Which reactions take place near the sensing electrode of an electrochemical NO analyzer?

a. 2H2O ? 4H+ + 4e– + O2 c. 4NO + 2H2O + 2O2 ? 4HNO3
b. O2 + 4H+ + 4e– ? 2H2O d. NO + 2H2O ? HNO3 + 3H+ + 3e–

ANS: D
An electrochemical NO analyzer consists of three electrodes immersed in an electrolyte solution that contains a highly conductive concentrated acid or alkali solution. The electrodes are sepa-rated from the gas sample to be analyzed by a semipermeable membrane. NO and NO2, from the unknown gas sample, diffuse across a semipermeable membrane and react with the electrolyte solution near the sensing electrode. This generates electrons in the oxidation reaction: NO + 2H2O → HNO3 + 3H+ + 3e–.