Rate of stirring

Increases the speed of dissolving. The solid particles would otherwise soon be surrounded by saturated solution, which would stop more solid dissolving until fresh solvent particles diffused in. The energy input from the hand holding the spoon transfers to moving the particles through the solution. A stirred cup of coffee dissolves the sugar faster  

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Too fast a rate of addition may cause aggregation of the lithium. If this occurs, the addition should be interrupted and the rate of stirring diminished until the aggregate has disintegrated. 

The technique of hydrodynamic modulation voltammetry (HMV), in which the rate of stirring is pulsed between high and low values, is demonstrated in this experiment. The application of HMV for the quantitative analysis of ascorbic acid in vitamin C tablets using the method of standard additions also is outlined. 

Beaded polymeric supports are produced by a two-phase suspension polymerization in which microdrops of a monomer solution are directly converted to the corresponding microbeads. The size of a microdroplet is usually determined by a number of interrelated manufacturing parameters, which include the reactor design, the rate of stirring, the ratio of the monomer phase to water, the viscosity of both phases, and the type and concentration of the droplet stabilizer. 

Because all the variables that influence the properties of the final product are known, one can use a statistical design (known as a one-half factorial) to optimize the properties of the GPC/SEC gels. Factorial experiments are described in detail by Hafner (10). For example, four variables at two levels can be examined in eight observations. From these observations the significance of each variable as related to the performance of the gel can be determined. An example of a one-half factorial experiment applied to the production of GPC/SEC gel is set up in Table 5.2. The four variables are the type of DVB, amount of dodecane, type of methocel, and rate of stirring. 

The time required for the completion of reaction varies with the rate of stirring. When the stirring is very vigorous most of the water comes over within twelve hours. 

Raising the temperature, say to between 70 and 80 °C, often improves the physical properties of the deposit. This is due to several factors, which include the decrease in resistance of the solution, increased rate of stirring and of diffusion. 

Side reactions are avoided at the generating electrode provided there is not complete depletion (at the electrode surface) of the substance involved in the generation of the titrant. The concentration of the titrant depends upon the current through the cell, the area of the generating electrode, and the rate of stirring the concentration of the generating substance is usually between 0.01 M and 0.1 M. 

Influence of the Rate of Stirring on the Rate of Nitration of Toluene. 

Fig 4 Influence of the rate of stirring on the rate of nitration of toluene (Orlova, Ref 62)... 

The checkers heated the suspension for a total of 10 hours, 7-8 hours having been required to dissolve the solid completely. The reaction time may depend on the particle size of the hydrochloride salt and the rate of stirring. 

When increasing the rate of stirring the half-wave potential of a reduction reaction is shifted to more negative values. 

Obviously, in the case of a cathodic reaction, the process at more positive potentials (at the foot of the wave) is reversible while at more negative potentials irreversible. Increasing rate of stirring makes the irreversibility more pronounced. 

The dependence of the limiting current density on the rate of stirring was first established in 1904 by Nernst (N2) and Brunner (Blla). They interpreted this dependence using the stagnant layer concept first proposed by Noyes and Whitney. The thickness of this layer ( Nernst diffusion layer thickness ) was correlated simply with the speed of the stirring impeller or rotated electrode tip. 

It was apparent that when the two immiscible fluids were stirred droplets of chloroform formed in the aqueous phase. It was hypothesised that the response of the phototrode was dominated by light scattered back from the droplets without reaching the mirror and that, as the droplet size decreases, the intensity of the back scattered light increases. This was confirmed in tests by increasing the rate of stirring and so decreasing the droplet size. 

The rate of stirring must be kept fairly constant since an abrupt increase in speed may cause the ether to reflux at an undesirably rapid rate. The rate of addition of the nitrite is also governed by the rate of the refluxing. 

Toward the end of the addition of the alcohol, foaming may occur but may be subdued by reducing the rate of stirring. 

In both regimes, speeding up electrolysis requires diminishing the time constant of the cell [equation (2.32)] by decreasing the volume-to-surface area ratio and/or the thickness of the diffusion layer by increasing the rate of stirring or of electrolyte circulation. 

Reaction temperatures (and therefore rates) may be controlled by the rate of stirring. 

Kinetics show that the reaction is pseudo-first order in the RX concentration and that there is a linear correlation in the rate of consumption of RX with the concentration of the catalyst. The need for a high rate of stirring indicates that, as discussed in Chapter 1, the base-initiated formation of the cobalt tetracarbonyl anion results from an interfacial exchange process. It is significant that, when preformed NaCo(CO)4 is used, the extractability of the anion by benzyltriethylammonium cation into diisopropyl ether is three times less efficient than it is into benzene or dichloromethane, but kinetic studies show that, in spite of the lower concentration of the anion in the ether, the rate of reaction with RX in that solvent is generally higher [3]. 

Consideration of stirring. In stripping voltammetry, it is normal to employ a stationary electrode and a solution which is gently stirred. An alternative method is to have a still solution and an electrode that is rotated. If the solution is stirred, then the rate of stirring should be reproducible and controlled. Exhaustive electrolysis can be performed without stirring but the time required for deposition is likely to be quite long. 

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