Report 9 - Determination of pKa of Bromocresol Green An...

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Determination of pKa of Bromocresol Green, An Acid-Base Indicator Vincent Gallo CHM3120C-903 April 4 th 2016 1
I. Introduction An acid-base indicator is a liquid or substance that accurately represents the acidity or basicity of a solution; the pH of the solution. The indicator functions by changing color as the concentration of H + and OH - ions changes in solution. Bromocresol green is a dye that is commonly used in analytical settings to measure pH. In a solution with a pH less than 3.8, bromocresol green is yellow in color. If the pH of a solution rises above 5.4, the dye moves towards the blue end of the color spectrum. The pKa of a certain chemical can be found via spectrophotometry techniques. In this experiment, the pKa of bromocresol green is found by relating the log of the ratio of acid concentration and conjugate base concentration to pH and pK HIn . The indicator is a monoprotic organic acid that dissociates in the following way: ¿ + ¿ + ¿ ¿ HIn + H 2 O ↔H 3 O ¿ This dissociation gives an equilibrium expression containing the concentration of the hydronium ion and the conjugate base over the concentration of the acid (HIn). The K a is a quantitative measure of the strength of an acid in solution, and also represents the equilibrium constant for a chemical reaction known as dissociation in the context of acid– base reactions. The chemical species are said to be in equilibrium when their concentrations do not change with the passing of time. The equilibrium expression can be manipulated so that the pKa of the indicator can be extrapolated from a linear function. Beginning with the equilibrium expression, the log of both sides of the equation can be taken, and logarithmic rules can be applied so that the new function reads as: 2
¿ ¿ ¿ ¿ ¿ + ¿ + log ¿ log K HIn = log [ H 3 O ] ¿ Rearrangement of this equation gives a relationship of pH and pK HIn added to the long of the Ka of the solution. A final rearrangement gives a linear function (y = mx +B) with the log of the Ka becoming the y, m is equal to 1, and b represents –pK Hin . The ratio of conjugate base to acid can be found spectrophotometrically by preparing two solutions, one that is very acidic, and contains the indicator in the HIn form. An absorption spectrum can then be taken through a range of wavelengths and then plotted vs. wavelength. From this graph, the wavelength of maximum absorbance can be

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