# Nitric acid

Nitric acid identity, aqueous and fuming forms, assay terminology, product-specific properties, and limits of safety and lot evidence.

Reviewed: 2026-10-30

Tags: chemicals, acids

## Identity and forms

Sigma-Aldrich **225711** lists **70% nitric acid** and **≥99.999% on a trace-metals basis**. Fisher A200 lot 256937 instead reports **69.5% w/w HNO₃** against a 68.0–70.0% assay specification. These are distinct quantities. Nitric acid is HNO₃, CAS 7697-37-2, 63.0128 g/mol. [NIST Chemistry WebBook, nitric acid](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7697372&Mask=4) [Sigma-Aldrich 225711](https://b2b.sigmaaldrich.com/US/en/product/aldrich/225711)

NIOSH describes fuming nitric acid as concentrated acid with dissolved nitrogen dioxide. Color alone cannot quantify dissolved nitrogen oxides or acid assay. [NIOSH Pocket Guide, reviewed October 30, 2019](https://www.cdc.gov/niosh/npg/npgd0447.html)

Nitric acid and nitrate are different chemical identities. The European Commission's SPECIEUROPE entry identifies the nitrate ion as NO₃⁻, CAS 14797-55-8, with molecular weight 62.0049. **Nonprocedural chemical interpretation:** a result expressed as nitrate describes a different reporting basis from HNO₃ assay. Nitrate content alone cannot establish free nitric acid content, water content, or the identities of other dissolved constituents. [SPECIEUROPE nitrate entry](https://source-apportionment.jrc.ec.europa.eu/Specieurope/specie.aspx?id=613); [NIST nitric acid identity](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7697372&Mask=4)

## Assay, grade, and lot evidence

These terms answer different questions about a product.

| Evidence | Product-specific meaning | Limit |
| --- | --- | --- |
| Acid assay | Merck 100456, named nitric acid 65%, lists an alkalimetric assay of at least 65.0%. | A lower bound is not an exact measured concentration. |
| Grade and impurity specification | The same EMSURE Reag. Ph Eur, ISO product lists separate limits for chloride, sulfate, metals, and residue. | One assay number does not describe all impurities. |
| Trace metals basis | Sigma-Aldrich 225711 lists both 70% acid and at least 99.999% on a trace metals basis. | The latter is not 99.999% HNO₃ in the supplied liquid. |
| Certificate of analysis | Fisher A200 lot 256937 reports an assay of 69.5% w/w against a 68.0–70.0% specification. | The result belongs to that lot, released March 26, 2026. |

Sources: [Merck 100456 specification](https://www.merckmillipore.com/TW/en/product/Nitric-acid-650-0,MDA_CHEM-100456); [Sigma-Aldrich 225711 product data](https://b2b.sigmaaldrich.com/US/en/product/aldrich/225711); [Fisher A200 lot 256937 certificate, both pages](https://assets.fishersci.com/api/ecertificate/9af0b53d-4aa6-4804-9a87-1e89d4945a72).

Fisher's certificate separates specification limits from reported results. For example, chloride has a limit of 0.08 ppm and a reported result below 0.08 ppm. That result is not zero chloride. The certificate applies to all package sizes of this lot and notes that some data may come from third parties. It provides no measured assay for a different or subsequently supplied lot. [Fisher A200 lot 256937 certificate](https://assets.fishersci.com/api/ecertificate/9af0b53d-4aa6-4804-9a87-1e89d4945a72)

Fisher's US SDS includes A200 and reports 65–70% nitric acid and 30–35% water by weight. These ranges are not lot measurements. [Fisher SDS, revision 14, sections 1 and 3](https://www.fishersci.com/store/msds?partNumber=A200212&productDescription=NITRIC+ACID+ACS+PLUS+2.5L&vendorId=VN00033897&countryCode=US&language=en)

**Nonprocedural interpretation:** % w/w expresses constituent mass per total mixture mass. A water balance by subtraction requires a binary acid–water model. Additional dissolved constituents invalidate that assumption.

