Dead-stop end-point titration

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.

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. ... 

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. 

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. 

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 ... 

In other words, the small excess of HN02 present at the end-point can be detected visually by employing either starch-iodide paper or paste as an external indicator. Thus, the liberated iodine reacts with starch to form a blue green colour which is a very sensitive reaction. Besides, the end-point may also be accomplished electrometrically by adopting the dead-stop end-point technique, using a pair of platinum electrodes immersed in the titration liquid. 

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. 

Figure 14.1 illustrates a simple dead-stop end-point assembly or a Karl Fischer titration apparatus. The titration vessel is fitted with a pair of identical platinum electrodes, a mechanical stirrer with adjustable speed, and a burette. It will be observed that absolutely little or no current may flow unless and until the solution is totally free from any polarizing substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the anode and cathode respectively. However, the current shall flow only when the two electrodes... 

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]. 

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)... 

From these equations it can be seen that each mole of water requires one mole of I2. In a visual endpoint Karl Fischer titration, a sample is titrated with the Karl Fischer reagent until a permanent iodine color (indicating that all water has been reacted) is observed. Because of other reaction products, the color change is usually from a yellow to a brownish color, which may be difficult to detect visually. Highly colored samples may affect the visual end point as well. A much sharper end point, known as the dead stop end point, can be obtained if the titration is done electrometrically. Here, two small platinum electrodes dip into the titration cell, a small constant voltage is impressed across these electrodes, and any current that flows is measured with a galvanometer. At the end point of the titration the current either goes to a minimum or else increases suddenly from nearly zero. Commercially available Karl Fischer instruments incorporate semiautomatic microprocessors based on this principle. 

R. A. Verdini and C. M. Lagier, Voltametric iodometric titration of ascorbic acid with dead-stop end-point detection in fresh vegetables and fruit samples, J. Agric. Food. Chem., 48 (2000) 2812-2817. 

---