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Vapour pressure

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Reference
Endpoint:
vapour pressure
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
See QMRF document "MPBPVPWIN.pdf" attached to this record.
Qualifier:
no guideline followed
Principles of method if other than guideline:
This parameter varies as the composition of the mixture changes during evaporation. For Type 1 NCSs with known constituents the “initial” vapour pressure can be calculated as the sum of the partial pressure of the known constituents. Also a range of the vapour pressure can be given. Therefore, the first approach will be a calculation of the vapour pressure based on constituents. Figure (attached as picture) shows that the measured values are very similar to the estimated values for various citrus oil constituents. Only for constituents with low measured vapour pressure, the estimation method overestimates the vapour pressure. However, it is considered that these extreme non-volatile constituents (<1 Pa) are not relevant for the volatility of the NCS. It is recommended in the NCS protocol to calculate the initial vapour pressure based on the vapour pressures of the individual constituents. From the comparison between estimated and measured data in the figure, it can be concluded that the epiwin calculation method can be considered as reliable. Therefore, the epiwin estimation method is considered as relevant to estimate vapour pressures for this group of constituents. For the initial vapour pressure, the estimated values are used in the calculation. The “initial” vapour pressure is calculated as the sum of the partial pressure of the constituents (based on molecular fraction).
GLP compliance:
no
Type of method:
other: Calculation by estimation
Key result
Temp.:
25 °C
Vapour pressure:
186.4 Pa
Remarks on result:
other: Initial vapour pressure
Substance CAS Estimated vapour pressure (Pa at 25 degrees Celsius) Molecular weight
D-limonene 5989-27-5 193 136.24
Citral 5392-40-5 12.2 152.24
beta-Myrcene 123-35-3 320 136.24
Linalool 78-70-6 11.1 154.25
Alpha-Terpineol 98-55-5 2.62 154.25
Alpha-pinene 7785-70-8 536 136.24
Citronellal 106-23-0 33.9 154.25
Decanal  112-31-2 31.4 156.27
Octanal 124-13-0 199 128.22

Weight percentages of constituents are converted to molar percentages using the molecular weight (MW)

. To correct for the 4% unknown constituents (assuming the average vapour pressure for the unknown constituents is similar to tha average vapour pressure of the known constituents), this number is multiplied by 1 / 0.96.

MPBPWIN v1.43 model details

Reference to the type of model used

This program (MPBPWIN) estimates the boiling point (at 760 mm Hg), melting point and vapor pressure of organic compounds. MPBPWIN requires only a chemical structure to make these predictions. MPBPWIN estimates vapor pressure (VP) using the estimated boiling point.

 

Description of the applicability domain

Currently there is no universally accepted definition of model domain. However, users may wish to consider the possibility that property estimates are less accurate for compounds outside the

Molecular Weight range of the training set compounds, and/or that have more instances of a given fragment than the maximum for all training set compounds. It is also possible that a compound may have a functional group(s) or other structural features not represented in the training set, and for which no fragment coefficient was developed. These points should be taken into consideration when interpreting model results.

 

Training Set Molecular Weights:

Minimum MW: 16.04

Maximum MW: 943.17

Average MW: 194.22

Description and results of any possible structural analogues of the substance to assess reliability of the prediction

Internal validation with a dataset containing 3037 substances resulted in a correlation coefficient (r2) of 0.914, a standard deviation of 1.057 and an average deviation of 0.644.

Predictivity assessment of the internal validation set:

Validation Set Estimation Error:

within <= 0.10 - 34.1%

within <= 0.20 - 45.6%

within <= 0.40 - 60.0%

within <= 0.50 - 64.9%

within <= 0.60 - 69.0%

within <= 0.80 - 75.0%

within <= 1.00 - 80.0%

Uncertainty of the prediction

All constituents for which estimations were made fall within the applicability domain of the model.

Mechanistic domain

This program (MPBPWIN) estimates the boiling point (at 760 mm Hg), melting point and vapor pressure of organic compounds. MPBPWIN requires only a chemical structure to make these predictions. MPBPWIN estimates vapor pressure (VP) by three separate methods:

(1) the Antoine method,

(2) the modified Grain method, and

(3) the Mackay method.

 

All three use the normal boiling point to estimate VP. Unless the user enters a boiling point on the data entry screen, MPBPWIN uses the estimated boiling point from the adapted Stein and Brown method (For more information see: Stein, S.E. and Brown, R.L.   1994.   Estimation of normal boiling points from group contributions. J. Chem. Inf. Comput. Sci. 34: 581-7). MPBPWIN reports the VP estimate from all three methods. It then reports a "suggested" VP. For liquids and gases, the suggested VP is the average of the Antoine and the modified Grain estimates. The Mackay method is not used in the suggested VP because its application is currently limited to its derivation classes.

Conclusions:
The initial vapour pressure of orange oil is 186.4 Pa at 25ºC.
Executive summary:

The initial vapour pressure of orange oil was estimated by calculation. Vapour pressures for the known constituents were estimated using the QSAR MPBPWIN v 1.43. The sum of the vapour pressures multiplied by the molecular fraction of the substance in the NCS was taken as an initial estimate for the vapour pressure of the mixture. The number obtained was then corrected for the unknown constituents (assuming the average vapour pressure for the unknown constituents is similar to the average vapour pressure of the known constituents).

The initial vapour pressure of orange oil was found to be 186.4 Pa at 25 ºC. The vapour pressure for the constituents ranges from 2.62 to 536 Pa.

Description of key information

Vapour pressure: 186.4 Pa at 25ºC

Key value for chemical safety assessment

Vapour pressure:
186.4 Pa
at the temperature of:
25 °C

Additional information

In a key study, the vapour pressure of orange oil was calculated using estimates of the vapour pressures of its' constituents, in line with the NCS protocol. The initial vapour pressure of orange oil was found to be 186.4 Pa at 25 ºC. The vapour pressure for the constituents ranges from 2.62 to 536 Pa.