Endpoint detection amperometric

Christian, G. D. A Sensitive Amperometric Endpoint Detection System for Microcoulometric Titrations. Microchem. J. 9, 484 (1965). 

The amperometric approach to endpoint detection provides considerable latitude in the selection of the best conditions for the most specific and sensitive endpoint response. Furthermore, the response signal is directly proportional to the concentration of the observed species, whereas potentiometric responses are a logarithmic function of the concentration. Another attractive feature of amperometric endpoint detection is that the most important data are obtained prior to and after the equivalence point, whereas in potentiometric titrations the most important data occur at the equivalence point, which is the most unstable condition of the titration. With amperometric titrations an extrapolation of the straight-line portion of the curve, either prior to or after the equivalence point, to an intercept will provide an accurate measure of the equivalence point. 

For this titration the dual-polarized amperometric endpoint detection system provides good sensitivity and rapid response. 

Figure 4.9 Coulometric titration cell with generator [II (generator anode, 0.7 x 0.7 cm)] and isolated auxiliary [I (generator cathode, 0.7 x 0.7 cm)] electrodes on the left side and a pair of identical platinum electrodes [III, IV (1.4 x 1.8 cm and 2.5 X 1.8 cm)] on the right for dual-polarized electrode amperometric endpoint detection.

In the final step of the analysis, the iodine is titrated with thiosulphate. The iodine is reduced to iodide, and the thiosulphate in turn is oxidized to the tetrathionate ion. The concentration of the thiosulphate solution used for the titration must be known precisely. The endpoint of the redox titration is commonly indicated by a starch indicator or by photometric or amperometric endpoint detection. The starch indicator forms an enclosure compound with iodine. The large electron cloud of the iodine interacts with the hydroxo dipoles in the starch helix resulting in an intensely blue colour of the iodine starch complex. Nevertheless, the iodine molecules can leave the starch hehx easily and thus can be reduced by thiosulphate. The endpoint of the titration is clearly marked by the change from blue to colourless. 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

A computer-controlled automated titration unit with a remote controlled burette of the above specifications and photometric (UV) endpoint detection may be used (Williams and Jenkinson, 1982). Amperometric endpoint detection is also possible (Grasshoff, 1981). 

Constant-current potentiometry seems to have the advantage of error-free operation, as compared to the conventional amperometric endpoint detection in the case of chlorine determinations in water. Barbolani et al. [7] employed 1 jxA DC between two identical platinum electrodes and measured the potential difference between them to detect the endpoint of the titrations. Chlorine was titrated with phenylarsine oxide at pH 7 chlorine and chlorine dioxide were titrated analogously in the presence of iodide ions, and all three components were titrated at pH 2 in the presence of iodide. The method was used for water samples (taken from a water purification plant) containing both chlorine and chlorine dioxide. 

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. 

In the iodimetric titration procedure, the combustion gases are bubbled through a diluent solution containing pyridine, methanol, and water. This solution is titrated with a titrant containing iodine in a pyridine, methanol, and water solution. In automated systems, the titrant is delivered automatically from a calibrated burette syringe and the endpoint detected amperometrically. The method is empirical, and standard reference materials with sulfur percentages in the range of the samples to be analyzed should be used to calibrate the instrument before use. Alternative formulations for the diluent and titrant may be used in this method to the extent that they can be demonstrated to yield equivalent results. 

As with amperometric titrations, to have straight-line portions of the titration curve dilution corrections must be made because the response is directly dependent on the concentration of the ionic species. Also, the important data are taken before and after the equivalence point rather than precisely at the equivalence point. The general conditions for effective conductometric measurements of solutions are discussed in Chapter 5 and are directly applicable when the system is used as the endpoint detection method. A particularly complete review of the subject has been presented.8... 

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

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. 

Assuming impurities can be satisfactorily pretitrated, the lower limit of the amount of sample that can be titrated is governed primarily by the sensitivity of the available endpoint detection system. Very small currents, such as 0.1 /xA, can be measured accurately (actually, currents smaller than 60 electrons per second have been measured and the time of electrogeneration can be measured accurately. With conventional amperometric and potentiometric endpoint indication, coulometric titrations in typical solution volumes cannot be accurately made at generating currents of less than about 100 A. 

