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Due to the almost complete dissociation and subsequent rapid hydrolysis, (photo)redox processes and polymerisation of the iron species, the soluble iron salts have been treated as a chemical category (OECD 2007). This makes sense for the environmental fate and effects and for some toxicological endpoints, but most physical and chemical endpoints for the individual iron salts cannot be treated as a category (e.g. appearance, melting point, boiling point, density, water solubility, pH and so on). Nonetheless read across between the different ferric salts makes sense as the counter-anions are inert with regard to e.g. oxidizing properties (auto) flammability or explosiveness. And finally a number of waiving arguments apply to the whole soluble iron salts category of ferric and ferrous salts (vapour pressure, partition coefficient, surface tension).

Briefly, however category considerations are mainly used in environmental fate, ecotoxicology and toxicology, the submission item fits into the category also for some of the physicochemical properties. Therefore category considerations are given here (for physicochemical properties, environmental fate, ecotoxicology) and with different considerations in the summary on toxicokinetics.

Available data on physiochemical properties have been reviewed and discussed in the peer-reviewed published SIARs. Ten iron salts and their hydrates were evaluated in SIAM 24, April 2007 (OECD 2007, section 1.3). The critical information from the SIARs is as follows: “All the iron salts considered have high melting and boiling points, with degradation occurring at very high temperatures. The vapour pressure of the salts is low and in aqueous solution the vapour pressure observed is that of water and is governed by the colligative properties of the salts. All the salts are highly soluble in water (apart from ferric sulphate, which is slowly soluble but rapidly soluble in the presence of a trace of ferrous sulphate). These salts have been known and studied by chemists for a considerable time and much of the data concerning their physico-chemical properties are to be found in tables and compendia of the properties of inorganic chemicals rather than in referenced papers.”

  • OECD Organisation for Economic Co-operation and Development (2007). SIDS Initial Assessment Report for SIAM 24. Chemical Category: Iron Salts. Self-published OECD, Paris, France, in April. 138 p.

Chemical Category Reporting Format according to ECHA Guidance

The following definition complies with the ECHA (2008, chapter R.6) Guidance on QSARs and grouping of chemicals. It is more comprehensive and more structured and should be used in the discussions of some physical/chemical properties, environmental fate and ecotoxicology. The most similar example in the above mentioned guidance is given in section R.6.2.7 (Case study using phosphonic acid compounds and alkali metal salts, p 126). For the toxicological evaluation a category approach applies also, but requires different justification. Therefore the soluble iron salt category with regard to the toxicological endpoints is given in the section on toxicokinetics.

Table: Chemical Category of dissociating, inorganic and non-toxic iron compounds, Reporting Format according to ECHA Guidance R.


Category definition and its members: Dissociating, inorganic and non-toxic iron compounds


Category Definition


Category Hypothesis

This category covers inorganic ferric and ferrous compounds with counter-ions, which are ubiquitous and non-toxic (all acute LC50 and EC50 > 100 mg/L & chronic NOEC > 1 mg/L) and do not require additional consideration, i.e. the sulphate (CAS 14808-79-8) and chloride (CAS 16887-00-6) anions.

Accordingly the first main assumption of the category hypothesis is that the sulphate and chloride anions are not significant in respect of the properties under consideration. Sulphate and chloride are mass anions omnipresent in biota and natural media. Irrelevantly low toxicity can be assumed. Thus they can be disregarded so that only the fate and effects of the iron is subject to the assessment. The impact of hydronium (CAS 13968-08-6) and hydroxide (CAS 14280-30-9) ions is anyhow restricted to pH effects. Such effects do not represent actual intrinsic toxicity and were generally excluded in the environmental fate and hazard assessment. In the respective OECD test guidance documents generally the test pH has to be in a range suitable for the test organisms or the test system. During the pH adjustment the hydroxide ion reacts with the hydronium ion to water (CAS 7732-18-5).

The second main assumption of the category hypothesis is that with regard to the environmental fate & behaviour and the effects to environmental organisms the category comprises a directly analogous group of Iron III and II (ferric and ferrous) compounds. The formation of a category is justified on the basis of a self-consistent model of the behaviour and properties of these substances. Under normal environmental conditions of pH, ambient temperature and dissolved oxygen, all of the salts will dissociate immediately in aqueous media to the respective anions and ferric (CAS 20074-52-6) and ferrous (CAS 15438-31-0) kations. Then they will be subject to rapid hydrolysis revealing the hydroxides. The following precipitation, change of oxidation state and further speciation will take place according to the geochemical conditions and identically for all category members (salts and hydroxides). Similar processes are likely in pore water of soils or due to the moisture in the atmosphere. The composition of this equilibrium depends on these conditions rather than the ferric or ferrous character or the counter-anions of the originally released iron compounds as the iron species transform readily, aerobic conditions provided. Anaerobic conditions are generally not expected until the equilibrium is reached. Nonetheless it should be noted that iron (II) is stable in anaerobic media and the reduction of iron (III) may be slow. This is important for the interpretation of MEC (Measured Environmental Concentration) data and the establishment of EQS (Environmental Quality Standards), but does not play a role in the hazard assessment as the released species are not determining the environmentally present species unless in case acute release of iron (II) directly in an anaerobic environmental compartment, e.g. the anoxic ocean water layer, which is unlikely according to the exposure scenario. Analytical differentiation in environmental samples is challenging. Generally analytical quantification in the literature is based on total iron. Therefore it would even be hardly achievable to treat iron (II) and (III) separately.

