Flow potentiometry

Studies of the Modulated Flow Technique for Flow Potentiometrie... 

In potentiometry the potential of an electrochemical cell is measured under static conditions. Because no current, or only a negligible current, flows while measuring a solution s potential, its composition remains unchanged. For this reason, potentiometry is a useful quantitative method. The first quantitative potentiometric applications appeared soon after the formulation, in 1889, of the Nernst equation relating an electrochemical cell s potential to the concentration of electroactive species in the cell. ... 

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

Cyanide in Waters etc. (by Reflux Distillation followed by either Potentiometry using a Cyanide Selective Electrode or Colorimetry, or Continuous Flow Determination of Cyanide or Determination by Microdiffusion), 1988... 

In measurements of conductivity, no electrochemical reactions occur. Differences in conductivity are due to differences in the ionic strengths of solutions. An alternating potential is applied to the solution at a known potential. The current is measured and the conductivity in Siemens/cm calculated.16 In potentiometry, the analyte is presumed to undergo no electrochemical reaction. The potential at the electrode changes due to changes in potential across the surface of the membrane in a membrane electrode or at the electrode surface of a solid electrode. The most familiar example of a potentiometric electrode is the pH electrode. In amperometry, current does flow, due to reduction or oxidation of the substance being analyzed. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

Most of the reported methods of analysis of valproic acid and its sodium salt in biological fluids rely on the use of chromatography, especially gas chromatography, although high performance liquid chromatography (HPLC) is also reported. Other methods, such as flow injection analysis, enzyme-immunoassay, fluorescence-polarization capillary electrophoresis, and potentiometry are sometimes used. The reported methods can be classified as follows. 

The two classifications are potentiometry and amperometry. Potentiometry is a technique in which electrode potential is measured and related to concentration, while amperometry is a technique in which the current flowing at an electrode is what is measured and related to concentration. 

Potentiometry is a technique traditionally employed for the quantification of ions in a liquid solution. It is a static electroanalytical method, that is, there is no current flow inside the measurement cell (f = 0). The measurement cell is constituted by two electrodes which are immersed in the solution containing the analytes. A voltmeter measures the potential difference between the two electrodes, which is a fimction of the concentration (actually, the activity) of the analytes, as described by the well-known Nerst s equation (Kissinger and Heineman, 1996). 

Potentiometry is a method of obtaining chemical information by measuring the potential of an indicator electrode under zero current flow. It is based on the Nernst equation, which expresses the electrode potential as a function of the activity (or activities) of the chemical species in solution. The information obtained varies with indicator electrode, from the activity (concentration) of a chemical species to the redox potential in the solution. The potential of the indicator electrode is measured against a reference electrode using a high inptit-impedance mV/pH me-... 

When three-electrode devices are used, reference electrodes similar to those in potentiometry (Section 6.1.2) are applicable, because no appreciable current flows through them. The reference electrodes used in non-aqueous solutions can be classified into two groups [1, 2, 5, 10]. Reference electrodes of the first group are prepared by using the solvent under study and those of the second group are... 

Chow, C.W.K., Kolev, S.D., Davey, D.E. and Mulcahy, D.E. (1996) Determination of copper in natural waters by batch and oscillating flow injection stripping potentiometry. Anal. Chim. Acta, 330, 79-87. 

Ultrafiltration was applied to examine the size fractionation of Al, Ca, Cu, Fe, K, Na, and Pb in white and red wines [91]. Metal determinations were performed on the unfiltered wine, the 0.45 p,m membrane-filtered wine and each ultrafiltrate fraction. Aluminum was determined by ET-AAS, while FAAS was employed for Cu and Fe. An electroanalytical technique, stripping potentiometry, was selected for Pb measurement, whereas flame photometry was chosen for K and Na quantification. Fractionation patterns were evaluated and discussed. Castineira et al. [92] combined on-line tangential-flow multistage ultrafiltration with a home-built carbon analyzer and ICP-MS for size fractionation of nonvolatile dissolved organic compounds and metal species in three German white wines. The study showed that the major part of the elements investigated (up to 25) were dissolved in the size fraction of < 1 kDa, with the exception of Ba, Pb, and Sr, which also appeared in other fractions. 

The different methods - potentiometry conductometry turbidimetry vis-cometry " calorimetry , kinetira, sedimentation , dynamic flow birefringence, light scattering , high resolution H-NMR spectrometry , chromatography, spectroscopy , electron microscopy , and others have been used for the investigation of the formation and composition of polymer-polymer complex. ... 

Electrochemical detection is based on the electrical signal arising between two electrodes immersed in a sample solution. Electroanalytical techniques fall into two main categories, potentiometric and Faradaic techniques. Potentiometry is the measurement of a potential difference between two electrodes under equilibrium conditions (i.e., no current flow). The potential is then related to concentration of the analyte species. Faradaic processes are based on the oxidation or reduction of the analyte, where a specific potential waveform is applied and the current is used to extract information about the sample. Many different techniques have been developed to gain quantitative and... 

