An instrumentation system monitors ambient air to determine whether hydrazine vapor is present in sufficient concentration to be harmful to humans or equipment. The system can measure hydrazine concentrations as low as 10 parts per billion (ppb); this level of concentration is denoted the threshold limit value (TLV) in a revised safety standard proposed by the American Conference of Governmental Industrial Hygienists.
The system includes plumbing, electronic, and mechanical subsystems that function together to implement an electrochemical detection principle. The overall function of the system is to trap hydrazine from air in an acidic solution, adjust the pH to 10.2 for electrochemical detection, feed the solution to an electrochemical cell in a commercial process analyzer, and measure the electric current in the cell (see Figure 1).
The system includes a plastic sampling block, into which a small flow of dilute sulfuric acid is pumped. In the sampling block, the acid is dripped through an incoming flow of ambient air that could contain hydrazine vapor. The resulting mixture of air bubbles and acid is drawn from the sampling block into a sampling tube, wherein the prolonged air/acid contact results in scrubbing of hydrazine vapor from the air into the acid solution. The mixture is then drawn into a liquid/gas separator, from which the air is vented and the solution is sent for further processing.
A flow of dilute NaOH is mixed into the solution to raise the pH to >10.2, as required for the chosen electrochemical detection process. In this process, hydrazine is oxidized on the surface of a platinum anode in the reaction
N2H4+ 4OH¯ → 4H2O + N2 + 4e¯,
while water is reduced to hydrogen at a stainless-steel cathode in the reaction
4H2O + 4e¯ →2H2 + 4OH¯.
The electrochemical cell is operated in an amperometric mode; this means that the cell current is measured while the potential applied to the working electrode (the anode in this case) is held constant with respect to a reference electrode. The cell current is directly proportional to the concentration of hydrazine (see Figure 2); the constant of proportionality is established initially and verified from time to time by use of a commercial toxic-vapor-generator and flow-control equipment that generates a calibration flow of air containing a known concentration of hydrazine at known temperature and humidity.
The basic operational concentration range of the system, denoted the "TLV range," is 0 to 1,000 ppb. The system can also be operated in a range of 0 to 10 parts per million (ppm), denoted the "leak range," in which the sensitivity of detection is reduced by introducing stream of pure water to dilute the acid/hydrazine sample solution stream. Laboratory and field prototypes of the system have exhibited response times of 10 to 12 minutes in the TLV range and <2 minutes in the leak range.
The system includes reservoirs of concentrated H2SO4 and NaOH solutions and of deionized water. By use of automatic level-sensing and flow-control equipment, ingredients from these reservoirs are mixed as needed to obtain the dilute acidic and basic solutions for sampling and electrochemical detection. The reservoirs are sized to provide for continuous, unattended operation of the system for 3 months. To minimize the generation of waste, all effluent liquid streams generated by the system are cleaned of acidic, basic, and hydrazine residues by use of ion-exchange cartridges, then reused in the system.
This work was done by Dale Lueck of Kennedy Space Center and Barry J. Meneghelli, Clyde Parrish, and Ron Barile of Dynacs Engineering Co., Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 152on the TSP Order Card in this issue to receive a copy by mail ($5 charge).KSC-11920