The reaction continues and current passes until all the iodide is used up. At this point some means of endpoint detection is needed. Two methods are commonly adopted. The first uses an amperometric circuit with a small imposed voltage that is insufficient to electrolyze any of the solutes. When the mercury ion concentration suddenly increases, the current will rise because of the increase in the concentration of the conducting species. The second method involves using a suitable indicator electrode. An indicator electrode may be a metal electrode in contact with its own ions or an inert electrode in contact with a redox couple in solution. The signal recorded is potentiometric (a cell voltage vs. a stable reference electrode). For mercury or silver we may use the elemental electrodes, because they are at positive standard reduction potentials to the hydrogen/hydrogen ion couple. 

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. 

Chlorine gas may be identified readdy by its distinctive color and odor. Its odor is perceptible at 3 ppm concentration in air. Chlorine may be measured in water at low ppm by various titrimetry or colorimetric techniques (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington DC American Pubhc Health Association). In iodometric titrations aqueous samples are acidified with acetic acid followed by addition of potassium iodide. Dissolved chlorine liberates iodine which is titrated with a standard solution of sodium thiosulfate using starch indicator. At the endpoint of titration, the blue color of the starch solution disappears. Alternatively, a standardized solution of a reducing agent, such as thiosulfate or phenylarsine oxide, is added in excess to chlorinated water and the unreacted reductant is then back titrated against a standard solution of iodine or potassium iodate. In amperometric titration, which has a lower detection limit, the free chlorine is titrated against phenyl arsine oxide at a pH between 6.5 and 7.5. 

The entire subject of amperometric titrations has been reviewed in a number of monographs on electrochemistry 4-6 a definitive work on this subject also has been published.7 Because the amperometric titration method does not depend on one or more reversible couples associated with the titration reaction, it permits electrochemical detection of the endpoint for a number of systems that are not amenable to potentiometric detection. All that is required is that electrode conditions be adjusted such that either a titrant, a reactant, or a product from the reaction gives a polarographic diffusion current. 

The endpoint may be detected by addition of colored indicators, provided the indicator itself is not electroactive. Potentiometric and spectrophotometric indication is used in acid-base and oxidation-reduction titrations. Amperometric procedures are applicable to oxidation-reduction and ion-combination reactions especially for dilute solutions. 

A square-wave amperometric titration has been used for the determination of the total available heavy metals in water samples. This method involves the direct anodic oxidation of mercury in the presence of excess EDTA. The resulting mercury wave is used to detect the endpoint of the amperometric titration by running a polarogram after each successive addition of an aliquot of EDTA. The successful utilization of this method lies in the ability to discriminate between Ca(n) and heavy metals, such as Cu(II) and Zn(II). Thus, in practice, it involves 1 1 dilution of samples with 0.2 mol 1 acetate buffer (pH 4.8), prior to the amperometric titration. At this pH, heavy metals, such as Fe(III), Hg(II), Ni(II), Cu(II), Pb(n), Zn(II), Cd(II), Co(II), and Al(III), are completely ( 99%) converted to EDTA complexes. Furthermore, the presence of Ca(II) does not interfere with the determination of the available heavy metals under these conditions. As little as 1 pmol 1 of available heavy metals has been successfully determined in water samples by this method. 

The means of detecting the endpoint will be dictated by the type of reaction employed. Acid-base titrations are most easily followed using a glass pH electrode while redox reactions lend themselves to amperometric detection (only a small fraction of the species detected is consumed at the indicator electrode). Other options are ion-selective electrodes and conductometric detection. 

The simplest method available for the determination of sulfur dioxide in foods is iodimetric titration in the presence of starch after addition of acetone, glyoxal, or formaldehyde. Acidification before the titration allows the determination of both free and reversibly bound sulfur dioxide. Amperometric methods for detecting the endpoint allow the method to be extended to colored samples. 

The 21 formed in the second reaction is determined either by visual chemical titration with a reagent such as sodium thiosulfate in the presence of a suitable endpoint indicator or by amperometric, coulometric, or photometric titration methods. The most sensitive KF methods for the measurement of iodine are coulometric. For both the volumetric-amperometric and coulometric methods the endpoint is detected by a pair of platinum electrodes called the indicator electrodes. An electrical potential (100-400 mV) is applied across the electrodes to balance the circuit and the endpoint is reached when the concentration of I2 ( 50pmoll ) depolarizes the cathode deflecting a galvanometer. The volumetric method measures the amount of standardized reagent necessary to depolarize the platinum electrodes. The coulometric method utilizes, in addition to the indicator electrodes, a second pair of platinum electrodes (generator electrodes) that electrolytically convert the 1 to I2. The current consumed in this process is used to calculate the amount of water using the equation that describes Faraday s laws of electrolysis. 

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. 

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