/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 42 Selenium (IV) in natural waters ... [FREE SOLUTION] | 91影视

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Chapter 10: Problem 42

Selenium (IV) in natural waters is determined by complexing with ammonium pyrrolidine dithiocarbamate and extracting into \(\mathrm{CHCl}_{3}\). This step serves to concentrate the \(\mathrm{Se}(\mathrm{IV})\) and to separate it from \(\mathrm{Se}(\mathrm{VI})\). The \(\mathrm{Se}(\mathrm{IV})\) is then extracted back into an aqueous matrix using \(\mathrm{HNO}_{3} .\) After complexing with 2,3 -diaminonaphthalene, the complex is extracted into cyclohexane. Fluorescence is measured at \(520 \mathrm{nm}\) following its excitation at \(380 \mathrm{nm}\). Calibration is achieved by adding known amounts of \(\mathrm{Se}(\mathrm{IV})\) to the water sample before beginning the analysis. Given the following results what is the concentration of \(\mathrm{Se}(\mathrm{IV})\) in the sample. \begin{tabular}{cc} {\([\) Se (IV)] added (nM) } & emission intensity \\ \hline 0.00 & 323 \\ 2.00 & 597 \\ 4.00 & 862 \\ 6.00 & 1123 \end{tabular}

Short Answer

Expert verified
Concentration found using the equation: \( y = 137x + 323 \).

Step by step solution

01

Understand the Experiment

The experiment involves analyzing the concentration of Selenium (IV) in water by using a fluorescent method. The sample undergoes various steps, including complexation, extraction, and fluorescence measurement. Calibration is done with known concentrations of Selenium.
02

Create a Calibration Curve

This will involve plotting the given concentration of Se(IV) that has been added ( M in nM) against the emission intensity. The provided data are: (0.00, 323), (2.00, 597), (4.00, 862), (6.00, 1123).
03

Calculate the Calibration Line Equation

Using the data points M (0.00, 323), (2.00, 597), (4.00, 862), and (6.00, 1123), we fit a linear equation. The line equation has the general form: \( y = mx + c \)Performing calculations to find the slope \( m \) and the y-intercept \( c \), we start with:- Slope \( m = \frac{y_2 - y_1}{x_2 - x_1} = \frac{597 - 323}{2.00 - 0.00} = 137 \)This process suggests a linear relationship given by the slope (137). Now, for the intercept, we'll use one of the data points: - Use \( (0.00, 323) \), thus the intercept is \( c = 323 \)Thus, the linear equation is: \( y = 137x + 323 \)
04

Determine the Concentration of Se(IV)

Find the concentration of Se(IV) using the calibration line equation we derived: \( y = 137x + 323 \). Since the emission intensity in this context is y, if we have a specific intensity value we are trying to calculate for, we solve for \( x \):Rearrange the formula to find \( x \) as follows: \( x = \frac{y - 323}{137} \) Assume we want to find the unknown concentration corresponding to an unknown emission intensity to observe relativity.
05

Interpreting Results

By plugging intensity values into the formula \( x = \frac{y - 323}{137} \), other concentrations (as needed) can be deduced based on their respective measured emission intensities.

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Key Concepts

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

Selenium Analysis
Selenium analysis in natural waters is critical due to its relevance in environmental monitoring and public health. Selenium exists primarily in two oxidation states in water: Selenium (IV) and Selenium (VI).
Selenium (IV) is significant as it is more bioavailable and potentially toxic than the other states.
Determining its concentration involves a detailed process where we initially focus on isolating Selenium (IV) from other components present in water.
This requires complexation鈥攁 chemical procedure that stabilizes its structure.
The next step is precision extraction to understand its exact levels in natural water.

Fluorescence spectroscopy is often used in this analysis due to its sensitivity in detecting low concentration levels. By measuring the fluorescence at specific wavelengths, analysts can accurately determine how much Selenium (IV) is present.
This entire process helps in assessing environmental impacts and ensuring water safety to meet regulatory standards.
Complexation and Extraction
In the context of Selenium (IV) analysis, complexation and extraction are crucial steps for accurate measurement. Complexation involves the formation of a complex molecule, where Selenium (IV) binds with other chemical agents to form a stable compound.
One common method uses ammonium pyrrolidine dithiocarbamate to form a complex with Selenium (IV).
Once the selenium is complexed, it is extracted into solvents like chloroform ( CHCl_3 ), ensuring its separation from Selenium (VI).
After this extraction, it is back-extracted into an aqueous solution using HNO_3 .
This method is carefully designed to enhance the concentration of Selenium (IV), improve its detectability and separate it cleanly from any other forms of selenium present.

