Ostwald

German silver, platinoid, or manganin, the last is the best on account of its low temperature coefficient. The wire must be calibrated. For high resistances (itooo-200,000 Ohms) the special form of resistance 

The apparatus is arranged in the form of Kirchhoff s modification of the Wlieatstone bridge (Fig. 8), the connections being made with stout copper wire. The induction coil is enclosed in a sound tight case, or is 

The value of k is determined by measuring the conductivity of N/50 solution of potassium chloride, for which Kohlrausch found the values  

A method has been described for determining the basicity of acids based on the alterations which they exhibit in electrolytic conductivity on the addition of alkali. Instead of the telephone and induction coil, a double 

A neat form of apparatus much smaller than Kohl-rausch s, consists of an ebonite cup and rod fitted with platinum electrodes prepared in the usual manner. The rod can be moved vertically over the cup by means of micrometer screws, and the distance between the electrodes read off on a divided scale by a vernier. Quantities of solutions as small as 3 cc maybe employed. 

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Ostwald ripeniDg A process of crystal growth in which a mixture of coarse and fine crystals of a substance are left in contact with a solvent. This results in a growth of the large crystals and the ultimate disappearance of the fine crystals. 

At equilibrium, crystal growth and dissolving rates become equal, and the process of Ostwald ripening may now appear, in which the larger crystals grow at the expense of the smaller ones. The kinetics of the process has been studied (see Ref. 103). 

In principle, then, small crystals should show a higher solubility in a given solvent than should large ones. A corollary is that a mass of small crystals should eventually recrystallize to a single crystal (see Ostwald ripening. Section IX-4). 

A3.3.4 LATE-STAGE GROWTH KINETICS AND OSTWALD RIPENING... 

Eigen M 1996 Die unmessbar schnellen Reaktionen (Ostwalds Klassiker der exakten Naturwissenschaften) vol 281 (Thun und Frankfurt Harri Deutsch)... 

A definition of catalysis similar to that given above was stated first in about 1895 by Wilhelm Ostwald, whose work on catalysis was recognized with a Nobel prize. Sixty years before, Jakob Berzelius had coined the tenn... 

Since zeolites are metastable crystallization products tliey are subject to Ostwald s mle which states tliat metastable phases are initially foniied and gradually transfonii into tlie tlieniiodynaniically most stable product. The least stable zeolitic phase (tliat witli tlie lowest framework density) is tlierefore foniied first and consumed with furtlier syntliesis time at tlie expense of a more stable phase due to a continuous crystallization/redissolution equilibrium. 

Ostwald W 1917 Theoretical and Applied Colloid Chemistry (London Chapman and Hall)... 

A common choice of functional relationship between shear viscosity and shear rate, that u.sually gives a good prediction for the shear thinning region in pseudoplastic fluids, is the power law model proposed by de Waele (1923) and Ostwald (1925). This model is written as the following equation... 

The basic design is that of the Ostwald viscometer a U-tube with two reservoir bulbs separated by a capillary, as shown in Figure 24a. The Hquid is added to the viscometer, pulled into the upper reservoir by suction, and then allowed to drain by gravity back into the lower reservoir. The time that it takes for the Hquid to pass between two etched marks, one above and one below the upper reservoir, is a measure of the viscosity. In U-tube viscometers, the effective pressure head and therefore the flow time depend on the volume of Hquid in the instmment. Hence, the conditions must be the same for each measurement. 

Fig. 24. (a) Ostwald glass capillary viscometer, (b) Cannon-Fenske viscometer, and (c) Ubbelohde viscometer. 

The original Ostwald viscometer has been modified in many ways, and a number of different versions are on the market (Table 4) (21). Most are available with a wide choice of capillary diameters and therefore a number of viscosity ranges. A number of viscometers are described in ASTM D445, which also Hsts detailed recommendations on dimensions and methods of use. 

Measured ia 95% ethanol at 25°C usiag an Ostwald-Cannon-Fenske viscometer. 

A catalyst is a substance that iacreases the rate of approach to equiUbrium of a chemical reaction without being substantially consumed itself. A catalyst changes the rate but not the equiUbrium of the reaction. This definition is almost the same as that given by Ostwald ia 1895. The term catalysis was coiaed ia ca 1835 by Ber2eHus, who recognized that many seemingly disparate phenomena could be described by a single concept. For example, ferments added ia small amounts were known to make possible the conversion of plant materials iato alcohol and there were numerous examples of both decomposition and synthesis reactions that were apparendy caused by addition of various Hquids or by contact with various soHds. 

E. Jacobson, Basic Color. An Interpretation of the Ostwald Color System, P. Theobald, Chicago, lU., 1948. 

A further problem is possible if the reinforcements are very small. Coarsening of the particles or whiskers may occur driven by Ostwald ripening, in which large particles grow through diffusional transport at the expense of smaller ones. This can be minimized by choosing matrices in which the reinforcement elements have very low solid solubilities and diffusion coefficients. Platelets, however, have been shown to be more resistant to coarsening than particles or whiskers. 

P. E. Ostwald, Engineering Cost Estimating 3rd ed., Prentice-Had, Englewood Cliffs, N.J., 1992. 

There aie a number of major indusuial problems in the operation of the steam reforming of metlrane. These include the formation of carbon on the surface of the catalyst, the sulphidation of the catalyst by the H2S impurity in commercial natural gas, and die decline of catalytic activity due to Ostwald ripening of the supported catalyst particles by migration of catalyst atoms from the smaller to tire larger particles, as the temperamre is increased. A consideration of tire thermodynamics of the principal reaction alone would suggest that the reaction shifts more favourably to the completion of the reaction as the temperature is increased. 

Greenwood (1956) described the behaviour of an assembly of n groups of particles undergoing Ostwald ripening by solution-diffusion controlled transfer between particles according to a general relationship... 

Figure 6.7 The model for atom transfer during Ostwald ripening, showing the flux of atoms from the smaller particle to the larger...

In the total particle size distribution, some particles of small diameter decrease in radius, and those in the larger diameter range increase in radius during Ostwald ripening. There will therefore be a radius at which particles neither decrease nor grow in size and if [Pg.210]

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