Dimethyl itaconate

Henry's Law constant

Description of key information

Calculated based on vapour pressure and water solubility; not volatile from watery solutions and wet surfaces; dimensionless Henry's Law Constant Kaw = 7.5 · 10^-6,  Log Kaw = -5.1 

Key value for chemical safety assessment

Henry's law constant (H) (in Pa m³/mol):

0.02

at the temperature of:

20 °C

Additional information

The transfer of a substance from the aqueous phase to the gas phase (e.g. stripping in the aeration tank of a Sewage Water Treatment Plant (STP), volatilisation from surface water or moist surfaces in general) is estimated by means of its Henry's Law constant. The Henry's Law constant and the air-water partition coefficient (also known as the “dimensionless” Henry's Law constant, Kaw) can be estimated using measured vapour pressure and water solubility data, but for water miscible compounds direct measurement is recommended (ECB 2003, part II, chapter 2.3.5.2, p 45). Henry’s Law constant (H) is thus an important parameter determining the potential for a substance to be lost to the atmosphere by evaporation. It is a measure of its equilibrium between an ideal solution phase and the vapour pressure.

Conventional Henry's Law constant

The molecular weight, water solubility and the vapour pressure can be used to estimate Henry’s Law Constant according to the TGD (ECB 2003, part II, equation 21, p 45).

H = Pvp · M / S

Where:

H = Henry's law constant [Pa·m³/mol] (to convert to [atm·m³/mol] divide by 101325)

Pvp = Vapour pressure [Pa], here 4.05 Pa (at 20 °C, Brekelmans 2010)

M = Molar mass of test item [g/mol], here 158.1519 g/mol (standard formula weight, while considering the natural isotope composition 158.152179 g/mol results)

S = Water solubility [g/m³ = mg/L], here 35.1 g/L = 35'100 mg/L (at 20 °C, Walker 2014)

Accordingly the Henry's Law constant of the submission item is calculated to

H = (4.05 Pa · 158.152179 g/mol) / 35'100 mg/L = 0.018248328 Pa·m³/mol.

The calculation based on measured vapour pressure and water solubility resulted thus in (rounded):

H = ca. 0.02 Pa·m³/mol = 1.8 · 10^-7 atm·m³/mol at 20 °C

The Henry's Law constant was additionally estimated with HENRYWIN v3.2 (March 2011) from the EPI Suite software package U.S. EPA (2011). The estimation worked for the bound contribution method only and gave 0.035 (Pa · m³)/mol = 3.45 · 10^-7 atm · m³/mol at 25 °C, which is in good agreement with the calculation above. This demonstrates that the QSPR approach worked with sufficient accuracy for the submission item.

Generally if H [Pa · m³/mol] < 2, then a substance can be regarded as not volatile from watery solutions and wet surfaces. This applies thus to the submission item, which falls by far below this reference value.

Dimensionless Henry's Law Constant

The Henry's Law Constant (H) can be made unitless by invoking the ideal gas law. Correction for the molar gas volume is required and the value becomes a function of the temperature. The resulting dimensionless air/water partition coefficient Kaw = (concentration of solute in air)/(concentration of solute in water) can be calculated by (ECB 2003, equation 22, p 46):

Kaw = H /( R · T)

Where:

R = Universal gas constant 8.314 Pa·m³/(K·mol) = 8.205 · 10^-5 atm·m³/(K·mol)

T = System temperature [K], with °C + 273.15 = K, here 20 °C = 293.15 K

Accordingly the dimensionless Henry constant of the submission item is calculated to

Kaw = 0.018248328 Pa·m³/mol / (8.314 Pa·m³/(K·mol) / (20 + 273.15) K) = 7.487 · 10^-6

The calculation based on measured vapour pressure and water solubility resulted thus in (rounded):

Kaw = 7.5 · 10^-6, Log Kaw = -5.1 at 20 °C

The HENRYWIN software estimated the air-water partition coefficient Kaw = 1.41· 10^-5 (dimensionless) at 25 °C, which is less accurate than the estimation of H, as the temperature effect is low for the 5 °C difference.

According to a scheme (White 2009, table 2 p 9) this Kaw places the submission item in the volatility class of substances, which are “Non-volatile from a water surface”.

• ECB European Chemicals Bureau (2003). Technical Guidance Document in support of Commission Directive 93/67/EEC on Risk Assessment for new notified substances. Commission Regulation (EC) No. 1488/94 on Risk Assessment for existing substances and Directive 98/8/EC of the European Parliament and of the Council concerning the placing of biocidal products on the market. Part II. Second edition, self-published, Ispra, Italy. 328 p.
• US EPA United States Environmental Protection Agency (2011). Estimation Program Interface (EPI) Suite. Version 4.10. October, 2011. Available from, as of April 25, 2012. URL http://www.epa.gov/oppt/exposure/pubs/episuitedl.htm
• White K (2009). Guidance for Reporting on the Environmental Fate and Transport of the Stressors of Concern in Problem Formulations for Registration Review, Registration Review Risk Assessments, Listed Species Litigation Assessments, New Chemical Risk Assessments, and Other Relevant Risk Assessment. U.S. EPA, Office of Pesticide Programs, Environmental Fate and Effects Division, published 14 December 14. 19 p. URL http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/endangered_species_reregistration_workgroup/esa_reporting_fate.pdf