Consequently the third main assumption of the category hypothesis is that due to the kinetics of the aerobic transformation processes and as the intended uses the category members do not directly reach the environment but after sufficient equilibration time (ca. 12 h or less) under predominantly aerobic conditions.

Hence some properties (measured or expressed in aqueous media, e.g. ecotoxicity) for a category salt or hydroxide can be directly read-across (with suitable mass correction) to another salt or hydroxide and vice versa.


Applicability Domain (AD) of the category

The category is applicable to all possible compounds equilibrating under aerobic conditions in comparable time to iron (II) and (III) kations, provided that the related anions can reasonably be considered non-toxic to the aquatic life in the relevant concentrations (all acute LC50 and EC50 > 100 mg/L & chronic NOEC > 1 mg/L). Therefore e.g. iron carbonates may fit in the category as defined here while nitrite will probably not.

Compounds released directly in the environment are generally not covered by the AD (i.e. preparations used as herbicides to control moss).


List of endpoints covered (numbers refer to the IUCLID 5 sections)

A) Physical and Chemical Properties of environmental relevance
(not true category endpoints as only common waiving applies)

4.6 Vapour pressure

4.7 Partition coefficient

4.10 Surface tension

4.21 Dissociation constant

B) Environmental fate and pathways

5.2.1 Biodegradation in water: screening tests

5.2.2 Biodegradation in water and sediment: simulation tests

5.2.3 Biodegradation in soil

5.3.1 Bioaccumulation: aquatic / sediment

5.3.2 Bioaccumulation: terrestrial

5.4.1 Adsorption / desorption

5.4.2 Henry's Law constant

5.4.3 Distribution modelling

5.4.4 Other distribution data

5.5.1 Monitoring data

C) Ecotoxicological Information

6.1.1 Short-term toxicity to fish

6.1.2 Long-term toxicity to fish

6.1.3 Short-term toxicity to aquatic invertebrates

6.1.4 Long-term toxicity to aquatic invertebrates

6.1.5 Toxicity to aquatic algae and cyanobacteria

6.1.6 Toxicity to aquatic plants other than algae

6.1.7 Toxicity to microorganisms

6.1.8 Toxicity to other aquatic organisms

6.2 Sediment toxicity

6.3.1 Toxicity to soil macroorganisms except arthropods

6.3.2 Toxicity to terrestrial arthropods

6.3.3 Toxicity to terrestrial plants

6.3.4 Toxicity to soil microorganisms

6.3.5 Toxicity to birds

6.3.6 Additional ecotoxicological information


Category Members apart from Hydronium jarosite (EC 940-441-4)

Common name

EC number

CAS number

Ferric chloride



Ferrous chloride



Ferric sulphate



Ferrous sulphate



Ferric chloride sulphate



Ferric hydroxide



Ferrous hydroxide



The definition of these salts also covers a number of chemical identities relating to specific hydrates. Under REACH all hydrates are covered by the registration of the anhydrous salt. As the hydrates influence the molecular weight, correction should apply as the hydrate weight is considered in CLP according to the ECHA (2013, IV.7.4 Example D: Hazard classification of a soluble metal salt: the case of rapid environmental transformation through speciation in the water column, table on p 640) Guidance on the Application of the CLP Criteria.

