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Chapter 10: Problem 24

The following table lists molar absorptivities for the Arsenazo complexes of copper and barium. \({ }^{27}\) Suggest appropriate wavelengths for analyzing mixtures of copper and barium using their Arsenzao complexes. $$ \begin{array}{ccc} \text { wavelength }(\mathrm{nm}) & \varepsilon_{\mathrm{Cu}}\left(\mathrm{M}^{-1} \mathrm{~cm}^{-1}\right) & \varepsilon_{\mathrm{Ba}}\left(\mathrm{M}^{-1} \mathrm{~cm}^{-1}\right) \\ \hline 595 & 11900 & 7100 \\ 600 & 15500 & 7200 \\ 607 & 18300 & 7400 \\ 611 & 19300 & 6900 \\ 614 & 19300 & 7000 \\ 620 & 17800 & 7100 \\ 626 & 16300 & 8400 \\ 635 & 10900 & 9900 \\ 641 & 7500 & 10500 \\ 645 & 5300 & 10000 \\ 650 & 3500 & 8600 \\ 655 & 2200 & 6600 \\ 658 & 1900 & 6500 \\ 665 & 1500 & 3900 \\ 670 & 1500 & 2800 \\ 680 & 1800 & 1500 \end{array} $$

Short Answer

Expert verified
Use 611 nm to analyze copper and 641 nm to analyze barium.

Step by step solution

01

Understand Molar Absorptivity

Molar absorptivity (\(\varepsilon\)) is a measure of how strongly a substance absorbs light at a particular wavelength. Higher values indicate greater absorption. We seek wavelengths where the difference in absorptivity between copper and barium is significant, allowing distinctions in their complex formations.
02

Look for Copper Absorption Peak

A good wavelength for analyzing copper is where\(\varepsilon_{\mathrm{Cu}}\)is high while\(\varepsilon_{\mathrm{Ba}}\)is low. From the table, we observe the highest extinction coefficient for copper at 611 nm (\(\varepsilon_{\mathrm{Cu}} = 19300\)) while barium's absorption at this wavelength is considerably lower (\(\varepsilon_{\mathrm{Ba}} = 6900\)).
03

Select Wavelength for Barium Peak

Identify a wavelength where barium's absorption is high while copper's is relatively low. At 641 nm,\(\varepsilon_{\mathrm{Ba}}\ = 10500\)is high compared to\(\varepsilon_{\mathrm{Cu}} = 7500\). This represents an advantageous wavelength for barium measurement due to the contrast.
04

