Constant-current

Measurement by quasi - constant current (steady - state value of pulse current) providing a compete tuning out from the effect of not only electric but also magnetic material properties. 

Figure Bl.19.1. Principle of operation of a scanning tiimrelling microscope. The x- andj -piezodrives scan the tip across the surface. In one possible mode of operation, the current from the tip is fed into a feedback loop that controls the voltage to die z-piezo, to maintam constant current. The Ime labelled z-displacement shows the tip reacting both to morphological and chemical (i.e. electronic) inliomogeneities. (Taken from [213].)...

Figure Bl.19.2. The two modes of operation for scanning tunnelling microscopes (a) constant current and (b) constant height. (Taken from [214], figure 1.)...

Figure Bl.19.6. Constant current 50 mn x 50 mn image of a Cu(l 11) surface held at 4 K. Tliree monatomic steps and numerous point defects are visible. Spatial oscillations (electronic standing waves) with a...

This method relies on the simple principle that the flow of ions into an electrolyte-filled micropipette as it nears a surface is dependent on the distance between the sample and the mouth of the pipette [211] (figure B 1.19.40). The probe height can then be used to maintain a constant current flow (of ions) into the micropipette, and the technique fiinctions as a non-contact imaging method. Alternatively, the height can be held constant and the measured ion current used to generate the image. This latter approach has, for example, been used to probe ion flows tlirough chaimels in membranes. The lateral resolution obtainable by this method depends on the diameter of the micropipette. Values of 200 nm have been reported. 

To a 250-ml not-partitioned electrochemical cell, 135 ml of CH3CN, 15 ml ofHiO, 6.20 g of NaBr and 2.82 g of olefin ( ) is added. The mixture, kept at 2(f C, is electrolysed by using the same electrodes as of Example 1, but with a constant current density of 1.7 A being used,until through the cell 4,000 Coulombs have been passed. The reaction mixture is then processed as described in Example 4.2.56 g is obtained of ketone (III), with a yield of 83.2%, as computed relatively to the olefin (I) used as the starting material. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

A second approach to coulometry is to use a constant current in place of a constant potential (Figure 11.23). Controlled-current coulometry, also known as amperostatic coulometry or coulometric titrimetry, has two advantages over controlled-potential coulometry. First, using a constant current makes for a more rapid analysis since the current does not decrease over time. Thus, a typical analysis time for controlled-current coulometry is less than 10 min, as opposed to approximately 30-60 min for controlled-potential coulometry. Second, with a constant current the total charge is simply the product of current and time (equation 11.24). A method for integrating the current-time curve, therefore, is not necessary. 

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

A titration in which the equivalence point is the time required for a constant current to completely oxidize or reduce the analyte. 

The purity of a sample of Na2S203 was determined by a coulometric redox titration using as a mediator, and as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample. 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

Directions are provided for constructing an inexpensive constant-current source and demonstrate its use in determining the concentration of HGl coulometrically. 

A 50.00-mL sample of water is placed in a coulometric cell, along with an excess of KI and a small amount of starch as an indicator. Electrolysis is carried out at a constant current of 84.6 mA, requiring 386 s to reach the starch end point. Report the concentration of H2S in the sample in parts per million. 

Curran, D. J. Constant-Current Coulometry. Chapter 20 in Kissinger, P. T. Heineman, W. R., eds. Eaboratory Techniques in Electroanalytical Chemistry. Marcel Dekker, Inc. New York, 1984, pp.539—568. 

Fig. 4. Operational modes for stm. (a) Constant height mode, (b) Constant current mode (18).

The constant height mode of operation results in a faster measurement. In this analysis, the tip height is maintained at a constant level above the surface and differences in tunneling current ate measured as the tip is scaimed across the surface. This approach is not as sensitive to surface irregularities as the constant current mode, but it does work well for relatively smooth surfaces. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

Several types of secondary pyrometer are available. In addition to those that measure by varying lamp current, some pyrometers maintain the lamp at constant current but interpose a wedge of graduated neutral density, whose position is a measure of temperature. Also, automatic pyrometers are available in which the eye is replaced by a detector and the measuring element is operated by a servo. In general, the accuracy of the automatic pyrometer is somewhat less than that achieved manually by a skilled operator. 

Ampere. The ampere is that constant current which, if maintained in two straight, parallel conductors of infinite length, of negligible circular cross section, and placed one meter apart in a vacuum, would produce between these conductors a force equal to 2 x 10 newton per meter of length. 

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

Fig. 11. Constant-current charge curve for a high rate Ag—Zn ceU at room temperature. Charging carried out at the 10-h rate.

Isoelectric focusing takes along (from ca 3 to 30 h) time to complete because sample compounds move more and more slowly as they approach the pH in the gel that corresponds to their isoelectric points. Because the gradient ampholytes and the samples stop where they have no mobiHty, the resistivity of the system increases dramatically toward the end of the experiment, and the current decreases dramatically. For this reason, isoelectric focusing is usually mn with constant voltage. Constant current appHcation can lead to overheating of the system. 

The bot-wire anemometer consists essentially of an electrically heated fine wire (generally platinum) exposed to the gas stream whose velocity is being measured. An increase in fluid velocity, other things being equal, increases the rate of heat flow from the wire to the gas, thereby tending to cool the wire and alter its electrical resistance. In a constant-current anemometer, gas velocity is determined by measuring the resulting wire resistance in the constant-resistance type, gas velocity is determined from the current required to maintain the wire temperature, and thus the resistance, constant. The difference in the two types is primarily in the electric circmts and instruments employed. 

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