In a spectrophotometry lab conducted to determin the concentration of iron in an
ID: 508701 • Letter: I
Question
In a spectrophotometry lab conducted to determin the concentration of iron in an unknown solution, and calculate the mass FE (in mg). This will be compared to the one in the previous lab (~56.00 mg). (Note: I believe the true value should be around 65 mg).
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The data and procedure is provided below.
We had to determine a regression line in excel:
y = 0.0713x + 0.0226
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Here is the table for determining peak absorbances (used 505 nm as the the wavelength)
Wavelength (nm)
Absorbance
400 nm
0.145
420 nm
0.221
440 nm
0.271
460 nm
0.325
480 nm
0.359
490 nm
0.368
500 nm
0.374
503 nm
0.377
504 nm
0.378
505 nm
0.378
506 nm
0.378
507 nm
0.378
508 nm
0.375
510 nm
0.370
520 nm
0.348
540 nm
0.183
560 nm
0.074
580 nm
0.035
600 nm
0.018
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Using wavelegnth 505 nm as the peak we determined the absorbances for the other samples, as well was our own.
Sample number
Wavelength (nm)
Absorbance
1
505 nm
0.062
2
505 nm
0.135
3
505 nm
0.251
4
505 nm
0.378
5
505 nm
0.474
Our unknown sample
505 nm
0.332
The regression line given by Excel was:
y = 0.0713x + 0.0226
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Procedure
Iron supplement tablet sample solution Use the Iron supplement solution from Lab 6. Base on the pre-lab calculation of the concentrations of standard iron solutions, one can design a procedure what concentration of unknown solution you have to make to be comparable to the calibration series.
Iron solution for spec 20: Quantitatively transfer 1 mL unknown solution (from lab 6) into a 1 L volumetric flask, carefully add, 4 ml of the hydroxylamine HCl solution, 40 ml of the 0.lM sodium acetate solution, and 40 ml of the 1,10 phenanthroline solution. Carefully dilute these solutions to volume with deionized water. Mix thoroughly. The concentration of Ferrous in this solution should fall within calibration range, i.e between C1 mg/L and C5 mg/L. Allow the solutions to stand for at least 5 minutes to fully develop the color. Mix well again.
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Measurement of Absorption Spectrum Determine the absorptions of all your solutions and your unknown by using the SPEC 20.
1. Determine the maximum absorbance wavelength of the iron-phenanthroline complex
Fill a clean, dry glass tube at about two-thirds depth for each solution.
Use the same blank solution for standard iron solutions and unknown solutions.
Measure and record absorption absorbance values of one of the standard solutions (better choose a middle concentration) at 20-nm intervals over the spectral range of 400 to 600 nm. Calibrate the absorbance reading to 0 with the blank solution every time you change the wavelength.
Make more measurements around the peak value (like 10 nm interval, 5 nm interval) to find an accurate maximum wavelength max.
Plot absorbance Absorption (vertical Y-axis) against wavelength (horizontal Xaxis) in your laboratory notebook or spreading sheet.
Select the maximum wavelength from this graph which to be used for measurement of the calibration and unknown solutions.
2. Obtain absorption spectrum of the iron-phenanthroline complex
Measure the absorbance of each standard solutions and unknown solution at the wavelength (max) of maximum absorbance.
Using your absorbance measurements for the standards to make a calibration plot and use regression analysis to get a linear equation that relates concentration to absorbance.
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To include In the Lab Report
1. Use the regression line to find the concentration of iron of your unknown solution, and then calculate the mass Fe (in mg) in one tablet.
2. Use that to compare results to the ones obtained in Lab 6 (~56 mg)
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Extra INformation:
Beer’s Law is a very simple relationship: A = bC, where A is the absorbance of a substance at a specified wavelength , in units of nm for light in the ultraviolet and visible regions of the electromagnetic spectrum; b is the length of the light path through the sample, usually in cm; is the molar absorptivity of the absorbing species at , when the concentration is in M; and C is the concentration in molar units, M. The molar absorption coefficient () of the ferrous complex, [(C12H8N2)3Fe]2+ so obtained is 11,100 L mol-1 cm-1 at the wavelength of maximum absorbance, max = 508 nm. The large value is indicative of strong absorption by the complex and forms the basis of the quantitative determination of iron (II).
Wavelength (nm)
Absorbance
400 nm
0.145
420 nm
0.221
440 nm
0.271
460 nm
0.325
480 nm
0.359
490 nm
0.368
500 nm
0.374
503 nm
0.377
504 nm
0.378
505 nm
0.378
506 nm
0.378
507 nm
0.378
508 nm
0.375
510 nm
0.370
520 nm
0.348
540 nm
0.183
560 nm
0.074
580 nm
0.035
600 nm
0.018
Explanation / Answer
For five absorption values of calibration standard , concentration of the solution is not provided. Hence concentration of unknown solution can not be determined.