Question

Calcium in flour was analyzed by the following procedure. First, 0.1170 g of flour were digested in 10 mL of boiling concentrated nitric acid for 30 minutes before adding 5 mL of concentrated perchloric acid and boiling for another 30 minutes. The resulting solution was transferred to a 100.0 mL volumetric flask and diluted to the line with deionized water. From this, 20.00 mL were pipetted into each of four separate 50-mL volumetric flasks. To these were added 0, 5.00, 10.00 and 15.00 mL of 6.47 mg/L calcium standard solution before diluting to the line with deionized water. Atomic absorption of calcium was measured for all four of the resulting solutions, giving the data shown below. A blank measurement gave an absorption of zero.

Description of solution		Atomic absorption
Unspiked solution (50.00 mL)		0.112
Spiked with 5.00 mL 6.47 mg/L Ca		0.188
Spiked with 10.00 mL 6.47 mg/L Ca		0.264
Spiked with 15.00 mL 6.47 mg/L Ca		0.339

(a) What is the calcium concentration added to the "5.00 mL spiked" solution?
mg/L Ca

(b) What is the calcium concentration of the unspiked solution?
mg/L Ca

(c) What is the calcium content of the flour in mg/g (parts per thousand)?
mg/g Ca

Answer 1

Ans. #a. [Ca] in 5.0 spiked aliquot from standard solution (spiking) = 0.6470 mg L^-1

            [See data table above the graph]

# [Ca] in 5.0 spiked aliquot from acid-digest solution = 0.9590 mg L^-1

            [See #b]

# Now, [Ca] in the spiked solution is contributed by both the [Ca] from original sample as well as [Ca] from the standard addition.

So,

            Total [Ca] in 5.0 mL spiked aliquot = 0.6470 mg L^-1 + 0.9590 mg L^-1

= 1.6060 mg L^-1

#b.

#c.