why do you need to construct a calibration curve when doing spectrophotometric determinations? Is it not possible to obtain the concentration of the unknown solution by simply getting the extinction coefficient of the species under study and then determine its concentration in the unknown solution by using Beer's law? Thanks!

Many or most spectrometers split a beam of light in two and pass it through two different "arms" ot the spectrometer, and then take the ratio of the measured signals. One of the arms contains the sample material, in a holder.

There can be differences in the intensities of the two beams because of an imperfect beam splitter or different losses in the two beams, including absorption by windows of the empty cell. This can be evaluated by means of a calibration scan of the empty sample holder.

The ratio of the signals with full and empty cell should be used for the sample material transmittance or absorptance.

The absorption at a given wavelength may be due more than one species. I assume that you are not trying to deduce the concentration of one species in a mixture of species with overlapping bands.

It has been a long time since I calibrated a spectrophotometer, and there must be newer and better kinds around these days.


This article may help.
http://www.wisegeek.com/what-is-spectrophotometer-calibration.htm

Let me add something to the answers provided by drwls. Knowing the extinction coefficient and using Beer's Law may work IF you assume that the extinction coefficient doesn't change with concentration. Sometimes it does and the calibration curves are not always linear. Some curve up, some down, some are straight lines.

Constructing a calibration curve is necessary when doing spectrophotometric determinations because it helps relate the absorbance of a solution to its concentration. While it is true that Beer's law can be used to determine the concentration of a solution using the extinction coefficient, constructing a calibration curve provides a more accurate and reliable method.

Here's how a calibration curve works:

1. The first step is to prepare a series of standard solutions with known concentrations. These solutions should cover a range of concentrations that are relevant to your analysis.

2. Each of these standard solutions is then measured using a spectrophotometer to determine their absorbance at a specific wavelength. The absorbance values are recorded.

3. Next, the known concentrations of the standard solutions are plotted on the x-axis of a graph, while the corresponding absorbance values are plotted on the y-axis.

4. By plotting the points and drawing a best-fit line through them, a calibration curve is obtained. The line represents the relationship between absorbance and concentration.

5. Finally, once the calibration curve is established, the absorbance of an unknown solution can be measured, and its concentration can be determined by interpolating it on the calibration curve using the corresponding absorbance value.

The calibration curve compensates for any deviations or non-linearities in the relationship between absorbance and concentration. It accounts for factors like instrumental limitations, sample impurities, and matrix effects, which can affect the accuracy of the quantitative analysis.

Overall, by constructing a calibration curve and using Beer's law in combination, you can determine the concentration of an unknown solution with improved accuracy and reliability.