HShin_Hoon_Shin_-_Lab_Report_5

HShin_Hoon_Shin_-_Lab_Report_5 - Author: Hoon Shin TA:...

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Author: Hoon Shin TA: Eileen Chen Section: A05 Class: Chem100A, UCSD(University of California San Diego) Date: 5/12/08 Buffers and Indicators Abstract The goal of this experiment is to observe the pK HIn of the bromocresol green by obtaining transmittance and absorbance of several solutions with different pH. For the fist part of this experiment where one must find the maximum absorbance for two solutions, H In and In−, are determined as 440nm and 610nm respectively. At this wavelength, the other solutions with different pH are measured to determine pK HIn . Respectively, at wavelength of 440nm and 610nm, pK HIn of bromocresol green was 4.3353(no unit) ± 1.052865 and 4.7904(no unit) ± 1.569909 Introduction The main purpose of this experiment is to find the pKa value of bromocresol green which is acid-base indicator. This indicator is a monoprotic organic acid which can be dissociates with following reaction: HIn + H 2 O   H 3 O + + In - Equilibrium expression for this dissociation can be calculated using following formula: [ ] - + HIn In log HIn pH pK = Using the graph observed by the experiment and the equation above can be expressed as y = mx + b where y = log ([In−] / [H In ]), x = pH, m = slope of a line, and b= pK HIn . Using following items, graph can be obtained as following figure:
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Using spectrophotometer, one can determine the ratio of log ([In ] / [H In ]). First of all, bromocresol green indicator of low pH is prepared where the entire indicator is H In form. Measuring a maximum absorbance of this low pH bromocresol green, concentration of H In can be obtained. Also, with bromocresol green of high pH, concentration of In can be determined by obtaining maximum absorbance of this solution. Using these values above, following graph can be obtained: Where Aa = absorbance of HIn, Ab= absorbance of In , and A = absorbance of the specific mixture being measured. Aa and Ab are fixed number since those two absorbance are
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measured in the first part of experiment. Thus an only value that moves is “A” from flask to flask. The graph above is the case where wavelength used is when HIn shows highest absorbance. If maximum absorbance of In−, graph should be mirror image of above graph. Experimental The bromocresol green indicator was prepared by the stockroom. By using specific amounts of specific species, 20 different solutions are made. Table 1 – Amounts and species needed for 8 basic solutions Flask Solution of Acid Solutions of base Indicator 1 48 mL of 0.1 M HCl - 2 mL of bcg 2 48 mL of 0.1 M HAc - 2 mL of bcg 3 46 mL of 0.1 M HAc 2 mL of 0.2 M NaAc 2 mL of bcg 4 44 mL of 0.1 M HAc 4 mL of 0.2 M NaAc 2 mL of bcg 5 31 mL of 0.1 M HAc 17 mL of 0.1 M NaOH 2 mL of bcg 6 13 mL of 0.1 M HCl 35 mL of 0.2 M NaAc 2 mL of bcg 7 7 mL of 0.1 M HAc 41 mL of 0.2 M NaAc
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This note was uploaded on 10/16/2008 for the course CHEM 100A taught by Professor Dai during the Spring '06 term at UCSD.

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HShin_Hoon_Shin_-_Lab_Report_5 - Author: Hoon Shin TA:...

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