Next, the complex undergoes another extraction with cyclohexane after interacting with 2,3-diaminonaphthalene.
This chemical reaction prepares the selenium for precise fluorescence measurement.
The complexity of these steps ensures that only the desired form of selenium, Selenium (IV), is measured, thus providing reliable results.
This technique illustrates the intricate yet critical role that chemical manipulation plays in accurate selenium analysis.
Calibration Curve
A calibration curve is a graphical representation used to deduce the concentration of an unknown sample by comparing it to a series of known concentrations. It is essential in the spectroscopic determination of Selenium (IV) in water samples.
To create a calibration curve, known concentrations of Selenium (IV) are added to samples, and their fluorescence emission is recorded.
In the provided exercise, the calibration curve is plotted with concentration on the x-axis and emission intensity on the y-axis.
This plotted data helps derive a linear relationship鈥攊n this case, the mathematical equation of the line is found to be \( y = 137x + 323 \).
This equation is crucial as it allows determination of unknown Selenium (IV) concentrations by rearranging the formula to solve for x\.
Simply inputting the emission intensity of an unknown sample into this equation gives the concentration.

Implementing a calibration curve ensures high accuracy in measurements, vital for consistent and reliable environmental analysis.
  • Helps visualize the linear relation between concentration and intensity.
  • Assists in estimating unknown concentrations with confidence.
  • Ensures the process is reproducible in different lab environments.
This makes calibration a foundational tool in chemical analysis through fluorescence spectroscopy.

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Most popular questions from this chapter

The stoichiometry of a metal-ligand complex, \(\mathrm{ML}_{n}\), is determined by the method of continuous variations. A series of solutions is prepared in which the combined concentrations of \(\mathrm{M}\) and \(\mathrm{L}\) are held constant at \(5.15 \times 10^{-4} \mathrm{M}\). The absorbances of these solutions are measured at a wavelength where only the metal-ligand complex absorbs. Using the following data, determine the formula of the metal-ligand complex. $$ \begin{array}{ccc} \text { mole fraction } \mathrm{M} & \text { mole fraction } \mathrm{L} & \text { absorbance } \\ \hline 1.0 & 0.0 & 0.001 \\ 0.9 & 0.1 & 0.126 \\ 0.8 & 0.2 & 0.260 \\ 0.7 & 0.3 & 0.389 \\ 0.6 & 0.4 & 0.515 \\ 0.5 & 0.5 & 0.642 \\ 0.4 & 0.6 & 0.775 \\ 0.3 & 0.7 & 0.771 \\ 0.2 & 0.8 & 0.513 \\ 0.1 & 0.9 & 0.253 \\ 0.0 & 1.0 & 0.000 \end{array} $$

A second instrumental limitation to Beer's law is stray radiation. The following data were obtained using a cell with a pathlength of \(1.00 \mathrm{~cm}\) when stray light is insignificant \(\left(P_{\text {strav }}=0\right)\). $$ \begin{array}{cc} \text { [analyte] }(\mathrm{mM}) & \text { absorbance } \\ \hline 0.00 & 0.00 \\ 2.00 & 0.40 \\ 4.00 & 0.80 \\ 6.00 & 1.20 \\ 8.00 & 1.60 \\ 10.00 & 2.00 \end{array} $$ Calculate the absorbance of each solution when \(P_{\text {stray }}\) is \(5 \%\) of \(P_{0},\) and plot Beer's law calibration curves for both sets of data. Explain any differences between the two curves. (Hint: Assume \(P_{0}\) is \(\left.100\right)\).

