Problem 39 - Analytical Chemistry

Question

A solution of \(5.00 \times 10^{-5} \mathrm{M} 1,3\) -dihydroxynaphthelene in \(2 \mathrm{M} \mathrm{NaOH}\) has a fluorescence intensity of 4.85 at a wavelength of \(459 \mathrm{nm}\). What is the concentration of 1,3 -dihydroxynaphthelene in a solution that has a fluorescence intensity of 3.74 under identical conditions?

Step-by-Step Solution

Answer

The concentration is \( 3.86 \times 10^{-5} \text{ M} \).

Step 1: Understanding the Problem

We are given the fluorescence intensity of a solution with a known concentration of 1,3-dihydroxynaphthelene and asked to find the concentration of another solution with a different fluorescence intensity under the same conditions.

Step 2: Identifying the Relationship

Fluorescence intensity is usually proportional to concentration within a certain range. This proportionality can be expressed as \( I = kC \), where \( I \) is the intensity, \( C \) is the concentration, and \( k \) is a proportionality constant.

Step 3: Calculate The Proportionality Constant \( k \)

Using the initial data, we know that \( 4.85 = k \times 5.00 \times 10^{-5} \). Solving for \( k \), we find:\[ k = \frac{4.85}{5.00 \times 10^{-5}} \]

Step 4: Calculate \( k \) Value

Compute \( k = \frac{4.85}{5.00 \times 10^{-5}} = 97000 \).

Step 5: Setting Up Equation for Unknown Concentration

For the unknown concentration \( C' \) with intensity 3.74, the equation is:\( 3.74 = 97000 \times C' \).

Step 6: Solve for Unknown Concentration \( C' \)

Rearrange to solve for \( C' : \)\[ C' = \frac{3.74}{97000} \]

Step 7: Compute Concentration \( C' \)

Perform the calculation:\( C' = \frac{3.74}{97000} = 3.86 \times 10^{-5} \text{ M} \).

Key Concepts

Proportionality Constant

In fluorescence spectroscopy, the relationship between fluorescence intensity and concentration is often linear. This can be mathematically expressed using the equation \(I = kC\), where \(I\) is the fluorescence intensity, \(C\) is the concentration of the fluorescent species, and \(k\) is the proportionality constant. This constant \(k\) represents the efficiency with which the fluorescent molecule converts absorbed light into emitted fluorescence.
For a given system, \(k\) may depend on factors such as the molecular structure of the fluorescent species and the solvent used. When working under identical conditions, this constant remains unchanged for a series of measurements. This allows us to use the known values to determine unknown concentrations, as demonstrated in the problem.
To find \(k\), you can rearrange the equation as \(k = \frac{I}{C}\). For instance, if the fluorescence intensity \(I\) is 4.85 for a concentration \(C\) of \(5.00 \times 10^{-5}\, \mathrm{M}\), \(k\) is calculated by dividing the intensity by concentration, resulting in \(k = 97000\). This \(k\) can then be used for subsequent calculations involving the same type of solution under the same conditions.

Concentration Calculation

Calculating the concentration of a substance using fluorescence spectroscopy relies on the linear relationship between fluorescence intensity and concentration. For a solution with an unknown concentration, you start with the known proportionality constant \(k\). Using the relationship \(I = kC\), you rearrange it to solve for the unknown concentration \(C'\): \(C' = \frac{I}{k}\).
In the given problem, the fluorescence intensity of another sample is 3.74. Knowing that the \(k\) value is 97000, you can find the unknown concentration \(C'\) by substituting the values: \(C' = \frac{3.74}{97000}\). When you perform this division, you get \(C' = 3.86 \times 10^{-5} \text{ M}\).
This calculation shows how you can determine the concentration of a solution by measuring its fluorescence intensity, provided you have a previously determined proportionality constant from a standard solution.

Fluorescence Intensity

Fluorescence intensity is a measure of the light emitted by a substance when it returns to its ground state after being excited by absorption of light. In fluorescence spectroscopy, intensity serves as a critical parameter for analyzing the concentration of fluorescent molecules in a solution.
The intensity depends on several factors including the concentration of the fluorescent molecules, the efficiency of the fluorescence process, and external conditions like the solvent and temperature. However, under controlled experimental conditions with consistent parameters, the intensity can be directly related to the concentration of the substance.
By maintaining consistent experimental conditions, fluorescence intensity measurements become a reliable tool for quantifying the concentration of substances. Thus, if you know the intensity of a solution with a known concentration, you can use it to calculate unknown concentrations of the same substance by proportion. This principle is illustrated by the given exercise, where the intensity measurement was used to derive the concentration of a solution.