Henry’s Law and Its Significance to Pesticide Movement in Environmental Compartments

December 23, 2016

On December 23, 1802 a 23-year-old scientist presented his experiments to the most prestigious scientific society of the day. The experiments were designed to better understand the fundamental nature of gases and liquids. He was at the forefront of science in his time. His contributions to modern-day environmental safety could not have been imagined. How much of what we do today will survive the same test of time?


Who was Henry?

HENRY, WILLIAM (1774–1836), chemist, son of Thomas Henry, F.R.S. [q. v.], born at Manchester on 12 Dec. 1774, was educated at the Manchester academy under the Rev. Ralph Harrison [q. v.] After five years spent with Dr. Thomas Percival he removed, in the winter of 1795–6, to the university of Edinburgh, where he attended, among other lectures, those of Dr. Black on chemistry. He afterwards assisted his father in general medical practice at Manchester, but returned to Edinburgh in 1805, and took the degree of M.D. in 1807, the title of his inaugural dissertation being ‘De Acido Urico et Morbis a nimia ejus secretione ortis.’ Meanwhile he had communicated to the Royal Society a paper on carbonated hydrogenous gas (1797), another on muriatic acid (1800), and the results of important experiments he had carried on with regard to the quantity of gases absorbed by water at different temperatures and under different pressures (1803).*

Perhaps his most significant contribution can be found from Phil. Trans. R. Soc. Lond. 1803 93, 29-274, published 1 January 1803, which was verbally presented on a December 23, 1802 to The Royal Society of London. This publication is generally attributed to being the origin of Henrys’ Law. Page 1 of the publication is reproduced here:

In this publication Henry also describes constructing an apparatus not much different in principle to those used today to measure static vapor pressures.

So how does this two-century-old publication contribute to current pesticide dissipation models for soil and aquatic systems?

For any flux determination we can derive a kinetic constant K to explain the rate of partitioning. In this example, from the EPA guidance document, we find the the dimensionless (Kh) which is the partitioning coefficient of a chemical between air and moist soil or air and a water body. 

HLC in the above equation is the actual Henrys’ Law constant derived from the ratio of vapor pressure and water solubility.

As we know, vapor pressure and water solubility are two key parameters required by regulatory authorities globally.


Risk assessments that focus on pesticide movement in air, soil, and aquatic systems generally require use of the Henry’ Law constant. As an example, the EPA issued the following guidance in 2015:

Guidance for Using the Volatilization Algorithm in the Pesticide in Water Calculator and Water Exposure Models

Prepared by: Gabe Rothman, Meridith Fry, Chuck Peck, Jim Lin, Dirk Young, Faruque Khan, and Jim Hetrick, Environmental Fate and Effects Division, December 8, 2015


This document provides guidance for using the volatilization algorithm in the Pesticide in Water Calculator (PWC). The release of the PWC (Version 1.5) includes additional inputs needed to execute the volatilization algorithm. These include the Diffusion in Air Coefficient, Heat of Henry, and Henry’s Law Constant. Additional volatilization-related outputs are also now available, including pesticide mass distribution within the soil profile and the amount of pesticide lost due to volatilization. This document describes these new additions and provides guidance for assessing pre-emergent and bare soil applications in the PWC with the volatilization algorithm.

Applicable Use of the Volatilization Algorithm

The volatilization algorithm calculates the daily pesticide mass flux from soil over the simulation period. It should only be used to evaluate aquatic exposure associated with bare soil and pre-emergent applications of fumigant and conventional pesticides. In addition, the impact of volatilization is not large for aquatic exposure estimates for semi-volatile chemicals with Henry’s Law Constants less than 10-7 atm•m3/mol. As such, the volatilization algorithm should not be used to evaluate the following at this time:

1. Aquatic exposure associated with foliar applications. The portion of the volatilization algorithm associated with the crop canopy has not been verified at this time. As such, one critical input for crops, the foliar volatilization dissipation rate constant, is not available in the current PWC. This variable parameterizes the contribution of off-gassing of residues from crop surfaces.

2. Aquatic exposure associated with compounds possessing Henry’s Law Constants less than 10-7 atm•m3/mol.

3. Inhalation exposure or other terrestrial exposure resulting from vapor-phase concentrations resulting from volatilization. Daily volatilization fluxes estimated from the PWC do not provide the precision required for addressing shorter-term inhalation exposure with external air exposure modeling tools. The daily average volatilization flux values potentially underestimate peak flux values, which can spike over short time scales, on the order of hours.

*Photo and bio of William Henry from Wikipedia


About The Author | Dr. Paul Reibach

Dr. Reibach is currently the Technical Director of Chemistry at Smithers Viscient in Wareham MA where he is responsible for identifying and implementing new technologies for conducting a broad range of regulated analytical chemistry studies. He received a BA in Biology from The University of Texas at Austin and a PhD in Plant Physiology from Texas A&M University. Prior positions have included Rohm and Haas Research Laboratories, Cerexagri Inc., United Phosphorus, and JRF America. At Smithers Viscient the chemistry department conducts analytical chemistry research in support of global pharmaceutical, pesticide, and specialty chemical registrations. Dr. Reibach has developed numerous residue analytical methods which have been utilized for trace level analysis of soils, crops, and animal tissues. Dr. Reibach has issued over 200 technical reports and publications that have been submitted to various regulatory agencies worldwide.

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