3(2000) As part of a study into the trace metals found in the natural waters in

ID: 1034193 • Letter: 3

Question

3(2000) As part of a study into the trace metals found in the natural waters in Eastern Europe. the iron content of Popradské pleso tarn in Slovakia was determined by atomic absorption spectroscopy at 248.3 nm. For the analysis, a standard solution of iron was prepared with a concentration of 0.204 mg/L. A series of standard addition samples was then prepared by adding 1.00 mL of the natural water into a 10.00 mL class A volumetric flask and adding a known volume of standard solution as shown in the table below. The solution was then made up to the mark in the volumetric flask using distilled water. Samples were then taken from the diluted solutions and the absorbance was measured at 248.3 Sample Volume of Sample (mL)Volume of Standard AddedAbsorbance @ mL 0.00 1.00 2.00 3.00 248.3 nm 0.042 0.113 0.156 0.225 No. 2 3 4 1.00 1.00 1.00 1.00 Determine the concentration of natural iron in the water sample.

Explanation / Answer

Determine the concentration of the added standard in the samples as below (take sample 2 as an example). We add 1.00 mL of 0.204 mg/L iron standard and make up the volume to 10.00 mL. Use the dilution equation.

C1*V1 = C2*V2

where C1 = 0.204 mg/L; V1 = 1.00 mL and V2 = 10.00 mL.

Therefore,

C2 = C1*V1/V2 = (0.204 mg/L)*(1.00 mL)/(10.00 mL) = 0.0204 mg/L.

Prepare the following table.

Sample No.

Volume of 0.204 mg/L iron standard added (mL)

Concentration of iron standard in the final sample (mg/L)

Absorbance at 248.3 nm

0.042

1.00

0.0204

0.113

2.00

0.0408

0.156

3.00

0.0612

0.225

Plot absorbance vs concentration of iron standard added as below.

It is assumed that the natural water sample contains iron in such low concentration that the absorbance for the sample alone (without added standard) is zero. Therefore, put y = 0 in the regression equation and obtain

0 = 2.902x + 0.0452

=====> -2.902x = 0.0452

=====> x = -0.0452/2.902 = -0.01557 ? -0.0156

The concentration cannot be negative; hence, ignore the negative sign and the concentration of iron in the natural water sample is 0.0156 mg/L (ans).