Nitric acid measurement techniques

Several formal and informal intercomparisons of nitric acid measurement techniques have been carried out (43-46) these intercomparisons involve a multitude of techniques. The in situ measurement of this species has proven difficult because it very rapidly absorbs on any inlet surfaces and because it is involved in reversible solid-vapor equilibria with aerosol nitrate species. These equilibria can be disturbed by the sampling process these disturbances lead to negative or positive errors in the determination of the ambient vapor-phase concentration. The intercomparisons found differences of the order of a factor of 2 generally, and up to at least a factor of 5 at levels below 0.2 ppbv. These studies clearly indicate that the intercompared techniques do not allow the unequivocal determination of nitric acid in the atmosphere. A laser-photolysis, fragment-fluorescence method (47) and an active chemical ionization, mass spectrometric technique (48) were recently reported for this species. These approaches may provide more definite specificity for HN03. Challenges clearly remain in the measurement of this species. 

Whole ash analyses were performed using flameless atomic absorption spectrophotometric techniques. The lithium metaborate—nitric acid fusion techniques was used in the preparation of the sample for analysis of Ca, Mg, Na and K. Hydrofluoric—nitric acid dissolution was employed in the preparation of the samples for measurement of Fe, Mn, Cu, Pb, Zn and Cd. 

Application of gas-liquid chromatography (GLC) to determine selenium in biological samples allows for the elimination of interference from the biological matrix. GLC requires prior decomposition of organic matter with nitric acid. GLC techniques are based on measurement of the amount of piazselenol formed... 

Other Methods. Ion chromatography using conductance detection can be used to measure low (<1%) levels of nitrite, chloride, sulfate, and other ions in nitric acid. Techniques for ion chromatographic analysis are available (93). 

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

A number of workers have described methods for the determination of mercury in which the mercury is first reduced to the element or collected as the sulfide on a cadmium sulfide pad. It is then volatilized into a chamber for measurement. These techniques are extremely sensitive. Thillez108) recently described a procedure for urinary mercury in which the mercury is collected on platinum and then volatilized into an air stream. Rathje109) treated 2 ml of urine with 5 ml of nitric acid for 3 min, diluted to 50 ml, and added stannuous chloride to reduce the mercury to the element. A drop of Antifoam 60 was added and nitrogen was blown through the solution to carry the mercury vapor into a quartz end cell where it is measured. Six nanograms of mercury can be detected. Willis 93) employed more conventional methods to determine 0.04 ppm of mercury in urine by extracting it with APDC into methyl-n-amyl ketone. Berman n°) extracted mercury with APDC into MIBK to determine 0.01 ppm. 

The metal may he analyzed hy atomic absorption or emission spectrophotometry (at trace levels). Other techniques include X-ray diffraction, neutron activation analysis, and various colorimetric methods. Aluminum digested with nitric acid reacts with pyrocatechol violet or Eriochrome cyanide R dye to form a colored complex, the absorbance of which may be measured by a spectrophotometer at 535 nm. 

Elemental composition H 4.11%, Mo 48.94%, N 14.29% O 32.65. (NH4)2Mo04 is digested with nitric acid and the molybdenum metal is analyzed by atomic absorption or emission spectrophotometry. It is dissociated to ammonia, which may be measured by titration or by an ion-specific electrode technique (see Ammonia). Ammonium molybdate reacts under acid conditions with dilute orthophosphate solution to form molybdophosphoric acid which, in the presence of vanadium, forms yellow vanadomolybdophosphoric acid the intensity of the yeUow color may be measured by a spectrophotometer at 400 to 490 nm and is proportional to the trace amount of ammonium molybdate. 

Elemental composition Bi 81.29%, S 18.71%. The metal may be determined by digesting the compound in nitric acid followed by instrumental analysis (see Bismuth). Sulfur may be measured in the acid extract by ICP/AES technique. 

Cadmium in acidified aqueous solution may be analyzed at trace levels by various instrumental techniques such as flame and furnace atomic absorption, and ICP emission spectrophotometry. Cadmium in solid matrices is extracted into aqueous phase by digestion with nitric acid prior to analysis. A much lower detection level may be obtained by ICP-mass spectrometry. Other instrumental techniques to analyze this metal include neutron activation analysis and anodic stripping voltammetry. Cadmium also may be measured in aqueous matrices by colorimetry. Cadmium ions react with dithizone to form a pink-red color that can be extracted with chloroform. The absorbance of the solution is measured by a spectrophotometer and the concentration is determined from a standard calibration curve (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC American Public Health Association). The metal in the solid phase may be determined nondestructively by x-ray fluorescence or diffraction techniques. 

Elemental composition Cd 77.81%, S 22.91%. In crystalline state, it may be identified by x-ray diffraction measurement. In aqueous acid extract following digestion with nitric acid, cadmium may be measured by various instrumental techniques, (see Cadmium). Warming with dilute mineral acids liberates H2S, which may be identified by its odor or by browning of a white paper soaked in lead acetate solution. 

Elemental composition Ca 51.33%, F 48.67%. The compound may be analysed nondestructively by x-ray techniques. Calcium may be measured in acid extract by AA or ICP spectrophotometry. The insoluble salt is digested in concentrated nitric acid and the acid extract diluted for analysis. 

Cerium may be analyzed in solution by AA or ICP techniques. The metal or its compounds are digested in nitric acid, diluted appropriately prior to analysis. Also, it may be measured by ICP/MS at a still lower detection level (low ppt). The metal may be analyzed nondestructively by x-ray techniques. 

