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Secondary school chemistry students can have a satisfying introduction to colorimetry by obtaining reasonably close results for the determination of iron in various foods. The classic reaction between the iron(III) ion and the thiocyanate ion to yield a reddish-brown complex ion works quite well for this activity. The various ions that interfere with the formation of this complex are silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury, zinc, cadmium, bismuth, phosphate, arsenate, fluoride, oxalate, tartrate, and sulfate (1). Most of these ions either are absent or in low concentrations in food.
ChemCom (2) has a visual comparison activity related to the iron content in food, and this procedure can be used as an extension. This experiment works very well in other chemistry courses as an experience in spectrophotometric analysis and its concomitant skills. It is precisely in this latter context that it is useful.
At high concentrations of thiocyanate ion, the following equilibrium is pushed far to the right:
(Equation omitted)
The food sample is heated until it is reduced to a fine ash. Hydrochloric acid is added to dissolve the iron in the residue and to prevent the hydrolysis of the iron(III) ion. After filtering, thiocyanate solution is added and the absorbance is determined with a spectrophotometer. Using a previously prepared iron(III) complex concentration-absorbance graph, the result can be obtained with the help of interpolation or extrapolation. (Graph omitted)
The solution made when the iron(III) complex is mixed with the thiocyanate ion is not stable under normal temperature and light conditions. The third dilution (described below) lost 5% of its absorbance within 10 min and 10% of its absorbance in 30 min. Spectrophotometric readings should be made without delay.
Equipment and Supplies List
* spectrophotometer-meter or digital readout (digital readout is preferred). Colorimetric probes with associated software are available for attachment onto computers (IBM, Mac, Apple II). These will gather the data and graph results.
* crucible or evaporating dish
* pipets or burets
* .0.0010 M FeCl sub 3 stock solution (at least 11 mL/group) dissolve 0.270 g of FeCl sub 3 X 6H sub 2 O in about 500 mL of distilled water. Acidify with 5 mL of concentrated HCl. Dilute to 1L with distilled water. Mix well. This is more than...