Boiling point stem corrections

The comparatively inexpensive long-scale thermometer, widely used by students, is usually calibrated for complete immersion of the mercury column in the vapour or liquid. As generally employed for boiling point or melting point determinations, the entire column is neither surrounded by the vapour nor completely immersed in the liquid. The part of the mercury column exposed to the cooler air of the laboratory is obviously not expanded as much as the bulk of the mercury and hence the reading will be lower than the true temperature. The error thus introduced is not appreciable up to about 100°, but it may amount to 3-5° at 200° and 6-10° at 250°. The error due to the column of mercury exposed above the heating bath can be corrected by adding a stem correction, calculated by the formula ... 

Tn addition to the error due to the exposed stem, ordinary chemical thermometers of low cost are subject to errors due to irregularities in the bore and sometimes the scale graduations may not be very accurate. " It is therefore essential to check the thermometer at several temperatures against the melting points of pure solids or the boiling points of pure liquids as described below. The application of an exposed stem correction will of course be unnecessary if the thermometer is calibrated in this way. A calibration curve may then be drawn upon graph " paper from the data thus obtained. Temperatures at intervals of about 20° are marked as abscissae and the corrections to be added or subtracted as ordinates the points thus obtained are then connected by a smooth curve. The thermometer correction at any temperature may be read directly from the curve. 

P° = 101.325 kPa (= 1 atm). At reduced pressures the solvent boils at Twater boils at 97°C), by certain weather conditions, or on the application of a partial vacuum. Since the boiling point is strongly pressure dependent, and since the usual determination with a mercury-inglass thermometer suspended in the vapour depends on the position of the thermometer, corrections to P° and for the thermometer stem-length have... 

It is seen that the correction may be quite large when the boiling point is high. Whenever a boiling point is given, the pressure at which it was taken should be stated, and an indication given as to whether or not a correction for the exposed stem has been applied. 

The methods of determining boiling-points at atmospheric pressure are described in all the practical manuals 7 a simple apparatus consists of a distilling flask in which the liquid is boiled, connected with a condenser (if necessary, a reflux condenser), and with a thermometer in the neck, the bulb being in the vapour suitable corrections for the variation of atmospheric pressure from 760 mm. Hg ( 14.VIIIJ) and the emergent stem corrections ( 3.VI A) must... 

Most laboratory thermometers have a mark around the stem that is three inches (76 mm) from the bottom of the bulb. This is the immersion line the thermometer will record accurate temperatures if immersed to this line. If the thermometer happens to be of the total immersion type then the readings will need to be corrected when taking melting and boiling points... 

The boiling point is one of the most important physical constants of a liquid. It is also easily determined with sufficient accuracy for most purposes if a reasonable quantity of a pure substance is available, for then it is only necessar to carefully distill the sample, noting both the vapor temperature and the barometric pressure. A particularly convenient apparatus for distillation of small quantities is shown in Fig. 1-10. The shape of the flask here is a desirable one because it confines the liquid to a smaller area than does a round-bottom flask. Thermometers with standard taper joints (1-in. immersion) are very convenient and reduce the possibility of contamination, and in vacuum distillation, of leakage. They are also calibrated for partial immersion, thus making emergent stem corrections (page 83) unnecessary. They... 

If a very accurate determination of a boiling-point is to be made, the substance should be distilled from a tube of such a length that the thermometer can be heated by the vapor up to the point where the top of the mercury column stands when the substance is boiling. When determined in this way no correction for stem exposure is necessary. 

As changes in the pressure of the atmosphere affect the boiling points of liquids, both the temperature and pressure are frequently recorded, thus, 220° (750 mm.). When no pressure is specified, the normal pressure, 760 mm., is understood. When the boiling point is taken at a pressure not far from this, a close approximation of the boiling point at 760 mm. may be obtained by applying a correction of 0.1° for each 2.7 mm. of pressure. If the observed pressure is less than 760 mm. the correction should be added, and if greater it should be subtracted. A correction for stem-exposure should be made in the way described under the determination of melting points. 

Nomogram 9 (Fig. 115) is used as follows. The temperature difference s — iJo is marked on scale A and this point is joined to the scale reading for a. The point of intersection of this line with scale C is then connected by a straight line to the barometric pressure on scale D. The point where this straight line intersects the line corresponding to d /dp for the substance in question gives the final correction in the boiling point for 760 mm pressure and a completely immersed thermometer stem. 

It should be noted, however, that the boiling point should be corrected to 760 mm (see Figs. 114, 115) and that temperatures should be corrected for emergent stem. 

When measuring temperatures above 150°C, thermometer errors can become significant. For an accurate boiling point with a high-boiling liquid, you may wish to apply a stem correction to the thermometer, as described in Section 13.3, or to calibrate the thermometer, as described in Technique 9, Section 9.9. 

Boiling-point determination and distillation are two techniques in which an accurate temperature reading may be obtained most easily with a partial immersion thermometer. A common immersion length for this type of thermometer is 76 mm. This length works well for these two techniques because the hot vapors are likely to surround the bottom of the thermometer up to a point fairly close to the immersion line. If a total immersion thermometer is used in these applications, a stem correction, which is described later, must be used to obtain an accurate temperature reading. 

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