Thermometers complete-immersion

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 ... 

The thermometer is held in the flask by means of a I clean bored cork the bulb of the thermometer must be completely immersed in the vapour and hence must be below the junction of neck and side tube. 

The reaction mixture is then cooled by an ice-salt mixture. The separatory funnel is replaced by a two-hole rubber stopper containing a thermometer (bulb immersed in the reaction mixture) and a glass tube drawn out to make a fine capillary (Note 5). When the temperature of the reaction mixture has reached — 7°, the condenser is replaced by an entry tube, 10 mm. in diameter and adjusted so the end is about 50 mm. above the surface of the reaction mixture (Note 6). The reaction mixture is stirred, and dry carbon dioxide is added through this tube (Note 7). The rate of flow of the carbon dioxide is regulated so that the temperature of the reaction mixture does not rise above —2°. The time required for the completion of the reaction varies from one and one-fourth to one and one-half hours. When the reaction is complete, the temperature falls below —7° and does not rise on increasing the rate of flow of carbon dioxide. 

Complete-immersion thermometers. Thermometers that require the... 

The use of complete-immersion thermometers is fairly obvious. However, it is not always possible or practical to completely immerse a thermometer. For example, if the solution is not transparent, it is not possible to see the temperature. 

The thermometer should be inserted so that the thermometer is immersed to the 0.0°C level (if calibrating a complete-immersion thermometer) or inserted to the immersion line (if calibrating a partial-immersion thermometer). The thermometer is likely to require support to maintain its proper position. Let the entire apparatus sit for 15 to 30 minutes, to reach equilibrium. Periodically add more ice, as needed, and remove any excess water with the siphon. If the ice is kept clean and... 

Introduce the stirrer-heater unit and thermometer into the calorimeter, making sure that the thermometer bulb (or any other temperature sensor) is completely immersed (level with or just below the heating coil). The irmer vessel should be held in a hole in the calorimeter cover by a split stopper so that it does not rest on the bottom of the Dewar flask. 

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 that all the thermometers listed above except the ground-joint type (Fig. 363) are calibrated for total immersion in other words, they read correctly only when the whole of the mercury thread is at the measured temperature. Short thermometer wells can thus give rise to appreciable errors. According to Piatti and Marti [20] the error is small only if the mercury bulb is just immersed in the oil in the well. In practice, total immersion thermometers should be mounted so as to be completely immersed in the vapour or liquid, although the ease of reading is thereby adver.sely affected. Otherwise a correction for emergent stem has to be applied (r/. 

This pressure is substantially greater than when only air is placed in the cylinder and thus the precision is superior. However, the design of the thermometer is much more difficult due to the possibility of vapor condensation in the Bourdon tube or in the tubing connecting the two. Furthermore, the lines connecting the tube to the gauge should also be completely immersed in the measured fluid to reduce ambiguity with respect to possible temperature variations. 

The advantage of the complete immersion thermometer is that the entire glass casing is at the same temperature as the system to be measured the correction factor is easily calculated. Calculation of volume expansion of the glass casing for partial and total immersion thermometers is less obvious since the bulb and part of the stem will measure the system temperature whereas the upper part of the stem will be at a different temperature. When the temperature to be measured is higher than the calibration temperature, the liquid level attained will be lower than anticipated since the glass will expand. When total immersion thermometers are only partially immersed in the medium to be measured, a correction factor is applied ... 

The discussion of some general principles of temperature measurements and thermometers is summarized in Fig. 3.3. The general method of temperature measurement is easily understood. One must first bring the thermometer, a system of known thermal properties, into intimate contact with the unknown system. For a mercury-in-glass thermometer this usually means complete immersion into the unknown system. Next, equilibration must be awaited. Finally, one must check whether a temperature correction is necessary... 

Proceed as iit the cloud point test with the difference that the thermometer bulb is just completely immersed in the oil, instead of touching the bottom (Figure 4.3). 

FIG. M-1 Partial-, total-, and complete-immersion thermometers. (Source Demag Delaval.)... 

A complete-immersion thermometer is designed to indicate the temperature correctly when used with the bulb, the entire liquid column in the stem, and the gas above the liquid exposed to the temperature being measured. 

Dissolve 15 ml. (15-4 g.) of aniline in a mixture of 40 ml. of concentrated hydrochloric acid and 40 ml. of water contained in a 250 ml. conical flask. Place a thermometer in the solution, immerse the flask in a mixture of ice and water, and cool until the temperature of the stirred solution reaches 5°. Dissolve I2 5 g. of powdered sodium nitrite in 30 ml. of water, and add this solution in small quantities (about 2-3 ml. at a time) to the cold aniline hydrochloride solution, meanwhile keeping the latter well stirred by means of a thermometer. Heat is evolved by the reaction, and therefore a short interval should be allowed between consecutive additions of the sodium nitrite, partly to allow the temperature to fall again to 5°, and partly to ensure that the nitrous acid formed reacts as completely as possible with the aniline. The temperature must not be allowed to rise above 10°, otherwise appreciable decomposition of the diazonium compound to phenol will occur on the other hand, the temperature... 

Dichlorobutane. Place 22-5g. of redistilled 1 4-butanediol and 3 ml. of dry pyridine in a 500 ml. three necked flask fitted with a reflux condenser, mechanical stirrer and thermometer. Immerse the flask in an ice bath. Add 116 g. (71 ml.) of redistilled thionyl chloride dropwise fix>m a dropping funnel (inserted into the top of the condenser) to the vigorously stirred mixture at such a rate that the temperature remains at 5-10°. When the addition is complete, remove the ice bath, keep the mixture overnight, and then reflux for 3 hours. Cool, add ice water cautiously and extract with ether. Wash the ethereal extract successively with 10 per cent sodium bicarbonate solution and water, dry with anhydrous magnesium sulphate and distil. Collect the 1 4-dichloro-butane at 55-5-56-5°/14 mm. the yield is 35 g. The b.p. under atmospheric pressure is 154 155°. 

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