Precipitation titrations amperometric

Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory for example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed (acid-base titration). This is because the readings near the equivalence point have no special significance in amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which points the solubility or hydrolysis is suppressed by the Mass Action effect the point of intersection of these lines gives the equivalence point. 

In the indirect amperometric method [560], saturated uranyl zinc acetate solution is added to the sample containing 0.1-10 mg sodium. The solution is heated for 30 minutes at 100 °C to complete precipitation. The solution is filtered and the precipitate washed several times with 2 ml of the reagent and then five times with 99% ethanol saturated with sodium uranyl zinc acetate. The precipitate is dissolved and diluted to a known volume. To an aliquot containing up to 1.7 mg zinc, 1M tartaric acid (2-3 ml) and 3 M ammonium acetate (8-10 ml) are added and the pH adjusted to 7.5-8.0 with 2 M aqueous ammonia. The solution is diluted to 25 ml and an equal volume of ethanol added. It is titrated amperometrically with 0.01 M K4Fe(CN)6 using a platinum electrode. Uranium does not interfere with the determination of sodium. 

Advantages and Limitations of Radiometric Titrations. Radiometric detection of the equivalence point is a general method that does not depend on the chemical reaction employed. This contrasts with other methods of detection, which depend on specific chemical or physical transitions at the equivalence point. Amperometric titrations are applicable only to electrochemically active systems conductometric titrations apply only to ionic solutions, and so on. In principle, any titration system in which a phase separation can be effected is amenable to radiometric detection, provided there exist suitable radioactive labels. The major limitation of the method is the requirement for phase separation. In precipitation titrations, the phase separation is automatic and the method is well suited to this class of titrations. For other classes of titrations, special phase-separation methods, such as solvent extraction, need to be applied. At the present time, the method suffers from a lack of phase-separation techniques suitable for continuous monitoring of the titration curves. 

Amperometric titrations have a number of advantages over other titri-metric methods determinations can usually be carried out in very dilute solutions (about 10 N or even, in certain cases, 10 N) and they are rapid since only a few measurements of current, before and after the end-point, at a constant applied voltage, need be made. It is often possible to carry out titrations in cases where potentiometric or visual-indicator methods are unsuitable such as in acid-base titrations in which the reaction product is hydrolysed or in precipitation titrations in which the precipitate has a significant solubility. There is, too, no interference from foreign electrolytes. 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

A second method which is now probably the most widely used method in the Pediatric Laboratory is to use amperometric titration. In this connection, a constant current flows through the solution. The silver dissolves and reacts stolchlometrlcally with chloride, precipitating silver chloride. When all of the chloride has reacted, there is a sharp increase in conductivity which is read as an end point. This instrument, therefore, measures the amount of time a current flows. Instruments are now available for which 5 microliters can be used routinely, rapidly, titration being of the order of about 20 seconds. 

Polarisation titrations are often referred to as amper-ometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidisable or reducible at the working electrode surface. In general, the polarisation titration method is applicable to oxidation-reduction, precipitation and complex-ation titrations. Relatively few applications involving acid/base titration are found. Amperometric titrations can be applied in the determination of analyte solutions as low as ICE5 M to 10-6 M in concentration. 

Of much greater importance, however, is the amperometric method for precipitation and complex-formation titrations. Here the advantages are as follows ... 

Lemahieu, et al. (89) have reported an amperometric method for the determination of hydrochlorides of several organic bases, including pyridoxine. Titration, with AgN03 solution in dimethyl sulphoxide, of such hydrochlorides in dimethyl sulphoxide gives an end-point corresponding to the formation of AgCl2 and a less sharp end-point for the precipitation of AgCl. 

Precipitation titrimetry — A method for the - titration of species by a - precipitation reaction. Commonly, the - end point of precipitation reactions is monitored by chemical, potentiometric or amperometric methods. A chemical method involves an -> indicator that usually has a change in color at the -> endpoint, while the other methods can be implemented as a -> potentiometric titration or -> amperometric titration, respectively. An important precipitating reagent is silver nitrate, i.e., silver ions Ag+. Such titrations are called argentometric titrations [i], and silver - electrodes are useful as indicator electrodes. 

Table 5 Applications of amperometric titrations with precipitation products...

Several different types of amperometric titration curves are possible. For example, one can titrate a metal ion that shows a voltammetric wave (e.g., Pb " ) with a titrant that causes its precipitation (e.g., Cx20 ). If the potential is held at the plateau of the voltammetric wave, the current will decrease during the titration and remain at the residual current level for/> 1. 

Suliide collectors such as xanthate can react with various metal ions to form various stable complexes and precipitates. Therefore, potentiometric and amperometric titration can be used to test the concentration of sulffde collectors. C. du. Rietz studied the adsorption of xanthate, aeroffoat and fatty acid by means of potentiometric titration. Meantime, hydrolysis and oxidation of xanthate, as well as solubility products of metal compounds of xanthate and fatty acid were determined [2, 3]. In the course of test, the concentration of xanthate was titrated using AgN03 solution. Titration endpoint was calculated according to the electric potentials of silver electrode and calomel electrode. 

The most frequently used titrating agents in polarometric and amperometric titrations are silver and mercury compounds and some heteropoly acids. It should be stressed once more that little selectivity, especially for precipitation reactions, can be expected and thus in the analysis of mixtures the application of polarometric titrations must be preceded by separation techniques. 

Decomposition with HsS liberates sulfuric acid which may be harmful on evaporation. Among others, Cd also forms a fairly insoluble mercaptide with GSH. Ag forms highly insoluble precipitates with nearly all mer-captans. The foregoing reaction and also the one using Hg ions have been used for an amperometric titration of GSSG (41, 42). Disulfides suffer hydrolysis in aqueous solution expressed in the following way by Cecil (43) ... 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

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