Citric acid

Partition coefficient

Link to relevant study record(s)

Reference

Endpoint:

partition coefficient

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

The study was well documented and meets generally accepted scientific principles, but was not conducted in compliance with GLP.

Principles of method if other than guideline:

Liquid-liquid extraction - flow injection analysis using coaxial segmentors to prevent phase separation.

GLP compliance:

no

Type of method:

other: Liquid-liquid extraction

Partition coefficient type:

octanol-water

Analytical method:

photometric method

Key result

Type:

log Pow

Partition coefficient:

-1.55

Remarks on result:

other: Low dispersion, ±0.05

Key result

Type:

log Pow

Partition coefficient:

-1.8

Remarks on result:

other: Zero dispersion, ±0.07

Key result

Type:

log Pow

Partition coefficient:

-1.61

Remarks on result:

other: High dispersion (mean of three measurements), ±0.09

Details on results:

Although, no further details are available on the pH at which the results are measured; the measured log Kow is used for assessment purposes since the results are very low. Applying any further correction for ionisation will result in log Kow values that are much lower.

Conclusions:

Log Kow values of -1.6 to -1.8 were determined in a reliable study conducted according to generally accepted scientific principles.

Description of key information

Log K_ow: -1.6 to -1.8

Key value for chemical safety assessment

Log Kow (Log Pow):

-1.6

Additional information

A number of log K_ow studies are available which indicate that citric acid has a very low log K_ow. A reliable study conducted according to generally accepted scientific principles gave values in the range -1.6 to -1.8 (Kuban 1991). Similarly, in another reliable study conducted according to generally accepted scientific principles, ion-pair octanol water partition coefficients of -0.76±0.10 for H2A- and -0.21±0.11 for H3A were determined for citric acid (Avdeef 1992).

In available secondary sources to which reliability could not be assigned, log K_ow values of -1.64 and -1.76 were reported for the substance.

Citric acid is a weak acid with pKa values for the three carboxylic acid groups of 3.13, 4.76 and 6.4 at 25°C. In aqueous solution, in the environmentally relevant pH range, two or three of the carboxylic acid groups will typically be dissociated.

In the Kuban 1991 study, the pH of the solution is not stated and it is not possible to tell which species the result relates to. Therefore, no correction for ionisation is applied to this value.

In the Avdeef 1992 study, the log K_ow of unionised citric acid was found to be -0.21. This log K_ow for the unionised form was corrected for ionisation using the equation CORR = 1/1 +10A(pH-pKa) [where A = 1 for acids, -1 for bases; pH = pH-value of the environment; pKa = acid/base dissociation constant]. Log K_ow values of -0.2 at pH 2, -1.1 at pH 4, -2.1 at pH 5, -4 at pH 7 and -4 at pH 9 were obtained (applying a lower limit of -4 to the estimated values).

An estimated log K_ow of -1.7 for the unionised species was predicted using KOWWIN v1.69 [US EPA. [2012]. Estimation Programs Interface Suite™ for Microsoft® Windows, v 4.11]. Using the above correction, this corresponds to log Kow values of -1.7 at pH 2, -2.6 at pH 4, -3.5 at pH 5, -4 at pH 7 and -4 at pH 9.

Therefore, estimates for the log K_ow of citric acid at pH 7 range from -1.6 to -4. The value of -1.6 is considered to be most conservative in terms of the PBT assessment. Therefore, this value is selected as key. The remaining uncertainty in the log K_ow value is not considered critical as the log K_ow value is very low.