vogel_practical_ochem_3.pdf

72 PRACTICAL ORGANIC CHEMISTRY [II.
11,9. CALIBRATION OF THERMOMETERS
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 :
Stem correction (in degrees) = KN(tx — t2)
where K = the apparent expansion coefficient of mercury in glass ;
N = the length, measured in degrees, of the part of the thermometer
not heated to the temperature of the bulb, i.e., the
length of the exposed column ;
tx = the observed temperature ; and
t2 = the mean temperature of the exposed mercury column
(determined on an auxiliary thermometer placed alongside
with its bulb at the middle of the exposed thread).
The value of K for normal glass is 0-000168 from 0° to 150° ; 0-000169 at
200°; 0-000161 at 250°; and 0-000164 at 300°. For borosilicate
(Pyrex) glass the values of K are: 100°, 0-000164; 150°, 0-000165;
200°, 0-000167; 250°, 0-000170; 300°, 0-000174; 350°, 0-000178;
400°, 0-000183 ; and 460°, 0-000188. Owing to the difficulty of determining
t2 with any accuracy, the above correction cannot be applied with
any degree of precision, but is sufficiently accurate for all ordinary purposes.
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.
The stem correction may be avoided by employing a thermometer with
a short scale (sometimes termed an Anschutz thermometer) and of such size
that the entire column of mercury is immersed in the vapour or in the
liquid. In practice, it is generally more convenient to use thermometers
which have been graduated for partial immersion for a short and con-
* For advanced work, it is advisable to use a thermometer of better quality and which
is provided with a small mercury bulb; the latter rapidly attains the temperature of the
bath or vapour. Satisfactory small bulb thermometers are manufactured inter alia by
H. J. Elliott Ltd. and by Short and Mason Ltd.