Problem 19 - Analytical Chemistry

Question

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.

Step-by-Step Solution

Answer

The sample concentration is approximately 88.23 mg/100 mL.

Step 1: Determine Absorbance Changes

Identify how the absorbance of the sample changes with pH adjustment. The sample absorbance decreases from 0.115 at pH 13 to 0.023 at pH 10. Similarly, the standard's absorbance decreases from 0.295 to 0.002.

Step 2: Calculate Absorbance Ratio for Sample

The absorbance ratio due to pH change for the sample is calculated as follows:\[\text{Ratio (Sample)} = \frac{0.023}{0.115} = 0.2\]

Step 3: Calculate Absorbance Ratio for Standard

Similarly, calculate the absorbance ratio for the standard:\[\text{Ratio (Standard)} = \frac{0.002}{0.295} \approx 0.0068\]

Step 4: Determining Barbital Concentration

Calculate the concentration of barbital in the sample by comparing the absorbance ratios:\[\text{Concentration Ratio} = \frac{0.2}{0.0068} \approx 29.41\]This ratio implies that the concentration of barbital in the sample is 29.41 times higher than that in the standard.

Step 5: Adjust for Standard Concentration

Convert the concentration ratio to mg barbital per 100 mL by multiplying with the standard concentration. Since the standard concentration is 3 mg/100 mL, the sample concentration is:\[\text{Sample Concentration} = 29.41 \times 3 \approx 88.23 \text{ mg/100 mL}\]

Key Concepts

Absorbance Measurement

Absorbance measurement is a crucial technique in analytical chemistry for determining the concentration of compounds like barbital. This process involves using a spectrophotometer, which measures how much light is absorbed by a solution at a specific wavelength. The higher the absorbance, the more concentrated the solution.

The exercise involves measuring the absorbance of barbital in blood at two different pH levels. Initially, at a pH of 13, an absorbance of 0.115 is recorded. When the pH is adjusted to 10, the absorbance drops to 0.023. This change indicates how barbital's absorption characteristics vary with pH, which can be used to deduce the concentration of barbital by comparing these values to a standard solution.

pH Adjustment

pH adjustment is an essential step in many chemical analyses because the solubility and absorbance of substances often depend on the pH of their environment. In this exercise, the pH of the barbital solution was modified twice: first, the solution was made highly basic (pH 13) using sodium hydroxide, and later, the pH was lowered to 10 using ammonium chloride.

Each pH level affects the ionization state of barbital, influencing its absorbance. By adjusting the pH, we can optimize the conditions for accurate measurement and ensure that the absorbance data reflects only the changes in barbital concentration.

Barbital Concentration Determination

Determining the concentration of barbital in a sample involves comparing its absorbance with that of a standard solution. In this problem, absorbance measurements were taken before and after pH adjustment for both the sample and a standard solution.

The changes in absorbance give insight into the concentration differences. By calculating the absorbance ratios for both the sample and the standard, the concentration ratio is determined. This ratio is then multiplied by the known concentration of the standard to find the concentration of barbital in the sample, yielding a result of approximately 88.23 mg per 100 mL.

Extraction Techniques

Extraction techniques are fundamental in isolating specific analytes from complex matrices, such as blood. In the scenario presented, chloroform was used to extract barbital from the blood sample.

These techniques rely on the differing solubility of substances in various solvents. Barbital, being more soluble in chloroform than in blood, is efficiently separated into the organic layer during extraction. This separation allows for subsequent analysis and quantification in a simpler mixture.

Chloroform Extraction

Chloroform extraction is a common method employed to separate compounds from aqueous solutions due to its effective solvating properties. In this exercise, it serves to isolate barbital from blood by mixing a specific volume of chloroform with the sample.

Following the extraction, barbital dissolves preferentially in chloroform, creating an organic layer that can be separated from the aqueous phase. This step is crucial for eliminating other components that may interfere with further analyses, like absorbance measurement.