The Measurement Cell

In potentiometry, the concentration of analyte in the cathodic half-cell is generally unknown, and the measured cell potential is used to determine its concentration. Thus, if the potential for the cell in Figure 11.5 is measured at -1-1.50 V, and the concentration of Zn + remains at 0.0167 M, then the concentration of Ag+ is determined by making appropriate substitutions to equation 11.3... 

Activity Versus Concentration In describing metallic and membrane indicator electrodes, the Nernst equation relates the measured cell potential to the concentration of analyte. In writing the Nernst equation, we often ignore an important detail—the... 

The concentration of Ca + in a water sample was determined by the method of external standards. The ionic strength of the samples and standards was maintained at a nearly constant level by making each solution 0.5 M in KNO3. The measured cell potentials for the external standards are shown in the following table. 

Ohmic Drops. Another irreversible contribution to the measured cell voltage is the ohmic or JR drop across the electrolyte, separator, and cell hardware. The JR drop across the hardware can be estimated from Ohm s law and the relationship... 

High refractory properties, extremely strong sensitivity to moisture and exceptionally high chemical activity of fluoride melts, especially of those containing ions of polyvalent metals, make spectral measurements of such melts extremely complicated. In order to obtain reliable results, the measurement cell must comply with three main requirements ... 

The logarithmic response of ISEs can cause major accuracy problems. Very small uncertainties in the measured cell potential can cause large errors. (Recall that an... 

Self-Test 12.12A Calculate the molar concentration of Y1H in a saturated solution of YF3 by using a cell constructed with two yttrium electrodes. The electrolyte in one compartment is 1.0 M Y(NO ),(aq). In the other compartment you have prepared a saturated solution of YF3. The measured cell potential is +0.34 V at 298 K. 

A glass electrode, a thin-walled glass bulb containing an electrolyte, is much easier to use than a hydrogen electrode and has a potential that varies linearly with the pH of the solution outside the glass bulb (Fig. 12.11). Often there is a calomel electrode built into the probe that makes contact with the test solution through a miniature salt bridge. A pH meter therefore usually has only one probe, which forms a complete electrochemical cell once it is dipped into a solution. The meter is calibrated with a buffer of known pH, and the measured cell emf is then automatically converted into the pH of the solution, which is displayed. 

In the measurements, one commonly determines the impedance of the entire ceU, not that of an individual (working) electrode. The cell impedance (Fig. 12.13) is the series combination of impedances of the working electrode (Z g), auxiliary electrode (Z g), and electrolyte (Z ), practically equal to the electrolyte s resistance (R). Moreover, between parallel electrodes a capacitive coupling develops that represents an impedance Z parallel to the other impedance elements. The experimental conditions are selected so that Z Z g Z g. To this end the surface area of the auxiliary electrode should be much larger than that of the working electrode, and these electrodes should be sufficiently far apart. Then the measured cell impedance... 

Figure 1.5 The slope of E ath versus log /orr through the fuel-cell-relevant potential range has an apparently constant value near RT/F (measured current density, here designated i, is corrected for hydrogen crossover current, designated i and the measured cell voltage is ir-corrected to provide the cathode potential E) [Neyerlin et al., 2006].

From eqn. 2.44 it follows that when using the measuring cell 2.41 one finds... 

The modulation index is related to the transmission spectra of the gas-filled reference cell, TRef( ), the measurement cell, with its unknown gas concentration, Ty Ca,(/.), and the optical filter, F(/.), all of which are shown in Equation 1. If required, the source spectra can also be taken into account, by using additional spectral functions (although this will usually have little spectral variation over the linewidth of the gas band) ... 

Figure 10. Predicted modulation index (m), as a function of C02 gas concentration (%v/v) in the measurement cell. Reference and measurement cells were of lm length and the reference cell contained 100% C02 gas at lBar/20 °C. A choice of optical filter, having a bandwidth of 100 nm and a centre wavelength of 2.004 pm, was assumed.

Figure 10 shows the dependency of the modulation index on the measurement gas cell concentration (%v/v), assuming dilution by nitrogen gas, at a pressure of 1 Bar and a temperature of 20 °C. This shows that there is a significant non-linearity in the modulation index response, particularly at higher CO2 gas concentrations in the measurement cell. As before, an optical filter bandwidth of 100 nm was assumed. 

The SNR of the detected signal is defined as the ratio of the signal change (produced as a result of the intensity modulation in the measurement cell) to the noise equivalent power (NEP) of the detection system for a given average received light intensity. In order to derive a figure for the NEP, various assumptions about the optical receiver must first be made. 

The effects of changing the temperature of the measurement cell were modelled, over a large range from -20 °C to 80 °C, assuming the... 

Figure 14. Expected variation of modulation index with temperature of the measurement cell, assuming both cells contained 100% C02 at 1 Bar and were 1 m long. The reference cell was held at 20 °C and the optical filter had a centre wavelength of 2.004 pm and a bandwidth of 100 nm.

The cross-sensitivity of the C02 sensor to water vapor is shown in Figure 16, where the expected modulation index, with the measurement cell filled... 

Figure 16. The variation (crosstalk) of modulation index with filter bandwidth, when the measurement cell contains a high concentration (0.05 Bar partial pressure) of H20 vapour impurity (both cells are 1 m in length cell at 1 Bar and 20 °C, and the reference cell contains 100% C02 gas).

Figure 17. Modulation index as a function of applied C02 concentration. 0%, 15.6%, 33.0%, 52.5%, 74.7% and 100% C02 concentration was applied to the measurement cell. The 90 cm long reference cell, contained 100% C02, and the 30 cm long measurement gas cell, contained 100% C02, were at 1 Bar and 20 °C. The optical LED emission spectrum was centred at 2.04 pm and had a 150 nm FWHM bandwidth.

The 3He melting pressure thermometer has been chosen to extend the ITS 90 for several reasons, such as the good sensitivity over three temperature decades, except around the pressure minimum at 315.24 mK. On the other hand, such a minimum is a reference point in the calibration of the pressure transducer in fact, the pressure must be measured in situ since, below 315.24 mK, the entrance of the measurement cell is blocked by solid 3He. 

Although anodic stripping voltammetry is one of the few techniques suitable for the direct determination of heavy metals in natural waters [310,756-764], it is not readily adaptable to in situ measurements. Lieberman and Zirino [623] examined a continuous flow system for the anodic stripping voltammetry determination of zinc in seawater, using a tubular graphite electrode predeposited with mercury. A limitation of the approach was the need to pump seawater to the measurement cell, while the method required the removal of oxygen with nitrogen before measurements. 

Irradiation of the photosensitizer solution takes place in a vessel (1) by means of a lamp (2). During measurements, a continuously irradiated solution is transferred with a peristaltic pump (5) from the vessel (1) into the measuring cell (6) of a chemiluminometer (7). Control of the unit, signal recording, and data processing are performed by a computer (8). The tube (4) serves for injection of various components during measurement. 

Electrochemical sensors with a liquid electrolyte are widely used for the detection of corrosive or toxic gases in the workplace. Portable monitors are used in short time measurements of exhaust gases as well. These sensors work amperometri-cally - an external voltage supply is connected with the electrode on both sides of the measuring cell. 

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