Stopped method

When we compare Figs. 3.80 and 3.81 with Fig. 3.79, it does not seem entirely logical to call the titration in Fig. 3.79 a dead-stop method, although this has been done the term differential amperometric titration might be more useful. 

The principle of the short-stop method of determining Y.xr, in cationic polymerisations is that to the polymerising solution a basic reagent Q is added which combines with the growing centre and thus forms an end-group in the dead polymer ... 

The short-stop method of measuring [Pn ] was developed to produce an easily determinable end-group in systems in which the natural end-groups are unsuitable for accurate, quantitative determination. It was first used by Jaacks et al., (1968) to determine the concentration of tertiary oxonium ions during the polymerisation of 1,3-dioxolan, and by Saegusa et al., (1968) for similar studies on tetrahydrofuran. For the polymerisation of alkenes it has only been used on two occasions. 

The residues of the organic portion of (he molecule remaining could be oxidized by the potassium dichromate used in (he back titration method and so introduce errors. The dead stop method or the colorimetric method (below) might be preferred in such cases... 

In contrast to the recoil-stopping methods, as used in He-jet systems or mass separators, where ion sources are utilized, recoil-separation techniques use the ionic charge and momentum of the recoiling fusion product obtained in the reaction process. Spatial separation from the projectiles and other reaction products is achieved by combined electric and magnetic fields. The... 

The equilibrium is shifted completely to the right. Thanks to the presence of the phenolate group at the chain ends, the macromolecules could be counted by UV spectrophotometry (polytetramethylene oxide is transparent in the UV range). With the counter-ion B = BF termination was slow with B- = AlCl it was rapid. The active centres can be counted using the Saegusa and Matsumoto method and, together with a determination of the degree of polymerization and the concentration of macromolecules, the elementary constants can be determined (the dead-stop method). 

Ill) face quite slowly. The difference in etch rates is 300 1. The wafer is immersed in the etch and, by carefully controlling the etch time or using an etch stop method, a deep pit is generated. Depending on the thickness of the wafer and the size of the etch window, a thin membrane of single-crystal silicon can be formed at the bottom. 

At present, the biamperometric (dead-stop) method is used virtually exclusively for indicating the end-point in Karl Fischer titrations. In this way the water content may be measured with appropriate accuracy down to a concentration limit of 0.001%. Such a sensitivity is unquestionably necessary, as a water content of 0.01%, for instance, corresponds to a water concentration of 5.56 x 10 M. In a spectroscopic examination of the solvent effect, the concentration of the solvate may... 

A special case in amperometric indication is the "Dead-Stop Method". Current can only flow at the indicator electrodes if both oxidizable and reducible substances are present. The potential difference may be very low (<100 mV). The equivalent point is characterized by the fact that the indicator current is zero, as no reversible ion pairs are present here (e.g. coulometric titration of Fe + with Ce "). 

Start-stop methods — Manual remote automatic... 

Fig. 8. Titration curves obtained when titrating 20 ml of 0.0004 M EDTA with 0, 20, 40, 60, and 80 ml of Lab-trol added to the titrate. The curves are parallel and equally spaced, therefore the concentration of calcium can be estimated from the spacing, provided the titrant concentration is known. The linear portions of the curves provide a convenient guide in determining the control-pointer setting when the automatic-stop method is used to obtain more accurate results.

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