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EXPERIMENT VII

DETERMINATION OF THE pKaOF AN INDICATOR (A WEAK ACID)

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INTRODUCTION

Bromothymol blue is a common indicator used in acid-base titrations. Aswith many acid-base indicators, bromothymol blue is itself a weak acid anddissociates in the following manner;

Structure I Structure III

Structure II

C3H

7

C3

H7

CH3CH3

SO3-

OH+

BrBr

HO

C3 H7 C3 H7

CH3 CH3SO3

-

BrBr

C3 H7 C3 H7

CH3CH3

SO3-

BrBr

HO

O-O

O

Red (pH < 1) Blue (pH > 8)

Yellow (3 < pH < 6)

The above reaction can be written in a more general and convenient from as:

HIn + H2O In- + H3O+

HIn represents the protonated form of the indicatorIn- represents the unprotonated form of the indicator.


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In order to be able to select an indicator for use in a titration a knowledge ofits pKa is necessary. In this experiment the pKa for bromothymol blue will bedetermined. A spectrophotometric method is particularly useful because adirect knowledge of [HIn] and [In-] is not necessary only the ratio of theconcentrations. The data from spectrophotometric measurements will providethe information necessary to calculate the ratio of these two forms.

PART I

THE DETERMINATION OF THE VISIBLE SPECTRUM OF[HIn] AND [In-]

THEORY

Bromothymol blue is a weak acid and in aqueous solution the followingreaction occurs:


In a very acidic solution bromothymol blue will be present as the protonatedform HIn(Le Chtletiers Principle). This is solution 1 in in the Experimental Sectionbelow. In very basic solution bromothymol blue will be present as theunprotonated from In-(Le Chtletiers Principle). This is solution 2 in the Experimental Sectionbelow. Both bromothymol blue species [HIn] and [In-], strongly absorbelectromagnetic radiation in the visible region. At any particular wavelengththe amount of electromagnetic radiation absorbed (A) is related to the molar

concentration (C) of the absorbing species in solution through Beers Law:A = bC

Where:

= molar absorbtivity and is a constant at any wavelength

b = path length in cm of the solution through which theelectromagnetic radiation

passes; it is determined by the dimensions of the cell used inthe experiment.

A measure of absorbance A then is related directly to the concentration Cof the absorbing species in solution.

EXPERIMENTAL

1. Carefully pipet 4 mL of bromothymol blue stock solution into a clean 100-mL volumetric flask, add ~ 10 mL of deionized water to the flask, thenadd 16 drops of concentrated hydrochloric acid and dilute to the markwith deionized water and mix well. This will give you a pH = 1 solutioncontaining only HIn.


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2. In a second 100-mL volumetric flask add 16 drops of hydrochloric acidand dilute to the mark with deionized water. This is your blank solution.


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3. Carefully pipet 4 mL of the bromothymol blue stock solution into a third,clean, 100- mL volumetric flask, add ~ 10 mL of deionized water to theflask, then add 2 mL of a 4M sodium hydroxide solution and dilute to themark with deionized water, and mix well. This will give you a pH = 13solution containing only In-.

4. Into a fourth 100-mL volumetric flask add 2 mL of 4M sodium hydroxideand dilute to the mark with deionized water. This is your blank solution.

5. Use the solutions prepared above to obtain two spectra of bromothymolblue, one of the protonated form of the indicator HIn (Structure I) andthe other of the unprotonated form of the indicator In- (Structure III) overthe wavelength range 350 - 750 nm (Visible region of theelectromagnetic spectrum). For this, use a scanning spectrophotometer.Your instructor will give you instructions on the use of thespectrophotometer.

CALCULATIONS AND RESULTS

From your spectra select two wavelengths, one where the absorbance ofHIn is large and the absorbance of In- is small and the other where theabsorbance of In- is large the absorbance of HIn is small. These twowavelengths are critical and will be used in the second part of theexperiment.

PART II

DETERMINATION OF THE ABSORBANCE OF HIn

AND In

-

AS A FUNCTION OF pH

In the second part of the experiment the absorbance of HIn and In- at twowavelength selected in PART I. will be determined in solutions of varying pHusing a spectrophotometer.

THEORY

Bromothymol blue in aqueous solution is a weak acid and its dissociation aswe saw previously can be represented as:


or mathematically

[ ]

-

3

a

In H OK

HIn

+ =

taking the -log and rearranging


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-

a

InpH pK log

HIn

= +

The concentrations [In-] and [HIn] are related to the absorbencies (A) (BeersLaw) at the two wavelengths determined in Part I. The absorbencies willvary with pH as the concentrations of [In-] and [HIn] are pH dependent. Aplot of pH vs. Absorbance (A) at each wavelength gives a s shaped curvemuch like a titration curve. The point of inflexion in the curve represents thesituation where [In-] = [HIn] and the pH is numerically equal to the pKa.

EXPERIMENTAL

1. Into each of eight clean 100-mL volumetric flasks pipet carefully 4.00 mL

of bromothymol blue stock solution. Dilute to the mark using a differentbuffer solution (provided) for each flask.

2. Using a pH meter, determine the exact pH of each of the eight solutionsprepared in (1) above.

3. Measure the absorbance spectrum of each of the eight solutions from350 to 750nm using the appropriate buffer solution as a blank.

4. From your spectra, determine for each solution the absorbance at thetwo wavelengths of interest you selected in Part 1.

CALCULATIONS AND RESULTS

Determination of the ratio

Hln

-In

Combine the data from your spectra at pH = 1 and 13 (from Part I) andthe eight intermediate pH solutions. You should have ten absorbancemeasurements at each of two wavelengths and ten different pH's. Using

Excel construct two graphs, one graph at each wavelength of absorbance vs.pH. Take the derivative of each curve

pH

A

and plot the derivative vs. pH.

The peak in the curve represents the situation where [HIn] = [In-] and from

pH = pKa + log[ ][ ]lnHIn

pH = pKa

You have now two values at pKa compare the two values and then compare

them with the literature value.


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