Determining Molar Absorptivity of Iron-Dye Complex at 660nm

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In summary, the procedure for determining the molar absorptivity of an iron-dye complex at a wavelength of 660nm involves creating two solutions - one acidic and one neutral - with varying ratios of iron standard, acidic buffer, and neutral buffer. The concentration of the chromophore must be precisely known and a baseline corrected absorbance measurement must be taken at the λmax for a range of concentrations. The Beer-Lambert Law can then be used to calculate the molar absorptivity by plotting absorbance against concentration and performing a linear least squares fit. Care must be taken to keep the pH of the solutions acidic to prevent the oxidation of Fe(II).
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scantor145
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I need help in devising a procedure to determine the molar absorptivity of an iron-dye complex in two solutions at a wavelength of 660nm. I have a UV-VIS spectrophotometer.

Solution 1 is an acidic buffer (R1) containing the iron-binding dye Chromazurol B and ferric chloride. Some amount of an iron standard will be added to this solution, hopefully forming a colored complex with the dye.

Solution 2 is Solution 1 with the addition of a neutral buffer (R2) raising the pH. I believe the molar absorptivity will change under varying pH. Accounting for dilution of Solution 1 would have to be taken into account.

The ratios of the volumes of iron standard to R1 to R2 are 16:200:60.
 
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You need to know the concentration of your chromophore precisely (level of precision depends on needs/goals of course). Get a baseline corrected absorbance measurement at the λmax for a range of concentrations. Plot absorbance as a function of concentration. Do a linear least squares fit. According to the Beer-Lambert Law, the slope of this line is the extinction coefficient (molar absorptivity) mulitplied by the path length.

A very simple procedure. The most difficult part most times is being able to know the concentration of the analyte very precisely.

Also why would you add Iron to solution 1 if you already have ferric chloride? Do you need to add Fe(II) for the complex to form?

Also why not just make a new solution 2 instead of diluting with buffer? If the extinction coefficient is not so high, you may dilute the analyte below a measurable absorbance.

Typically you want to measure absorbances between 0.1-1.0 AU (and your calibration curve should cover atleast that much range), but this depends on the instrument you are using.

I just thought of another thing, if you are indeed using Fe(II) in aqueous solutions, you need to keep the pH pretty acidic. Fe(II) is not very stable above ~pH 4 (IIRC), quickly oxidizes to Fe(III) and precipitates out as rust. Chelators help to stop this but pH affects chelation depending on what you are using. Just a few things off the top of my head for you to keep in mind.
 
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1. What is molar absorptivity and why is it important in this experiment?

Molar absorptivity, also known as molar extinction coefficient, is a measure of how strongly a substance absorbs light at a specific wavelength. In this experiment, it is important because it allows us to quantify the amount of light absorbed by the iron-dye complex at 660nm, which is crucial for determining the concentration of the complex in a sample.

2. What is the principle behind determining molar absorptivity in this experiment?

The principle behind determining molar absorptivity is based on the Beer-Lambert Law, which states that the absorbance of a substance is directly proportional to its concentration and the path length of the light through the sample. In this experiment, we use a spectrophotometer to measure the absorbance of the iron-dye complex at 660nm, and then use the Beer-Lambert Law to calculate the molar absorptivity.

3. How is the concentration of the iron-dye complex calculated using the molar absorptivity?

The concentration of the iron-dye complex can be calculated using the molar absorptivity by rearranging the Beer-Lambert Law equation: A = εlc, where A is the absorbance, ε is the molar absorptivity, l is the path length, and c is the concentration. By plugging in the absorbance and path length values, we can solve for the concentration of the complex.

4. What factors can affect the accuracy of the molar absorptivity value obtained in this experiment?

Several factors can affect the accuracy of the molar absorptivity value obtained in this experiment. These include the purity and concentration of the iron-dye complex, the path length of the light through the sample, the precision of the spectrophotometer, and any interferences from other substances present in the sample.

5. How can the molar absorptivity value be used in other experiments or applications?

The molar absorptivity value obtained in this experiment can be used in other experiments or applications that involve the same iron-dye complex at 660nm. It can be used to determine the concentration of the complex in different samples, or to compare the absorbance of the complex at different wavelengths. It can also be used to monitor the progression of a reaction involving the complex, or to determine the purity of the complex in a sample.

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