experiment 4c

experiment 4c - EXPERIMENT 4C Analysis of Iron in an...

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Experiment 4C- Page 1 EXPERIMENT 4C Analysis of Iron in an Iron(III) Oxalate Complex INTRODUCTION Spectrophotometry In the previous experiment, an iron(III) oxalate complex was analyzed for its oxalate content. In this experiment, the iron content in this complex will be determined using spectrophotometry , one of the most widely used analytical methods of analysis today. The use of this technique, particularly in the visible region, is routine in clinical, biochemical, and environmental laboratories. Multiple samples can be quickly analyzed. In a spectrophotometric method of analysis, the sample solution absorbs electromagnetic radiation from an appropriate light source, and the amount of radiation absorbed can be related to the concentration of material of interest (analyte) in the solution. The energy of radiation absorbed, although often in the visible region, may be in any of the regions of the electromagnetic spectrum. If visible light is absorbed, we can see the results. A copper(II) solution appears blue to our eyes because it absorbs the complementary color yellow from white light and transmits the remaining blue light to our eyes. Spectrophotometric methods can be easily quantitated. The amount of light absorbed is directly related to the amount of analyte through which the light is passed. This is determined by both the concentration of the analyte (c) and the path length of the sample (b). The ratio of the intensity of light entering the cell, I 0 , and that leaving the cell, I, is related to these two factors by Beer's Law. The transmittance (T) or fraction of radiant energy that reaches the other side of the cell is given by I/I 0 . Beer's Law states that: ln 1/T = ln I 0 /I = k b c The constant k often includes the conversion from natural logarithms to base ten logs and is called the molar absorptivity, ε , with units of M 1 cm 1 . log 1/T = log I 0 /I = ε b c The term log I 0 /I is renamed absorbance (A), and A = ε b c, where A = log 1/T. Note that absorbance is directly proportional to concentration. Instruments used for spectrophotometric measurements often have scales that indicate both absorbance and percent transmittance (%T = 100T). The conversion between these is: A = log T % % 100 Since the absorbance unit is linearly related to concentration, it is the unit of preference. However, the scale on the instrument is linear in %T. Hence, the %T scale is usually recorded for ease and accuracy of readings and the values are mathematically converted to absorbance values (See examples in Table I).
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Experiment 4C- Page 2 TABLE I %T T A B (cm) ε (M 1 cm 1 ) C (M) 60 0.60 0.22 2.0 2.0 × 10 4 5.5 × 10 6 45 0.45 0.35 1.0 1.3 × 10 3 2.7 × 10 4 37 0.37 0.43 1.0 1.2 × 10 3 3.6 × 10 4 %T values are read directly from the instrument. The conversions to T and A are mathematical. There are two methods used to determine concentration values from this data. In the first method, the b value or path length can be determined from the cell in use; sample cells may be purchased that have precisely known lengths. The molar
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experiment 4c - EXPERIMENT 4C Analysis of Iron in an...

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