Phosphoric acid, mono- and di-C16-18-alkyl esters

Dissociation constant

Administrative data

Endpoint:

dissociation constant

Type of information:

calculation (if not (Q)SAR)

Remarks:

Migrated phrase: estimated by calculation

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

2015

Report date:

2015

Materials and methods

Principles of method if other than guideline:

According to REGULATION (EC) No 1907/2006 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 18 December 2006 Column 2, 7.16 - The study does not need to be conducted if - it is scientifically not possible to perform the test for instance if the analytical method is not sensitive enough. Based on compositional information and experimental data of the water solubility and ultraviolet/visible absorbance properties of the test substance provided by the Sponsor, it was concluded that it was not feasible to complete an experimental determination of the dissociation constants in water as detailed within the OECD Guidelines for Testing of Chemicals, Section 1, No. 112: “Dissociation Constants in Water” adopted May 12, 1981.
The analytical test methods under consideration were those specified in the internationally accepted OECD Guidelines for Testing of Chemicals.

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

154518-38-4

Cas Number:

154518-38-4

IUPAC Name:

154518-38-4

Constituent 2

Reference substance name:

Phosphoric acid, isotridecyl ester

EC Number:

258-261-3

EC Name:

Phosphoric acid, isotridecyl ester

Cas Number:

52933-07-0

Constituent 3

Reference substance name:

Phosphoric acid, mono- and di-C11-14 (linear and branched) alkyl esters

IUPAC Name:

Phosphoric acid, mono- and di-C11-14 (linear and branched) alkyl esters

Test material form:

liquid: viscous

Results and discussion

Dissociating properties:

yes

Dissociation constant

Dissociation constant 1

No.:

#1

pKa:

1.9

Temp.:

25 °C

Remarks on result:

other: pka1 monoalkyl ester

Dissociation constant 2

No.:

#2

pKa:

6.5

Temp.:

25 °C

Remarks on result:

other: pka2 monoalkyl ester

Dissociation constant 3

No.:

#3

pKa:

1.5

Temp.:

25 °C

Remarks on result:

other: pka1 dialkyl ester

Any other information on results incl. tables

The purpose of this evaluation was consideration of the feasibility of undertaking an experimental determination of the dissociation constants in water for this test item, at the request of the Sponsor.

 

The dissociation of a chemical in water may be of importance in assessing its impact upon the environment. It governs the form of the substance, which in turn may influence its behavior and transport, for example it may affect the adsorption of the chemical on soils and sediments, or adsorption into biological cells. 

 

The experimental procedures under consideration were those specified in the regulatory document OECD Guidelines for Testing of Chemicals, Section 1, No. 112: “Dissociation Constants in Water” adopted May 12, 1981. The three options specified within this guideline are the potentiometric titration method, the spectrophotometric method and the conductometric method. However it is also important to note two critical constraints of these regulatory methodologies. The first is that the sample solution concentrations should not exceed half the water solubility of the test item and secondly, testing should be performed on the purest possible form of the substance under investigation.

 

With respect to the water solubility of the test item, the supplied water solubility report detailed that the excess, undissolved test item dispersed in water to form a micro-emulsion. Such behaviour is as expected as the proposed structures for the test item are analogous to a number of recognized classes of anionic surfactants. Subsequently only an estimation of solubility could be provided from an arbitrary turbidity cut-off value. To assist further in identification of the valid sample concentration for any experimental determination of dissociation constants in water, computer estimations of water solubility were performed for the dominant C₁₃monoalkyl ester and dialkyl ester components using the specialized predictive software WSKOWWIN, version 1.42, September 2010, © 2000 U.S. Environmental Protection Agency.Estimations were performed using C₁₃ R groups ofn-tridecane, 11-methyl-dodecane and 3,5,7-trimethyl-decane to assess for any influence of branching. These estimations are presented as Appendix 1 (Attachment 1 of this Summary) of this report and resulted in predicted solubilities of 0.60 to 0.93 mg/L and 3.9 x 10^-7to 9.2 x 10^-7mg/L for the monoalkyl ester and dialkyl ester components respectively. As such, the working concentration of any sample solutions prepared for the determination of dissociation constants would need to limited to less than half that of the less soluble component (to ensure dissolution of the test item as a whole), so less than 1.9 x 10^-7mg/L.

 

The second above method constraint also highlighted the unsuitability of this substance to the available regulatory methods. The test methods are intended for evaluation of pure substances only, as opposed to the complex UVCB mixture presented by this substance.  In this case, the complex composition of the test item would prevent the generation of sample solutions of accurate known molarity as required for the titration method, as well as introducing difficulties in evaluating and assigning data to individual dissociation constants, in particular due to the presence of residual orthophosphoric acid, irrespective of the method employed.

