Electrometric end-point detection

An end point in a Karl Fischer titration can be observed visually based on the brown color of the excess reagent. More commonly, however, end points are obtained from electroanalytical measurements. Several instrument manufacturers offer automatic or semiautomatic instruments for performing Karl Fischer titrations. All of these are based on electrometric end point detection. The details of operation of Karl Fischer titrators are discussed in Chapter 22. 

It is certainly clear that a coulometric titration, like any other type of titration, needs an end-point detection system in principle any detection method that chemically fits in can be used, be it electrometric, colorimetric, photoabsorptionmetric, etc. for instance, in a few cases the colour change of the reagent generated (e.g., I2) may be observed visually, or after the addition of a redox, metal or pH indicator the titration end-point can be detected photoabsorptiometrically by means of a light source and photocell combination. Concerning the aforementioned coulometric titration of Fe(II), it is... 

For end-point detection, any method usual in acid-base titration can be used with electrometric indication the precautions for protection against the... 

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. 

Electrical methods of determining the end-point of titrations are widely used some of the advantages of the technique are obvious, such as the ability to titrate coloured solutions where the change of a visual indicator would be difficult or impossible to detect and the ability to carry out titrations for which no suitable visual indicator exists. Electrometric endpoints may often be employed with greater accuracy than visual ones and with greater sensitivity. It should always be remembered, however, that where a suitable visual method of end-point detection is available, it is usually more rapid and more economical to use. Electrometric methods may be classified into potentiometric, conductometric and amperometric methods. 

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. 

Because the end point (formation of a permanent brownish coloration) is not very sharp, the back titration method (Ref 4) was used successfully by Dr Fedoroff at the Keystone Ordn Works, Meadville, Pa and improved at PicArsn (Ref 9a). More recently, an electrometric method for detection of the end point was proposed (Refs 5, 6, 7, 8 and 11 Refs 6 7 are discussions of instrumental design and not primarily about FeS04 to detn nitrate). 

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. 

Electrometric methods are employed to detect the end points of conventional titrations, as well as those of the coulometric titrations described in Section 11.4. These detection methods typically involve two small electrodes in an indicator circuit that is electrically separate from the generating circuit that would be present for a coulometric titration. All electrometric methods are based on measurements of either the potential difference between the two electrodes in the indicator circuit (potentiometric methods) or the current passing in that circuit (amperometric methods). Further classification is based on the... 

Anhydrous ammonia is normally analyzed for moisture, oil, and residue. The ammonia is first evaporated from the sample and the residue tested (86). In most instances, the amount of oil and sediment in the samples are insignificant and the entire residue may be assumed to be water. For more accurate moisture determinations, the ammonia can be dissociated into nitrogen and hydrogen and the dewpoint of the dissociated gas obtained. This procedure works well where the concentration of water is in the ppm range. Where the amount of water is in the range of a few hundredths of a percent, acetic acid and methanol can be added to the residue and a Kad Fischer titration performed to an electrometrically detected end point (89—92). 

Aminosalicylic acid and its salts may be determined either by bromin-ation or by titration with nitrite. The acid itself, official in the t/.5.P., is determined by titration with nitrite, the end-point being detected externally with starch-iodide paste. A more satisfactory method is to determine the end-point electrometrically by the dead-stop technique (see p. 867). A suitable method is as follows ... 

Titrimetric (or volumetric) methods depend on the detection of an end point of the stoichiometry of a reaction. Strictly interpreted, the classical methods are limited to visual indicators of the end point. There are, however, other instrumental modes that include a variety of electrometric, spectrophotometric, spectrofluorimetric, and enthalpic techniques. These latter methods are also amenable to... 

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