Absorption spectroscopy and acetic acid

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 inter act 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.