Absorption spectroscopy and acetic acid

The optical density of visible radiation, wavelength 632nm, was measured in an index solution at changing pH, and changing concentration, leting for a Beer-Lambert secret plan to be constructed. This was so used to mensurate acetic acid uptake at the surface of deionised H2O and octan-1-ol coated H2O, leting pH, and therefore concentration, to be calculated from optical density of the liquid.


Wetting agents are molecules which are able to organize a surface across a liquid, and halt the interaction of foreign molecules with the solution without interacting with these molecules foremost. These are highly utile since they frequently contain a hydrophobic and hydrophilic facet, which interact otherwise to different molecules. Wetting agents are used in the industry of paper, fabrics and building among others. [ 1 ] They are the chief ingredient of detergents and they allow non-polar molecules to fade out in polar molecules, such as oil into H2O.

On the surface of the liquid, the wetting agent will interact somewhat otherwise. It will make a surface of hydrophobic ‘tails ‘ . This will halt polar molecules from come ining the liquid, since the liquid will look to be a hapless solution for the polar molecule to interact with. They besides increase decrease tenseness of the liquid. [ 4 ]

This barrier is expected to halt the acetic acid, used in portion 3 of the experiment, interacting with the H2O dissolver. If it does interact, the pH of the solution will take down due to acetic acids presence, and the index will demo a alteration in coloring material. If no acetic acid enters the solution, no alteration should be observed or measured.


Using de-ionised H2O, a mention light strength was recorded. A 250ml solution ( 1 ) of 0.005 % wt bromocresol viridity was so prepared, and optical density was measured. 100ml was removed, and the pH adjusted utilizing 0.1M sodium hydrated oxide and glacial acetic acid, and optical density was noted at pH ‘s between 3-6 at 0.3 increases. 50ml of staying solution ( 1 ) was farther 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 so spectroscopically analysed and it ‘s approximative concentration determined. This solution was so enclosed in a container with acetic acid, and spectroscopic readings taken every 30 seconds. This was repeated with fresh solution, with the add-on of 0.2ml of octan-1-ol to the surface of the cuvette.


The consequences for the pH alteration showed a curve, traveling from lower pH on the left to high pH on the right.

This is a more quantifiable manner of demoing that as the Bromocresol turned blue at higher pH. This shows soaking up toward the terminal of the spectrum of lower energy, ( ie higher wavelength ) . So as pH increased, the optical density of Bromocresol at 632nm increased excessively as it became bluish.

The following facet of the experiment was to analyze how concentration affected the optical density of Bromocresol green. As concentration of bromocresol viridity was altered, it was possible to pull a Beer-Lambert secret plan detailing how the soaking up of the visible radiation changed with concentration of the Bromocresol Green.

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

The Bromocresol solution of unknown concentration transmitted 0.222, doing 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 find the sourness of the solution utilizing the Beer-Lambert secret plan as given above. Using an original pH, it is so possible to find the concentration of the acetic acid on top of this, utilizing simple equations associated with pKa and pH.

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

Mistake analysis

Using mistake analysis and standard mistakes of instrumentality used, it is possible to build the same graphs as above but with mistake bars. These are shown below.


The computations and graphs suggest that surfacing a dissolver in octan-1-ol would promote consumption of acetic acid, instead than suppress it. This may be due to dimerzation or trimerzation of acetic acid ( 1 ) as it evaporates from the surface, doing it more soluble in the partly polar octan-1-ol solution.

Individual carbon-oxygen bonds display less polarization than carbonyl bonds do, and so it is likely that in this dimerised agreement acetic acid more readily dissolved in the oil, in add-on to acetic acid readily fade outing in organic dissolvers. Because of these grounds it readily crossed over from the comparatively non-polar octanol to the polar H2O dissolver, diminishing the pH of the Bromocresol incorporating solution in both the uncoated and octanol coated solutions.

It is, nevertheless, most likely that the experiment was non successful. Alternate indexs, such as NH3, would hold readily dissolved in H2O and increased the pH of the solution. It would besides non hold been able to fade out in the octanol due to the higher mutual opposition and handiness of the N lone brace. Because of this it would hold been a better index of the presence of a wetting agent than acetic acid.


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


  1. R. J. Farn, Chemistry and Technology, Blackwell Publishing ( 2006 ) pp. 6.
  2. L. L. Schramm, Wetting agents: basicss and applications in the crude oil 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 chemical science, 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 chemical science: understanding our chemical universe, John Wiley & A ; Sons ( 2004 ) pp. 225.
  7. V. H. Agreda, J. R. Zoeller, Acetic acid and its derived functions Volume 49 of Chemical industries, CRC Press ( 1993 ) pp. 96.

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