Method dead stop

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 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... 

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). 

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 "). 

The so-called dead-stop method of end-point detection is a special case and one which is of considerable importance in pharmaceutical work. Two identical platinum electrodes are placed in the solution to be titrated and a current of a few millivolts is applied across them. The end-point of the titration is shown by a sudden change in the current flowing between the two electrodes. In the titration of iodine with a thiosulphate solution both electrodes are depolarised during the titration and hence a current flows through the cell but at the end-point only one of the electrodes remains depolarised and the current ceases to flow. This has given rise to the term dead-stop end-point to describe this type of titration. In practice, however, the converse usually applies no current passes until the equivalence-point when depolarisation of the electrodes occurs and a current flows, as shown by a sudden permanent deflection on a galvanometer in series. Details of a suitable circuit and procedure are as follows ... 

The specimen is added to a solvent and titrated with electrolytically generated bromine at room temperature. The end point is determined by a dead-stop method. The time of titration is proportional to the bromine added to the specimen. 

The reverse titration of thiosulphate with iodine is depicted in Fig. 3.81 and is often called the "reversed dead-stop end-point method . 

One of the most important applications of the dead-stop end-point method is the Karl Fischer titration of water the titrant usually consists of I2 amd S02 with pyridine in methanol, which reacts with H20 as follows ... 

This technique with two indicator electrodes, proposed in 1956 by Dubois and Walisch146, is an intermediate between bipotentiometry and biamperometry, because neither the current nor the potential across the electrodes of the same metal are kept strictly constant. The authors opinion, e.g., in the titration of iodide with bromate in hydrochloric acid, that the AE value does not matter much, has been contradicted by Kies (ref. 141a, p. 18). As a method of alkalimetry or acidimetry it cannot be preferred, like any other dead-stop technique to the usual glass electrode methods147. Nevertheless, the fact that the apparatus permits a choice of adjustment towards constant current or constant potential can be useful, but then the method approaches either bipotentiometry or biamperometry. 

By this method the weighed dry product is dissolved in methanol and titrated with the Karl Fischer solution until the color changes from brown to yellow. The visual observation can be replaced by an ammeter, which shows an steep increase in current, when the titration is terminated (dead-stop-titration). The samples can be two to four times smaller than for the gravimetric method. To avoid the visual observation completely, iodine can be produced by electrolyzation and the water content is calculated by Coulomb s law. Such an apparatus (e. g. Fig. 1.97.1 and 1.97.2) is available commercially. The smallest amount of water to be detected by such instruments is 10 pg. Wekx and De Kleijn [1.84) showed, how the Karl Fischer method can be used directly in the vial with the dried product. The Karl... 

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 ... 

End-point Detection The end-point of the Karl Fischer titration may be determined quite easily by adopting the electrometric technique employing the dead-stop end-point method. When a small quantum of e.m.f. is applied across two platinum electrodes immersed in the reaction mixture, a current shall tend to flow till free iodine exists, to remove hydrogen and ultimately depolarize the cathode. A situation will soon arise when practically all the traces of iodine have reacted completely thereby setting the current to almost zero or veiy close to zero or attain the end-point. 

Hi) Amperometric titrations with twin-polarized microelectrodes (or Biamperometric Titrations or Dead-stop-end-point method). 

AMPEROMETRIC TITRATIONS WITH TWIN-POLARIZED MICROELECTRODES (BIAMPEROMETRIC TITRATIONS OR DEAD-STOP-END-POINT METHOD)... 

Dead-stop-end-point method was first introduced by Foulk and Bawden in 1926. Evidently, this particular technique is a modification of the classical amperometric titration. This technique is specifically applicable to only such systems where the phenomenon of oxidation-reduction exists both before as well as after the equivalence point has been duly accomplished. 

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

Amperometric Titrations with Twin-Polarized Microelectrodes (Biamperometric Titrations or Dead-Stop-End-Point Method)... 

The oxidative titration of chlorpromazine with ceric sulfate or KBrOj-KBr in acid solution has been described, with the end point being determined by a dead-stop end point technique [55]. A similar method involving visual or potentiometric detection of the end point was also reported [56]. 

A colorimetric method based on the violet color produced by ferrous sulfate in sulfuric acid in the presence of N03 was announced by English in 1947 (Ref 9) and applied to the determination of NA and nltrosylsulfuric acid (NSA) in spent mixed acid. Since then a number of papers (Refs 13, 14, 15, 16 and 17) have extended the method to NOa, organic nitrates and RDX -HMX mixtures Analytical Methods The visual determination of the endpoint (appearance of a permanent brown color) in the dead-stop titrimetric method is reported accurate to 0.03ml (Ref 3) and was used recently by Frejacques and LeClercq (Ref 12) for analysis of Pentolites, Tetryls, NGu, NG prepns, and Nitroethane-EGDN expls. The endpoint has been criticized as difficult to see, and if problems arise, electrometric methods are available for detection (Refs 6,... 

Detn of N in NC by ferrous-titanous titrimetric method) 37)S.Sandi G.Flanquart, ChimAnal(Paris) 39, 20-4(1957) CA 51, 7943 (1957)(Detn of N in NC, NG, PETN, etc by titration with ferrous sulfate soln using the dead-stop end point method) 38)Y.Lacroix er al, MP 39, 459-68(1957) CA 52, 19688(1958)... 

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