Browning temperature coefficients

This method of determining AG was proposed by Bronsted (Z. phj-sikal. Chem. 1911, 77, 284) who attempted the measurements using hydrogen electrodes, but the hydrogen reduced the picric add. By the use of a more indirect electrometric method he obtained — AG = 2.05 kcal mole at 20 °C with a temperature coefficient d AG/dr = — AS = 4 cal deg mole. Brown (J. Chem. Soc. 1925, 345), from cryoscopic measurements in nitrobenzene derived an approximate value of 2.08 kcal mole at 3 C. 

The rate of browning increases with increasing tanperature or time. Since these reactions have been shown to have a high temperature coefficient, lowering of the tanperature during the processing or the storage of food products can help to minimize these reactions. 

Brown and Cohen " state that the temperature coefficient is proportional to <7. However, the equation of Benedek et a/. shows it to be proportional to and hence where K is not expected to be constant between complexes [but is... 

Natural convection occurs when a solid surface is in contact with a fluid of different temperature from the surface. Density differences provide the body force required to move the flmd. Theoretical analyses of natural convection require the simultaneous solution of the coupled equations of motion and energy. Details of theoretical studies are available in several general references (Brown and Marco, Introduction to Heat Transfer, 3d ed., McGraw-HiU, New York, 1958 and Jakob, Heat Transfer, Wiley, New York, vol. 1, 1949 vol. 2, 1957) but have generally been applied successfully to the simple case of a vertical plate. Solution of the motion and energy equations gives temperature and velocity fields from which heat-transfer coefficients may be derived. The general type of equation obtained is the so-called Nusselt equation hL I L p gp At cjl... 

The use of ethylene dichloride as solvent was extended by Brown et al. 11 to the determination of the kinetics of benzoylation of other aromatics, using benzoyl chloride catalysed by aluminium chloride, and the data are included in Table 109 the relative reactivities are thus benzene, 1.0 toluene, 117 o-xylene, 1,393 m-xylene, 3,960 and p-xylene, 243 and these values are closely similar to those obtained with nitrobenzene as solvent. No exact comparison of the coefficients with those of Corriu et al. 16 is possible because of the different temperatures employed, but the rates appear to be comparable for the two sets of data after allowing for reasonable temperature dependencies. 

The kinetics of the decarboxylation of hydroxybenzoic acids have been studied. Brown et a .639 used resorcinol as an acidic solvent, since it was liquid over a wide range of temperatures and was similar to the reaction products the first-order rate coefficients are given in Table 210. The significant decrease in the log A factor... 

By applying this analysis at discrete 10 K temperature intervals to the NMR thermal analysis data for the 102 coals (subdivided into brown coals (<75% C), lower rank (80-85% C) and higher rank (85-90% C) bituminous coals), regression coefficients could be obtained as functions of temperature and hence the average 2T of the three maceral groups were generated (Figures 3-... 

Wu and Farges (1999) have made use of eqn (9.17) relating bond valence to the coefficient of thermal expansion to confirm that it is possible to resolve the different thermal expansions of the long and short Th-O bonds in thorite (o -ThSi04) from XAFS spectra measured between room temperature and 1700K. They also use this relation to estimate the anharmonic corrections needed for the bond lengths determined from XAFS (Brown et al. 1995, pp. 358-9). 

Fig. 20. The dependence on temperature of the coefficient of diffusion of H atoms in H2 molecules at 1 atm ( 2 ) D> Sancier and Wise [79] Wise [82] X, Browning and Fox [83] , Cheng and Blackshear [84] +, Schiff and co-workers [85] I, Chery and Villermaux [67] , Lede and Villermaux [68]. (Courtesy Lede and Villermaux [68].)...

Figure 38 shows the absolute rate coefficients plotted as log o(7Z T - ) versus 10 IT, with the solid line corresponding with the expression (71). Rejecting the measurements of Avramenko and Lorentso [198] and Schott [150] from the evaluation a priori, the results of Browne et al. [203], Eberius et al. [168], and Westenberg and de Haas [208] all lie systematically below this predicted line. However, the above expression for recommended by Baulch and Drysdale [215] also lies above the measurements of Westenberg and de Haas [208] on that reaction, and for the single case where the measurements of the latter authors on reactions (i) and (xxiii) have been carried out at nearly the same temperature, their ratio ki/ki 3 is close to the evaluation of Fig. 37. All their results may therefore be systematically low.

