Cooling determination

The observation of CST with practical precision is usually very simple. The two liquids are placed in a test tube and are stirred with a thermometer while heating or cooling until the liquids just mix (while heating) or just cloud (on cooling). Determinations of the cloud point are usually more precise than determinations of the temperature of disappearance of two phases. There is very little risk of subcooling a liquid mixture below the CST, and having it remain homogeneous. When the upper layer becomes small before it disappears, more of the major component of the upper layer is added, and the observation is repeated until the interface disappears near the middle of the system. This is necessary in order to... 

The rate of cooling determines the degree of crystallinity of the extrudate and consequently its dimensions and properties. Very rapid cooling, especially of large-diameter rods, will produce a high internal stress in them. Such parts have to be annealed before they can be machined to close tolerances. 

As quickly as possible after they have cooled, determine the absorbance at 260 nm of the solution in each tube. For a blank, use a sample of dilute saline-citrate (1/10X) that contains no DNA. 

Rate of Cooling Determined from the Initial Linear Decrease in Temperature to 34°C for pNMR, Turbidimetry, and Light-Scattering Spectroscopy ... 

Solar heating/radiative cooling (determined by cloud cover, surface cover, latitude from the equator, time of day, and time of year)... 

A typical study by Simon and Suverkrup [ 170] compared techniques used to characterise the crystallisation behaviour of fatty acid bases. In addition to DSC, isothermal DSC, NMR, oscillation rheometry and thermorheography were used. The crystallisation of hard fats was a biphasic process [170]. The formation of crystal nuclei was controlled by the partial glyceride content the degree of super-cooling determined the formation of crystal nuclei the final solidification level depended on the crystallisation temperature. Similar studies [171] showed that the recrystallisation rates and the degree of supercooling before crystallisation occurred depended on the molecular weight of PEG. 

In Fig. 6.27, the flue gas is cooled to pinch temperature before being released to the atmosphere. The heat releaised from the flue gas between pinch and ambient temperature is the stack loss. Thus, in Fig. 6.27, for a given grand composite curve and theoretical flcune temperature, the heat from fuel amd stack loss can be determined. 

The experimental conditions used to determine the CFPP do not exactly reflect those observed in vehicles the differences are due to the spaces in the filter mesh which are much larger in the laboratory filter, the back-pressure and the cooling rate. Also, research is continuing on procedures that are more representative of the actual behavior of diesel fuel in a vehicle and which correlate better with the temperature said to be operability , the threshold value for the Incident. In 1993, the CEN looked at two new methods, one called SFPP proposed by Exxon Chemicals (David et al., 1993), the other called AGELFI and recommended by Agip, Elf and Fina (Hamon et al., 1993). 

Fig. XVII-5. Schematic detector response in a determination of nitrogen adsorption and desorption. A flow of He and N2 is passed through the sample until the detector reading is constant the sample is then cooled in a liquid nitrogen bath. For desorption, the bath is removed. (From Ref. 28. Reprinted with permission from John Wiley Sons, copyright 1995.)...

The acconunodation coefficient for Kr on a carbon filament is determined experimentally as follows. The electrically heated filament at temperature 72 is stretched down the center of a cylindrical cell containing Kr gas at 7. Gas molecules hitting the filament cool it, and to maintain its temperature a resistance heating of Q cal sec cm is needed. Derive from simple gas kinetic theory the expression... 

By working In the above order, time will not be wasted by having to allow the apparatus to cool between consecutive determinations. 

The oxime is freely soluble in water and in most organic liquids. Recrystallise the crude dry product from a minimum of 60-80 petrol or (less suitably) cyclohexane for this purpose first determine approximately, by means of a small-scale test-tube experiment, the minimum proportion of the hot solvent required to dissolve the oxime from about 0-5 g. of the crude material. Then place the bulk of the crude product in a small (100 ml.) round-bottomed or conical flask fitted with a reflux water-condenser, add the required amount of the solvent and boil the mixture on a water-bath. Then turn out the gas, and quickly filter the hot mixture through a fluted filter-paper into a conical flask the sodium chloride remains on the filter, whilst the filtrate on cooling in ice-water deposits the acetoxime as colourless crystals. These, when filtered anddried (either by pressing between drying-paper or by placing in an atmospheric desiccator) have m.p. 60 . Acetoxime sublimes rather readily when exposed to the air, and rapidly when warmed or when placed in a vacuum. Hence the necessity for an atmospheric desiccator for drying purposes. 

Anilides, (a) To 1 ml. of aniline in a small conical flask add very slowly and carefully about i ml. of acetyl chloride. A vigorous reaction occurs and a solid mass is formed. Add just sufficient water (about 15 ml.) to dissolve the solid completely on boiling. On cooling, crystals of acetanilide separate out filter and determine the m.p. 

This method of Molecular Weight determination should be used only with solvents in which the particular substance is freely soluble, since it is essential that, on cooling, the solvent, and hot the solute, should crystallise out. 

Benzene. Pure benzene (free in particular from toluene) must be used, otherwise the freezing-point is too low, and crystallisation may not occur with ice-water cooling alone. On the other hand, this benzene should not be specially dried immediately before use, as it then becomes slightly hygroscopic and does not give a steady freezing-point until it has been exposed to the air for 2-3 hours. Many compounds (particularly the carboxylic acids) associate in benzene, and molecular weights determined in this solvent should therefore be otherwise confirmed. 

Cyclohexane. An excellent sohent for many determinations, particularly as, owing to the high value of K, a large fall in the freezing-point is obtained, and the accuracy of the determination is therefore correspondingly increased. Care should be taken to avoid super-cooling, however, as it has a marked effect on the true freezing-point of cyclohexane solutions. 

The absorption tubes are then detached from each other, transferred to their stand, and polished, matured and weighed exactly as at the beginning of the experiment, taking care that no particles of rubber or wax are left in the side-arms. They must be repolished, matured and reweighed before they are used for a further determination. Finally when the tube is cool, the tap Tj of the purification train may be closed and the oxygen supply shut off. 

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