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Cyanides guidance for reporting to the National Pollutant Release Inventory

Background

Cyanides are chemical compounds that contain the cyano group (–CN). Many industrial processes utilize cyanides, such as:

According to the Screening assessment – cyanides, in Canada, cyanides are both imported and produced incidentally. Imports, particularly of sodium cyanide, are mainly used for metal extraction and by surface finishing industries as analytical reagents and chemical intermediates. Domestic incidental production occurs in a few industrial sectors, notably integrated iron and steel mills, with emissions mainly to air and surface water. Among cyanides, sodium cyanide is imported in the largest quantities, primarily for mining applications. Tetrasodium ferrocyanide is commonly used in road salt treatments as an anticaking agent, with potential release into the environment through runoff. 

Cyanides exist in several forms, including:

Under the NPRI, if a facility meets the manufacture, process or otherwise use (MPO) threshold, they must report releases, disposals and transfers of:

NA - 07 includes salts of hydrogen cyanide but excludes organic cyanides, nitriles, and organometallic cyanides. NA - 49 covers nine specific cyanide substances identified by their Chemical Abstracts Service Registry Numbers (CAS RNs).

As per the information compiled from the BC Cyanide Water Quality Guidelines (PDF) and the Screening assessment – cyanides, cyanide compounds encompass several categories:

Free cyanide

Free cyanide refers to the total concentration of molecular cyanide (hydrogen cyanide, HCN) and the cyanide ion (CN⁻) present in an aqueous solution. The equilibrium between these two species is influenced by pH and, to a lesser extent, by temperature. Due to its volatility, hydrogen cyanide (HCN) tends to partition into the air.

Simple Cyanides - salts

Compounds with the general formula:


A ( CN ) x

Where:

Examples include NaCN, KCN, Pb(CN)2, or Zn(CN)2. These substances contain the cyanide moiety bonded to any chemical through ionic bonds.

Complex cyanides (also known as metallocyanides)

Formed by the coordination of cyanide ions with metals such as nickel, silver, copper, cobalt, iron, mercury, zinc, and cadmium. These compounds are typically represented by the formulas:


A y M ( C N ) x ⇒ e . g . K 3 F e ( C N ) 6
M y ( M ( C N ) x ) ⇒ e . g . F e 4 ( F e ( C N ) 6 ) 3

Where:

Upon initial dissociation, these complex cyanides release an anionic complex, M(CN)ₓʸ⁻. It may further dissociate under certain conditions, releasing free cyanide ions (CN⁻).

Metal cyanide complexes may also form salt type compounds such as the following three listed substances under NA - 49:

Organic cyanides, or nitriles

Compounds in which the cyano group is bonded to a carbon atom within an organic molecule. Examples include acrylonitrile (CAS RN 107-13-1) and acetonitrile (CAS RN 75-05-8), both of which are listed under the NPRI Part 1 substance list.

Additional forms of cyanide found in aquatic environments include cyanates (containing the –OCN group) and thiocyanates (containing the –SCN group). These species fall outside the scope of NPRI reporting requirements.

Reporting of cyanides to the NPRI

Overview of NPRI listings for cyanides

Starting in the 2025 reporting year, there were changes to the NPRI reporting requirements for cyanides: 

The reporting of ionic cyanides (NA - 07) remained unchanged.

Threshold calculations

This guide aims to help facilities determine if they meet NPRI reporting thresholds and the quantities they must report for the following cyanides:

Hydrogen cyanide (CAS RN 74-90-8)

The NPRI lists hydrogen cyanide (CAS RN 74-90-8) as a Part 1B substance with:

The MPO threshold also includes all quantities:

NA – 07 (cyanides, ionic)

The 2025-2027 Guide for reporting to the NPRI states that NA - 07:

The NPRI lists NA – 07 as a Part 1A substance with:

The MPO threshold also includes all quantities:

NA – 49 (free cyanide, cyanide salts, and cyanide complexes)

NA – 49 is limited to nine cyanide substances identified by their CAS Registry Numbers:

  1. 143-33-9
  2. 506-61-6
  3. 13967-50-5
  4. 13601-19-9
  5. 13746-66-2
  6. 13943-58-3
  7. 14038-43-8
  8. 25869-00-5
  9. 25869-98-1

The NPRI lists NA - 49 as a Part 1B substance with: 

The MPO threshold: 

The basis for the MPO threshold calculation for these NPRI substances is presented in Table 1. 