The concentration of the barbiturate barbital in a blood sample is determined by extracting \(3.00 \mathrm{~mL}\) of blood with \(15 \mathrm{~mL}\) of \(\mathrm{CHCl}_{3}\). The chloroform, which now contains the barbital, is extracted with \(10.0 \mathrm{~mL}\) of \(0.45 \mathrm{M} \mathrm{NaOH}(\mathrm{pH} \approx 13)\). A 3.00-mL sample of the aqueous extract is placed in a 1.00 -cm cell and an absorbance of 0.115 is measured. The \(\mathrm{pH}\) of the sample in the absorption cell is then adjusted to approximately 10 by adding \(0.50 \mathrm{~mL}\) of \(16 \% \mathrm{w} / \mathrm{v} \mathrm{NH}_{4} \mathrm{Cl}\), giving an absorbance of 0.023 . When \(3.00 \mathrm{~mL}\) of a standard barbital solution with a concentration of \(3 \mathrm{mg} / 100 \mathrm{~mL}\) is taken through the same procedure, the absorbance at \(\mathrm{pH} 13\) is 0.295 and the absorbance at a \(\mathrm{pH}\) of 10 is 0.002. Report the mg barbital/100 mL in the sample.

The concentration of acetylsalicylic acid, \(\mathrm{C}_{9} \mathrm{H}_{8} \mathrm{O}_{4},\) in aspirin tablets is determined by hydrolyzing it to the salicylate ion, \(\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{2}^{-},\) and determining its concentration spectrofluorometrically. A stock standard solution is prepared by weighing \(0.0774 \mathrm{~g}\) of salicylic acid, \(\mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}_{2}\), into a 1-L volumetric flask and diluting to volume. A set of calibration standards is prepared by pipeting \(0,2.00,4.00,6.00,8.00,\) and 10.00 \(\mathrm{mL}\) of the stock solution into separate \(100-\mathrm{mL}\) volumetric flasks that contain \(2.00 \mathrm{~mL}\) of \(4 \mathrm{M} \mathrm{NaOH}\) and diluting to volume. Fluorescence is measured at an emission wavelength of \(400 \mathrm{nm}\) using an excitation wavelength of \(310 \mathrm{nm}\) with results shown in the following table. $$ \begin{array}{cc} \text { mL of stock solution } & \text { emission intensity } \\ \hline 0.00 & 0.00 \\ 2.00 & 3.02 \\ 4.00 & 5.98 \\ 6.00 & 9.18 \\ 8.00 & 12.13 \\ 10.00 & 14.96 \end{array} $$ Several aspirin tablets are ground to a fine powder in a mortar and pestle. A 0.1013 -g portion of the powder is placed in a 1-L volumetric flask and diluted to volume with distilled water. A portion of this solution is filtered to remove insoluble binders and a 10.00 -mL aliquot transferred to a 100 -mL volumetric flask that contains \(2.00 \mathrm{~mL}\) of \(4 \mathrm{M}\) \(\mathrm{NaOH}\). After diluting to volume the fluorescence of the resulting solution is 8.69 . What is the \(\% \mathrm{w} / \mathrm{w}\) acetylsalicylic acid in the aspirin tablets?

A chemical deviation to Beer's law may occur if the concentration of an absorbing species is affected by the position of an equilibrium reaction. Consider a weak acid, HA, for which \(K_{\mathrm{a}}\) is \(2 \times 10^{-5}\). Construct Beer's law calibration curves of absorbance versus the total concentration of weak acid \(\left(C_{\text {total }}=[\mathrm{HA}]+\left[\mathrm{A}^{-}\right]\right),\) using values for \(C_{\text {total }}\) of \(1.0 \times 10^{-5}, 3.0 \times 10^{-5}, 5.0 \times 10^{-5}, 7.0 \times 10^{-5}, 9.0 \times 10^{-5}, 11 \times 10^{-5}\), and \(13 \times 10^{-5} \mathrm{M}\) for the following sets of conditions and comment on your results: (a) \(\varepsilon_{\mathrm{HA}}=\varepsilon_{\mathrm{A}^{-}}=2000 \mathrm{M}^{-1} \mathrm{~cm}^{-1} ;\) unbuffered solution. (b) \(\varepsilon_{\mathrm{HA}}=2000 \mathrm{M}^{-1} \mathrm{~cm}^{-1} ; \varepsilon_{\mathrm{A}^{-}}=500 \mathrm{M}^{-1} \mathrm{~cm}^{-1} ;\) unbuffered solution. (c) \(\varepsilon_{\mathrm{HA}}=2000 \mathrm{M}^{-1} \mathrm{~cm}^{-1} ; \varepsilon_{\mathrm{A}^{-}}=500 \mathrm{M}^{-1} \mathrm{~cm}^{-1} ;\) solution buffered to a \(\mathrm{pH}\) of 4.5 Assume a constant pathlength of \(1.00 \mathrm{~cm}\) for all samples.
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