Goals
Use the quantitative technique of spectrophotometry to determine the amount of iron in a vitamin pill.
Continue to practice dilution calculations.
To learn the process of error propagation through a multi-step calculation using lab data.
Background
The average adult human body contains 4 to 6 grams of iron. The majority of this iron is found in the red blood cell protein called hemoglobin. The function of hemoglobin is to transport oxygen from the lungs to the various tissues in the body where it is used to produce energy. Humans obtain iron from their diet in foods such as meats and leafy green vegetables. Dietary supplements of iron can be taken to alleviate and/or prevent anemia, a condition caused by lack of iron, which results in low energy and pale skin tone. Most vitamin tablets contain iron in the form of ferrous fumarate, [Fe2+(C4H2O4)2-], yet we will be using chemicals today to ensure that any form of iron in the pill (Fe solid, Fe3+, or Fe2+) is completely converted into Fe2+ which can then react completely with our 1,10-phenanthroline to make our orange colored complex, Fe(Phen)3.
This week, you will use the standard curve you made last week, Beer's law, and the class average extinction coefficient for the orange colored complex, Fe(Phen)3 to determine the amount of iron present in a vitamin pill.
Do not pour solutions down the drain. Pour all chemical waste into the appropriate disposal containers provided.
**Ferrous ammonium sulfate and sodium acetate solutions are mild irritants. Please rinse any affected area with water.
**0.002 M 1,10-Phenanthroline solution is toxic if ingested. It's harmful to the environment, so please dispose of this chemical in the appropriate waste container.
** 2% Hydroxylamine hydrochloride solution is harmful if inhaled, ingested, or absorbed through the skin. Possible mutagen. Please rinse any affected area with copious amounts of water.
**Please dispose of used fluted filter paper and vitamin pill solid waste in the labeled solid waste container.
Please clean up after yourselves and leave your bench space exactly as you found it when you came into lab!
Rinse your beakers and graduated cylinders and let them dry on the glassware rack above the sinks.
Volumetric flasks get rinsed and dry in the test tube rack at your bench
Volumetric pipettes should be placed in the "pipet tub" with their TIPS UP!
Rinsed disposable pipettes and cuvettes can go in the trash
3M HCl
2% hydroxylamine hydrochloride, HONH2• HCl
10% sodium acetate, NaOCOCH3
0.002 M 1,10-phenanthroline, C12N2H8
iron vitamin capsule
Ocean Optics spectrophotometer and laptop computer
watch glass
long-stemmed glass funnel
fluted filter paper
50.00 mL & 100.00 mL volumetric flasks
1.00 mL volumetric pipet
10, 25, and 100 mL graduated cylinders
Students will work in groups of 2 unless otherwise specified by your instructor.
Preparation of the sample solution
Record the full brand name of the vitamin that you take and the mg of iron stated on the bottle. Take your assigned vitamin pill and open the capsule. Quantitatively transfer the powder inside the pill into a 250 mL beaker; discard the capsule shell.
Add approximately 25 mL of 3M HCl to the beaker containing the pill contents. Use a glass stir rod to stir the contents for the beaker.
Take your vitamin solution and gravity filter out any solids, using a a 100.00 mL volumetric flask, a long-stem funnel and fluted filter paper. Filter into the 100.00 mL volumetric flask and rinse out the beaker with deionized water into the flask as well.
(NOTE: A 100.00 ml total volume is used providing your vitamin pill label states that it contains18 mg. If your pill contains much more than 18 mg, please consult your instructor to determine if you need to use a different final volume.)
When it's done filtering, fill to the 100.00-mL mark with deionized water until the bottom of the miniscus of your solution is resting ON the mark. Please do NOT go over the 100.00-mL mark in the neck of the flask. Stopper and invert the flask several times to create a homogeneous solution.
Pipet 1.00 mL of this dilute solution into a 50.00 mL volumetric flask Now, treat it EXACTLY as the standards were treated in Part 2 from last week's lab, i.e.-add the same amounts of hydroxylamine hydrochloride, sodium acetate, and 1,10-phenanthroline, in that order. Fill the flask to the line with deionized water and stopper and invert the flask. Before you measure the absorbance of this solution, make a blank according to the instructions below.
Some of the vitamins may be colored with dyes that absorb at the λmax, you used to create your standard curve in last week's Spectroscopy lab. We can correct for this by making a blank that includes vitamin solution but is not reacted to form the iron-phenanthroline complex. To make this blank solution, take a 50.00 mL volumetric flask and add 1.00 mL of your vitamin pill stock solution (from the 100.00 mL flask). Add the usual amounts of hydroxylamine hydrochloride and sodium acetate (in that order) and fill to the line with deionized water. Do not add 1,10-phenanthroline to your "blank" since we do not want the orange iron-phenanthroline complex to form! If possible, make sure to use the same spectrophotometer as last week. Use this blank solution to zero your spectrophotometer. Then measure the absorbance of your vitamin pill solution with the 1,10-phenanthroline to get the amount of absorbance that is due to only the phenanthroline complex.
For your results, you will be using your class's average extinction coefficient for the iron-phenanthroline complex. This value, along with the absorbance of the phenanthroline complex from your vitamin, will be used to calculate the concentration of your vitamin pill solution. Finally, you will use the concentration to calculate the amount of iron in your vitamin pill. Don't forget that you never measured the absorbance of ALL the iron in your vitamin pill, but only a small amount from your diluted original 100.00 mL vitamin pill solution ( or adjusted volume, if applicable.)
Laura Muller, “Spectrophotometric Determination of Iron in a Vitamin Tablet,” General Chemistry Lab, Wheaton College, 2000.
For more information:
MEDLINEplus Medical Encyclopedia: Serum iron
http://www.nlm.nih.gov/medlineplus/ency/article/003488.htm
Lab Tests Online: Iron Tests:
http://www.labtestsonline.org/understanding/analytes/serum_iron/test.html
The method used in clinical chemistry labs for determination of serum iron:
Iron Panel of the International Committee for Standardization in Hematology. Revised recommendations for the measurements of the serum iron in human blood. British Journal of Hematology 75:615-616, 1990.
Iron Panel of the International Committee for Standardization in Hematology. Recommendations for the measurements of the serum iron in human blood. British Journal of Hematology 38:291-294, 1978.