Exp 5 - Lab Manual Buffers and Indicators 1 Buffers and...

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Lab Manual: Buffers and Indicators 1 Buffers and Indicators INTRODUCTION & THEORY In this experiment, spectrophotometry is employed to measure the p K a of bromocresol green, an acid-base indicator. The indicator (HIn) is a monoprotic organic acid and we can represent its dissociation as follows: 23 HIn H O H O In     the equilibrium expression for such a dissociation can be written [] og HIn In pH pK HIn     In this experiment og In HIn (i.e., the y-axis) is plotted against pH and a linear relationship, ym xb , is expected. Based on the graph, at the point where [ ] I nH I n , og 0 In l HIn , and HIn p Kp H . In terms of the linear equation, HIn b pK m  (See Fig. 1). . Figure 1 Plot to determine HIn pK
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Lab Manual: Buffers and Indicators 2 The ratio [] In HIn can be determined spectrophotometrically. First, a solution of bromocresol green is prepared in acidic solution (low pH ) where essentially all the indicator is in the HIn form. The absorption spectrum is then taken over a range of wavelengths. From a plot of Absorbance vs , the wavelength of maximum absorbance, max , for HIn is determined. Similarly, a second solution is prepared in basic solution (high pH ) where essentially all the indicator is in the In form. Again, an absorption spectrum is taken over a range of wavelengths from which a max for In is determined. Buffered solutions with pH values on either side of the HIn pK of bromocresol green are then prepared and the absorbances measured at the two selected wavelengths. The solutions contain the same total concentration of indicator, [ ] HIn In , but the ratios vary with pH . Figure 2 shows a typical plot of absorbance vs pH at the wavelength of maximum absorbance for the In species. The terms used in this figure are as follows: a A = absorbance of HIn b A = absorbance of In A = absorbance of the specific mixture being measured Flask1 Flask2 Flask3 Flask4 Flask6 Flask7 Flask8 Figure 2 Plot of absorbance of In vs p H at max From the graph it is evident that a b AA In HIn A A Note that a A and b A , the extrapolated values from the graph are the same for all calculations at max for In . Only A (the absorbance of a specific mixture) changes from flask to flask. If the wavelength used is the one at which HIn shows maximum absorbance, the curve will approximately be the mirror image of that shown in Fig. 2.
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Lab Manual: Buffers and Indicators 3 MATERIALS & METHODS The stockroom will prepare the indicator solution (0.10 g of bromocresol green (C 21 H 14 Br 4 O 5 S) is dissolved in approximately 100 mL of 95% ethanol, neutralized to p H of 7.0 with 1 M NaOH and quantitatively diluted in a 250-mL volumetric flask to the mark with the ethanol solvent).
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Exp 5 - Lab Manual Buffers and Indicators 1 Buffers and...

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