The following hydrates are identified in the public domain though not all are necessarily commercially relevant:

Chemical name (CAS name)

CAS number

Molecular formula

Iron chloride (FeCl3), monohydrate (9CI)


Cl3 Fe . H2O

Iron chloride (FeCl3), hydrate (2:3) (9CI)


Cl3 Fe . 3/2 H2O

Iron chloride (FeCl3), dihydrate (9CI)


Cl3 Fe . 2 H2O

Iron chloride (FeCl3), trihydrate (9CI)


Cl3 Fe . 3 H2O

Iron chloride (FeCl3), hexahydrate (8CI, 9CI)


Cl3 Fe . 6 H2O

Iron chloride (FeCl3), nonahydrate (9CI)


Cl3 Fe . 9 H2O

Iron chloride (FeCl3), dodecahydrate (9CI)


Cl3 Fe . 12 H2O

Iron chloride (FeCl3), hydrate (8CI, 9CI)


Cl3 Fe . x H2O

Iron chloride (FeCl2), hydrate


Cl2 Fe . x H2O

Iron chloride (FeCl2), monohydrate


Cl2 Fe . H2O

Iron chloride (FeCl2), dihydrate


Cl2 Fe . 2 H2O

Iron chloride (FeCl2), tetrahydrate


Cl2 Fe . 4 H2O

Iron chloride (FeCl2), hexahydrate


Cl2 Fe . 6 H2O

Sulphuric acid, iron(3+) salt (3:2), nonahydrate


Fe2(SO4)3 . 9H2O

Sulphuric acid, iron(3+) salt (3:2), hydrate


Fe2(SO4)3 . xH2O

Sulphuric acid, iron(3+) salt (3:2), hexahydrate


Fe . 3/2 H2SO4 . 3 H2O

Sulphuric acid, iron(3+) salt (3:2), heptahydrate


Fe . 3/2 H2SO4 . 7/2 H2O

Sulphuric acid, iron(3+) salt (3:2), tetrahydrate


Fe . 3/2 H2SO4 . 2 H2O

Sulphuric acid, iron(2+) salt (1:1), hexahydrate


Fe . H2SO4 . 6 H2O

Sulphuric acid, iron(2+) salt (1:1), trihydrate


Fe . H2SO4 . 3 H2O

Sulphuric acid, iron(2+) salt (1:1), tetrahydrate


Fe . H2SO4 . 4 H2O

Sulphuric acid, iron(2+) salt (1:1), monohydrate


Fe . H2SO4 . H2O

Sulphuric acid, iron(2+) salt (1:1), hydrate


Fe . H2SO4 . x H2O

Sulphuric acid, iron(2+) salt (1:1), dihydrate


Fe . H2SO4 . 2 H2O

Sulphuric acid, iron(2+) salt (1:1), heptahydrate


Fe . H2SO4 . 7 H2O


Purity / Impurities

Iron itself is not hazardous to the environment and generally only substances with comparable purity or impurity profiles can be assessed together. Technical grade materials may contain relevant quantities of impurities, which exceed the low ecotoxicological effects of the pure compounds comprised in this category. In cases where the sum of the impurities triggers the toxicity, the assessment has to be based on these impurities considering their concentration and the category assessment is restricted to the iron salt components.

The purity of the category members is generally > 80 % w/w. Deviations below this purity value would not normally be acceptable since this would suggest that the substance should not be considered as a mono-constituent substance.

Impurities comprise salts of other metals up to ≤ 1 % w/w and free acid up to ≤ 2 % w/w.


Category justification

Due to their rapid transformation and the formation of insoluble precipitating species the causation in effect studies is complicated. The Hill (1965, Wess 2016) criteria for causation can hardly be met. Therefore precise effect measurement under exclusion of fouling effects accompanied by detailed documentation allowing reconstructing is the exemption in the literature and study reports. In consequence reliable data to compare equimolar iron effects from different species are not available and cannot be produced with reasonable efforts. Testing of the category members in concentrations exceeding the natural background levels and/or the solubility of the iron kations is thus considered technically not feasible and the wide range of published data is likely to represent test artefacts rather than differences in toxicity. Published effect level data differ even for the same salt if equimolar nominal iron concentrations are compared.

Vangheluwe & Versonnen (2004) performed a comprehensive literature search and evaluation of acute and chronic aquatic ecotoxicity data. The authors state that “it should be noted that most of the effects observed are due to the particulate nature of the formed iron hydroxide (ferric form) rather than the toxic properties of the dissolved Fe (II) ion as such. No evidence of systemic toxicity by iron to aquatic organisms has been found. 16 studies on standard test species were found to be reliable. Six studies were performed on iron (II) and 10 studies were performed on iron (III). The results of the fish studies are all in the same order of magnitude. The LC50 values of the acute experiments (96 h) with standard species have a narrow range from 16.6 to > 27.9 mg Fe/L. Chronic LOECs for fish are in the same order of magnitude. For the daphnids, acute survival (24-48 h) results in EC50 values of 1.29-36.9 mg Fe/L. Chronic NOEC, LOEC and EC50 values range from 0.63 to 5.9 mg Fe/L.” Notwithstanding the methodological objections these data support the category hypothesis.

Additionally sufficient knowledge on iron chemistry is in the literature available to assume reasonably that comparable exposure to ferric and ferrous kations originate from initially equimolar category member exposure.


