Question

Design an experiment to find the iron concetration (Fe3+) in spinach using a spectrophotometer

Answer 1

Determination of Fe³⁺ concentration in spinach

1. A few grams of spinach (2 − 5 g is required, depending on iron content of the sample) was weighed into a crucible.
2. The crucible was heated over a bunsen burner until the sample is reduced completely to ash.
3. When the sample and crucible have cooled, the ash was crushed with a stirring rod to a fine powder.
4. 10 mL of 1 M hydrochloric acid was added to it and stirred for 5 minutes, making sure that all the ash is soaked.
5. 5 mL of distilled water was then added and then the solution was filtered into a 100 mL conical flask and the filtered solution was used for colorimetric analysis.

Colorimetric analysis

1. 10 mL of the sample solution was taken into a clean, dry boiling tube with a pipette.
2. 10mL of each Fe3+ standard solution into separate boiling tubes in order of increasing concentration, beginning with the 2 × 10−5 mol L−1 standard.
3. Then 10 mL of 1M ammonium thiocyanate solution was added to each iron solution in sequence and mixed, with 2 minutes between each addition. These additions must be carefully timed so that all samples react for the same period of time.
4. A stable red colour will appear over the next few minutes.
5. As near as possible to 15 minutes after adding thiocyanate, the absorbance was measured at a wavelength of 490 nm for each coloured solution using a 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 solution’s colour.

Calculations

1. Using the absorbance results obtained for Fe³⁺ standard solutions , a graph was constructed with [Fe3+] (in mol L−1) as the horizontal axis and absorbance (at 490 nm) as the vertical axis.

2. The best fit line was drawn through the data points that goes through the origin (because absorbance must be zero when Fe³⁺ concentration is zero).

3. Then that point is identified on the line of best fit which corresponds to the absorbance measured for the unknown iron sample. By drawing a vertical line to the horizontal axis the concentration of Fe³⁺ in the unknown solution can be determined.

4. This concentration is used to calculate the mass of iron (in mg) in the sample (molecular weight of iron is 55.8 g mol−1).