To determine the mass of iron in one adult dose of either a ferrous sulfate or. ferrous gluconate iron supplement using a colorimetric technique.

Transcription

1 Lab: Colorimetric Analysis of Iron in Iron Supplements Purpose To determine the mass of iron in one adult dose of either a ferrous sulfate or ferrous gluconate iron supplement using a colorimetric technique. Theory Iron is one of the minerals required by the body. It is used in the manufacture of the oxygen-carrying proteins hemoglobin and myoglobin. Without sufficient supplies of iron, a person can feel tired and listless. Many of the foods we eat contain small quantities of iron, however if iron is low, iron supplementation may be required. Iron supplements are available in different forms. This experiment compares the amount of element iron found in two different supplements: Ferrous Sulfate and Ferrous Gluconate. In order to quantify the amount of iron found in a solution, a series of five FeCl3 solutions will be prepared of decreasing concentrations. To make the presence of the Fe3+ ions in solution visible, thiocyanate (SCN ) ions are added. These react with the Fe3+ ions to form Fe(SCN)2+ ions, which give the solution a red colour: Fe3+(aq) + SCN (aq) Fe(SCN)2+(aq) The red colour will deepen in intensity as the concentration of Fe3+ ions increases. In other words, the more concentrated the solution, the more light it will absorb. The colour will be quantified using a colourimeter. A colourimeter is a device that can pass light of a desired wavelength through a

2 sample of a solution. It measures the quantity of light that is absorbed will give a quantitative value of absorbance which is directly proportional to the quantity of iron present in the solution. If graph of absorbance versus concentration is plotted for a series of solutions with a known iron concentration, the graph can be used to estimate the concentration of an unknown solution. This is a powerful analytical technique known as colorimetry. Safety Procedures 1. Follow all instructions for using the equipment in this activity. 2. Wear safety glasses at all times. 3. Potassium thiocyanate and hydrochloric acid may cause irritation to the skin and eyes. Avoid contact. 4. Discard all waste in the containers provided. Materials and Apparatus Part A: 10 ml of mol/l FeCl3 25 ml of 2.0 mol/l HCl 2 x 5 ml pipettes with bulbs 1 x 1 ml pipette (if available) 5 test tubes Test tube rack

3 Part B: Ferrous Sulfate iron supplement (liquid form may be used) Ferrous Gluconate iron supplement 100 ml of 2.0 M HCl 2 x 100 ml beakers 5 ml pipette Electronic balance Mortar and pestle Test tubes test tube racks Part C: Colorimeter set to read at approximately 475 nm 7 4 ml cuvettes 15 ml of 1.0 mol/l KSCN 10 ml pipette with bulb

4 Procedure: Part A: Preparing the Standard Solutions You are provided with 10 ml of mol/l aqueous Iron (III) Chloride solution. This is your stock solution. Your task is to prepare 5 solutions of decreasing FeCl3 concentration by diluting this sample. Instead of diluting the sample with water, you will dilute it with 2.0 mol/l HCl. 1. Develop a procedure to prepare 5 ml each of FeCl3 solutions of the following concentrations: mol/L, mol/l, mol/l, mol/l and mol/l. (Hint: You will need to use the equation C1V1= C2V2 in order to do this.) 2. Record the procedure in the Initiate and Plan section of your lab report, and fill in the chart (Table 1) below to help you to organize your calculations: Table 1: Volume of mol/l FeCl3 stock solution and 2.0 M HCl needed to prepare standard solutions of5 different concentrations. Concentration of Fe3+(mol/L) ml of FeCl3 stock solution ml of 1.0 M HCl Final Volume (ml) Prepare the solutions as described in your procedure. Part B: Preparing the Vitamin Supplement Samples ***The iron supplement solutions may be prepared ahead of time by the teacher to save time*** Choose either the ferrous sulfate or ferrous gluconate iron supplement to analyze. If preparing from a liquid supplement: 1. Measure out 25% of an adult dose of the liquid with a pipette according to the dosage on the label (should be about 1.5 ml). 2. Place the liquid supplement in a 50 ml beaker and dilute with 2.0 M HCl to a final volume of 100 ml. 3. Place 5 ml of the diluted iron supplement solution into a labeled test tube. If preparing from a tablet form: 1. Take the mass of 1 tablet. 2. Crush the tablet using a mortar and pestle. 3. Place the crushed tablet into a 100 ml volumetric flask and dilute to 50 ml using 2.0 M HCl. 4. Place 5 ml of the diluted iron supplement solution into a labeled test tube.

5 Part C: Data Recording 1. Add 2.5 ml of 1.0 mol/l KSCN to each of your standard samples from Part A. Do the same to your iron supplement solution from Part B. Mix the solutions. 2. Place approximately 3 ml of each of your samples into each of 6 cuvettes. 3. Place approximately 3 ml of 2.0 mol/l HCl into a 7th cuvette. This is your blank. 4. Set up the colourimeter as described in Appendix A. This step may have already been done for you. 5. Record the absorbance of each of your 5 standard solutions, your blank, and your iron supplement sample into an observation table. 6. Discard your solutions in the waste containers supplied by your teacher. What to include in the Lab Report: ***The Lab Report is to be assessed according to the rubric provided*** Initiate and Plan: Include in this section the procedure for how to prepare the standard solutions by dilution. Make sure the procedure is clear and easy to follow. Include one sample calculation. Perform and Record: Create a table of observations that includes all relevant data (qualitative and/or quantitative). The data should be presented clearly, allowing for easy interpretation. Your observation table should include absorbance data for both the ferrous sulfate and ferrous gluconate iron supplements. Some class data sharing is necessary to do this. Analyze and Interpret: a. Create a Standard Curve, which is a graph of Absorbance vs. Concentration for your 5 standard solutions. Use a line of best fit to represent the trend in the data. b. Determine the concentration in both iron supplement samples by either or the following methods: interpolating or extrapolating off of your standard curve. Show your work clearly, directly on the graph. Determine the equation of the line in the form of y=mx+b, and find the concentration in your supplement sample by substituting into the equation. c. Use the concentration of each iron supplement solution to determine the mass of iron (in g) in one dose of both the ferrous sulfate and ferrous gluconate supplements. Conclude and Evaluate: Conclusion: Briefly summarize the technique. State the mass of iron found in one dose of each iron supplement. Based on this, and any information obtained in the iron supplement fact sheet, discuss the differences between the supplements. Is one better than the other? Evaluation: Evaluate the procedure used to collect the data. Identify any limitations, weaknesses, and sources of error. Make sure to discuss how they may have

6 affected the results. Appendix A: Preparation of Solutions The following should prepare enough materials for 10 groups of students to perform the lab. Adjust the quantities as needed depending on the number of students. To Prepare 200 ml of 1.0 M KSCN: Formula weight of KSCN=97.2 g/mol Add 19.5 g of solid KSCN to a 200 ml volumetric flask and dilute to 200 ml with distilled water. To Prepare 1.5 L of 2.0 M HCl using 12.0 mol/l stock solution: Dilute 51 ml of 12.0 M stock solution of HCl to 1.5 L using distilled water. To Prepare of 100 ml mol/l FeCl3 Add g of solid FeCl3 to a 100 ml volumetric flask and dilute to 100 ml with distilled water