Flow-through potentiometric cells

FIGURE 5-18 Flow-through potentiometric cell-cap design. A, reference electrode B, iodide electrode C, flow-through cap D, inlet E, outlet. (Reproduced with permission from reference 49.)... 

A potentiometric determination of saccharin was proposed by Fatibello-Filho et al. [86]. In this method, saccharin was potentiometrically measured using a silver wire coated with a mercury film as the working electrode. With this, the main difficulty was the presence of a precipitate (mercurous saccharinate) that could adsorb on tube walls and the electrode surface. To avoid these undesirable effects, a relocatable filter unit was placed before the flow-through potentiometric cell and a surfactant was added to the carrier solution (Figure 24.12). The same investigation team reported the construction and analytical evaluation of a tubular ion-selective electrode coated with an ion pair formed between saccharinate anion and toluidine blue O cation incorporated on a poly(vinyl chloride) matrix [87]. This electrode was constructed and adapted in a FIA system. The optimum experimental conditions found were an analytical path of 120 cm, an injection sample volume of 500 pL, a pH of 2.5, a flow rate of 2.3 mL/min, and a tubular electrode length of 2.5 cm. 

FIGURE 24.12 FIA manifold for potentiometric determination of saccharin with a relocatable filter. C carrier solution (0.2 M sodium nitrate in 1 mM nitric acid) F filter FC flow-through potentiometric cell Rl, R2, and R3 mercury(I) nitrate, 1 mM nitric acid, and 0.2 M sodium nitrate in 1 mM nitric acid, respectively S sample THR tubular helicoidal reactor W waste. 

Contrary to reversible cells, this reaction proceeds, though at a very limited rate, even when the electrodes are not connected and no current is flowing through the cell. If its EMF is compensated in a potentiometric connection by an equal potential of opposite direction, the zinc dissolution continues. Should the compensating potential exceed the EMF of the cell, the above mentioned reaction will not proceed in the opposite direction but a new process appears instead, during which the copper is dissolved and the hydrogen is evolved on the zinc electrode ... 

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

Fibre optic-based flow-cell To a flow-through potentiometric sensor... 

Fig. 4 The flow-through electrochemical cell for potentiometric measurements at temperatures up to 400°C A - flow-through Ptfhh) electrode, B - flow-through YSZ(Hg/HgO), C -flow-through external Ag/AgCl reference electrode [13, 16].

Pt(H2) electrode was maintained at ambient temperature with the flow of both reference and test solutions coming from opposite ends of the cell past each electrode. The authors used the flow-through thermocell to measure the acid dissociation constant for phenol to 400 °C and 34.9 MPa [26], Hence, it was firmly proved that the flow-through potentiometric technique could be used at temperatures up to 400 °C and densities down to 0.2 g m-3. 

The hydrogen concentration cells described above cannot be used at temperatures above 300 °C because Teflon loses its thermal stability. However, a flow-through potentiometric system can be used [13,... 

The potentiometric measurements most often are carried out at no or negligible current flowing through the cell, i = 0. In this case, the measuring cell is actually a galvanic cell, composed of two half-cells. The potential difference is called the electromotive force (emf). 

Most electrochemical detectors, such as amperometric and potentiometric detectors, are surface detectors. They respond to substances that are either oxidizable or reducible and the electrical output results from an electron flow caused by the chemical reaction that takes place at the surface of the electrodes (Rao et ah, 2002 Mehrvar and Abdi, 2004 Trojanowicz, 2009). Successful operation of a surface detector requires a reproducible radial concentration distribution. There are several types of flow-through detection cells, each type being characterized by parameters such as the length, diameter, and shape of its detection channel, which determine the laminar character of the liquid flow under the given experimental conditions and the predominant mode of the mass transport within the cell. 

Flow-cell (or flow-through cell) — is a device through which fluid may be driven using an external force. Often of rectangular or tubular form for electrochemical applications, a cell may incorporate a variety of different - sensors, e.g., voltammetric [i] or potentiometric [ii], these devices can also be used in conjunction with spectroscopic analysis. Flow-through cells are also used extensively in synthetic and technical applications - Swiss-role cell. 

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. 

To completely eliminate changes in reference electrode half-cell potentials, a three electrode potentiostat is often employed. In simple terms, the potentiostat appHes a voltage to the working electrode, which is measured versus a reference electrode via a zero current potentiometric type measurement, but the current flow is between the working electrode and a third electrode, called the counter electrode. Thus if reduction takes place at the working electrode, oxidation would occur at the counter electrode but no net reaction would take place at the surface of the reference electrode, since no current flows through this electrode. A potentiostat circuit is relatively simple to construct using modern operational amplifiers. 

In order to determine wither potassium fertilization is needed or not, it is important to determine the exchangeable potassium in soil samples [53]. The ion-selective potassium electrode proved applicable for these measurements [54]. A potentiometric flow injection manifold and method was worked out for solving that task by Almeida and coworkers [55]. They used the ion-selective potassium electrode built in a flow through cell. 

Furthermore, it was shown that it is possible to integrate and miniaturize indicator and reference electrodes while maintaining the characteristics of macro-conventional potentiometric sensors [21]. In consequence, it was shown that such electrodes can be placed together on a flat surface. In this way the miniature galvanic cells for flow-through modules, for direct measurements in small sample volumes can be obtained. 

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