Table 1: MPO threshold calculation for HCN, NA - 07 and NA - 49
SubstanceFootnote 1 MPO Threshold Threshold calculation
HCN (CAS RN 74-90-8) 1,000 kg (0.1% w/w) N/A
NA - 07 10 tonnes (1% w/w)

The threshold is based on the quantity of the cyanide ion itself, not the full compound (e.g., CN⁻ portion in NaCN or KCN).

Requires calculating the quantity of the CN⁻ ion present in any compound after dissociation, typically in aqueous solutions.
NA - 49 1,000 kg (0.1% w/w)

The threshold applies to the cumulative total of the 9 substances, rather than applying to each substance individually.

The threshold is based on the entire quantity of the compound (e.g., all 1,000 kg of NaCN, not just the CN⁻ content).

Some industrial processes with partial combustion of materials that contain both nitrogen and carbon produce incidental releases of hydrogen cyanide. These releases are considered as by-products and are you are required to include them in MPO calculations and reported, regardless of concentration.

When calculating if your facility meets the reporting threshold for NA - 07, you may need to calculate the equivalent weight of cyanide (CN), using its molecular weight of 26 g/mol, for each cyanide substance that dissociates. NA - 07 includes a range of dissociable cyanide compounds, such as simple cyanide salts. As a reference, Table 2 lists the properties of “hydrogen cyanide” and the substances that fall under the “free cyanide, cyanide salts, and cyanide complexes” listings, which may be useful for these calculations.

Table 2: Identity of hydrogen cyanide and individual cyanide substances listed under “free cyanide, cyanide salts, and cyanide complexes”
CAS RN DSL NameFootnote 2 Common name(s) Molecular formula Molecular weight (g/mol) % CN Subgroup
74-90-8 Hydrocyanic acid Hydrogen cyanide HCN 27.03 96.3% Free and simple cyanides
143-33-9 Sodium cyanide Sodium cyanide NaCN 49.01 53.10% Free and simple cyanides
506-61-6 Argentate(1-), bis(cyano-C)-, potassium Potassium dicyanoargentate K[Ag(CN)2] 199.00 26.20% Metal cyanide complexes – gold or silver
13967-50-5 Aurate(1-), bis(cyano-C)-, potassium Potassium dicyanoaurate K[Au(CN)2] 288.11 18.10% Metal cyanide complexes – gold or silver
13601-19-9 Ferrate(4-), hexakis(cyano- C)-, tetrasodium, (OC-6-11) Tetrasodium ferrocyanide (Yellow prussiate of soda) Na4[Fe(CN)6] 303.91 51.40% Metal cyanide complexes – single-iron
13746-66-2 Ferrate(3-), hexakis(cyano-C)-, tripotassium, (OC-6-11)- Tripotassium ferricyanide K3[Fe(CN)6] 329.25 47.40% Metal cyanide complexes – single-iron
13943-58-3 Ferrate(4-), hexakis(cyano- C)-, tetrapotassium , (OC-6-11)- Tetrapotassiu m ferrocyanide (Yellow prussiate of potash) K4[Fe(CN)6] 368.35 42.40% Metal cyanide complexes – single-iron
14038-43-8 Ferrate(4-), hexakis(cyano-C)-, iron(3+) (3:4), (OC-6- 11)- Ferric ferrocyanide or Prussian blue, insoluble Fe4[Fe(CN)6]3 859.31 54.50% Metal cyanide complexes – multiple-iron
25869-00-5 Ferrate(4-), hexakis(cyano-C)-, ammonium iron(3+) (1:1:1),(OC-6-11)- Ferric ammonium ferrocyanide NH4Fe[Fe(CN)6] 285.83 54.60% Metal cyanide complexes – multiple-iron
25869-98-1 Ferrate(4-), hexakis(cyano-C)-, iron(3+) potassium (1:1:1), (OC-6- 11)- Potassium ferric ferrocyanide (Turnbull's blue or Prussian blue, soluble) KFe[Fe(CN)6] 306.90 50.90% Metal cyanide complexes – multiple-iron

Example of MPO calculation for NA - 07, NA - 49 and hydrogen cyanide

During the reporting year, a mining facility used 20,000 kg of Sodium Cyanide Briquettes (98% purity of NaCN, CAS RN 143-33-9). The facility mixed the briquettes with water to create an aqueous ionic cyanide that they added to a leaching tank to extract gold from the ore. 

