Answer to Question #85340 in General Chemistry for Damola Olawepo

Question

The signal from a chemiluminescene analyser for NO is proportional to the concentrations. The instrument gave a signal of 135uA for an air sample that was passed over a thermal converter and 49uA with the converter out of the stream. A standard sample containing 0.23A gave a signal of 80uA. What is the concentration of No2 in the sample?

Answer 1

The chemiluminescence – reference method for the measurement of nitrogen dioxide (NO2) and oxides of nitrogen (NOx) is based on the reaction of nitric oxide (NO) with Ozone (O3): NO molecules react with O3 to form excited NO2* molecules. If the volumes of sample gas and excess ozone are carefully controlled, the light level in the reaction chamber is proportional the concentration of NO2 in the gas sample.

NO + O3 -> NO2* + O2

The excited NO2* molecules returns to their fundamental state by emitting a light radiation “hv”, measured by the instrument: :

NO2* -> NO2 + hv.

With a chemiluminescence based analyser, NO2 measurement is the result of a calculation process cycle as follow:

• In a first time, sample gas goes straight to the reaction chamber where NO will react with O3 to give the “NO measure”

• In a second time, sample gas pass through a heated catalytic converter where in addition to the NO present in the sample gas, NO2 will be reduced in NO before going to the reaction chamber for the reaction with O3. It gives the “NOx measure”

• Calculation of the NO2 concentration (NO2 = NOx – NO)