Dead-stop titration

A dead-stop titration curve is produced if Ag+ is titrated with a halide using a pair of identical silver electrodes. Only whilst both Ag+ and Ag are present will a current flow in the cell, and this is linearly related to the Ag+ concentration. Bi-amperometric titrations require only simple equipment but generally give poorer precision because the currents measured are not necessarily on the limiting current plateau. 

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... 

By this method the weighed dry product is dissolved in methanol and titrated with Karl Fischer solution until the color changes from brown to yellow. The visual observation can be replaced by an ammeter, which shows a steep increase in current when the end-point of the titration is reached (dead-stop titration). The samples can be two to four times smaller than for the gravimetric method. To avoid the visual observa-... 

Biamperometry — Whereas in amperometry the -> current is limited by the electrode process proceeding at one indicator electrode (and the -> counter electrode has no effect), in biamperometry the current flowing between two indicator electrodes is measured, i.e., both electrodes can limit the overall current. This approach is useful in following some -> titrations, and it may lead to zero current (dead-stop) at the equivalence point (dead-stop titration). Example iodine in an iodide solution is titrated with As(III). Two platinum electrodes with a potential difference of around 100 mV prompt iodine to be reduced on one electrode and iodide being oxidized at the other. The two processes maintain an almost constant current until the endpoint when iodine is exhausted. 

A dead-stop titration curve is produced if Ag is titrated with a halide using a pair of identical silver electrodes. Only whilst both Ag and Ag arc... 

Utility-grade pH meters usually cost about 100-300. Most are battery operated, and thus portable generally they offer enough sensitivity to be used in many quality-control applications and out in the field. Their relative accuracy is about +0.1 pH unit, and they have taut-band meter movements. General-purpose pH meters are more often line operated, and cost about 300-700. For the extra cost, they usually offer better stability and accuracy ( 0.05 pH or 3 mV), larger taut-band scales, and extra features such as a recorder output, mV scales, and a constant-current jack for performing polarized electrode measurements or dead-stop titrations such as the Karl Fischer titration for water determination. 

The shape of the amperometric titration curve in this case, where both the titrant and the substance titrated undergo reversible redox reactions, is illustrated in Figure 3.21A. In the case where the substance titrated does not have a reversible voltammetric wave, the titration curve will have the shape illustrated in Figure 3.2IB. Prior to the equivalence point, the applied voltage is too small to cause both oxidation and reduction of the redox couple of the substance titrated. If the titrant has an irreversible wave, the titration curve will look like that in Figure 3.21C. This type of titration is commonly called a dead-stop titration, because the indicator current falls to zero at the equivalence point. 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

Larson and Jenness (89,90) have adapted the apparatus used for the dead stop titration (40,130) to the amperometric determination of —SH groups with o-iodosobenzoic acid. The —SH groups are oxidized by o-iodosobenzoate at pH 7 a mixture of acidified potassium iodide and sodium thiosulfate is then added. o-Iodosobenzoate is added to the first appearance of free iodine as determined amperometrically. Iodine can be used directly but this reagent might oxidize —SH groups beyond the —SS— stage and might react with tyrosine and tryptophane residues. The procedure of Larson and Jenness may be of particular value in the analysis of turbid suspensions such as milk or in the presence of denatured proteins. With such suspensions evaluation of the end point with starch in conventional titrations with sodium thiosulfate is difficult. 

Figure 14.2(a) is a schematic diagram of a suitable circuit for coulometric titration with internal generation of titrant and using the dead-stop or... 

TITRATION OF THIOSULPHATE WITH IODINE ( DEAD-STOP END POINT )... 

Discussion. Dead-stop end point titrimetry may be applied to the determination of nitrate ion by titration with ammonium iron( II) sulphate solution in a strong sulphuric acid medium ... 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

Fig. 3.79. Dead-stop end-point titration, i.e. measuring the current across two Pt-IE s with constant potential difference AE (differential amperometric titration), curves being obtained from Fig. 3.78.

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

Instrumentally it is common to apply a "magic eye as a very sensitive and easily perceptible indication of the dead-stop transition of the titration endpoint. 

Fig. 3.80. Dead-stop end-point titration of I3 with thiosulphate.

Fig. 3.81. Reversed dead-stop end-point titration of thiosulphate with I3. ... 

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. 

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 ... 

Fig. 3.82. Dead-stop end-point titration of (5 ml 0.0983 M) I with (0.01662 M) I03 in 1 iV HC1 by Kies (titration reactions according to Andrews).

Instead of following a dead-stop end-point titration as is usual in amperometry, it is often attractive to do so potentiometrically, i.e., at constant current, for two reasons ... 

In fact, this has already been illustrated in Fig. 3.73 for the differential electrolytic potentiometric titration of Ce(IV) with Fe(II), both being reversible systems. This technique can be usefully applied, for instance, to the aforementioned KF titration of water and its reverse titration (cf., Verhoef and co-workers preference for bipotentiometric detection) in these instances the potentiometric dead-stop end-point titration and the reversed potentiometric dead-stop end-point titration, respectively, yield curves as depicted in Fig. 3.83. 

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. 

Fig. 3.83. Potentiometric dead-stop (1) and reversed dead-stop (2) end-point titrations, (a) Metrohm Polarecord 626 PARC Model 174A Polarographic Analyzer, (b) Radiometer ISS 820 Ion Scanning System, (c) Tacussel PRG4 Polarograph. 

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