Permeation Tubes for Nitric Acid Vapour Used to Calibrate Air Monitors

Trace concentration mixtures of nitric acid vapour are particularly difficult to prepare and store due to the aggressive chemical properties of nitric acid. Even with conventional permeation or diffusion tubes, establishing the nitric acid emission rate of the device can be problematic. Reagent grade nitric acid is relatively stable but contains thirty percent water and emission is the azeotrope. Pure nitric acid spontaneously decomposes even at room temperature liberating large amounts nitrogen dioxide. In either case, traceable weight-loss certification is not possible.

KIN-TEKs (USA) Trace Source Permeation Tubes use a mixture of reagent grade nitric and sulphuric acids as the analyte source. The sulphuric acid binds the water, reducing water emission to negligible levels. Emission of the sulphuric acid is extremely low compared to that of nitric acid, so the emission is primarily nitric acid. Formation of nitrogen oxides in the vapour is minimised by dynamically blending the emission to form trace concentration mixtures that are then used immediately.

Tubes are available for concentrations from low ppb to to about 10 ppm in 1 l/min dilution flow.