Beer’s plot

Samples from these aliquots were then colormetrically analyzed with the spectrophotometer and a Beer s plot drawn (see Figure 2). 

Therefore, the solute-concentration present in an unknown solution can be estimated conveniently from the Beer s Plot or sometimes referred to as the Standard Curve, merely by measuring the absorbance value of the solution and then finding the concentration value that corresponds to the measured absorbance value as is illustrated in the following Figure 2.2. 

Salama et al. [34] developed and validated a spectrophotometric method for the determination of omeprazole and pantoprazole sodium via their metal chelates. The procedures were based on the formation of 2 1 chelates of both drugs with different metal ions. The colored chelates of omeprazole in ethanol were determined spectrophotometrically at 411, 339, and 523 ran using iron(III), chromium(III), and cobalt(II), respectively. Regression analysis of Beer s plots showed good correlation in the concentration ranges 15-95, 10-60, and 15-150 /ig/ ml of pure omeprazole using iron(III), chromium(III), and cobalt(II), respectively. 

Calculate the absorbance of each solution when Pstray is 5% of Pq, and plot Beer s law calibration curves for both sets of data. Explain any differences between the two curves. (Hint Assume that Pq is 100). 

For solutions which do not follow Beer s Law, it is best to prepare a calibration curve using a series of standards of known concentration. Instrumental readings are plotted as ordinates against concentrations in, say, mg per lOOmL or lOOOmL as abscissae. For the most precise work each calibration curve should cover the dilution range likely to be met with in the actual comparison. 

Now, starting with 0.05 mL toluene, repeat the procedure to obtain five working solutions l -5 and use solution 5 to plot the absorption curve of toluene again record the Amax values for the peaks of the curve. There is a well-developed peak at approximately 270 nm, and using the five test solutions, measure the absorbance of each at the observed peak wavelength and test the application of Beer s Law. Measure solution 5 also at the wavelength used for benzene, and solution 5 at the wavelength used for toluene. 

Prepare a benzene-toluene mixture by placing 0.05 mL of each liquid in a 25 mL graduated flask and making up to the mark with methanol. Take 1.5 mL of this solution, place in a lOmL graduated flask and dilute to the mark with methanol this solution contains benzene at the same concentration as solution 5, and toluene at the same concentration as solution 5. Measure the absorbances of this solution at the two wavelengths selected for the Beer s Law plots of both benzene and toluene. Then use the procedure detailed in Section 17.48 to evaluate the composition of the solution and compare the result with that calculated from the amounts of benzene and toluene taken. 

With a fixed amino acid concentration of 0.02 M, the rate constant proved independent of the concentration of BrO" over the range (0.38-3.09) x 10"3M for N-Br-aminoisobutyric acid and N-Br-Proline. The plot of the obtained initial absorbance values against the initial N-Br-amino acid cone tration shows that Beer s law is obeyed, and the values for the molar absorptivity of the studied N-bromoamino acids are listed in Table 2. 

For concentrated or bulk samples a transmission experiment is both the simplest and the most effective. In essence, one measures the X-ray intensities incident and transmitted through a thin and uniform film of the material. Careful analysis of signal-to-noise ratio considerations indicates that optimal results are obtained when the sample thickness is of the order of 2.5 absorption lengths. Since in this case a simple Beer s law applies, the data are usually plotted as In(7//0) versus E. The intensities are measured using ionization chambers in conjunction with high-gain electrometers (see Fig. 11). 

Walash et al. [14] described a kinetic spectrophotometric method for determination of several sulfur containing compounds including penicillamine. The method is based on the catalytic effect on the reaction between sodium azide and iodine in aqueous solution, and entails measuring the decrease in the absorbance of iodine at 348 nm by a fixed time method. Regression analysis of the Beer s law plot showed a linear graph over the range of 0.01 0.1 pg/mL for penicillamine with a detection limit of 0.0094 pg/mL. 

Thus, the band at the lower wavelengths exhibits perfect linearity, but the one at the higher wavelengths does not. Therefore, even though the underlying spectra follow Beer s law, the measured spectra not only show nonlinearity, they do so differently at different wavelengths. This is clearly shown in Figure 27-2, where absorbance versus concentration is plotted for the two peaks. 

Effect of bandpass and choice of wavelength on a Beer s law plot. Curve A represents a calibration curve using a narrow bandpass monochromator at k. Curve B represents a calibration curve using a wide bandpass filter at X or a narrow bandpass monochromator at. ... 

From this reference point, accurate absorbances can then be determined. This data is then plotted (absorbance vs. known concentrations) and the unknown concentration then extrapolated from the Beer s law plot. Factors that limit this technique include 1) sensitivity of the instrument being used — usually best between 10 and 90%T, 2) the magnitude of the molar absorptivity (e), 3) fluctuations due to pH changes, and 4) temperature changes. 

Various known concentrations of the stock solution of the complex ion were colormetrically analyzed and a Beer s law plot drawn (see Figure 2). The ASA that was produced was then treated as described earlier and then colorimetrically analyzed with the concentration determined through extrapolation of the Beer s law plot. 

From the Beer s law plot, determine the concentration of the ASA produced. 

MLR is based on classical least squares regression. Since known samples of things like wheat cannot be prepared, some changes, demanded by statistics, must be made. In a Beer s law plot, common in calibration of UV and other solution-based tests, the equation for a straight line... 

Most quantitative analyses by Beer s law involve preparing a series of standard solutions, measuring the absorbance of each in identical containers (or the same container), and plotting the measured... 

FIGURE 7.21 An example of data and standard curve for a quantitative spectrophotometric analysis. The standard curve in this case is also known as a Beer s law plot. 

Experiment 20 Designing an Experiment Determining the Wavelength at which a Beer s Law Plot Becomes Nonlinear... 

You should have noticed that the Beer s law plot constructed in Experiment 19 is linear between concentrations of 1 and 4 ppm Fe. At some point beyond 4 ppm, there will be a deviation from Beer s law. Design and conduct an experiment that will precisely determine the concentration at which this occurs. 

In order to plot the absorption spectrum of a compound or complex ion, we must be able to carefully control the wavelengths from the broad spectrum of wavelengths emitted by the source so that we can measure the absorbance at each wavelength. Additionally, in order to perform quantitative analysis by Beer s law, we need to be able to carefully select the wavelength of maximum absorption, also from this broad spectrum of wavelengths, in order to plot the proper absorbance at each concentration. These facts dictate that we must be able to filter out the unwanted wavelengths and allow only the wavelength of interest to pass. 

Deviations from Beer s law are in evidence when the Beer s law plot is not linear. This is probably most often observed at the higher concentrations of the analyte, as indicated in Figure 8.10. Such deviations can be either chemical or instrumental. 

Obtain the maximum absorbance value for each spectrum and prepare the standard curve (Beer s law plot) as in Experiment 18. Obtain the concentration of the unknown and the control sample. 

Quantitative analysis in flame atomic absorption spectroscopy utilizes Beer s law. The standard curve is a Beer s law plot, a plot of absorbance vs. concentration. The usual procedure, as with other quantitative instrumental methods, is to prepare a series of standard solutions over a concentration range suitable for the samples being analyzed, i.e., such that the expected sample concentrations are within the range established by the standards. The standards and the samples are then aspirated into the flame and the absorbances read from the instrument The Beer s law plot will reveal the useful linear range and the concentrations of the sample solutions. In addition, information on useful linear ranges is often available for individual elements and instrument conditions from manufacturers and other literature. 

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