Commercial nitric acid is supplied as aqueous solutions in which the product model denotes concentration, trace-metal profile, inhibitor package, and end-use grade rather than mechanical hardware. Common commercial model designations include technical grade 55–68 wt% HNO₃, ACS reagent grade 70 wt%, electronics grade 70–71 wt%, and white or red fuming grades above 90 wt%. The molecular formula is HNO₃ and the CAS registry number is 7697-37-2. Red fuming product is assigned UN 2032; non-fuming grades fall within UN 2031 when the regulatory concentration threshold is met. The density at 20 °C rises from approximately 1.34 g/cm³ at 55 wt% to 1.41 g/cm³ at 68 wt% and 1.50 g/cm³ near 98 wt%. The aqueous HNO₃ system forms a maximum-boiling azeotrope at 68.5 wt% and 121.9 °C under 101.3 kPa; this creates a process control boundary because distillation cannot produce concentrations above the azeotrope without a dehydrating agent. Concentrations above the azeotrope are produced commercially by extractive distillation with sulfuric acid or by dehydration with magnesium nitrate. Routine concentration verification is performed by density measurement under ISO 2990:1974. Vendor certificates of analysis for technical grade commonly specify HNO₃ assay by acid-base titration, chloride ≤10 ppm, sulfate ≤20 ppm, iron ≤5 ppm, arsenic ≤1 ppm, and residue on ignition ≤0.01 wt%. Electronics-grade material is controlled to trace-metal limits below 1 ppb for key elements such as Fe, Al, Ca, and Zn by ICP-MS.

Bulk storage and transfer impose process-specific constraints. For ≤68 wt% acid, 304L stainless steel tanks are standard, but field inspection records show vapor-space attack above the liquid line where condensed NOx-rich moisture forms; this is managed by nitrogen blanketing or continuous vent scrubbing rather than by alloy upgrade alone. Weld seams are a site-specific failure location unless post-weld pickling and passivation are completed after fabrication. Transfer skids on production sites typically use PTFE-lined centrifugal pumps with SiC mechanical seal faces and PTFE-lined pipe or 304L stainless steel; carbon steel, copper, and many elastomers are excluded. Fume scrubber systems are typically packed columns using water or dilute sodium hydroxide with continuous pH measurement and staged NOx oxidation. High-strength fuming acid requires vented storage at low temperature, commonly below 30 °C, to reduce NO₂ evolution and pressure accumulation. Aluminum equipment appears in selected aerospace ground-support systems for fuming acid above 80 wt%, but dilute grades attack aluminum rapidly.

What Distinguishes Nitric Acid from Sulfuric and Hydrochloric Acid in Process Selection?

Nitric acid is a strong mineral acid and an oxidizer; its anion participates directly in redox and nitration reactions. This differs from sulfuric acid, which at 96 wt% acts principally as a dehydrating and sulfonating agent, and from hydrochloric acid, which behaves as a non-oxidizing acid with chloride-driven metal complexation and pitting risk. In metal dissolution, 68% HNO₃ attacks carbon steel rapidly by nitrate reduction to NO₂ and NO; 96% H₂SO₄ can be stored in carbon steel at ambient temperature because a sulfate film passivates the surface, while 37% HCl corrodes carbon steel through hydrogen evolution and chloride attack. In stainless steel processing, nitric acid removes free iron and promotes chromium oxide enrichment; hydrochloric acid is avoided in passivation because residual chloride can initiate pitting and stress corrosion cracking. In organic synthesis, nitric acid supplies the nitronium ion in mixed acid for aromatic nitration, whereas sulfuric acid removes water and regenerates the nitronium ion from nitric acid.

