A spectrophotometric method for the analysis of iron has a linear calibration cu

Question

A spectrophotometric method for the analysis of iron has a linear calibration curve for standards of0.00,5.00,10.00,15.00, and 20.00 ppm. An iron ore sample with an expected iron content of 40-60% w/w Fe is to be analyzed by this method. An approximately 0.5-g sample is taken, dissolved in a minimum of concentrated HCl, and diluted to 1 L in a volumetric flask using distilled water. A 5.00-mL aliquot is removed with a pipet. To what volume, 10,25,50, 100, 250, 500, or 1000 mL. should the aliquot bc diluted for the analysis so that the concentration of iron in the diluted sample will fall within the range of the standards? EXPLAIN and show necessary calculations. (ans: 100.0 mL).

Explanation / Answer

In the 1 L volumetric flask: CFe3+ is close to 0.5 g/L x (40…60)/100 = 0.2…0.3 g/L (round to 0.25)

The aliquot have to be diluted to obtain a concentration close to the middle of the calibration curve:

10 ppm = 10x10-3 g/L

The dilution factor has to be close to

0.25 g/L / 10x10-3 g/L = 25

The best choice for the volumetric flask is

5.00 mL (aliquot) x 25 = 125 mL.   Choose 100 mL vol. flask.

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