Diiron tris(sulphate)

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Handbook of reliable peer-reviewed data

Principles of method if other than guideline:

No details are available on the guideline used to determine this result.

GLP compliance:

no

Water solubility:

4 400 g/L

Temp.:

20 °C

Remarks on result:

other: Reported as 440 g/100 g solution

The pH of the solution not stated, but concentrated solutions of ferric salts are known to be strongly acidic as the hexa-aquo kation is formed, which is strongly acidic with a pKa of 3.05 (Cotton & Wilkinson 1972, see discussion on dissociation constant)

• Cotton FA, Wilkinson G eds (1992). Advanced Inorganic Chemistry. John Wiley and Sons, New York, U.S.A

Conclusions:

Interpretation of results (migrated information): very soluble (> 10000 mg/L) Initial solubility, rapid transformation/speciation to be considered
A water solubility of 440 g/100 g solution is reported in a handbook of reliable data which has been subject to peer-review and in which the original data sources are traceable. The result is considered to be reliable.

Reference 2

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Handbook of reliable peer-reviewed data

Principles of method if other than guideline:

No details are available on the guideline used to determine this result.

GLP compliance:

no

Water solubility:

4 400 g/L

Remarks on result:

other: Reported as 440 g/100 g water

The pH of the solution not stated, but concentrated solutions of ferric salts are known to be strongly acidic as the hexa-aquo kation is formed, which is strongly acidic with a pKa of 3.05 (Cotton & Wilkinson 1972, see discussion on dissociation constant)

• Cotton FA, Wilkinson G eds (1992). Advanced Inorganic Chemistry. John Wiley and Sons, New York, U.S.A

Conclusions:

Interpretation of results (migrated information): very soluble (> 10000 mg/L) Initial solubility, rapid transformation/speciation to be considered

Description of key information

Initially very soluble (>>10 g/L), rapid transformation/speciation in environmental waters and body fluids
Initial solubilities are
Fe2(SO4)3: Anhydrous and Nonahydrate 4400 g/L

Additional information

Testing for this endpoint has been waived in accordance with column 2 restrictions. All soluble iron salts dissociate in aqueous media to the respective anions and kations, and are then subject to further change of oxidation and speciated state according to the conditions. Thus the salts are not stable and undergo fast hydrolysis (DT50 < 12 h) in water, particularly if oxygen is present and the pH is in the common environmental ranges. Nonetheless a number of generally high initial water solubilities are published, which are reported in the following paragraphs. Comparably aqueous solutions are initially (i.e. before transformation/speciation) unlimitedly miscible with water and water solubility is not a relevant endpoint for them, as they are unlimitedly miscible with water.

Ferric sulphate (Fe2(SO4)3)

The value 4400 g/L at 20 ºC has been reported for the anhydrous salt and also for the nonahydrate in a reliable source (Lide 2002). Other references state 448 g/L (Seidell 1940) for an unspecified form or qualitative data such as „slightly“ (Lewis 2002) or „slowly soluble in cold water“ (Budaravi et al 1996, The Merck Index). The anhydrous salt and hydrated forms are thus eventually highly soluble in water and the significantly different values may be due to the slow solubility.

Table: Initial water solubilities [g/L] of the registered category member salt forms

Salt	Anhydrous form	Solid hydrate form	Aqueous solution
Ferric sulphate, Fe2(SO4)3	4'400 at 25 °C	Nonahydrate (CAS 13520-56-4): 4400 at 20 °C	Unlimited miscibility

Ferric and Ferrous kations

In presence of oxygen ferrous kations are readily oxidized to ferric kations, which are thus more important for the environmental assessment of aerobic waters.

The following Table below shows the maximum dissolved concentration of the iron kations in pure water as it depends upon pH (see section on Hydrolysis).

Table: Calculated maximum solubility of iron in solution at 20 °C depending on pH (OECD 2007, table 13, p 33)

pH	Ferrous kation, iron (II)		Ferric kation, iron (III)
	[mg Fe/L]	[mmol Fe/L]	[mg Fe/L]	[mmol Fe/L]
4	>1E+06	>1.8E+04	6.16E-02	1.1E-03
5	>1E+06	>1.8E+04	6.16E-05	1.1E-06
6	>1E+06	>1.8E+04	6.16E-08	1.1E-09
7	89'600	1'600	6.16E-11	1.1E-12
8	896	16	6.16E-14	1.1E-15

In conclusion the limit of the ferric iron kation solubility is even at significantly acidified conditions in the order of ng Fe(III)/L.

• Budaravi S, O'Neil MJ, Smith A, Heckelman PE, Kinneary JF eds (1996). The Merck Index. An encyclopedia of Chemicals, Drugs, and Biologicals. 12th edn. ISBN 0911910-12-3. Whitehouse Station, NJ, U.S.A. Merck and Co. 1741 p.
• Lewis R J, Sr. (2002). Hawley's Condensed Chemical Dictionary (14th Edition). John Wiley & Sons.
• Lide DR ed (2002). CRC Handbook of Chemistry and Physics. 83rd edn. ISBN 0-8493-0483-0. Boca Raton, FL, U.S.A. CRC Press. 2664 p.
• OECD Organisation for Economic Co-operation and Development (2007). SIDS Initial Assessment Report for SIAM 24. Chemical Category: Iron Salts. Self-published, Paris, France, 17-20 April. 138 p.
• Seidell A, Link W F eds (1940). Solubilities of Inorganic and Metal Organic Compounds. 3rd edn. New York, NY, U.S.A. D Van Nostrand.