Potassium hexaiodorhenateflV) is a deep brown, nearly black, crystalline substance. It dissolves in methanol, ethanol, and acetone to give dark purple solutions. It can be recrystallized from hot hydriodic acid (57%). It is stable when kept in a desiccator, but in moist air it undergoes slow hydrolysis. The hydrolysis is rapid in neutral or alkaline medium. The magnetic moment at room temperature is 3.43 BM. The high 0 value suggests antiferromagnetic interaction. The absorption spectrum in methanol gives bands at 425, 465, and 510 nm and shoulders at 540 and 645 nm. The spectrum in hydriodic acid (57%) shows bands at 434(6980), 444(6920), 481(4700), 520(6850), 572(5090), and 659 (2280) nm and shoulders at 423(6280), 457(5850), 533(6140), 592(4310), and 690(1860)nm (the values in parenthesis are the molar extinction coefficients). 

An exponential expression with the quotient of alkaline contents and slagging coefficient fg is introduced for the extension of the evaluation to fouling of brown coals on heat exchanger surfaces in the temperature range of O < lOOO C after Lautenschlager (1 ) as well as Below and Rundgin (1 ), but predominantly after interpretation of operational experience with salt coals (Kluge (12)). 

The coefficient of variation of estimated half life were 29% and 46% for ethoprophos and bentazone, respectively. The principal cause of this important user subjective variability was the lack of guidance on the transformation rate dependency on soil temperature. Brown, C.D. et al. compared the outputs from three models operated by five modellers. Differences between the output data from the five modellers using the same model were of a similar magnitude to the variation associated with field measurements. They concluded that model development should seek to reduce subjectivity in the selection of input parameters, and improve the guidance available to users where subjectivity cannot be eliminated. 

Exists in two crystalline forms, a-SijN4 and (J-Si N4. a-Form occurs when powdered Si, N4 is heated to 1200 > 4 hr increasing the temperature to > I450"C for 2 hr produces the -form. White when pure but often brown or black powder, nip 2173 K. (sublimes), d 3.2 kg/dm1. Coefficient of thermal expansion K l 2.5 X 10. ... 

MnP Activity Assay MnP activity was measured by monitoring the oxidation of 2,6-dimethoxyphenol (2,6-DMP) at 469 nm [33]. The reaction mixtures contained 0.4 mM MnS04, 50 mM sodium malonate (pH 4.5), 0.1 mM 2,6-DMP, and MnP enzyme. The extinction coefficient, 4 9, for the orange-brown dimeric product 2,2, 6,6 -tetramethoxydibenzo-l,T-diquinone is 49,600 m -cm " at 469 nm. MnP activity is expressed as the micromoles of product formed per minute (units)/liter (U/1). The culture samples (1 ml) were centrifuged at 10,000xg for 5 min to pellet yeast cells and heme particles, and the clear culture supernatant (10-100 p.1) was added to the reaction mixture. Deionized water was added to a final volume of 1 ml. Addition of H2O2 to a concentration of 0.1 mM was used to initiate reactions at room temperature, and the absorbance at 469 nm was measured after 1 min. 

Despite widespread use of the ideal K-value concept in industrial calculations, particularly during years prior to digital computers, a sound thermodynamic basis does not exist for calculation of the fugacity coefficients for pure species as required by (4-85). Mehra, Brown, and Thodos discuss the fact that, for vapor-liquid equilibrium at given system temperature and pressure, at least one component of the mixture cannot exist as a pure vapor and at least one other component cannot exist as a pure liquid. For example, in Fig. 4.3, at a reduced pressure of 0.5 and a reduced temperature of 0.9, methane can exist only as a vapor and toluene can exist only as a liquid. It is possible to compute vl or f v for each species but not both, unless vl = vy, which corresponds to saturation conditions. An even more serious problem is posed by species whose critical temperatures are below the system temperature. Attempts to overcome these difficulties via development of pure species fugacity correlations for hypothetical states by extrapolation procedures are discussed by Prausnitz. ... 

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