Iron Colorimeter
Iron Colorimeter
Iron Colorimeter
Method
Preparation of Fe3+ standard solutions
NB: It may take several days to dissolve the Fe3+ salt used here, so carry out this preparation well in advance of the rest of the experiment. Weigh out about 3.0 g of ferric ammonium sulfate (FeNH4(SO4)212H2O). Use a mortar and pestle to grind the salt to a fine powder. Accurately weigh 2.41 g of the powder into a 100 mL beaker and add 20 mL of concentrated sulfuric acid (Caution: concentrated acids are highly corrosive wear safety glasses and rubber gloves and take care when handling). Leave powder to soak in acid overnight. The next day, carefully pour the acid/powder slurry into a 500 mL volumetric flask, rinsing the beaker into the flask a few times with water, then make up to the mark with distilled water. Let this solution stand for several days until the ferric ammonium sulfate powder has fully dissolved. If possible, insert a magnetic stirrer bar and stir the solution to speed up this dissolving process. Use a pipette to transfer 20 mL of ferric ion solution to a 200 mL volumetric flask and make up to the mark with distilled water. This gives a solution with [Fe3+] = 0.001 mol L1. To prepare a 2 105 mol L1 standard solution pipette 10 mL of the 0.001 mol L1 solution into a 500 mL volumetric flask, add 10 mL of 1 mol L1 sulphuric acid, and then make up to the mark with distilled water. Repeat this procedure in separate 500 mL volumetric flasks, pipetting in 20, 30, 40 and 50 mL of 0.001 mol L1 Fe3+ solution in turn, to obtain 4, 6, 8 and 10 105 mol L1 solutions respectively. (NB: if you do not have five 500 mL volumetric flasks you can use one flask to prepare each standard in turn. After preparing each standard, pour the solution into a labelled glass vessel which has a lid (eg: a glass bottle). Then rinse your 500 mL volumetric flask thoroughly with distilled water before using it to prepare your next standard solution.)
0.15 mol L potassium permanganate solution (see below for preparation) 100 mL beaker 100, 200, 250 and 500 mL volumetric flasks 5 mL pipette 10 mL measuring cylinder 100 mL conical flask
Weigh 38 g of solid ammonium thiocyanate into a 500 mL volumetric flask and make up to the mark with distilled water.
combusting your sample. Also beware that the crucible will become very hot during this process, so handle it only with crucible tongs or preferably not at all until it has cooled. 3. When the sample and crucible have cooled, use a stirring rod to crush the ash to a fine powder (see Figure 3). Use a measuring cylinder to add 10 mL of 1 mol L1 hydrochloric acid and stir for 5 minutes, making sure that all the ash is soaked. 4. Add 5 mL of distilled water and then filter the solution into a 100 mL conical flask to remove the ash. This filtered solution will be used for colorimetric analysis.
Colorimetric analysis
1. Accurately measure 10 mL of your sample solution into a clean, dry boiling tube. (NB: a boiling tube, or large test tube, is ideal; however any small vessel eg: a beaker, flask or glass vial may also be used). This measurement is most accurately made using a 10 mL pipette; however, it is possible to do this accurately enough (and with less hassle) using a clean 10 mL measuring cylinder if you measure carefully. 2. Next, measure 10 mL of each Fe3+ standard solution into separate boiling tubes (one standard per tube) in order of increasing concentration, beginning with the 2 105 mol L1 standard. It is a good idea to first rinse your pipette or measuring cylinder with a few mLs of the 2 105 mol L1 standard. NB: Make sure you label each boiling tube appropriately with the name of the sample or standard it contains. A test tube rack is very useful for holding and transporting your tubes (see Figure 4). Alternatively you can use a large beaker to hold them. 3. Using a 10 mL measuring cylinder, add 10 mL of 1 mol L1 ammonium thiocyanate solution to each iron solution in sequence, with 2 minutes between each addition. These additions must be carefully timed so that all samples react for the same period of time. 4. Mix the solutions by swirling. A stable red colour will appear over the next few minutes. 5. As near as possible to 15 minutes after adding thiocyanate, measure the absorbance at wavelength of 490 nm for each coloured solution using your colorimeter. These measurements will be made in sequence one sample every two minutes reflecting the timing of the thiocyanate additions above. The measured absorbance of light is a direct measure of the intensity of the solutions colour. Figures 4 and 5 illustrate the range of colour intensities that you can expect from your set of Fe3+ standards.
Figure 5. The same boiling tubes as in Figure 4 (prepared using Fe3+ standard solutions with concentrations of 2 12 105 mol L1) viewed from above, looking directly down the length of the tube. This view provides the most accurate eyeball comparison of the solutions colour intensities.
Calculations
1. Using only the absorbance results obtained for your Fe3+ standard solutions (not your unknown iron sample), prepare a graph with [Fe3+] (in mol L1) as the horizontal axis and absorbance (at 490 nm) as the vertical axis. 2. Draw a line of best fit for your data points that goes through the origin (because absorbance must be zero when Fe3+ concentration is zero. 3. Now identify the point on your line of best fit which corresponds to the absorbance measured for your unknown iron sample. By drawing a vertical line to the horizontal axis you will be able to determine the concentration of Fe3+ in your unknown solution. 4. Use this concentration to calculate the mass of iron (in mg) in your original tablet or food sample (NB: the molecular weight of iron is 55.8 g mol1). Remember to take into account any dilutions that you performed while preparing your sample solution. 5. If the absorbance value you measured for your unknown iron sample is greater than the absorbance value for your highest concentration Fe3+ standard you will need to modify the above procedure. In the case of an iron tablet, you should repeat the analysis with a more dilute solution of the dissolved iron tablet. In the case of a food sample, you should repeat the analysis using a smaller mass of your food.
Figure 2. Set up used for combusting a silverbeet sample directly in the bunsen burner flame.
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Figure 3. Left photo: ash remaining after combustion of a silverbeet sample. Right photo: silverbeet ash which has been ground to a fine powder using a stirring rod.
If you have any questions or comments relating to this experiment, please contact us: Outreach College of Science University of Canterbury Private Bag 4800 Christchurch New Zealand Phone: +64 3 364 2178 Fax: +64 3 364 2490 Email: outreach@canterbury.ac.nz www.outreach.canterbury.ac.nz
Figure 4. Blood-red coloured solutions produced by the ferric thiocyanate complex ion (Fe[SCN]2+). These solutions were prepared using the following range of Fe3+ standards: 2, 4, 6, 8, 10, 12 105 mol L1.