91Ó°ÊÓ

A few decades ago benzene was thought to be a harmless chemical with a somewhat pleasant odor and was widely used as a cleaning solvent. It has since been found that chronic exposure to benzene can causc health problems such as anemia and possibly leukemia. Benzene has an OSHA permissible exposure level (PEL) of 1.0 ppm (part per million on a molar basis, equivalent to a mole fraction of \(1.0 \times 10^{-6}\) ) averaged over an 8 -hour period. The safcty engincer in a plant wishes to determine whether the benzene concentration in a laboratory exceeds the PEL. One Monday at 9 a.m., 1 p.m., and 5 p.m., she collects samples of room air \(\left(33^{\circ} \mathrm{C}, 99 \mathrm{kPa}\right)\) in evacuated 2 -liter stainless steel containers. To collect a sample she opens the container valve, allows room air to enter until the container pressure equals atmospheric pressure, and then charges clean dry helium into the container until the pressure reaches 500 kPa. Next, she takes the containers to an analytical laboratory in which the temperature is \(23^{\circ} \mathrm{C}\), leaves them there for a day. and then feeds gas from each container to a gas chromatograph (GC) until the pressure in the container is reduced to \(400 \mathrm{kPa}\). In the order in which they were collected, the samples that pass through the GC are found to contain \(0.656 \mu \mathrm{g}\) (microgram), \(0.788 \mu \mathrm{g},\) and \(0.910 \mu \mathrm{g}\) of benzene, respectively. (a) What were the concentrations of benzene (ppm on a molar basis) in the original room air at the three collection times? (Assume ideal-gas behavior.) Is the average concentration below the PEL? (b) Why did the engineer add helium to the container after collecting the room air sample? Why did she wait a day before analyzing the container contents? (c) Why might a finding that the average benzene concentration is below the PEL not necessarily mean that the laboratory is safe insofar as exposure to benzene is concerned? Give several reasons, including possible sources of error in the sampling and analysis procedure. (Among other things, note the day on which the samples were taken.)

Step by step solution

01

Concentration Calculation

To calculate the concentration of benzene at each point in time, we will use the ideal gas law, \(PV = nRT\) where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature. By using this equation twice and some conversion factors, it's possible to find the number of grams per litre of air at each point in time which can be converted to ppm (parts per million). The average concentration can then be found by simply averaging these three values.

02

Purpose of Helium and Waiting

The safety engineer added helium to make sure there is sufficient pressure for the samples to reach the chromatograph from the containers. Waiting for a day gives the gases ample time to mix fully, ensuring a homogeneous sample for analysis.

03

Interpretation of Results

Although an average concentration below the permissible exposure limit (PEL) might initially seem safe, several factors could make the situation hazardous. For example, localised high concentrations, fluctuating concentrations, and exposure exceeding the 8-hour limit set by the PEL. Additionally, errors could arise from the method of sample collection, uneven mixing of gases, or inaccuracies in the gas chromatograph reading.

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Key Concepts

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

Benzene Exposure

Benzene is a volatile organic compound that was once thought safe but is now known to pose significant health risks. Chronic exposure to benzene can lead to severe health issues, including anemia and potentially leukemia.
It behaves as a carcinogen, which means it can contribute to the development of cancer over time. This is why monitoring benzene exposure is critical, especially in industrial and laboratory settings where benzene may still be used.

• Health Impacts: Persistent exposure can disrupt blood cell production, leading to various forms of blood disorders.
• Occupational Risk: Workers in industries using benzene must be particularly cautious and adhere to strict safety protocols.

Understanding benzene's impact is crucial for implementing protective measures and ensuring workplace safety.

Permissible Exposure Limit (PEL)

The PEL for benzene, set by OSHA, is 1.0 ppm. This limit signifies the maximum concentration of benzene in the air that employees can be exposed to during a standard 8-hour workday.
It's a measure to help prevent adverse health effects by regulating exposure levels.

• Standard Measurement: PEL is measured in parts per million ( ext{ppm}) on a molar basis, translating to a mole fraction of \(1.0 \times 10^{-6}\).
• Compliance is Key: Organizations are required to monitor and mitigate exposure levels to stay below this limit, ensuring a healthy working environment.

Adhering to these limits helps protect workers from long-term health consequences associated with benzene exposure.

Ideal Gas Law

In this exercise, the Ideal Gas Law (\(PV = nRT\)) is pivotal for calculating benzene concentrations at specific times. This law relates the pressure (P), volume (V), number of moles (n), the ideal gas constant (R), and temperature (T).
It's an essential tool in deriving the concentration of gases under ideal conditions.

• Variable Analysis: Adjusting for the conditions at the time of sampling helps in determining the number of moles of benzene present.
• Applications: By knowing the pressure and volume where benzene was measured, one can calculate the amount present using this formula.

This principle aids in quantifying gas concentrations in a given volume, allowing for precise monitoring and control.

Gas Chromatography

Gas Chromatography (GC) is an analytical method used to separate and analyze compounds from gaseous mixtures. In this case, it helps in identifying and quantifying benzene within collected air samples.
The process involves injecting the sample into a GC system, where components are separated based on their volatility.

• Precision Analysis: GC provides accurate readings of benzene since it separates it from other gases in the sample.
• Ensures Safety: Reliable results from GC help assess whether benzene levels exceed the PEL, crucial for making informed safety decisions.

Therefore, Gas Chromatography is indispensable for continuous monitoring of air quality and maintaining compliance with safety standards.

Sampling Error Analysis

Even sophisticated methods can have errors. Sampling for benzene, as highlighted in this exercise, involves potential sources of error that need consideration. These errors stem from the collection, transfer, and analysis stages.
Understanding these can help improve the reliability of benzene concentration measurements.

• Collection Errors: If a method is inconsistent or incomplete, it can result in inaccurate representation of air samples.
• Analysis Errors: Instrumentation and calibration discrepancies in the GC readings may introduce errors.
• Time-Related Variability: Sampling on a particular day doesn’t account for fluctuations over a longer period; daily variance can affect exposure assessments.

Recognizing and addressing these errors is essential for ensuring accurate reporting of environmental hazards and planning effective mitigation strategies.