Note that the NPRI lists NaCN (full substance) under NA – 49 and not under NA - 07. Therefore, the sodium cyanide from handling and storing the substance in a solid form should be considered under NA - 49 and not NA - 07.  

Note: dissociated cyanide resulting from the addition of sodium cyanide to water may be found in: 

and should be included under NA - 07. 

This mining facility also purchased 1,300 kg of tetrasodium ferrocyanide (CAS RN 13601-19-9) in bulk solid granular form with a purity of 99%. During the reporting year, the facility used this quantity for various purposes, including: 

When used as an anti-caking agent in road salts on haul roads and site roads, it can be released to the environment by runoff from deicing applications. Runoff from these salts can contribute to trace amounts of free cyanide to the local environment and should be considered when the facility reports NA – 07.  

In ore flotation and metal extraction steps, ferrocyanide can be present in the tailings and process waters. Poor containment or accidental discharges may release cyanide-bearing effluents into the environment. Storage and handling of tetrasodium ferrocyanide for multiple site uses increases the risk of spills or leaks. These spills or leaks can lead to the formation of free cyanide that should be considered when the facility reports NA - 07.  

MPO calculation for NA - 07

The facility needs to consider all NA - 07 including: 

  1. the use of sodium cyanide briquettes
  2. additional release or disposal quantities of cyanide (ionic) from other sources

1. Use of sodium cyanide briquettes:

NaCN dissociates in water to Na⁺ and CN⁻ following the chemical reaction:


NaCN ⇔ Na + + CN −
Molar mass NaCN = 49 g/mol Molar mass of CN − = 26 g/mol

Therefore, CN⁻ is 26/49 = 53% of NaCN by mass. That means 20,000 kg of 98% pure NaCN contains about 10,388 kg of cyanide (ionic). 10,388 kg should be used to assess if the MPO threshold for NA - 07 was met.

2. Additional release or disposal quantities of cyanide (ionic) from other sources:

The quantities from (1) and (2) should be added before comparing the total to the MPO threshold. 

MPO calculation for NA - 49

The NPRI defines NA - 49 as a total of nine specific cyanide substances identified by their CAS RNs:

  1. 143-33-9
  2. 506-61-6
  3. 13967-50-5
  4. 13601-19-9
  5. 13746-66-2
  6. 13943-58-3
  7. 14038-43-8
  8. 25869-00-5
  9. 25869-98-1

In this example, the facility uses the following substances listed under NA - 49: 

The quantity used during the reporting year for NA - 49 is:


( 20,000 kg NaCN briquettes × 98% purity ) + ( 1,300 kg tetrasodium ferrocyanide × 99% purity ) = 20,887 kg NaCN

20,887 kg should be used to assess if the MPO threshold for NA - 49 was met.

MPO calculation for Hydrogen cyanide

Hydrogen cyanide (CAS RN 74-90-8) may be generated and released to air if either:

In this case, hydrogen cyanide should be considered as a by-product and its releases to air should be included in the MPO calculation (in this case study without consideration for the 0.1% w/w concentration threshold). The facility can use mass balance or other estimation methods to estimate the potential release of hydrogen cyanide from the ore extraction process.

Reporting of releases, disposals and transfers

Cyanides (ionic), NA - 07

The reporting of releases, disposals and transfers of Ionic cyanides (NA - 07) to the NPRI is as CN⁻ as stated under:

Free cyanide, cyanide salts, and cyanide complexes, NA - 49

The reporting of releases, disposals and transfers of free cyanide, cyanide salts, and cyanide complexes (NA - 49) to the NPRI is as CN⁻ (moiety basis).

NPRI reporting is based on the CN⁻ moiety for NA - 49 for the following reasons:

Hydrogen cyanide (CAS RN 74-90-8)

Hydrogen cyanide should continue to be reported as a distinct substance, not based on CN⁻ equivalency, for the following reasons: 

The table below summarizes the reporting form of each of the three cyanide substances listed on the NPRI. 

