An organic complex (HIn) changes its color at a given pH when either its acidic form (color 1) or basic form (color 2) become dominant. The complex has an acid dissociation constant of 4.80 x 10 -6 at ordinary temperatures. The accompanying absorbance data are for 8.00 x 10 -5 M solution of the compound measured in 1.00-cm cells in strongly acidic and strongly alkaline media. Based on the figure that you will draw (combine the two sets of data in one figure), answer the following questions:
(1) What is color of the solution in a solution of pH 1.0 and what is color of the solution when pH is 13.0? The acid color (dominant by HIn species) and base color (dominant by In- species) of this indicator?
(2) Which species of this compound (acidic or basic) has the highest molar absorptivity at which wavelength? What is it? What is the molar absorptivity of the basic species at its maximum wavelength?
(3) What observations can you make from your figure and discuss the factors that can influence the absorbance of an organic compound?
The absorbance of an indicator at different wavelength and pH
| λ/nm | 420 | 445 | 450 | 455 | 470 | 510 | 550 | 570 | 585 | 595 | 610 | 650 |
| pH 1.00 | 0.525 | 0.657 | 0.658 | 0.656 | 0.614 | 0.353 | 0.119 | 0.068 | 0.044 | 0.032 | 0.019 | 0.014 |
| pH 13.00 | 0.050 | 0.068 | 0.076 | 0.085 | 0.116 | 0.223 | 0.324 | 0.352 | 0.360 | 0.361 | 0.355 | 0.284 |
