"experiment: spectrophotometric determination of the acid dissociation constant of methyl red"
1. Why are matched cells used in the experiment?
2. Why is pH plotted against log([MR-]/[HMR])?
3. Why is it right that HMR has a pH equal to 2 and MR- to have a pH equal to 8?
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1. Matched cells are used in the spectrophotometric determination of the acid dissociation constant of methyl red to ensure accurate comparisons between the different samples. These cells have similar geometric dimensions and optical properties, minimizing any discrepancies that could introduce errors into the experiment. By using matched cells, any differences in absorbance readings can be attributed solely to the changes in concentration or pH of the solution being measured.
2. The pH is plotted against log([MR-]/[HMR]) because it allows for the determination of the acid dissociation constant (Ka) of methyl red. The ratio [MR-]/[HMR] represents the concentration of the deprotonated form ([MR-]) over the protonated form ([HMR]) of methyl red. Taking the logarithm of this ratio helps to magnify the changes in concentration, making it easier to observe and analyze the relationship between pH and the extent of dissociation.
3. It is expected that HMR (protonated form) has a pH value of around 2, while MR- (deprotonated form) has a pH value of around 8, because these pH values correspond to the pKa value of methyl red. The pKa is the pH at which the concentration of the acidic and basic forms of a compound are equal, meaning they are half dissociated. In the case of methyl red, the pKa is around 4.9, indicating that at a pH below 4.9, the HMR form predominates, and above 4.9, the MR- form predominates. Thus, pH 2 corresponds to a mostly protonated form, while pH 8 corresponds to a mostly deprotonated form of methyl red. This pH range is chosen to ensure that significant dissociation occurs, providing a measurable signal for the spectrophotometric determination of the acid dissociation constant.
1. Matched cells are used in the spectrophotometric determination of the acid dissociation constant of methyl red to ensure accurate and reliable results. Matched cells are essentially cuvettes (small transparent containers) that have exactly the same dimensions and optical properties. By using matched cells, any differences in light path length or transmittance between the reference and sample solutions are minimized, allowing for more precise measurements of absorbance. This helps to eliminate errors caused by variations in the experimental setup and increases the accuracy of the determined acid dissociation constant.
To obtain matched cells, several cuvettes are usually manufactured and tested to find the pair that exhibits the most similar optical characteristics. These matched cells are then used for both the reference solution (without the analyte) and the sample solutions (with the analyte), ensuring that any differences in absorbance are due to the presence or absence of the analyte, rather than variations in the cuvettes themselves.
2. In the spectrophotometric determination of the acid dissociation constant of methyl red, pH is plotted against the logarithm of the ratio of the concentration of MR- (deprotonated form) to HMR (protonated form). This logarithmic plot is used to determine the pKa value, which is a measure of the acidity or basicity of an acid.
By plotting pH against log([MR-]/[HMR]), any changes in the concentrations of MR- and HMR due to changes in pH can be observed. At low pH, more of the analyte will be in the protonated form (HMR), resulting in a higher value of log([MR-]/[HMR]). As the pH increases, the concentration of H+ ions decreases, leading to a higher concentration of MR- and a lower value of log([MR-]/[HMR]). The pKa value can be determined by analyzing the point on the plot where the concentrations of MR- and HMR are equal, which corresponds to a pH value where log([MR-]/[HMR]) equals zero.
3. In the case of the acid dissociation constant of methyl red, it is not necessarily "right" for HMR to have a pH equal to 2 and MR- to have a pH equal to 8. Rather, these pH values are predicted based on the properties of the compound and the acid-base equilibrium involved.
Methyl red is a pH indicator that exhibits a color change at different pH values. At low pH (acidic conditions), the protons (H+) in the solution preferentially bind to methyl red, resulting in protonation and the formation of HMR. This protonated form appears red. On the other hand, at high pH (alkaline conditions), the methyl red molecules lose their protons and predominantly exist in the deprotonated form, referred to as MR-, which appears yellow.
Based on the observed color change, the pH at which the half-neutralization of the indicator occurs can be determined. In the case of methyl red, this happens around pH 5.1-5.4. Furthermore, at pH values much lower than the pKa (around 2), most of the indicator will be in the protonated form (HMR), while at pH values much higher than the pKa (around 8), most of the indicator will be in the deprotonated form (MR-). These pH values serve as reference points and guide the experimental determination of the acid dissociation constant of methyl red.