Passage VI

In a colorimeter, light of a specific wavelength is directed through a sample of solution onto a detector. If some of the components in the solution absorb some of the light, that light will not reach the detector. The solution's absorbance is related to the amount of light absorbed, and therefore, to the concentration of these components.

Dissolved iron (Fe) does not absorb visible light. However, when a certain indicator solution is added to an Fe solution, the Fe will react with the indicator solution to form Fe(SCN)₆^-3 ions, which absorb blue light.

Experiments were done to study the effects of boiling and canning on the Fe content of beans.

Experiment 1

Several 80 mL solutions with known concentrations (in mg/L) of Fe in H₂O were each mixed with 20 mL of the indicator solution. The absorbance of blue light by each solution was then measured in a colorimeter (see Figure 1).

Experiment 2

Raw black beans were ground and 5 g were heated until only ash remained. The ash was placed in 20 mL of indicator solution. The mixture was filtered and the solution was then diluted to 100 mL withH₂O. The absorbance was measured and the Fe concentration of the solution was determined using Figure 1. The Fe content of the beans (in mg/100 g) was then calculated. The procedure was repeated for boiled black beans and canned black beans, and then for other types of beans (see Table 1).

Table adapted from the U.S. Department of Agriculture, USDA Nutrient Database for Standard Reference, Release 13, 1999