5. (10) A dietary supplement claiming to contain 250 ppm Cr was analyzed to dete

Question

5. (10) A dietary supplement claiming to contain 250 ppm Cr was analyzed to determine if the claim was true. A representative sample of the tablets were ground into a powder using a mortar and pestle. Two portions of the powder, both weighing 0.9863 g, were dissolved in dilute acid and transferred to 50-mL volumetric flasks. To one of these, 10.00 mL of 75.0 ppm Crwas added, then both flasks were diluted to the mark with distilled water. When aspirated into the flame of an atomic absorption spectrometer set at a Cr absorption wavelength, the absorbances of the unknown and unknown plus standard were 0.374 and 0.641, respectively. Was the manufacturers claim correct?

Explanation / Answer

Estimation of Cr content in the tablet by standard addition method

0.9863 g sample dissolved in acid and diluted to 50 ml ----> Solution without standard

Absorbance = 0.374

0.9863 g sample dissolved in acid + Cr standard added diluted to 50 ml ----> Solution with standard

concentration of Cr standard in diluted solution = 75 ppm x 10 ml/50 ml = 15 ppm

Absorbance = 0.641

Using,

Ci/(Ci + Cs = Ai/(Ai + As)

with,

Ci = concentration of Cr in solution without standard = ?

Cs = concentration of Cr standard in diluted solution = 15 ppm

Ai = Absorbance of Cr in solution without standard = 0.374

(Ai + As) = Absorbance of solution with Cr standard = 0.641

So,

Ai/(Ai + 15) = 0.374/0.641

0.641Ai = 0.374Ai + 5.61

Ai = 21.01 ppm

So concentration of Cr in diluted solution = 21.01 ppm

Concentration of Cr in 0.9863 g tablet sample = 21.01 ppm

therefore,

concentration of Cr in two portions combined tablet sample = 2 x 21.01 = 42.02 ppm

Which is much less than claimed. The manufacturers claim was not correct

Related Questions

Navigate

• Browse (All)
• Browse 5
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.