Solution Concentration and Absorbance

Solution Concentration and Absorbance - He that will not...

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He that will not apply new remedies must accept new evils: for time is the great innovator --Francis Bacon Solution Concentration and Absorbance In your earlier studies you have already seen that the concentration of a solute in a solvent can be determined in a variety of ways: volumetric and gravimetric. Titration and recovery of precipitates are two very basic and useful techniques. It would also be possible to employ colligative properties to the same end. This experiment concerns another technique that is applicable to solutions which absorb visible light. [ transmittance is the ratio of the light intensity exiting a sample compared to the light intensity entering a sample; absorbance is the negative log of transmittance , i.e., A = -log T OR A = 2-log %T] Most solutions absorb electromagnetic radiation of some wavelength. Those which appear colored to the human eye absorb in the visible range of the spectrum. The theories behind the production of color in solution can be pretty complex but you might suspect that electron transitions of some sort are involved. In fact, these quantized transitions often account for relatively sharp absorption spectra (within narrow wavelength ranges). Consider the absorption spectrum of Ti(H 2 O) 6 3+ : Consulting a wavelength table you would find that the maximum absorption is in the yellow-green region. But the solution itself appears violet . This is because when yellow- green light is removed from the mixture of visible light we call "white", the remaining colors mix to give violet. In general, the apparent color of a solution is the complement of the color of light which is absorbed (see the following diagram on which complementary colors are opposite one another) Before you get the idea that the situation is fairly simple you should also know that many solutions absorb at more than one wavelength although they appear to be just one color. This should not be surprising since electrons exist on many energy levels in atoms and molecules and can be expected to make a variety of transitions involving different energies and so absorbing different wavelengths (remember, Δ E = hc/ λ ). This would just be very interesting if not for the fact that absorbance is proportional to concentration! Strictly speaking that proportionality is expressed as Beer's Law : A = ε bc y=mx+ b where A is the absorbance (unitless), ε is a proportionality constant known as the molar absorptivity coefficient , b is the path length of solution through which the light travels (in cm), and c is the concentration of the solution (in Molarity). This is a linear relationship and is generally true for most solutions. Like most "laws" there are exceptions and solutions which behave most ideally are those which have moderate concentrations and are examined at the wavelength of maximum absorbance . Because some ordinary solutions can deviate from Beer's Law, it is common practice to develop a calibration graph for a particular system before interpreting the concentration of a
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Solution Concentration and Absorbance - He that will not...

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