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Lab Report Pka of Methyl Red

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Determination of the pKa of Methyl Red
by UV-Vis Spectroscopy

February 16, 2010

The purpose of this experiment was to obtain the pKa for an acid-base indicator. A spectrometer was used to determine the pKa of methyl red. The calibration curves were created by analyzing samples of known concentration at pH’s well above and below the pH indicator range for methyl red. Then samples of known pH were analyzed. By, solving the system of equations using linear algebra, the concentrations of the indicator and the conjugate base were determined. The Henderson-Hasselbach equation predicted that the pH for these solution plotted against the log of the ratio of the two species would yield a line whose Y-intercept equaled the pKa for methyl red. The value obtained was 4.96(1), which has a percent error of 1.9. Perhaps, if this were corrected for temperature there would be even greater agreement with the accepted value.

The purpose of this experiment was to determine the pKa of an acid-base indicator (methyl red). All indicators are weak acids, and as such have a dissociation constant, or Ka. The subscript a signifies that the constant is for an acid. The lower case p indicates that the value is the negative log (base 10) of the constant. The Ka expression,
Ka=H+In-HIn [1] (1)
is the equilibrium for the dissociation of the weak acid, where HIn is the acid and In- is the conjugate base. This is represented by the chemical equation:
HIn ⇌H++ In-  
[1]Figure 1: Chemical equation for the dissociation of an acid-base indicator.

Methyl red is a monoprotic organic acid. Figure 2 is a structural model of methyl red.

Figure 2: Chemical structure of methyl red
By plotting the data as the pH versus the log term of the Henderson-Hasselbach Equation,
pH=pKa+logIn-HIn [2] (2)
the pKa was determined. The plot was linear with the Y-intercept equaling the value for the pKa. The pH was determined...

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