Ionization Constant of Weak Acid

PURPOSE

The purpose of this experiment is to determine the ionization constant of an organic acid, or indicator. The techniques to be used include using a pH meter and spectrophotometry.

DISCUSSION

An acid-base indicator is a weak acid and ionizes according to the equation:

The ionization constant may be expresses as:

If we let x represent the fraction of indicator in the ionized form, then:

Thus we may use this expression for K.

or

pH can be determined using a pH meter. The ratio can be determined using a spectrophotometer and Beer's Law, which states that absorbance is proportional to concentration.

In an acid solution most of the indicator will be in the form HIn. In a basic solution nearly all of the indicator will be in the form In^-. At intermediate pH there will be some HIn and some In^-.

If we choose a wavelength where either HIn or In^- absorbs strongly the total absorbance will be made up of absorbances contributed from each form.

If we let A_a represent the absorbance in the most acid solution and Ab the absorbance in the most basic solution, then at intermediate pH the absorbance is:

Solving for x we find:

If we substitute this into the equation for pK, we obtain:

If we plot vs. pH we should obtain a straight line.

At the intercept where is zero, the pH is pK.

Also, where pH = 0,

EQUIPMENT AND CHEMICALS

Spectrohotometer (visible, such as Spectronic 20, Turner 350, or Coleman 124)

pH meter

1 M HCl, small amount

1 M Sodium Acetate, 50 ml

Indicator stock solution to be made up as follows:

(a) approx. one-half gram indicator

(b) 15 ml 0.1 M NaOH

(c) Sufficient distilled water to make one liter.

Indicator may be methyl orange, methyl red, bromphenal blue, or bromcresol green.

PROCEDURE

Add 2 ml indicator to 50 ml sodium acetate in a 250 ml volumetric flask. Fill to the mark with distilled water.

Find the absorbance of this solution in the range 350 - 650 nm. Use a recording spectrophotometer, such as Coleman 124 or P. E. Lambda 3.

Determine the pH of the solution. (Be sure the pH meter is calibrated!)

Add a small amount of 1M HCl to reduce pH and again determine absorbance as a function of wavelength. Measure the pH of the solution.

Repeat the procedure over the range pH 2-6 for a total of at least five absorption curves. When using Coleman 124 all runs may be made using same recorder paper.

At a wavelength corresponding to one of the absorption maxima, plot against pH. The graph should be linear. At either intercept, determine pK. Note: The number of points on your graph will be the number of runs minus 2.