## Physical properties require composition and conditions

The following values describe the named products or source entries. They are reference data, not production settings.

| Source and form | Reported property | Qualification |
| --- | --- | --- |
| Merck 100456, nitric acid 65% | Density 1.39 g/cm³ at 20 °C. | The separate specification gives relative density d 20/20 of 1.384–1.416. |
| Merck 100456 | Boiling point 121 °C at 1013 hPa; melting point −32 °C. | The page does not supply measurement methods or a melting-point pressure. |
| Merck 100456 | Vapor pressure 9.4 hPa at 20 °C. | The page does not identify component partial pressures. |
| Sigma-Aldrich 225711, nitric acid 70% | Density 1.413 g/mL at 20 °C; boiling point 120.5 °C. | Both carry a literature qualifier. The boiling-point pressure is absent. |

Sources: [Merck 100456 physicochemical information and specification](https://www.merckmillipore.com/TW/en/product/Nitric-acid-650-0,MDA_CHEM-100456); [Sigma-Aldrich 225711 properties](https://b2b.sigmaaldrich.com/US/en/product/aldrich/225711).

Fisher's SDS instead lists −41 °C melting point, water miscibility, and 0.94 kPa vapor pressure at 20 °C. Its different product scope and missing melting-point conditions leave the discrepancy unresolved. [Fisher SDS, section 9](https://www.fishersci.com/store/msds?partNumber=A200212&productDescription=NITRIC+ACID+ACS+PLUS+2.5L&vendorId=VN00033897&countryCode=US&language=en)

**Nonprocedural synthesis:** 9.4 hPa equals 0.94 kPa, so these two vapor-pressure entries agree numerically after unit conversion. Agreement does not establish a common measurement method. Miscibility describes an absence of a finite water-solubility limit in the source; it does not establish ideal mixing, constant density, or an activity model. Relative density is a ratio, whereas g/cm³ and g/mL express mass per volume.

NIST's compound entry reports vaporization enthalpy of 38.6 kJ/mol at 312 K, attributed to Holeci, 1966, from data spanning 273–356 K. This is a compiled compound value, not an aqueous-product heat duty or a value valid at every temperature. The original article was not independently examined here. [NIST phase-change table, reference, and notes](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7697372&Mask=4)

## Hazard classification and evidence limits

Fisher's US SDS, revision 14 of December 18, 2025, uses OSHA Hazard Communication Standard 2024. It lists oxidizing liquid category 3, metal corrosion category 1, acute inhalation toxicity for vapors category 3, skin corrosion category 1A, and serious eye damage category 1. This is a regional mixture classification. [Fisher SDS, sections 1–2](https://www.fishersci.com/store/msds?partNumber=A200212&productDescription=NITRIC+ACID+ACS+PLUS+2.5L&vendorId=VN00033897&countryCode=US&language=en)

The SDS disclaims status as a quality specification. Its data do not establish another concentration's properties or global compliance. [Fisher SDS, section 16](https://www.fishersci.com/store/msds?partNumber=A200212&productDescription=NITRIC+ACID+ACS+PLUS+2.5L&vendorId=VN00033897&countryCode=US&language=en)

## Source scope and related reference

Manufacturer web pages were consulted October 30, 2026 PDT and display no technical revision dates. The dated SDS and lot certificate identify their own scope above. NIOSH's older review date and NIST's individual data attribution remain distinct from this article's review date. The sources do not provide a complete concentration-dependent property table or a specification for every commercial form. Current-lot composition, fuming-grade limits, full aqueous equilibrium behavior, materials compatibility, and customer capability remain outside this reference's evidence.

[Surface-finishing processes](../../processes/finishing/index.md) provides related manufacturing terminology. Chemical identity and generic properties alone do not establish a finishing process or its operating conditions.