 

Therefore considering the three analytical approaches to the experimental quantification of dissociation constants in water individually:

 

Potentiometric titration method

 

The potentiometric titration method requires the accurate, stepwise addition of an exact molar equivalent of acid or base to the test solutions, and monitoring of the change in solution pH throughout this addition. The dissociation constant is then derived from the solution pH and theoretical distribution of ionized and unionized species. Solutions are typically prepared at 0.01 M (2.80 g/L based on aC₁₃alkyl mono ester); however based on estimated solubility data available, the maximum permitted solution concentrations would be 2.1 x 10^-6M or 1.4 x 10^‑12 M based on the dominant C₁₃alkyl monoester computer predicted solubility and the less soluble C₁₃alkyl diester computer predicted solubility respectively; again using the dominant C₁₃alkyl monoester molecular weight of 280 to estimate molar concentrations. Such permitted concentrations would not be sufficient to achieve robust, reliable titration data. In addition, due to the nature of the composition of the test item, irrespective of solubility constraints, it would not be possible to readily identify accurate molar concentrations of the dissociating species under evaluation.This prevents the preparation of solutions at concentrations above the estimated water solubility of the test item, in a range of water and organic solvent mixtures of differing compositions, and extrapolation of dissociation constant data to pure water using theYasuda Shedlovsky extrapolation method.

 

Spectrophotometric method

 

The spectrophotometric method depends on differences in ultraviolet/visible spectra profiles between the ionized and unionized species of a substance, and where such behaviour exists, this method is typically the most suitable for substance of low water solubility. However, from spectral data generated under a number of pH conditions (supplied by the Sponsor), although a possible minor shift in relative intensity of the absorbance at approximately 240 nm was seen between acidic and basic conditions, actual total absorbance was limited in this region (approximately 0.15 absorbance units for a 1.7 g/L solution). On diluting test solutions to the permitted working concentration for the dissociation constant determination, no detectable absorbance response would be achieved on analysis of the solutions.

 

Conductometric method

 

The conductometric method can only be applied to water soluble acids for which the neutral sodium salt is also available. The method is dependent of preparing a number of serial aqueous dilutions (typically over a number of magnitudes of concentration) for both substances and deriving the dissociation constant from comparison of conductivity data from the unsalted and salted form. In this case, negligible solubility, the complexity of the composition and the absence of the relevant salted form all prevent the viability of this procedure.

Therefore, on the basis of the test item being a complex UVCB substance composed of multiple individual components which is considered to be essentially insoluble in water (when considering successful dissolution of all dominant components present in the substance), it was concluded that experimental determination of the dissociation constants in water was not feasible.

 

As an alternative source of dissociation constant data for this substance, computer estimations were performed for the proposed dominant structures within the test item using the specialized modeling software Advanced Chemistry Development, Inc. (ACD/Labs) pKa Algorithm Version: v12.1.0.50374. The results are presented in full as Appendix 2 (Attachment 2 of this Summary) and summarised in the table in Attachment 3 of this Summary.

Applicant's summary and conclusion

Conclusions:

As an alternative source of dissociation constant data for this substance, computer estimations were performed for the proposed dominant structures within the test item using the specialized modeling software Advanced Chemistry Development, Inc. (ACD/Labs) pKa Algorithm Version: v12.1.0.50374. The results are summarized in the table illustrated above.

Where “R” equals either a linear or branched alkyl group. Estimations were performed using C13 R groups of n-tridecane, 11-methyl-dodecane and 3,5,7-trimethyl-decane to assess for any influence of branching. Alkyl chain length variation between C11 and C14 were predicted not to influence the dissociation constant values of individual components as the local chemical environment of the dissociating functional groups remained essentially unchanged.

Executive summary:

Based on compositional information and experimental data of the water solubility and ultraviolet/visible absorbance properties of the test item provided by the Sponsor, it was concluded that it was not feasible to complete an experimental determination of the dissociation constants in water as detailed within theOECD Guidelines for Testing of Chemicals, Section 1, No. 112: “Dissociation Constants in Water” adopted May 12, 1981.

 

Primarily this conclusion was made on the basis of the test item being a complex UVCB substance composed of multiple individual components, which was also considered to be essentially insoluble in water (when considering successful dissolution of all dominant components present in the substance). Scientific justification for this decision is presented in full within any other results incl. table section of this Summary.

As an alternative source of dissociation constant data for this substance, computer estimations were performed for the proposed dominant structures within the test item using the specialized modeling softwareAdvanced Chemistry Development, Inc. (ACD/Labs) pKa Algorithm Version: v12.1.0.50374. The results are summarized in the table illustrated above.

Where “R” equals either a linear or branched alkyl group. Estimations were performed using C₁₃ R groups ofn-tridecane, 11-methyl-dodecane and 3,5,7-trimethyl-decane to assess for any influence of branching. Alkyl chain length variation between C₁₁and C₁₄were predicted not to influence the dissociation constant values of individual components as the local chemical environment of the dissociating functional groups remained essentially unchanged.