Absorption spectroscopy and acetic acid

The absorbance of light, wavelength 632nm, was measured in an indicator solution at varying pH, and varying concentration, allowing for a Beer-Lambert plot to be constructed. This was then used to measure acetic acid uptake at the surface of deionised water and octan-1-ol coated water, allowing pH, and hence concentration, to be calculated from absorbance of the liquid.

Introduction

Surfactants are molecules which are able to form a surface across a liquid, and stop the interaction of foreign molecules with the solution without interacting with these molecules first. These are extremely useful since they often contain a hydrophobic and hydrophilic aspect, which interact differently to different molecules. Surfactants are used in the manufacture of paper, textiles and construction among others.[1] They are the main ingredient of detergents and they allow non-polar molecules to dissolve in polar molecules, such as oil into water.

On the surface of the liquid, the surfactant will interact slightly differently. It will create a surface of hydrophobic ‘tails’. This will stop polar molecules from entering the liquid, since the liquid will appear to be a poor solution for the polar molecule to interact with. They also increase decrease tension of the liquid.[4]

This barrier is expected to stop the acetic acid, used in part 3 of the experiment, interacting with the water solvent. If it does interact, the pH of the solution will lower due to acetic acids presence, and the indicator will show a change in colour. If no acetic acid enters the solution, no change should be observed or measured.

Experimental

Using de-ionised water, a reference light intensity was recorded. A 250ml solution (1) of 0.005% wt bromocresol green was then prepared, and absorbance was measured. 100ml was removed, and the pH adjusted using 0.1M sodium hydroxide and glacial acetic acid, and absorbance was noted at pH’s between 3-6 at 0.3 increments. 50ml of remaining solution (1) was further diluted to solutions of 0.0025%, 0.00125%, 0.000625% and 0.0003125% concentration. Spectroscopic analysis of these concentrations was made, and a Beer Lambert graph plotted. A solution of unknown concentration was then spectroscopically analysed and it’s approximate concentration determined. This solution was then enclosed in a container with acetic acid, and spectroscopic readings taken every 30 seconds. This was repeated with fresh solution, with the addition of 0.2ml of octan-1-ol to the surface of the cuvette.

Results

The results for the pH change showed a curve, going from lower pH on the left to high pH on the right.

This is a more quantifiable way of showing that as the Bromocresol turned blue at higher pH. This shows absorption toward the end of the spectrum of lower energy, (ie higher wavelength). So as pH increased, the absorbance of Bromocresol at 632nm increased too as it became blue.

The next aspect of the experiment was to analyse how concentration affected the absorbance of Bromocresol green. As concentration of bromocresol green was altered, it was possible to draw a Beer-Lambert plot detailing how the absorption of the light changed with concentration of the Bromocresol Green.

As would be expected, there is a straight line relationship between Bromocresol concentration and Absorbance except at higher concentrations, where the solution plateaus and becomes non-linear. Excluding this end point it is possible to derive the gradient, and hence the value of ?L. This was determined to be 36600.

The Bromocresol solution of unknown concentration transmitted 0.222, making a LOG(Io/I) value of 0.67. Dividing this by the gradient gave the Bromocresol solution concentration to be 4.57×10-6moldm-3.

From this it is possible to determine the acidity of the solution using the Beer-Lambert plot as given above. Using an original pH, it is then possible to determine the concentration of the acetic acid on top of this, using simple equations associated with pKa and pH.

From the information of Ka and pH, it is possible to calculate the concentration of acetic acid in the solvent.

Error analysis

Using error analysis and standard errors of instrumentation used, it is possible to construct the same graphs as above but with error bars. These are shown below.

Discussion

The calculations and graphs suggest that coating a solvent in octan-1-ol would encourage uptake of acetic acid, rather than inhibit it. This may be due to dimerzation or trimerzation of acetic acid (1) as it evaporates from the surface, making it more soluble in the partially polar octan-1-ol solution.

Single carbon-oxygen bonds display less polarisation than carbonyl bonds do, and so it is likely that in this dimerised arrangement acetic acid more readily dissolved in the oil, in addition to acetic acid readily dissolving in organic solvents. Because of these reasons it readily crossed over from the relatively non-polar octanol to the polar water solvent, decreasing the pH of the Bromocresol containing solution in both the uncoated and octanol coated solutions.

It is, however, most likely that the experiment was not successful. Alternative indicators, such as NH3, would have readily dissolved in water and increased the pH of the solution. It would also not have been able to dissolve in the octanol due to the higher polarity and availability of the nitrogen lone pair. Because of this it would have been a better indicator of the presence of a surfactant than acetic acid.

Acknowledgements

I would like to thank my demonstrators M. Azwani Mat Lazim and Miss Olesya Myakonkaya for their advice on the experiment.

References

1. R. J. Farn, Chemistry and Technology, Blackwell Publishing (2006) pp. 6.
2. L. L. Schramm, Surfactants: fundamentals and applications in the petroleum industry, Cambridge University Press (2000) pp. 7.
3. R. J. Farn, Chemistry and Technology, Blackwell Publishing (2006) pp. 6.
4. K. S. Birdi, Handbook of surface and colloid chemistry, CRC Press (1997) pp. 338.
5. P. Atkins, J. De Paulo, Atkins Physical Chemistry 8th Edition, Oxford Publishing (2006) pp. 432.
6. P. M. S Monk, Physical chemistry: understanding our chemical world, John Wiley & Sons (2004) pp. 225.
7. V. H. Agreda, J. R. Zoeller, Acetic acid and its derivatives Volume 49 of Chemical industries, CRC Press (1993) pp. 96.