Baths, heating

For temperatures up to 100°, a water bath or steam bath is generally employed. The simplest form is a beaker or an enamelled iron vessel mounted on a suitable stand water is placed in the vessel, which is heated by means of a flame. This arrangement may be used for non-inflammable liquids or for refluxing liquids of low boiling point. Since numerous liquids of low boiling point are highly inflammable, the presence of a naked flame will introduce considerable risk of fire. For such liquids a steam bath or an electrically-heated water bath, provided with a constant-level device, must be used. If the laboratory is equipped with a 

The beaker and thermometer should be removed from the metal bath before the latter solidifies. Metal baths have the advantage that they do not smoke or catch fire they are, however, solid at the ordinary temperature and are usually too expensive for general use. 

A satisfactory bath suitable for temperatures up to about 250° may be prepared by mixing four parts by weight of 85 per cent, ortho-phosphoric acid and one part by weight of meta-phosphoric acid the mixed components should first be heated slowly to 260° and held at this temperature until evolution of steam and vapours has ceased. This bath is liquid at room temperatures. For temperatures up to 340°, a mixture of two parts of 85 per cent, ortho-phosphoric acid and one part of meta-phosphoric acid may be used this is solid (or very viscous) at about 20°. 

A shallow metal vessel containing sand, the so-called sand bath, heated by means of a flame, was formerly employed for heating flasks and other glass apparatus. Owing to the low heat conductivity of sand, the temperature control is poor the use of sand baths is therefore not 

The advantages of the above air bath are (1) simplicity and cheapness of construction (2) ease of temperature control (3) rapidity of cooling of the contents of the flask effected either by removing the asbestos covers or by completely removing the air bath and (4) the contents of the flask may be inspected by removing the asbestos covers. 

Hard hydrogenated cotton-seed oil (m 40-60°) or a 1 1 mixture of cotton-seed oil and castor oil containing about 1% of hydroquinone. 

A mixture of 85% orthophosphoric acid (4 parts) and metaphosphoric acid (1 part) 

In using metal baths, the container (usually a metal crucible) should be removed while the metal is still molten. 

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There is one special class of reaction systems in which a simplification occurs. If collisional energy redistribution of some reactant occurs by collisions with an excess of heat bath atoms or molecules that are considered kinetically structureless, and if fiirthennore the reaction is either unimolecular or occurs again with a reaction partner M having an excess concentration, dien one will have generalized first-order kinetics for populations Pj of the energy levels of the reactant, i.e. with... 

In a molecular dynamics calculation, you can add a term to adjust the velocities, keeping the molecular system near a desired temperature. During a constant temperature simulation, velocities are scaled at each time step. This couples the system to a simulated heat bath at Tq, with a temperature relaxation time of "r. The velocities arc scaled bv a factor X. where... 

Yield, 22 -23 g. This product has m.p. 152-155 " when heated from room temperature, and 155 156° when immersed in a heating-bath at 140 . 

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 ... 

For a constant temperature simulation, a molecular system is coupled to a heat bath via a Bath relaxation constant (see Temperature Control on page 72). When setting this constant, remember that a small number results in tight coupling and holds the temperature closer to the chosen temperature. A larger number corresponds to weaker coupling, allowing more fluctuation in temper-... 

Table 11.10 Substances Which Can Be Used for Heating Baths...

Langevin dynamics a technique to reduce the total number of equations of motion that are solved. Utilize the Coupled Heat Bath, wherein the method models the solvent effect by incorporating a friction constant into the overall expression for the force. 

The purification of Hquid nitro alcohols by distillation should be avoided because violent decompositions and detonation have occurred when distillation was attempted. However, if the distillation of a nitro alcohol cannot be avoided, the utmost caution should be exercised. Reduced pressure should be utilised, ie, ca 0.1 kPa (<1 mm Hg). The temperature of the Hquid should not exceed 100°C hot water should be used as the heating bath. A suitable explosion-proof shield should be placed in front of the apparatus. At any rise in pressure, the distillation should be stopped immediately. The only commercially produced Hquid nitro alcohol, 2-nitro-1-butanol, is not distilled because of the danger of decomposition. Instead, it is isolated as a residue after the low boiling impurities have been removed by vacuum treatment at a relatively low temperature. 

For heating baths, see Table 4. For distillation apparatus on a micro or semi-micro scale see Aldrich and other glassware catalogues. Alternatively, some useful websites for suppliers of laboratory glassware are www.wheatonsci.com,www.sigmaaldrich.com and www.kimble-kontes.com. 

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