Iron citrate

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods with acceptable restrictions

Qualifier:

equivalent or similar to guideline

Guideline:

EU Method A.6 (Water Solubility)

Principles of method if other than guideline:

Ferric citrate monohydrate is used in the SPS5019 process, where the material is dissolved in -4 volumes of water at 90 - 100'C to form a solution.
It is known that higher temperature closer to the boiling point of water help in achieving a complete solution.

GLP compliance:

no

Type of method:

flask method

Specific details on test material used for the study:

Testing was carried out using ferric citrate monohydrate from Jost (23685005).
The raw material itself was a free flowing powder and did not appear to be hygroscopic.

Key result

Water solubility:

< 0.05 g/L

Conc. based on:

test mat.

Temp.:

23.5 °C

pH:

5.1

Conclusions:

The solubility in water of ferric citrate monohydrate has been tested at 20 - 25'C and was found to have a solubility of <0.05 g/L.
Sparingly soluble at a natural pH of 5.1

Reference 2

Endpoint:

water solubility

Type of information:

other: assessment

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Justification for type of information:

The substance ferric citrate (iron III citrate) is typically supplied as a hydrated solid and is considered typically to be highly soluble in water. However, examinations during the work to Register the substance, it was noted that solubility in water was relatively low under dilute conditions.
Many metal salts demonstrate a high level of pH dependant solubility characteristics with many metal salts appearing soluble under low pH, but precipitating at higher pH. This is especially true with weak acids (eg citrates, acetates, oxalates etc).

Qualifier:

no guideline followed

Principles of method if other than guideline:

The substance ferric citrate (iron III citrate) is typically supplied as a hydrated solid and is considered typically to be highly soluble in water. However, examinations during the work to Register the substance, it was noted that solubility in water was relatively low under dilute conditions.
Many metal salts demonstrate a high level of pH dependant solubility characteristics with many metal salts appearing soluble under low pH, but precipitating at higher pH. This is especially true with weak acids (eg citrates, acetates, oxalates etc).
This review of behaviour in water explains the solubility characteristics of ferric citrate and the effect of dilution of the weakly acidic substance. Estimates are also made as to environmental solubility in soft and hard water where the presence of other anions such as carbonates will have an impact on the solubility of the iron.

GLP compliance:

no

Type of method:

other: Assessment of data

Water solubility:

ca. 500 g/L

Conc. based on:

test mat.

Temp.:

20 °C

pH:

< 2

Remarks on result:

other: Soluble at high levels in concentrated acidic solution

Water solubility:

> 1 g/L

Conc. based on:

test mat.

Temp.:

20 °C

pH:

ca. 3.5

Remarks on result:

other: At below 1 g/l, solublity decreases unless acidity is maintained

Details on results:

Although the effect of pH seems less important than with some other organo-metallic salts, it is clear that at high concentrations (typically > 10 g/l) in distilled water, there is good solubility and even up to 50% appears stable in solution at environmental temperatures.
However, at lower concentrations (ca 300 mg/l), precipitation is seen and this is still apparent at 50 mg/l in distilled water at pH 5.1.

Information from reliable review documents suggest that the solubility in water is approximately 0.5 mg/l at pH 3. However, suppliers report solubility in terms of percentage (> 10 g/l) figures with a pH 3 - 4
The main influence on 'solubility' is the formation of complexes with other ions and with itself; this is influenced both by pH and presence of other ions.
The solubility of iron (III) carbonate is reported as 60 mg/l (pH not reported) with iron at ca 25 mg/l. This implies a higher solubility than ferric citrate, so the complexation of the citrate is also key to the final figure.

Conclusions:

It is not possible to cite a single water solubility figure for ferric citrate.
For worker risk assessment, cleaning of spills, engineering controls etc, it can be considered as 'soluble' in water with a solubility up to 50% (500 g/l). However, in the level of < 1% (at about 1 - 10 g/l), the substance will start to precipitate.
The theory is that as the weakly acidic substance is diluted, the impact of the acidity is reduced and the substance can complex itself and effectively polymerise into a citrate-iron complex of low solubility.
Under environmental conditions, this is compounded by the presence of other cations (metals) and anions (carbonates, sulphates, phosphates etc) and the complexes formed will reduce solubility to below 1 mg/l. At lower concentrations, the dilution factors outweigh the ability to complex and the iron will again be in soluble form.
The citrate rapidly biodegrades in the environment and can be disregarded for purposes of environmental risk assessment.

Description of key information

It is not possible to cite a single water solubility figure for ferric citrate.

At low concentrations and pH values of >3.5, the solubility appears to be low (<50 mg/l). Iron salts at pH values higher than 3.5 generally hydrolyse to form insoluble hydroxides. At pH values lower than this, solubilisation is possible and is dependent on pH and temperature.

For worker risk assessment, cleaning of spills, engineering controls etc, it can be considered as 'soluble' in water with a solubility up to 50% (500 g/l). However, in the level of < 1% (at about 1 - 10 g/l), the substance will start to precipitate unles acidity is maintained