Paramagnetism vs diamagnetism in continuous flue-gas analysis A continuous flue-gas analyzer that uses magnetism relies on paramagnetic components moving toward the strongest magnetic field, displacing diamagnetic gases. Which of the following gases is diamagnetic?

Introduction / Context:
Some continuous emissions monitoring systems (CEMS) measure oxygen by exploiting magnetic properties: oxygen is strongly paramagnetic and is attracted to a magnetic field. In such instruments, diamagnetic components are displaced. To interpret the readings and understand interferences, you should know which common flue-gas constituents are para- or diamagnetic.

Given Data / Assumptions:

• Paramagnetic gases are attracted to magnetic fields.
• Diamagnetic gases are weakly repelled.
• Typical stack gases include O2, CO2, NO, NO2, N2, H2O (vapour), etc.

Concept / Approach:
Molecular oxygen (O2) is a classic paramagnetic gas due to its unpaired electrons. Nitric oxide (NO) and nitrogen dioxide (NO2) also have unpaired electrons and are paramagnetic. Carbon dioxide (CO2), with all electrons paired and a linear, closed-shell electronic configuration, is diamagnetic. Thus, in paramagnetic analyzers, CO2 behaves as the diamagnetic component displaced by O2’s magnetically induced motion.

Step-by-Step Solution:

Verification / Alternative check:
Instrument manuals for paramagnetic O2 analyzers list CO2, N2, and others as diamagnetic, while O2 is the paramagnetic target species.

Why Other Options Are Wrong:

• O2 / NO / NO2: All paramagnetic, not diamagnetic.

Common Pitfalls:
Assuming all oxides are diamagnetic; many nitrogen oxides are paramagnetic due to odd-electron configurations.

Final Answer:
CO2