CHEM 140 - Sample Final Evaluation

Please read the following instructions carefully. You may use either Excel OR Mathcad to answer the following problem. NOT BOTH! Not all required items are specified in the in the instructions. You must decide what is important and should be included in the solution of this problem. For example, items such as standard deviations, error bars, correlation coefficients, etc, must be included where appropriate and/or necessary. You may be required to refer to the CHEM 140 manual for some pertinent equations/formulas.

The method of standard addition is used in instrumental analysis to determine the concentration of a substance (analyte) in an unknown sample. In this method, a known amount of the analyte is added to the unknown solution. A measured response of this solution is determined before and after the addition of the standard. From the measured response increase due to the increased analyte concentration, the original concentration can be determined.  Since the measured response (signal) is proportional to analyte concentration, we can say that

concentration of analyte in unknown            signal from unknown

concentration of analyte + standard in mixture      signal from mixture

The standard addition equation is given by Equation 1,

where [X]i  is the unknown initial concentration of the analyte, which yields a signal of IX.  After the standard was added, [X]f  is the final concentration of the unknown analyte and [S]f  is the  final concentration of the standard.  After the standard was added, IX+S  is the signal measured.

If we define the initial volume of the unknown as V0, the volume of the standard added as VS, and final volume of the mixture as V (Note, V does not necessarily equal V0  + VS),the final concentrations of the unknown analyte and standard are given by Equations 2 and 3, respectively.

By substituting the expressions for [X]i  (Equation 2) and [S]i  (Equation 3) into the standard addition equation (Equation 1) and rearranging, we obtain the expression given by Equation 4.

An experimental analysis was conducted to determine theiron (Fe) content in drinking water. The drinking water was analyzed using the standard addition method, described above.  The concentration of theiron (Fe) standard solution was 11.1 ppm.  All solutions were diluted to a   final volume of 50.0 mL.  The iron in the drinking water samples were experimentally analyzed  by using an instrument called an atomic absorption spectrophotometer (AAS).  The experiment results are given in Table 1, below.

Using the given experimental data, determine theiron (Fe) concentration in the drinking water sample.