PKa of Bromothymol Blue Indicator Lab Report

Posted by Winnie Melda on May 6th, 2019

Page one

            Figure 26.3 shows the graph of Bromothymol blue spectra at 295.6 K. The graph has the absorbance values (AU) versus the wavelength values (nm). From the graph, the spectra of bromothymol blue versus PH at room temperature helps to explain the effect of increasing wavelength on the PH and absorbance. At a wavelength of 500nm, the curves appear to cross, and the absorbance value remains constant regardless of the PH. However, for the other sides of the point, the absorbance changes according to change in PH. For a low wavelength value, the PH sample has a low absorbance too, and the vice versa.

            Table 26.2 shows the data obtained for the observations of ph, Absorbance, and Color of bromothymol blue at 296 k and 425 nm. The data shows that there were nine beakers and the solution in each of them had a different PH ranging between 11.02 and 2.66. The colors observed the change from one form to another, and none of the beakers showed a similar color to another. The data obtained was helpful in estimating the pka value of bromothymol blue using the experimental conditions.

            Table 26.3 shows the data obtained from the observations made for the PH and Absorbance of bromothymol blue at 296 K and 600 nm. The PH values and the Absorbance values obtained were used in the determination of the pka value of bromothymol blue indicator at a temperature of 296 K and a wavelength of 600 nm.

Second & Third Pages

1. As shown in the Tables 26.2 and 26.3
2. The inconsistency in pKa values over the range of PH values arises due to the acidity range of the solution in which the indicator is expected to change color. A change of PH means that either the solution becomes a stronger or a weaker acid based on the concentration of the indicator in the solution. For the calculated pKa values, inconsistencies arise due to the lower and imprecise absorbance readings.

3. The mean pKa determined at the two wavelengths should be the same, but since the absorbance levels vary which in turn affects the absorbance at high PH (A_H) and the absorbance at low PH (A_L). The two terms are critical in the determination of pKa value which tends to vary depending on the wavelength. The wavelength of maximum absorbance on either side of the isosbestic point would result in great sensitivity and turn different pKa value.

4. The uncertainty can have at most two decimal places in this experiment since PH can only be determined to two decimal places and PH is necessary for the determination of the pK. The pK determined at low wavelength way have more deviation than that of high wavelength due to the lower reading of the absorbance readings which is usually less precise.

5. The determined pK at 273.15 K would be slightly higher than that at 298.15 K. In general, the pKa value is expected to decrease with an increase in temperature. The pKa value is a –log of ka, and hence does the opposite. An increase in Ka means a decrease in pKa. A change in temperature from 298.15 to 273.15 is a decrease, and hence the pk is expected to increase as the ka decreases.

7. The isosbestic point would not be a good wavelength for the determination of pKa since the value of wavelength remains constant for all the PH values which mean that the changes in PH which in turn affects the pka value would not be considered in the determination of pKa. There lacks a variation in the absorbance with changes in PH at the isosbestic point. However, on either side of the point, the absorbance values change according to the corresponding change in the PH of the indicator solution.

The pKa value can be determined using the formula:

The calculated pka value for the indicator is shown only for the PH at 6.52 since it is the only one corresponding with that of the PH values shown in figure 26.3. The other PH values do not match with the experimental values.

Table 26.2 pk of bromothymol blue at 296 K and 425 nm

Table 26.3 pk of bromothymol blue at 296 K and 600 nm

 Carolyn Morgan is the author of this paper. A senior editor at MeldaResearch.Com in best research paper writing services. If you need a similar paper you can place your order from non plagiarized research papers.