Question

Janusa and coworkers describe the determination of chloride by CZE. \(^{29}\) Analysis of a series of external standards gives the following calibration curve. $$ \text { area }=-883+5590 \times \text { ppm } \mathrm{Cl}^{-} $$ A standard sample of \(57.22 \% \mathrm{w} / \mathrm{w} \mathrm{Cl}^{-}\) is analyzed by placing \(0.1011-\mathrm{g}\) portions in separate \(100-\mathrm{mL}\) volumetric flasks and diluting to volume. Three unknowns are prepared by pipeting \(0.250 \mathrm{~mL}, 0.500 \mathrm{~mL},\) and \(0.750 \mathrm{~mL}\) of the bulk unknown in separate \(50-\mathrm{mL}\) volumetric flasks and diluting to volume. Analysis of the three unknowns gives areas of \(15310,31546,\) and \(47582,\) respectively. Evaluate the accuracy of this analysis.

Short answer

The analysis shows chloride concentrations very close to 578 ppm, indicating reasonable accuracy with slight variation.

Step-by-step solution

Calculate the Chloride Concentration of the Standard

First, we need to determine how much actual chloride is in the standard solution. We start with 0.1011 g of the substance, which is 57.22% chloride by weight. To find the weight of chloride:\[(0.1011 \text{ g}) \times 0.5722 = 0.0578 \text{ g of } Cl^-\]Since the solution is diluted to 100 mL:\[\text{ppm of } Cl^- = \frac{0.0578 \text{ g} \times 1000}{100 \text{ mL}}= 578 \text{ ppm}\]

Determine Chloride Concentrations for Each Unknown Solution

We have three samples with different amounts of the bulk unknown and varying volumes. First, calculate the chloride concentration for each unknown based on the given calibration curve.For the first unknown (area = 15310):\[15310 = -883 + 5590 \times \text{ppm }\text{ppm} = \frac{15310 + 883}{5590} = 2.88 \text{ ppm}\]For the second (area = 31546):\[31546 = -883 + 5590 \times \text{ppm}\text{ppm} = \frac{31546 + 883}{5590} = 5.77 \text{ ppm}\]For the third (area = 47582):\[47582 = -883 + 5590 \times \text{ppm}\text{ppm} = \frac{47582 + 883}{5590} = 8.69 \text{ ppm}\]

Back-Calculation to Determine Accuracy of Analysis

Use the prepared volumes and resulting concentrations to evaluate the accuracy. Each unknown is made from a bulk unknown, so calculate the relative concentration based on the 50 mL volumetric flask. - For 0.250 mL of bulk: \[\text{ppm}_{\text{bulk}} = 2.88 \text{ ppm} \times \frac{50 \text{ mL}}{0.250 \text{ mL}} = 576 \text{ ppm}\]- For 0.500 mL: \[\text{ppm}_{\text{bulk}} = 5.77 \text{ ppm} \times \frac{50 \text{ mL}}{0.500 \text{ mL}} = 577 \text{ ppm}\]- For 0.750 mL: \[\text{ppm}_{\text{bulk}} = 8.69 \text{ ppm} \times \frac{50 \text{ mL}}{0.750 \text{ mL}} = 579 \text{ ppm}\]Compare against the standard concentration value of 578 ppm. The results show that the method's accuracy is reasonable with slight variation around the expected value.

Key concepts

Chloride Determination

Chloride determination in capillary zone electrophoresis (CZE) relies on understanding the chloride content within a given sample. In this exercise, the sample begins with 0.1011 g containing 57.22% chloride by weight.
By multiplying, we find the actual weight of chloride to be 0.0578 g. Diluting this into 100 mL gives us a chloride concentration of 578 ppm. Chloride determination is integral to various chemical analyses because chloride ions are prevalent in many chemical and biological systems. The goal here is to accurately gauge the presence of these ions within a solution. CZE plays a role by separating ions in a sample, making it easier for analysts to pinpoint the concentration of chloride present. This is crucial for ensuring consistency in operations like food safety testing, water quality assessments, or any process where chloride concentration is monitored.

Calibration Curve

A calibration curve in the context of CZE is a valuable tool for translating experimental data into meaningful analysis outcomes. In this exercise, the calibration curve provided is: \[ \text{area} = -883 + 5590 \times \text{ppm} \ Cl^{-} \]This equation symbolizes the relationship between the measured area in the electrophoresis and the concentration of chloride ions present.
When you know the area, you can calculate ppm, which tells you the parts per million of chloride in your sample.Creating an accurate calibration curve is vital. It requires analysis of standards with known concentrations and plotting the response, such as the area under a peak from an electropherogram. When unknown samples are assessed, their response can be matched to this curve to gauge concentration levels.

• The calibration curve must be linear over the concentration range of interest for accuracy.
• Regular checks and recalibrations ensure validity since equipment may drift over time.

Calibration curves are a staple in quantitative analysis, converting data into a form that speaks to real-world concentrations.

Quantitative Analysis

In quantitative analysis, the aim is to determine the quantity or concentration of a substance in a sample. In this case, we're quantitatively analyzing chloride ions using CZE and calibration curve-derived equations.
Using the equation from our calibration curve, we compute the concentrations for unknowns by solving for ppm based on the area values obtained from CZE. Calculations for three samples gave ppm values of 2.88, 5.77, and 8.69 respectively. Such results help identify the bulk unknown's chloride concentration when volumes are back-calculated. Quantitative analysis facilitates the understanding of sample content that is necessary when scrutinizing substances for compliance with regulatory standards.

• Exactness is pivotal; even small inaccuracies can lead to significant deviations in critical fields like pharmaceuticals or environmental monitoring.
• It links experimental data to tangible quantities much needed for reliable and usable results.

It's about transforming qualitative observations into numbers we can compare, evaluate, and rely upon.