Nitric acid collection, gaseous

Filters. HN03 is efficiently trapped out on nylon filters. Typically, two or more filters are connected in series. A schematic of such a filter pack was shown in Fig. 11.22 (Anlauf et al., 1988). A Teflon filter first removes particles from the airstream and a nylon filter then removes gaseous HNO,. In this particular system, a third filter (Whatman 41 impregnated with an aqueous solution of glycerol and citric acid) was used to trap NH3. After sample collection, each of the filters is extracted separately and nitrate, ammonium, and additional particle components collected on the Teflon filter are measured by ion chromatography. The sensitivity of this method for nitric acid and the other species is determined in part by filter blank values (i.e., nitrate on unexposed filters) and by the total amount collected and hence the sampling time used. Times of... 

Potential interferences in the measurement of nitric acid using this method include removal of gaseous HN03 on the Teflon particle filter and/or volatilization of particle nitrate collected on this Teflon filter. As discussed in Chapter 7, NH4N03 is a common particle component, but exists in equilibrium with gas-phase NH3 and HN03 ... 

Filter packs. As shown in Fig. 11.22, NH3 can be collected on impregnated filters in filter packs designed to collect particles and gas-phase nitric acid. Oxalic acid or citric acid on Whatman filters is often used to absorb the gaseous ammonia, which is then measured by extraction into aqueous solution and ion chromatography or by a colorimetric method (e.g., see Anlauf et al., 1988 and Williams et al., 1992). 

One procedure that is widely used to circumvent these complications is to remove ambient ammonia from the sampled air without removing particles by inserting one of several types of diffusion denuders upstream from the filter(s). In fact, in a recent Environmental Protection Agency (EPA)-sponsored intercomparison of methods for determination of strong acid content of aerosols, all but one protocol utilized an ammonia denuder (63), and all used an impactor or cyclone to remove coarse particles. The presence of this denuder clearly prevents neutralization of acidic aerosols by ammonia but also disturbs the gas-aerosol equilibrium between sulfate-nitrate aerosols and gaseous species. Ammonia and nitric acid are released from the depositing particles (64, 65) and must be collected downstream if accurate particulate ammonium and nitrate determinations are to be made. If equal amounts of ammonia and nitric acid are released, then the absolute [H+] (neq/m3) will not be altered. No specific evidence is available in the literature to demonstrate alteration of the observed [H+] as the result of reequilibration, but this area deserves further study. 

First prepd and described in 1934 by Cady (Ref 4) by passing fluorine, dild with N, 0 or air, thru a specially constructed app made of Ni, Pt and Monel, metal contg approx 3N nitric acid cooled to 0° or below. Hie resulting gaseous product was collected and analyzed. Later, Ruff et al (Ref 5) showed that... 

The second step is to dissolve the metal oxide fuel using strong nitric acid. The object is to bring all the fission products, uranium, and transuranics, into solution to feed the extraction process. Some of the fission products exceed solubility limits and the fine solids formed must be removed before extraction. Provisions to recover nitrogen oxides and collect gaseous fission products released during this step must be in place. The stainless steel and zircaloy fuel jackets from the fuel assemblies do not dissolve and are separated from the solution, washed, checked for radioactivity, and packaged for disposal as low-level radioactive waste. 

Smaller acidic sulphate particles may lose chloride and nitrate ions in the form of gaseous hydrochloric and nitric acid. Thus, the chloride in airborne sea salt may be driven off as hydrochloric acid, which may be subsequently absorbed by larger, less acidic particles. Similar chemical reactions can also take place in samples of particles collected on filters, particularly if the coarse and fine particles are not separated. The pressure drop across the filter may also cause evaporation of the more volatile components. The chemical analysis of the collected particles may then give a distorted picture of the true airborne composition of the aerosol. 

The results of Spicer and Schumacher (1977) suggest that alkaline sites on many filters collect gaseous nitric acid and cause it to be reported as particulate nitrate. 

---