Electrometric endpoint detection

Aieta et al. [10] worked out electrometric titrations for the sequential determination of chlorine dioxide, chlorine, chlorite, and chlorate. Phenylarsine oxide or sodium thiosulfate titrants and potentiometric or amperometric endpoint detection are used in their method. 

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. 

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,... 

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. 

Titration of iodine with thiosulfate or phenylarsin oxide titrant is an everyday task in iodometric analysis. The iodine content of water samples, however, is much lower than the detection limit of this titration, even with amperometric endpoint location. Some of the highly sensitive electrometric inverse methods have been successfully used. 

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