Simple Iron in Food Determinations

The other procedures listed in this section for iron determination are good activities but may require equipment or chemicals not available in your lab. There is a less complex, but a bit less accurate method for the determination of iron in food. It consists of burning the food in a crucible, dissolving the residue in water or dilute hydrochloric acid, mixing with thiocyanate ion and comparing to a standard set of solutions visually or with a spectrophotometer. The text ChemCom: Chemistry in the Community by the American Chemical Society has a procedure in the foods unit that works reasonably well.

The following is the outline of a simple procedure that works with raisins,dates, cereal, and spinach.

Materials required:

iron III chloride (hexahydrate)
evaporating dishes, beakers, 100 mL grad., small test tubes
10 ml pipet
burner and ringstand

Standard preparation:

  1. Dissolve 1.70g of FeCl3 in 99 ml of distilled water. Mix well and label 1% iron
  2. Prepare a serial dilution in steps of 10 fold dilutions by using a pipet to measure 10 ml and add that to 90 ml of distilled water. Prepare concentrations of 0.1%. 0.01%, 0.001%, 0.0001%, 0.00001%, and 0.000001% of the original.
  3. Add 5 ml of each solution to a separate small test tube. To each tube add 5 ml of 0.1M KSCN solution.

Testing food for iron:

  1. Determine the mass of raisin or other high iron food (about 2 g)
  2. Heat strongly in an evaporating dish or crucible until a gray ash remains. The sample may burn
  3. After cooling add 5 ml of distilled water to the ash and stir well.
  4. Filter and transfer the filtrate to another test tube of the same size as the standards.
  5. Add 5 ml of 0.1M KSCN solution and compare the color to the standards.
  6. If you have a spectrophotometer you could test the absorbance of the standards and unknowns at 450 nm.

The orange-red color of the iron thiocyanate complex is not totally stable and is an equilibrium condition. If standard solutions are prepared in advance, it would be best to add the KSCN the day of the lab. A simple visual comparison is enough to get the range of iron in different foods.

The use of a light box and looking down the tubes length will increase the sensitivity as long as the tubes are all the same diameter and the same total volume is used in each tube.