Substrates coated with a precious metal salt KAuCl4 have been found to be useful for detecting hydrazine vapors in air at and above a concentration of the order of 0.01 parts per million (ppm). Upon exposure to air containing a sufficient amount of hydrazine for a sufficient time, the coating material undergoes a visible change in color. Although the color change is only a qualitative indication, it can serve as an alarm of a hazardous concentration of hydrazine or as advice of the need for a quantitative measurement of concentration. Detection of hydrazine vapors by this technique costs much less and takes less time than does laboratory analysis of sorbent tubes using high-performance liquid chromatography, which is the technique used heretofore to detect hydrazines at concentrations down to 0.01 ppm.

A substrate for use in this technique should be made of a chemically inert material (e.g., fiberglass filter paper). The substrate is uniformly coated with 1 to 10 weight percent of the preciousmetal salt in a solvent (e.g., dilute HCl) that does alter the physical characteristics of the substrate. After driving off the solvent by gentle heating and/or by use of a vacuum, the coated substrate is packed into an inert tube with openings at each end. (The dried preciousmetal coating is somewhat sensitive to light; the dried coated substrate should be handled accordingly and stored in the dark.)

The coated substrate is held in place with small quantities of inert wadding (i.e., borosilicate glass wool). A gas suction pump is attached to one end of the tube, and the air or other gas suspected to contain hydrazine vapor is drawn through the tube at a specified pumping rate for an amount of time sufficient to obtain a sufficient chemical change (and thus an observable color change) in the coating material. A semiquantitative relationship between the degree of chemical change and the quantity of vapor sampled can be established from observations of intensities of color changes and/or areas of color change in tests on similarly prepared substrates and tubes using known concentrations of hydrazine vapors.

In experiments, tubes containing KAuCl4-coated substrates prepared as described above were exposed to 40-liter flows of air containing, variously, hydrazine, monomethylhydrazine, or unsymmetrical dimethylhydrazine at concentrations of the order of 0.01 ppm. These exposures caused the colors of the substrates to change from yellow to various purplish colors and, in one case, to black.

No such color changes were observed upon exposure of the KAuCl4-coated substrates to flows of air that contained other gases (ammonia, isopropyl alcohol, NO2, and H2). Whether or not other precious-metal coating materials could be used as color-change indicators of collecthese or other nonhydrazine gases remains to be determined.

This work was done by Louis A. Dee and Benjamin Greene of Allied-Signal Aerospace Co. for Johnson Space Center. For further information, contact the Johnson Commercial Technology Office at (281) 483-3809. MSC-22870

NASA Tech Briefs Magazine

This article first appeared in the December, 2004 issue of NASA Tech Briefs Magazine.

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