Data matrix

Notwithstanding the methodological objections the following Acute toxicity data were deemed reliable according to the literature search and evaluation of Vangheluwe & Versonnen (2004). The values were used in ECHA (2013, Table IV.7.4-a, p 641, Example D) Guidance on the Application of the CLP Criteria. However the same principal objections apply as well, two algal studies performed by national institutes of South Korea (NIER 2001) and Japan (MOE 2002) were added to present all required trophic levels. The actual toxicity is considered significantly lower and these values should not be used as starting point for PNEC derivation.

Test substance

Test organism



[mg Fe/L]



Pimephales promelas

96 h



Birge et al 1985

Lepomis macrochirus

96 h




Oncorhynchus mykiss

96 h



Mattock (2002a)


Oncorhynchus mykiss

96 h



Mattock (2002b)


Daphnia pulex

24 h



Lilius et al 1995


Daphnia magna

24 h



Calleja et al 1994


Daphnia pulex

48 h



Birge et al 1985


Daphnia longispina

48 h



Randall et al 1999


Daphnia magna

48 h



Biesinger & Christensen 1972


Daphnia magna

24 h



Lilius et al 1995


Daphnia magna

48 h



LISEC 1999


Pseudokirchnerella subcapitata

72 h

Growth rate


NIER 2001


Pseudokirchnerella subcapitata

72 h

Growth rate


MOE 2002

Notwithstanding the methodological objections the following Chronic toxicity data were deemed reliable according to the literature search and evaluation of Vangheluwe & Versonnen (2004). The values were used in ECHA (2013, Table IV.7.4-b, p 641, Example D) Guidance on the Application of the CLP Criteria. However the same principal objections apply as well, two algal studies performed by national institutes of South Korea (NIER 2001) and Japan (MOE 2002) were added to present all required trophic levels. The actual toxicity is considered significantly lower and these values should not be used as starting point for PNEC derivation.

Test substance

Test organism



[mg Fe/L]



Salvelinus fontinalis

30 days





Smith & Sykora 1976


Oncorhynchus kisuth

30 days



Smith & Sykora 1976


Pimephales promelas

33 days


1.0/1.61 1.61/2.81

Birge et al 1985


Daphnia pulex

21 days

Total offspring,
Brood size

2.51/5.01 0.63/1.26 1.26/2.51

Birge et al 1985


Daphnia magna

21 days


5.9 EC50
4.4 EC16

Biesinger & Christensen 1972


Pseudokirchnerella subcapitata

72 h

Growth rate

1.06 NOEC

NIER 2001


Pseudokirchnerella subcapitata

72 h

Growth rate

10.2 NOEC

MOE 2002


Conclusions per endpoint for C&L, PBT/vPvB and dose descriptor

The category member with lowest molecular weight is ferric hydroxide, i.e. 89.862 g/mol. This gives the conversion factor of 1.6 to mg Fe/L values. The lowest reported NOEC is in iron basis 0.63 which is still results in a NOEC > 1 mg/L. All acute values are even on the basis of iron > 1 mg/L and thus higher for all category members. As rapid transformation is deemed, no classification is required as PBT and vPvB properties can also be excluded.

There is no starting point for PNEC derivation as the toxicity values given under point 3. (Data matrix) cannot be considered to be threshold values. Given the half-time for oxidation and precipitation being < 12 h, it is anticipated that a significant proportion of any ferrous salts added to oxygenated aqueous test media will have converted to ferric within the timescale of the standard OECD test protocols. The cited LC50, EC50 and NOEC for the substance, expressed in terms of iron concentration, significantly exceed the equilibrium concentrations of dissolved ferric iron given below. In conclusion the solute iron (III) and (II) equilibrium does not exhibit any toxicity up to the limit of its solubility.

Calculated maximum solubility of ferric iron in solution for a pH range of 4.0 to 8.0 and a temperature of 20 ºC:


Fe [mg/L]

















When reading across concentrations have to be recalculated on an equimolar basis to iron and eventually to the target substance. Some adjustments factors are given in the table below for convenience.

Table: Adjustment factors to convert numerical endpoints expressed as Fe concentrations to numerical endpoints for iron compounds

Chemical Species

Molecular Weight [g/mol]














(H3O)Fe3(SO4)2(OH)6  481  8.62

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  • Birge WJ, Black JA, Westerman AG, Short TM, Taylor SB, Bruser DM, Wallingford ED (1985). Recommendations on numerical values for regulating iron and chloride concentrations for the purpose of protecting warmwater species of aquatic life in the Commonwealth of Kentucky. Memorandum of Agreement no. 5429, Kentucky Natural Resources and Environmental Protection Cabinet.
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