at the intersection point of the graph, then 1= c1c2= In-HIn.

Thus, pH=pKa+log1 pH=pKa at the intersection of both curves. The procedure for determination of bromocresol green pKa appeared to have worked because the percent error was roughly around 10%. Although slightly high, these errors are unpreventable. For example, cuvettes with smudges on the sides would have increased the absorbance readings of the solutions. Moreover, the maximum wavelength is crucial because it is used as a standard to know where to record the absorbance levels of the other solutions. This is because it is where the maximum absorbance occurs for the particular color produced by the solution is. It is important to measure the absorbance levels at these standardized wavelengths to keep the data consistent and to have the ability to compare the absorbance levels of two solutions without the need to calibrate or adjust the readings. Finally, a limited number of data points when determining the pH graphically could have also added to the error in this experiment.

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Chemistry Lab Critique Essay. (2017, Feb 10). Retrieved from

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