Question

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

Answer 1

(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