Although electrochemistry has the stigma of being difficult to use, and therefore is often overlooked as an analysis option, potentiometric measurements are probably the most common technique encountered. Many analytical chemists make potentiometric measurements daily, whenever they use a pH meter. Potentiometry is based on the measurement of the potential between two electrodes immersed in a test solution. As the electrical potential of the cell is measured with no current flow between the electrodes, potentiometry is an equilibrium technique. The first electrode, the indicator electrode, is chosen to respond to the activity of a specific species in the test solution. The second electrode is a reference of known and fixed potential. The design of the indicator electrode is fundamental to potentiometric measurements, and should interact selectively with the analyte of interest so that other sample constituents do not interfere with the measurement. Many different strategies have been developed to make indicator electrodes that respond selectively to a number of species including organic ions. 

Any type of detector with a flow-through cell can be used for FIA. Photometric detectors are most often used in FIA (15-18, 25). However, many other analyses using fluorimeters (28, 29), refractometers (24), atomic absorption (30, 31), and inductively coupled plasma emission spectrometers (32) have been described. Electrochemical detectors based on potentiometry with ion-selective electrodes (15, 33), anodic stripping voltammetry (15, 34), potentiometric stripping (35), and amperometry (36) have also been used. 

In electrochemical detection, the potential of a working electrode can be measured versus a reference electrode, usually while no net current is flowing between the electrodes. This type of detection is referred to as potentiometry. Alternatively, a potential is applied to the working electrode with respect to the reference electrode while the generated oxidation or reduction current is measured. This technique is referred to as amperometry. When applying a negative po-... 

In ion-selective potentiometry the stability and reproducibility of Eceu depends on the type of ISE used. Carefully optimized liquid membrane electrodes in flow-through arrangements for clinical applications yield standard deviations in Eceu smaller than 0.1 mV. This corresponds to less than z, X 0.4% error in the determination of activities. Ion-selective microelectrodes have poorer characteristics due to technical (e.g., silanization) and electrical (high resistance see below) reasons. 

Several methods can be used for the determination of chloride in water [2], The argentometric and mercuric nitrate methods are based on the titration of chlorine in the presence of an indicator. Experimental procedures are easy, but many substances may interfere with the results. There are also other methods such as potentiometry, capillary electrophoresis and other automated methods (ferricyanide method or flow injection analysis). 

J.W.B. Stewart, J. Ruzicka, H. Bergamin-Filho, E.A.G. Zagatto, Flow injection analysis. Part III. Comparison of continuous flow spectrophotometry and potentiometry for the rapid determination of total nitrogen content in plant digests, Anal. Chim. Acta 81 (1976) 371. 

Quantitative precipitation may be observed after a few seconds, and can be confirmed by potentiometry, but turbidity is sometimes observed only after several weeks [9]. Therefore, the timing of turbidimetric procedures is generally a very important aspect of system design [55]. In this context, the rate of turbidity formation can be selected as the measurement basis for kinetic procedures, as elegantly demonstrated in the flow injection determination of immunoglobulin G in serum [56]. 

For simultaneous determinations and /or speciation, different separation/ concentration steps can be implemented in the same manifold, as in, e.g., the determination of nitrogen, phosphorus and potassium in fertilisers [314]. The sample was inserted and passed successively through a dialysis unit and a gas diffusion unit to a flow cell for the spectrophotometric determination of phosphate. The dialysed potassium ions and the diffused gaseous ammonia were collected in specific streams and determined by flame photometry and potentiometry, respectively. 

The choice is dictated by cost and apphcabihty to the problem. The most widely used methods include spectrophotometry to measure color, ultraviolet or infrared absorption, turbidity, film thickness, and the like electrochemistry, primarily potentiometry, for the measurement of pH and cation or anion activity and gas and liquid chromatography, especially in the petrochemical industry where complex mixtures from distillation towers are monitored. Spectrophotometiic and other measurements are often rapidly made using flow injection analysis. 

Potentiometric and refractive index detection are not affected by volume but are relatively insensitive in the nanolitre to picolitre range compared to amperometric detection (micro surface area) and fluorimetric detection (micro amount of material). At 1 pL, limits of detection are similar for potentiometry, amperometry and fluorescence. On-chip LC is very compatible with mass spectrometry due to the low volumes and flow rates required. Battery-operated ion trap MS has been reported but miniaturisation of MS offers no sensitivity or selectivity advantages. Electrospray ionisation (ESI) has been successfully integrated into a chip format allowing for many ESI nozzles on one chip. Arrays make pattern recognition possible. 

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