Melting points thermometer calibration

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 ... 

Table II, 9, A.—Reference Substances for the Calibration of Thermometers BY THE Melting Point Method...

After the melting point has been determined, the thermometer reading is corrected by reference to the calibration chart of the thermometer. Methods for calibrating a thermometer are described in Section 11,9. 

Repeat the boiling point determination with the following pure liquids (a) carbon tetrachloride, A.R. (77°) (6) ethylene dibromide (132°) or chlorobenzene (132°) (c) aniline, A.R. (184-6°) and (d) nitrobenzene, A.R. (211°). An air condenser should be used for (c) and (d). Correct the observed boiling points for any appreciable deviation from the normal pressure of 760 mm. Compare the observed boiling points with the values given in parentheses and construct a calibration curve for the thermometer. Compare the latter with the curve obtained from melting point determinations (Section 111,1). 

Determination of melting points (a-naphthylamine, a-naphthol, benzoic acid, succinic acid and p-nitrobenzoic acid). Use the apparatus shown in Fig. II, 10, 2, a. Construction of calibration curve for thermometer. Determination of m.p. of unknown compound. 

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. 

Full experimental details for the determination of melting and boiling points are given in Sections 11,10 and 11,11 respectively. The Tables II, 9, A and II, 9, B list suitable substances for the calibration of thermometers by melting point or boiling point determinations respectively. Substances which are bracketed are alternative to each other. It need hardly be emphasised that only compounds of the highest pimity should be employed. 

The 3He melting pressure thermometer has been chosen to extend the ITS 90 for several reasons, such as the good sensitivity over three temperature decades, except around the pressure minimum at 315.24 mK. On the other hand, such a minimum is a reference point in the calibration of the pressure transducer in fact, the pressure must be measured in situ since, below 315.24 mK, the entrance of the measurement cell is blocked by solid 3He. 

Table 1 Corrected Melting Points of Compounds Suitable as Reference Materials in the Calibration of Thermometers...

Corrected melting points of compounds suitable as reference materials in the calibration of thermometers... 

On the basis of the calibration by melting points and phase transitions, a number of shift thermometers have been developed for solid-state NMR spectroscopy in various temperature ranges. Wehrle et al. 144), for example, used the line splitting in the N CP/MAS NMR spectrum of the organic dye molecule tetra-methyldibenzotetraaza annulene (TTAA) in the temperature range 123—405 K. A high-temperature shift thermometer for temperatures of up to 790 K was developed by van Moorsel et al. 40) on the basis of Sn MAS NMR spectroscopic... 

Assume that your thermometer was not properly calibrated and showed only 95°C difference between the melting point and boiling point of water. How would that affect your Kf value ... 

The transition temperature at which a hydrated salt liberates all or part of its water and passes into another less hydrated form is almost as definite as an ordinary melting-point, and can therefore be used as a standard for the calibration of thermometers thus the transition temperature for Glauber s salt, Na2S04.10H2O, occurs at 32 383° C., whilst that of sodium bromide, NaBr. 2H20, is at 50-674° C.3... 

Comments. Methods for obtaining melting-points are not usually given in the specifications, but it is absolutely necessary that a definite procedure and definite apparatus be agreed upon. The following points should be taken into consideration calibration and stem correction of thermometers, size of tubes, amoimt of substance taken, rate of melting and vessel of liquid to be used as bath. 

Measuremaits. A thermostat bath containing water or glycerine (at temperatures above 95 °C) was used in the melting point determinations. The temperature was regulated to 0.02 C. A calibrated thermometer was used. The melting point was taken as the temperature at which the last crystals of solid became liquid or at which turbidity cleared. 

Your thermometer may be a major source of error in melting point measurement. Occasionally thermometers for routine laboratory use may not be accurate and may read up to 10 °C high or low. To avoid this problem you should always calibrate your thermometer to determine any error and be... 

The accuracy of the melting point depends on the accuracy of the thermometer, so the first exercise in this experiment will be to calibrate the thermometer. Melting points of pure, known compounds will be determined and deviations recorded so that a correction can be applied to future melting points. Be forewarned, however, that the thermometers are usually fairly accurate. 

Determine the melting point of standard substances (Table 1) over the temperature range of interest. The difference between the values found and those expected constitutes the correction that must be applied to future temperature readings. If the thermometer has been calibrated previously, then determine one or more melting points of known substances to familiarize yourself with the technique. If the determinations do not agree within rC, then repeat the process. 

If one considers the amount of time usually spent in the preparation of a new compound, the few minutes required to obtain a good melting point seems little enough to make it inexcusable to report carelessly determined melting points. For this purpose, the use of the Hershberg apparatus and calibrated Anschutz thermometers is strongly recommended. 

For most purposes, it is convenient to use partial-immersion thermometers, because the corrections required at any temperature will usually be smaller. However, regardless of which type is used, it should be calibrated against some thermometers for which the calibrations are known. The common practice of calibrating thermometers by using the melting points of known substances will often lead to considerable errors if the capillary method is used. 

Thermometer correction. The temperature which is read on the thermometric scale must be corrected because there are several errors in such determinations. One source of error arises from the construction and calibration of the thermometer. The bore of the capillary may not have the same diameter throughout further, the scale graduation and the calibration of low-priced thermometers are not very accurate. A second source of error is the method used in the common melting point apparatus. The common thermometer has been calibrated while totally immersed in a bath. In the melting-point apparatus described, only a part of the stem is immersed. The column of mercury above the oil bath has a lower temperature than that at which the thermometer was calibrated. Therefore either a thermometer calibrated by partial immersion should be used or a correction must be made for the unequal heating of the mercury in the stem of the thermometer. Although thermometers calibrated by partial immersion are available, the latter practice is the more common. 

Thus, it has been found that the high temperature scale defined by the platinum resistance thermometer calibrated in terms of the melting point of ice, 0°C., the boiling points of water, 100°C., and of sulphur, 444.6°C., at a pressure of 760 mm. Hg.,... 

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