Analyze Differences at Suggested Wavelengths

The wavelengths 611 nm and 641 nm clearly show discrimination between copper and barium absorptivities. At 611 nm, copper absorption is dominant, while at 641 nm, barium absorption is relatively higher. This difference aids in distinguishing and quantifying each metal in a mixture.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Arsenazo Complexes
Arsenazo is a type of organic dye that can form complexes with a variety of metal ions. These complexes help in quantifying and analyzing metals in solutions due to their specific absorption properties across distinct wavelengths. When metals like copper and barium form complexes with Arsenazo, the resulting complex shows a characteristic absorption spectrum. This means that depending on the metal involved, the Arsenazo complex will absorb light at different efficiencies and at specific wavelengths. The complexation aids in analytical techniques like spectrophotometry, where light absorption is measured and used for quantitative determinations. - Arsenazo's ability to create distinctive patterns of light absorption is what makes it so useful in chemical analysis. - The unique absorption patterns are due to the electronic interactions between the metal ions and the Arsenazo dye. Understanding the absorption characteristics of these complexes enables chemists to choose appropriate wavelengths for analysis, thereby allowing for efficient and accurate detection of metals in various mixtures.
Copper and Barium Analysis
Analyzing copper and barium requires careful selection of the wavelengths that allow us to differentiate the two metals based on their absorption properties. The molar absorptivity, or extinction coefficient (\( \varepsilon \)), provides the measure of the metal's ability to absorb light at a given wavelength. For copper and barium Arsenazo complexes:- Copper generally absorbs more strongly at shorter wavelengths, typically around 611 nm with an absorptivity of 19300.- Barium shows relatively higher absorption at wavelengths like 641 nm, with a molar absorptivity of 10500.The process involves measuring how much light is absorbed by the sample at selected wavelengths, highlighting:- High copper readings at wavelengths where its absorptivity significantly exceeds that of barium.- High barium readings where it stands out against copper. This differentiation allows chemists to not only detect the presence of each metal but also quantify their concentrations in a mixture. The key is selecting wavelengths that maximize the absorption differences, making the analysis clearer and more distinct.
Wavelength Selection for Analysis
Selecting the right wavelength for analysis is crucial in spectrophotometry as it determines the effectiveness of the measurement. For copper and barium Arsenazo complexes:1. **Copper Analysis**: At 611 nm, copper absorption (\( \varepsilon_{\text{Cu}} = 19300 \)) is significantly higher compared to barium (\( \varepsilon_{\text{Ba}} = 6900 \)). This high disparity makes 611 nm an ideal choice for measuring copper content in mixtures.2. **Barium Analysis**: At 641 nm, the situation is reversed. Here, barium's absorptivity of 10500 is notably higher than copper's 7500, making 641 nm optimal for barium analysis.Choosing these wavelengths is based on:- Looking for points where one metal's absorptivity is much higher than the other's.- Ensuring minimal cross-absorption, which leads to clearer distinction in measurements.With these selected wavelengths, researchers can effectively separate the signals from copper and barium, ensuring that each metal is accurately measured without interference from the other.

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Most popular questions from this chapter

The equilibrium constant for an acid-base indicator is determined by preparing three solutions, each of which has a total indicator concentration of \(1.35 \times 10^{-5} \mathrm{M}\). The \(\mathrm{pH}\) of the first solution is adjusted until it is acidic enough to ensure that only the acid form of the indicator is present, yielding an absorbance of \(0.673 .\) The absorbance of the second solution, whose \(\mathrm{pH}\) is adjusted to give only the base form of the indicator, is 0.118 . The \(\mathrm{pH}\) of the third solution is adjusted to 4.17 and has an absorbance of 0.439 . What is the acidity constant for the acid-base indicator?

Yan and colleagues developed a method for the analysis of iron based its formation of a fluorescent metal-ligand complex with the ligand 5-(4-methylphenylazo)-8-aminoquinoline. \({ }^{33}\) In the presence of the surfactant cetyltrimethyl ammonium bromide the analysis is carried out using an excitation wavelength of \(316 \mathrm{nm}\) with emission monitored at \(528 \mathrm{nm}\). Standardization with external standards gives the following calibration curve. $$ I_{f}=-0.03+\left(1.594 \mathrm{mg}^{-1} \mathrm{~L}\right) \times \frac{\mathrm{mg} \mathrm{Fe}^{3+}}{\mathrm{L}} $$ A 0.5113 -g sample of dry dog food is ashed to remove organic materials, and the residue dissolved in a small amount of \(\mathrm{HCl}\) and diluted to volume in a 50 -mL volumetric flask. Analysis of the resulting solution gives a fluorescent emission intensity of \(5.72 .\) Determine the \(\mathrm{mg} \mathrm{Fe} / \mathrm{L}\) in the sample of dog food.

