Question

# Iron in drinking water may be measured using the optical absorbance at 520 nm after treating...

Iron in drinking water may be measured using the optical absorbance at 520 nm after treating the sample like this: add 300-500 mg of ascorbic acid to reduce Fe3+ to Fe2+, add 20 mL of 1 g/L 2,2'-dipyridyl solution, dilute to 100.0 mL, wait 5 minutes and then measure absorbance. Standards were prepared by following this procedure with 5.00, 10.00, 15.00, 20.00 and 25.00 mL of 4.05 mg/L Fe. Then the procedure was followed using 50.00 mL of a tap water sample. The resulting data are tabulated below.

Procedure followed using    |Absorbance

5.00 mL of 4.05 mg/L Fe |0.169

10.00 mL of 4.05 mg/L Fe |0.318

15.00 mL of 4.05 mg/L Fe |0.501

20.00 mL of 4.05 mg/L Fe |0.642

25.00 mL of 4.05 mg/L Fe |0.826

50.00 mL of tap water |0.245

(a) What is the Fe concentration of the standard solution in which the procedure was followed using 5.00 mL of 4.05 mg/L Fe? mg/L

(b) What is the slope of the calibration line? (units = L/mg)

(c) What is the Fe concentration of the tap water? mg/L

(i)

5.00 mL of 4.05 mg/L Fe

V1 = 5 mL

C1 = 4.05 mg/L

Final volume, V2 = 100 mL

V1 C1 = V2 C2

C2 = 5 * 4.05 / 100 mg/L

= 0.2025 mg/L

(ii)

Similarly for other samples, Fe concentration can be obtained as shown below:

 Volume of sample, mL Initial [Fe], mg/L Final Volume, mL Final [Fe2+], mg/L Absorbance, A 5 4.05 100 0.2025 0.169 10 4.05 100 0.405 0.318 15 4.05 100 0.6075 0.501 20 4.05 100 0.81 0.642 25 4.05 100 1.0125 0.826

Slope = 0.809

(iii)

A = 0.8089 [Fe2+] – 0.0002

0.245 = 0.8089 [Fe2+] – 0.0002

[Fe2+] = 0.303 mg/L

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