Amperometric end-point

Ey and E2 are the indicator electrodes. These may consist of a tungsten pair for a biamperometric end point for an amperometric end point they may both be of platinum foil or one can be platinum and the other a saturated calomel reference electrode. The voltage impressed upon the indicator electrodes is supplied by battery B (ca 1.5 volts) via a variable resistance Rs N records the indicator current. For a potentiometric end point Ey and E2 may consist of either platinum-tungsten bimetallic electrodes, or Ey may be an S.C.E. and E2... 

Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the amperometric end-point with the Karl Fischer reagent. Quickly add 0.2 g of prednisolone sodium phosphate sample, stir for 1 minute and again titrate to the amperometric end-point with the Karl Fischer reagent. The difference between the two titrations gives the volume (v) of Karl Fischer reagent consumed by the sample. 

Explain how the amperometric end-point detector in Figure 17-8 operates. 

The potentiometric titration curves and indicators for the Fe(II)-dichromate reaction are discussed in Section 15-3. In general, from curves such as in Figure 15-2, the dichromate potential has been found to increase with acidity, as expected. The variations with the nature of the acid, however, have not yet been explained. In particular, the dichromate potential is so low in 0.1 M perchloric acid that the potential break is barely discernible. From the practical viewpoint it should be emphasized that the rate of attainment of electrode equilibrium, particularly near the end point and beyond it, becomes slower with increasing dilution. Nevertheless, the reaction itself proceeds quantitatively and reasonably rapidly even at extreme dilution. As little as 1 ng of chromium in 100 ml of solution ( 10 M dichromate) has been successfully titrated with an accuracy of 1% by use of an amperometric end point. 

The determination of catecholamines requires a highly sensitive and selective assay procedure capable of measuring very low levels of catecholamines that may be present. In past years, a number of methods have been reported for measurement of catecholamines in both plasma and body tissues. A few of these papers have reported simultaneous measurement of more than two catecholamine analytes. One of them utilized Used UV for endpoint detection and the samples were chromatographed on a reversed-phase phenyl analytical column. The procedure was slow and cumbersome because ofdue to the use of a complicated liquid-liquid extraction and each chromatographic run lasted more than 25 min with a detection Umit of 5-10 ng on-column. Other sensitive HPLC methods reported in the literature use electrochemical detection with detection limits 12, 6, 12, 18, and 12 pg for noradrenaline, dopamine, serotonin, 5-hydroxyindoleace-tic acid, and homovanillic acid, respectively. The method used very a complicated mobile phase in terms of its composition while whilst the low pH of 3.1 used might jeopardize the chemical stability of the column. Analysis time was approximately 30 min. Recently reported HPLC methods utilize amperometric end-point detection. 

Phenoxyacetic acid was separated from fermentation fluids, crude phenoxymethyl penicillin and finished pharmaceutical products. Determinations of the separated acid were made by ultraviolet spectrophotometry or a bromometric titration using an amperometric end point 68. a study of phenoxyacetic acid in penicillin V by ultraviolet spectroscopy has been described recently 6,... 

Three types of end points are encountered in titrations with silver nitrate (1) chemical, (2) potentiometric, and (3) amperometric. Three chemical indicators are described in the sections that follow. Potentiometric end points are obtained by measuring the potential between a silver electrode and a reference electrode whose potential is constant and independent of the added reagent. Titration curves similar to those shown in Figures 13-3, 13-4, and 13-5 are obtained. Potentiometric end points are discussed in Section 21C. To obtain an amperometric end point, the current generated between a pair of silver microelectrodes in the solution of the analyte is measured and plotted as a function of reagent volume. Amperometric methods are considered in Section 23B-4. 

ASP, anodic stripping polarography CRP, cathode-ray polarography DME, dropping mercury electrode CSP, cathodic stripping polarography Amp., amperometric end point. 

The third important source of error comes with the end point detection. In case of potentiometric detection, these errors are related mostly to cell temperature measurement and temperature differences between the two electrodes [8]. If amperometric end point detection is used, the temperature control and the aging of the diffusion barrier can be serious sources of error [9]. 

Bruckenstein S, Johnson DC (1964) Coulometric diffusion layer titrations using the ring-disk electrode with amperometric end point detection. Anal Chem 36 2186-2187... 

An amperometric end point may be used with the two-phase titration of amphoterics at low pH with dodecylsulfate. Iron(II) 1,10-phenanthroline is added as the redox indica-... 

These titrations can be done manually using a starch indicator for end point detection or more accurately by amperometric methods. 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

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. 

Apparatus. Set up the apparatus as in Section 14.10 with two small platinum plates connected to apparatus for the amperometric detection of the end point. 

Amperometry refers to measurement of current under a constant applied voltage and under these conditions it is the concentration of the analyte which determines the magnitude of the current. Such measurements may be used to follow the change in concentration of a given ion during a titration, and thus to fix the end point this procedure is referred to as amperometric titration. 

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. 

Constant- 2. Differential electrolytic 3. Amperometric 4. Dead-stop end-point... 

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.

In the first group the titrant is generated either directly from a participating or active electrode, or indirectly from an inert or passive electrode, in which case it is necessary to add previously an auxiliary substance that generates the titrant by either cathodic reduction or anodic oxidation the end-point detection is usually potentiometric or amperometric. The following selected examples are illustrative of the first group in non-aqueous media ... 

In this automatic system, the authors preferably used coulometric generation of titrant (cf., microcoulometric determination of deviations in the above end-point titration ), e.g., H, OH, Ag, Hg2+, Br2,12, Fe(CN) (cf., Table 1 in ref. 63). The detection method may be potentiometric (logarithmic signal), amperometric (linear signal), biamperometric, conductometric, oscillometric, etc. Moreover, the authors evaluated triangle programmed titration curves by... 

The above system of directly sensing a process stream without more is often not sufficiently accurate for process control so, robot titration is preferred in that case by means of for instance the microcomputerized (64K) Titro-Analyzer ADI 2015 (see Fig. 5.28) or its more flexible type ADI 2020 (handling even four sample streams) recently developed by Applikon Dependable Instruments20. These analyzers take a sample directly from process line(s), size it, run the complete analysis and transmit the calculated result(s) to process operation (or control) they allow for a wide range of analyses (potentiometric, amperometric and colorimetric) by means of titrations to a fixed end-point or to a full curve with either single or multiple equivalent points direct measurements with or without (standard) addition of auxiliary reagents can be presented in any units (pH, mV, temperature, etc.) required. 

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