Table 3: Key differences in MPO calculation and reporting with the three cyanide NPRI listed substances
NPRI substance MPO threshold calculation CAS RN based Releases/disposals/transfers
Hydrogen cyanide (HCN) full substance yes, CAS RN 74-90-8 report as HCN, not as CN⁻ moiety basis
NA – 07 : Cyanide (ionic) only quantity of CN⁻ ion (dissociated substances) no, it is a category report as CN⁻ moiety basis
NA - 49 (9 specific cyanide substances) quantity of total full substances yes, group of 9 specific CAS RNs report as CN⁻ moiety basis

Bases of estimate you can use to report to NPRI

When a facility estimates the quantity of a substance: 

they can use one of the following methods: 

For NPRI reporting purposes, if emissions are already monitored or measured under provincial or federal legislation or a municipal bylaw, those measurements must be used to report to the NPRI. However, all releases, disposals or transfers off site for recycling must be included in threshold calculations and reported, unless otherwise specified, not just those that are measured or monitored. An NPRI report is mandatory for any substances that meet the reporting thresholds, regardless of whether the substance is being measured or monitored for other jurisdictions. If emissions are not monitored or measured under provincial or federal legislation or a municipal bylaw, reasonable efforts must still be undertaken to gather information on releases, disposals and transfers of a substance.  

What is “reasonable” depends on individual circumstances but may include additional monitoring for NPRI substances. In deciding whether additional efforts should be undertaken to generate new information for the purposes of NPRI reporting, the following factors, among others, should be considered:

General cyanide classification and considerations in using cyanide analytical methods for reporting to the NPRI

General cyanide classification

The descriptions of the classification of cyanides in this section are based on the information contained in: 

Cyanide compounds are generally classified into three categories: 

In environmental measurements, cyanides can be quantified as:

Total cyanide represents the combined concentration of free cyanide, WAD cyanide, and the remaining strong acid dissociable complexes (SAD CN).

Free cyanide (Free CN)
Free CN refers to the sum of hydrogen cyanide (HCN) and cyanide ion (CN⁻) in a sample. Free cyanide is referred to as the amount of HCN liberated from solution at pH 6.0.
Weak Acid Dissociable Cyanide (WAD CN)

WAD cyanide refers to metal cyanide complexes (Zn, Cd, Cu, Hg, Ni, and Ag) that dissociate under weak acid conditions of pH 4.5 to 6. Weak to moderately strong metal-cyanide complexes are compounds that dissociate and release hydrogen cyanide gas under mildly acidic conditions (pH 3 to 6). Cyanide species within this category include:

  • simple cyanides–soluble/dissociable alkali metal and alkali earth metal-cyanide complexes such as:
    • NaCN
    • KCN
    • Ca(CN)2
  • weak metal cyanide complexes such as:
    • Zn(CN)42-
    • Cd(CN)3-
  • moderately strong metal-cyanide complexes such as:
    • Cu(CN)2-
    • Ni(CN)42-
    • Ag(CN)2-

WAD CN measured in the environment includes Free CN and WAD CN.

Total Cyanide (TCN)

TCN refers to the sum of all cyanide-containing compounds in a sample, including:

  • free cyanide
  • WAD cyanide compounds
  • strong metal-cyanide complexes

Strong acidic conditions (pH < 2) are required to dissociate strong metal-cyanide complexes and release free cyanide. Examples of strong metal-cyanide complexes include:

  • Fe(CN)62-
  • Fe(CN)64-
  • Co(CN)64-
  • Au(CN)2-

The strong acidic conditions used to dissociate these resistant metal-cyanide complexes readily dissociates all other cyanide species present in a sample.

Considerations in using cyanide analytical methods for reporting to the NPRI

Given the chemical characteristics of cyanide species, the general expectation is that their relative levels follow the order: Free CN < WAD CN < TCN. 

Facilities should ensure that the analytical method chosen reflects the reporting category (NA - 07 vs NA - 49) and avoids under, or over estimation. 

When using measurement data for reporting under NA - 07 or NA - 49, it is essential to recognize that the:

can affect the quality of the reported data. These factors may lead to false positives or false negatives. This can misrepresent the actual quantity of reportable cyanide. 

False positives can occur when using the TCN method for reporting under NA – 07. This is because it may overestimate the reported quantity by including cyanide from stable complexes that do not readily transform to CN⁻ under release conditions. 

False negatives may occur when using the WAD CN method for reporting under NA - 49 in the presence of ferrocyanide complexes. This is because this method does not fully measure or recover these more stable cyanide species. 

Therefore: 

is critical to ensure accurate reporting under NA - 07 and NA - 49.

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2026-05-08