Lozano-Calero and colleagues developed a method for the quantitative analysis of phosphorous in cola beverages based on the formation of the blue-colored phosphomolybdate complex, \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{PO}_{4}\left(\mathrm{MoO}_{3}\right)_{12}\right] .^{21}\) The complex is formed by adding \(\left(\mathrm{NH}_{4}\right)_{6} \mathrm{Mo}_{7} \mathrm{O}_{24}\) to the sample in the presence of a reducing agent, such as ascorbic acid. The concentration of the complex is determined spectrophotometrically at a wavelength of \(830 \mathrm{nm}\), using an external standards calibration curve. In a typical analysis, a set of standard solutions that contain known amounts of phosphorous is prepared by placing appropriate volumes of a 4.00 ppm solution of \(\mathrm{P}_{2} \mathrm{O}_{5}\) in a \(5-\mathrm{mL}\) volumetric flask, adding \(2 \mathrm{~mL}\) of an ascorbic acid reducing solution, and diluting to volume with distilled water. Cola beverages are prepared for analysis by pouring a sample into a beaker and allowing it to stand for \(24 \mathrm{~h}\) to expel the dissolved \(\mathrm{CO}_{2}\). A \(2.50-\mathrm{mL}\) sample of the degassed sample is transferred to a 50 -mL volumetric flask and diluted to volume. A \(250-\mu \mathrm{L}\) aliquot of the diluted sample is then transferred to a \(5-\mathrm{mL}\) volumetric flask, treated with \(2 \mathrm{~mL}\) of the ascorbic acid reducing solution, and diluted to volume with distilled water. (a) The authors note that this method can be applied only to noncolored cola beverages. Explain why this is true. (b) How might you modify this method so that you can apply it to any cola beverage? (c) Why is it necessary to remove the dissolved gases? (d) Suggest an appropriate blank for this method? (e) The author's report a calibration curve of $$ A=-0.02+\left(0.72 \mathrm{ppm}^{-1}\right) \times C_{\mathrm{P}_{2} \mathrm{O}_{5}} $$ A sample of Crystal Pepsi, analyzed as described above, yields an absorbance of \(0.565 .\) What is the concentration of phosphorous, reported as ppm \(\mathrm{P}\), in the original sample of Crystal Pepsi?

Saito describes a quantitative spectrophotometric procedure for iron based on a solid-phase extraction using bathophenanthroline in a poly(vinyl chloride) membrane. \({ }^{22}\) In the absence of \(\mathrm{Fe}^{2+}\) the membrane is colorless, but when immersed in a solution of \(\mathrm{Fe}^{2+}\) and \(\mathrm{I}^{-},\) the membrane develops a red color as a result of the formation of an \(\mathrm{Fe}^{2+}\) -bathophenanthroline complex. A calibration curve determined using a set of external standards with known concentrations of \(\mathrm{Fe}^{2+}\) gave a standardization relationship of $$ A=\left(8.60 \times 10^{3} \mathrm{M}^{-1}\right) \times\left[\mathrm{Fe}^{2+}\right] $$ What is the concentration of iron, in \(\mathrm{mg} \mathrm{Fe} / \mathrm{L},\) for a sample with an absorbance of 0.100 ?

The concentration of \(\mathrm{Na}\) in plant materials are determined by flame atomic emission. The material to be analyzed is prepared by grinding, homogenizing, and drying at \(103^{\circ} \mathrm{C}\). A sample of approximately \(4 \mathrm{~g}\) is transferred to a quartz crucible and heated on a hot plate to char the organic material. The sample is heated in a muffle furnace at \(550^{\circ} \mathrm{C}\) for several hours. After cooling to room temperature the residue is dissolved by adding \(2 \mathrm{~mL}\) of \(1: 1 \mathrm{HNO}_{3}\) and evaporated to dryness. The residue is redissolved in \(10 \mathrm{~mL}\) of \(1: 9 \mathrm{HNO}_{3},\) filtered and diluted to \(50 \mathrm{~mL}\) in a volumetric flask. The following data are obtained during a typical analysis for the concentration of \(\mathrm{Na}\) in a \(4.0264-\mathrm{g}\) sample of oat bran. $$ \begin{array}{lcc} {\text { sample }} & \mathrm{mg} \mathrm{Na} / \mathrm{L} & \text { emission (arbitrary units) } \\ \hline \text { blank } & 0.00 & 0.0 \\ \text { standard 1 } & 2.00 & 90.3 \\ \text { standard } 2 & 4.00 & 181 \\ \text { standard } 3 & 6.00 & 272 \\ \text { standard } 4 & 8.00 & 363 \\ \text { standard } 5 & 10.00 & 448 \\ \text { sample } & & 238 \end{array} $$ Report the concentration of sodium in the sample of oat bran as \mug Na/g sample.
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