  When measuring Henry's Law constants ($k_H$) using the phase ratio method via
headspace gas chromatography (GC), the value of $k_H$ of the compound under
investigation is calculated from the ratio of the slope to the intercept of a
linear regression of the the inverse GC response versus the ratio of gas to
liquid volumes of a series of vials drawn from the same parent solution. Thus,
an experimenter will collect measurements consisting of the independent
variable (the gas/liquid volume ratio) and dependent variable (the inverse GC
peak area). There is a choice of values of the independent variable during
measurement. A review of the literature found that the common approach is a
simple uniformly spacing of the liquid volumes. We present an optimal
experimental design which estimates $k_H$ with minimum error and provides
multiple means for building confidence intervals for such estimates. We
illustrate efficiency improvements of our new design with an example measuring
the $k_H$ for napthalene in aqueous solution as well as simulations on previous
studies. The designs can be easily computed using our open source software
optDesignSlopeInt, an R package on CRAN. We also discuss applicability of this
method to other fields.
