Time and temperature

The most crucial point for a successful microwave-mediated synthesis is the optimized combination of temperature and time. According to the Arrhenius equation, k = A exp(- a/RT), a halving of the reaction time with every temperature increase of 10 degrees can be expected. With this rule of thumb, many conventional protocols can be converted into an effective microwave-mediated process. As a simple example, the time for a reaction in refluxing ethanol can be reduced from 8 h to only 2 min by increasing the temperature from 80 °C to 160 °C (Fig. 5.1 see also Table 2.4). 

For many transformations, the reaction times are in fact significantly shorter than the Arrhenius equation would predict, probably because of the additional pressure that is developed, or arguably due to the involvement of microwave effects (see Section 2.5). 

When investigating completely new reactions for which no thermal protocol is available, a feasible starting point is approximately 30-40 °C above the boiling point of the solvent used [1]. Know-how on the way to proceed will improve with personal practical experience and varies with the type of chemistry to be accomplished. Per- 

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Although phenol gives these marked colour changes, the test is unsatisfactory with many other phenols, the precise tint obtained varying with the purity of the phenol, amount of reagents used, and temperature and time of heating. 

The physical constants of furfuryl alcohol are Hsted in Table 1. When exposed to heat, acid or air the density and refractive index of furfuryl alcohol changes owing to chemical reaction (51), and the rate of change in these properties is a function of temperature and time of exposure. 

The ethyl acetate is distilled at 70—100°C, leaving spherical particles. This graining operation requires ca 1 to 1.5 h. Grain density and size are determined by the concentration of salt in solution, the temperature and time of the dehydration, agitation speed, and the rate of distillation of the ethyl acetate. 

Sulfur. Sulfur in diesel fuel should be kept below set limits for both environmental and operational reasons. Operationally, high levels of sulfur can lead to high levels of corrosion and engine wear owing to emissions of SO that can react with condensed water during start-up to form sulfuric acids. From an environmental perspective, sulfur bums to SO2 and SO, the exact spHt being a function of temperature and time in the combustion chamber. 

Mercuric Sulfide. Mercuric s A ide[1344 8-5] HgS, exists ia two stable forms. The black cubic tetrahedral form is obtaiaed when soluble mercuric salts and sulfides are mixed the red hexagonal form is found ia nature as cinnabar (vermilion pigment). Both forms are very insoluble in water (see Pigments, inorganic). Red mercuric sulfide is made by heating the black sulfide in a concentrated solution of alkah polysulfide. The exact shade of the pigment varies with concentration, temperature, and time of reaction. 

Fig. 16. Influence of temperature and time on strength during aging where (—) corresponds to the optimum temperature, and (—...

Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

Eor most polymer applications the removal of the inhibitors from the monomer is unnecessary. Should it be requited, the phenolic inhibitors can be removed by an alkaline wash or by treatment with a suitable ion-exchange resia. Uninhibited MMA is sufftcientiy stable to be shipped under carehiUy controlled temperature and time restrictions. Uninhibited monomers should be monitored carehiUy and used promptiy. 

In industrial production of acid-modified starches, a 40% slurry of normal com starch or waxy maize starch is acidified with hydrochloric or sulfuric acid at 25—55°C. Reaction time is controlled by measuring loss of viscosity and may vary from 6 to 24 hs. For product reproducibiUty, it is necessary to strictly control the type of starch, its concentration, the type of acid and its concentration, the temperature, and time of reaction. Viscosity is plotted versus time, and when the desired amount of thinning is attained the mixture is neutralized with soda ash or dilute sodium hydroxide. The acid-modified starch is then filtered and dried. If the starch is washed with a nonaqueous solvent (89), gelling time is reduced, but such drying is seldom used. Acid treatment may be used in conjunction with preparation of starch ethers (90), cationic starches, or cross-linked starches. Acid treatment of 34 different rice starches has been reported (91), as well as acidic hydrolysis of wheat and com starches followed by hydroxypropylation for the purpose of preparing thin-hoiling and nongelling adhesives (92). 

The critical parameters of steam sterilization are temperature, time, air elimination, steam quaUty, and the absence of superheating. Temperature and time are interrelated, as shown in equation 8. The success of steam sterilization is dependent on direct steam contact which can be prevented by the presence of air in the chamber. The abiUty of steam to heat a surface to a given temperature is considerably reduced by the presence of air. Air elimination, therefore, is regarded as an absolute parameter. If the required amount of air has not been eliminated from the chamber and the load, no combination of time and temperature results in complete sterilization. 

Pitch. For the solvent analysis of pitch, a number of methods have been proposed. The solvents may be used sequentially or a fresh sample may be used with each solvent. Either the least or the most powerflil solvent may be used first. The ratio of solvent to pitch or pitch fraction and the temperature and time of extraction vary. 

Other Uses. The production of finished shapes from iron powder by compacting and sintering utilizes about 100,000 t of iron powder aimuaHy copper powder (2—10 wt %) is normally added as a sintering aid. Addition of 2% tin powder or equal amounts of tin and copper powder considerably lowers the sintering temperature and time of sintering at a cost saving. The tin addition also improves dimensional control. Iron powder plus 10 wt % powdered lead—tin metal is pressed and siatered to make pistons for use ia automotive hydrauHc brake cylinders. 

YeUowing of wool can occur during dyeing, depending on pH, temperature and time, and chlorinated wools ate especially sensitive. Bleaching agents that can be added to the dyebath have been developed based on sodium bisulfite and hydroxylamine sulfate (108). Addition of hydrogen peroxide to the dyebath after exhaustion can also be effective. 

Cocoa Powder Manufacture. When chocolate Hquor is exposed to pressures of 34—41 MPa (5000—6000 psig) in a hydrauHc press, and part of the fat (cocoa butter) is removed, cocoa cake (compressed powder) is produced. The original pot presses used in cocoa production had a series of pots mounted vertically one above the other. These have been supplanted by horizontal presses that have four to twenty-four pots mounted in a horizontal frame. The newer presses are capable of complete automation, and by careflil selection of pressure, temperature, and time of pressing, cocoa cake of a specified fat content can be produced. 

Molten chromic acid decomposes at its melting point at a significant rate. The lower oxides formed impart darkness and turbidity to the water solution. Accordingly, both temperature and time are important in obtaining a quaHty product. 

Avoid soldering of cable and wire terminal joinis at such installations, which may loosen with temperature and time. 

Preparation of paint, its viscosity, solvent, thickness of one coat, air pressure, curing temperature and time of curing will remain the same as for the primer (Table A13.4). 

The temperature and time of curing, as indicated in Table A 13.4, are indicative and for general guidance only. They may vary with the type and quality of paint and effectiveness of the furnace. For exact details, consult the paint manufacturer. The operator may also vary the given parameters slightly, based on his own experience and the end results. 

Moulding conditions, i.e moulding pressure, temperature and time. 

Coal-based pitches are predommantly byproducts of metallurgical coke operations in recovery-type coke ovens. The volatile products from the coke oven are recovered and processed, in simplest terms, into gas, light oils, and tar. The quantity and character of the materials are influenced by the type of coal charge, the design of the cokmg equipment, and the temperature and time profile of carboni2ation. Table 1 shows a typical yield of products from the... 

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