Spectroscopic Method of Determination

1 January 2017

The purpose of this lab experiment is to determine the Ka value for the indicator bromcresol green using spectroscopy. Bromocresol green is a weak organic acid; therefore, it doesn’t reacte to completion, rather it reacts to a dynamic equilibrium, the point at which the rate of reactants turning to products is equal to the rate of products changing back into reactants. An acid (in terms of the bromsted-lowry definition) is a substance that donates H+ ions in a reaction.

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In bromcresol green, the acidic proton is the that is attached to the SO3- group. The acid, in this lab will be represented by a HB and the conjugate base will be represented by B- . The Ka value for this reaction would be: [H3O+][B-] / [HB] In this lab, the concentration of [H30+] will be determined by adjusting to a known value by using a buffer solution of HOAc and it’s conjugate base OAc-. By adjusting the concentration of HOAc and it’s conjugate base in solution, we can control the amount of hydronium ion produced in solution.

This will be done by using the equilibrium constant: Keq [H2O] = [H3O+] [OAc? ] = Ka = 1. 75 x 10? 5(8) [HOAc] Since the concentration of HOAc and it’s conjugate base are known , we can use the ratio of acid to base and the known Ka value to solve for [H3O+]. Then through spectroscopic techniques, using the ratio of wavelength to absorbance. By using the absorbance values of bromcresol green in it’s basic form, acidic form and intermediate form, we can calculate the ratio of [B-]/[HB] using the formula: [B-] = (Aobserved ? AHB) (5) HB] (AB- – Aobserved) where the observed concentration refers to the intermediate concentration, HB refers to the absorbance of acidic concentration and B- refers to the absorbance of the bromocresol green of the basic solution. Then, using the ratio of base to acid and the hydronium ion concentration, the value of Ka can be calculated. Ka of the buffer solutions: 2: 5. 311×10- f 3: 6. 57 x10-7 4: 2. 34 x10-6 5: 7. 48 x10-6 6: 1. 00 x10-5 Average Ka value: 5. 15 x10-6 [pic]

The average value of Ka was calculated at 5. 5 x 10-6 . The standard deviation was calculated to be 5. 96 x10-6. Any possible sources of error may have been derived from any deviations from the the absorbancy caused by the fact that the cuvette absorbed some of the wavelength. Based on this, we can say that the pH for bromocresol green would be ideal within the range of 3-5. According to the graph, the wavelength at which the absorbance of the acid solution, base solution and the intermediate solution was around 500 nm, at which the absorbance was.

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