Metal ions potentiometric detection

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. 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

The now familiar alternatives of visual and potentiometric detection are available. A number of organic dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the eye at concentrations of 10 6-10 7 mol dm 3 and can function as visual indicators. Most metal ion indicators will also undergo parallel reactions with protons bringing about similar colour changes. Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator function. 

Table 5.8 gives an indication of the range of elements that may be determined. Most procedures will require an analyte concentration of 10-3 mol dm 3 or more, although with special conditions, notably potentiometric end-point detection, the sensitivity may be extended to 1(H mol dm 3. The analysis of mixtures of metal ions necessitates masking and demasking, pH adjustments and selective separation procedures. Areas of application are spread throughout the chemical field from water treatment and the analysis of refined food and petroleum products to the assay of minerals and alloys. Table 5.10 gives some selected examples. 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

Potentiometric Methods Potential measurements can be used for end point detection in the EDTA titration of those metal ions for which specific ion electrodes are available. Electrodes of this type are described in Section 21D-1. In addition, a mercury electrode can be made sensitive to EDTA ions and used in titrations with this reagent. 

P. R. Haddad, P. W. Alexander, and M. Trojanowicz, Ion chromatography of inorganic anions with potentiometric detection using a metallic copper electrode, /. Chromaiogr., 321,363,1985. 

As in amperometric applications, many experiments can be conducted in the close proximity mode where the tip is moved very close to the substrate surface and a perturbation is applied to the sample. This perturbation may take several forms, typically potentiostatic or galvanostatic excursions if the sample is acting as an electrode, but also optical illumination with a laser beam, change of solution, etc. The tip response is then recorded as a function of time following the application of the perturbation. In these conditions potentiometric detection offers two advantages over amperometric detection (1) the range of ions detectable is extended to nonelectroactive species such as alkali metals, and (2) the tip response is selective. There are, however, some drawbacks. Because of the high impedance of the electrometer, the response time is worse in potentiometric applications where the t90 is rarely below 30 s. This must be compared to the millisecond time scale available with amperometric responses (89). Ohmic drop may also affect the tip potential. 

The method of isothermal saturation consists of the addition of an excess quantity of oxide to the melt studied. Such an addition results in the formation of a saturated solution which is in equilibrium with the oxide precipitate. The fact that the equilibrium conditions have been achieved is detected in different ways among them we should mention potentiometric measurements using different oxygen electrodes [238, 326] and titrimetric determination of the concentration of metal ions in a sample of the melt [327, 328]. The sum of the concentrations of ionic and non-dissociated forms of the oxide according to equation (3.6.5) is the main result of these determinations. Taking into account the thermodynamic parameters which describe equilibria in the saturated solutions, equation (3.6.5) may be rewritten as... 

Other metallic electrodes has not been, so far, examined as detectors for ion-chromato-graphy. Flow-injection measurements for different anions with potentiometric detection using mercury film on a metal silver electrode indicate satisfactory detectability and dynamic properties (except iodide) for such a detection (Fig.6). ... 

Compared with separation-based techniques, potentiome-try is a difficult technique to use to detect multiple analytes because of the selectivity of the ion-selective electrode. Because of the influence of the separation voltage, it is difficult to measure the response of the electrode potential, which is correlated logarithmically to the concentration of analyte it has not yet been employed with microchip CE. Potentiometric detection has found application in conventional CE. Kappes and Hauser have fabricated a universal potentiometric sensor for CE by coating wire electrodes with a solid PVC membrane [10]. The sensor was of approximately the same diameter as the outside of the capillary and located at a distance of about 50 p.m from the capillary outlet. A reference electrode was located beside the detection electrode. These coated-wire electrodes with PVC membranes have been employed to determine alkali and alkaline earth metals, small inorganic anions, and lipophilic organic ions such... 

Chen, Z., De Marco, R., and Alexander, P.W., Flow—injection potentiometric detection of phosphates using a metallic cobalt wire ion-selective electrode. Analytical Communications 34, 93,1997. 

The techniques of titrimetry, using both visual and potentiometric end point detection, are used to measure species, particularly metal ions, in a wide range of samples. 

Wang J, Tian B, Rogers KR (1998) Thick-fihn electrochemical immunosensor based on stripping potentiometric detection of a metal ion label. Anal Chem 70 1682—1685... 

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