SIMPLE BOILING-POINT CORRECTION CREIG S. HOYT Grove City College, Grove City, Pennsylvania
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VERY student of organic chemistry, whose misfortune it has been to work a t altitudes considerably higher than sea level, has encountered the direction to preserve "the fraction distilling between 78 and 7g0," to find to his dismay that the whole preparation distilled below that temperature range. Even when the student realizes the reason for the discrepancy, he seldom possesses the ability to apply the necessary correction a t this stage of his development. Exact equations for the purpose are involved and not infrequently vapor pressure data for a particular substance are lacking. A simple equation for the purpose may be derived. Whiie it contains a number of approximations, which render it somewhat inaccurate, it is sufficiently exact for the purpose and its simplicity renders it p&icularly
and assuming P to be approximately 760 in all cases,
The correction to be applied to the boiling point for each mm. of deviation of the barometer from 760 mm. is, therefore, equal to the absolute boiling point divided by 8000 for all normal liquids. There are numerous exceptions to this equation in the case of polar liquids such as water, the lower alcohols, ketones, and acids. But the correction is made here by changing the value of M L J T to correspond to the experimental data on these substances. For water and the lower alcohols this averages 25.8, which makes the equ&ion
nwfnl.
The Clausius-Clapeyron equation in the differential form may be employed over a short range of temperature without serious error.
Here M is the molecular weight, L, is the latent heat of vaporization, P the vapor pressure, and T the absolute boiling point. Assuming from Trouton's rule, that for normal liquids
As the polarity decreases, the constant in the denominator decreases to the limiting value of 8000, the value for normal liquids. The following constants were computed from the values of Trouton's ratio as given in the International Critical Tables for liquids of various classes. The variation in the case of acids is too wide to permit of a single value.