Metal Finishing and Passivation Bath Parameters

Nitric acid is used in stainless steel passivation under ASTM A967-17. Typical passivation formulations are 20–50 vol% HNO₃ at 20–55 °C for 20–60 min, with the higher concentration range specified for free-machining grades and the lower temperature range used for austenitic alloys to avoid flash attack. The bath dissolves surface iron and supports chromium oxide enrichment. Bath control is by acid titration and specific gravity, with iron accumulation commonly limited below 2 g/L to reduce smut deposition. In stainless steel pickling lines, nitric acid is frequently combined with hydrofluoric acid at 8–15 vol% HNO₃ and 0.5–3 vol% HF at 40–60 °C; the nitric acid oxidizes the metal surface while HF complexes dissolved iron and chromium. This differs from hydrochloric acid pickling because chloride ions can remain in pits and promote localized corrosion after welding.

In ammonium nitrate and mixed-acid nitration plants, nitric acid consumption is dominated by neutralization and electrophilic substitution. Ammonium nitrate production uses 55–65 wt% HNO₃ reacted with ammonia in recirculating neutralizers at 140–180 °C; pH is maintained between 2.5 and 4.0 to control ammonium nitrate decomposition and ammonia slip. The resulting solution is evaporated to a melt for prilling or granulation. In mixed-acid nitration, a representative mononitrobenzene charge uses 30–35 wt% HNO₃, 55–60 wt% H₂SO₄, and 5–10 wt% water, with reaction temperature held at 60–90 °C in batch stirred reactors and near 120 °C in adiabatic systems; the spent sulfuric acid is recovered by vacuum distillation. Dinitrotoluene production for toluene diisocyanate precursor uses similar mixed-acid chemistry, with additional HNO₃ consumed in the second nitration stage. The process demonstrates the functional difference between nitric acid, which supplies the electrophilic nitronium ion, and sulfuric acid, which acts as a dehydrating catalyst and recycle medium.

In semiconductor fabrication, electronics-grade nitric acid is used in silicon etching and cleaning. Isotropic silicon etching is performed in mixtures of nitric acid, hydrofluoric acid, and acetic acid; HNO₃ concentration is selected between 20 vol% and 70 vol% with published formulation ratios from 3:1 to 20:1 HNO₃:HF by volume. The nitric acid oxidizes silicon to silicon dioxide, and HF dissolves the oxide; acetic acid or water moderates the oxidation rate. Etch bath temperature is typically held at 20–30 °C; higher temperature increases etch rate and reduces selectivity to photoresist. In cleaning, 70 wt% HNO₃ is used in photoresist stripping and trace-metal removal before furnace operations, with particle specifications typically below 10 particles/mL at 0.2 µm. Electronics-grade product differs from technical material primarily by metal ion content, which is specified by ICP-MS rather than by titration alone.

When Red Fuming or High-Strength Grades Replace 68% Technical Grade in Oxidizer Service

When a system designed for 68% technical grade is switched to red fuming nitric acid, density, vapor pressure, and compatibility assumptions change substantially. Red fuming nitric acid contains dissolved nitrogen oxides that increase density above 1.50 g/cm³ and lower the freezing point; inhibited grades may contain hydrogen fluoride or iodine compounds for compatibility with stainless steel or aluminum propellant tanks. The vapor hazard is more severe because NO₂ is continuously evolved; closed containers must be vented and maintained below 30 °C. In rocket propulsion ground testing, IRFNA is handled through passivated 304L or 316L systems with PTFE seals and is kept free of organic lubricants. Substitution of 68% acid into a fuming-acid system is not equivalent: lower density and different vapor composition alter pump net positive suction head, flowmeter calibration, and materials compatibility.

Because of its oxidizing strength, nitric acid must be segregated from combustible solvents, organic oils, reducing agents, and ammonia in storage and dosing areas. Reaction with acetone, methanol, or toluene can be violent and has been documented in waste-treatment incidents where spent acid was mixed with organic streams in unvented tanks; drainage and collection systems are therefore designed with separate acidic and organic lines. Spill neutralization with sodium hydroxide or soda ash is exothermic and requires cooling; the resulting nitrate salts remain oxidizing. The operational boundary for most aqueous technical grades is defined by the azeotrope at 68.5 wt% and by increasing NOx vapor pressure as temperature rises. Published data for high-temperature corrosion rates of high-silicon stainless steels in boiling nitric acid with condensate return is limited; vendor isocorrosion curves should be consulted before selecting alloys for service above 70 °C.