Elemental composition Cr 32.84%, Cl 67.16%. Chromium(HI) chloride may be solubilized in water by a reducing agent and the aqueous solution may be analyzed for chromium by AA, ICP, or other instrumental techniques. Alternatively, the compound may be digested with nitric acid, brought into aqueous phase, diluted appropriately, and analyzed for the metal as above. The aqueous solution (when a nonchloride reducing agent is used for dissolution of the anhydrous compound in water) may be analyzed for chloride ion by ion chromatography or chloride-selective electrode. The water-soluble hexahydrate may be measured in its aqueous solution as described above. 

Elemental composition Cr 33.57%, Cl 45.77%, O 20.66%. A trace amount may be dissolved in a suitable organic solvent and identified and measured quantitatively by GC-FID, GC-ECD, or by mass spectroscopy. For GC-ECD determination, use a nonchlorinated solvent. Chromium may be determined by AA or ICP techniques following thorough digestion in nitric acid. 

Elemental composition Co 49.55% C 10.10%, O 40.35%. Analysis of cobalt may be performed by digesting a measured amount of the compound in hot nitric acid followed by appropriate dilution and measurement by AA, ICP or other instrumental technique (see Cobalt). Also, treatment with hot acid liberates CO2 (with effervescence) which turns lime water milky. The CO2 may be analyzed by several tests (see Carbon Dioxide). 

Copper(I) sulfide may be analyzed by x-ray analyses. The copper concentration in nitric acid extract may be measured by various instrumental techniques (see Copper). 

Elemental composition Pb 90.40%, H 0.29%, O 9.30%. The hydroxide is digested with nitric acid, diluted and analyzed for lead by AA, ICP or other instrumental technique (See Lead). A weighed amount of the salt is heated in an oven at 145°C and water lost is measured by gravimetry. The residue lead monoxide also may be analyzed by x-ray, or its lead content can be measured by various instrumental methods. 

Elemental composition Pb 86.60%, S 13.40%. Both mineral and synthetic forms can be identified by x-ray measurements. Lead can be analyzed by various instrumental techniques after digestion with nitric acid and appropriate dilution of the acid extract (See Lead). 

Manganese in aqueous solution may be analyzed by several instrumental techniques including flame and furnace AA, ICP, ICP-MS, x-ray fluorescence and neutron activation. For atomic absorption and emission spectrometric determination the measurement may be done at the wavelengths 279.5, 257.61 or 294.92 nm respectively. The metal or its insoluble compounds must be digested with nitric acid alone or in combination with another acid. Soluble salts may be dissolved in water and the aqueous solution analyzed. X-ray methods may be applied for non-destructive determination of the metal. The detection limits in these methods are higher than those obtained by the AA or ICP methods. ICP-MS is the most sensitive technique. Several colorimetric methods also are known, but such measurements require that the manganese salts be aqueous. These methods are susceptible to interference. 

Molybdenum may be identified at trace concentrations by flame atomic absorption spectrometry using nitrous oxide-acetylene flame. The metal is digested with nitric acid, diluted and analyzed. Aqueous solution of its compounds alternatively may be chelated with 8—hydroxyquinobne, extracted with methyl isobutyl ketone, and analyzed as above. The metal in solution may also be analyzed by ICP/AES at wavelengths 202.03 or 203.84 nm. Other instrumental techniques to measure molybdenum at trace concentrations include x-ray fluorescence, x-ray diffraction, neutron activation, and ICP-mass spectrometry, this last being most sensitive. 

Elemental composition Ni 34.38%, C 28.13%, O 37.48%. The compound may be identified and measured quantitatively by GC/MS. An appropriately diluted solution in benzene, acetone, or a suitable organic solvent may be analyzed. Alternatively, nickel tetracarbonyl may be decomposed thermally at 200°C, the liberated carbon monoxide purged with an inert gas, and transported onto the cryogenically cooled injector port of a GC followed by analysis with GC-TCD on a temperature-programmed column. Nickel may be analyzed by various instrumental techniques following digestion of the compound with nitric acid and diluting appropriately (See Nickel). 

Rhenium can be analyzed by various instrumental techniques that include flame-AA, ICP-AES, ICP-MS, as well as x-ray and neutron activation methods. For flame-AA analysis the metal, its oxide, or other insoluble salts are dissolved in nitric acid or nitric-sulfuric acids, diluted, and aspirated directly into nitrous oxide-acetylene flame. Alternatively, rhenium is chelated with 8-hydroxy quinoline, extracted with methylisobutyl ketone and measured by flame-AA using nitrous oxide-acetylene flame. 

Silver metal and its contents in silver alloys and salts can be measured at trace levels by various instrumental techniques such as flame- and furnace-AA, ICP-AES, ICP/MS and x-ray fluorescence methods. It is solubilized by digestion with nitric acid prior to analysis. The AA measurement may be carried out at the wavelength 328.1 nm and ICP analysis at 328.07 nm. ICP/MS is the most sensitive technique while x-ray fluorescence is relatively less sen-... 

Perrino, C., F. De Santis, and A. Febo, Criteria for the Choice of a Denuder Sampling Technique Devoted to the Measurement of Atmospheric Nitrous and Nitric Acids, Atmos. Environ., 24A, 617-626 (1990). 

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