Conductometric Instrumentation

The apparatus required for making conductance measurements and performing conductance titrations is generally inexpensive and basically simple in detail. For these reasons, the measurement of conductance finds wide acceptance in industry as an analytical tool, both in the laboratory and in process control. 

Preparation of the electrode surfaces is very important. The electrodes should be cleaned in an acid cleaning solution, rinsed thoroughly with distilled water, and immersed in a platinizing solution. DC voltage is adjusted to give a moderate evolution of gas for about 20 seconds, after which the polarity is reversed. The reversal process is repeated until a gray (not black) deposit of platinum has formed on the surface. Too heavy a deposit should be avoided because spongy platinum will absorb unwanted chemical species. 

The electrodes are washed with distilled water, immersed in 1. iV H2SO4, electrolyzed with DC voltage using repeated polarity reversal to remove impurities, and finally washed and stored in distilled water. Hatinizing solutions as well as cells may be purchased from a number of supply houses. More detailed discussion and exact procedures of platinization are available in an article by Jones and Bollinger [3]. 

The signal generator may be a 60-Hz transformer, a 1000-Hz oscillator, or a variable-frequency oscillator. If earphones are used as a null, the 1000-Hz oscillator is preferred. The null indicator can also be a sensitive microammeter or more elaborate null-point indicators. 

Alternating currents are preferred to direct current because little or no polarization of platinized electrodes takes place. During electrolysis the platinum black 

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R. Dunn, New Developments in Membrane-Selective Conductometric Instruments for Total Organic Carbon Determination in Water, Am. Lab., September 2004, p. 22. 

As we shall see, the solution conductivity depends on the ion concentration and the characteristic mobility of the ions present. Therefore, conductivity measurements of simple, one-solute solutions can be interpreted to indicate the concentration of ions (as in the determination of solubility or the degree of dissociation) or the mobility of ions (as in the investigations of the degree of solvation, complexation, or association of ions). In multiple-solute solutions, the contribution of a single ionic solute to the total solution conductivity cannot be determined by conductance measurements alone. This lack of specificity or selectivity of the conductance parameter combined with the degree of tedium usually associated with electrolytic conductivity measurements has, in the past, discouraged the development of conductometry as a widespread electroanalyti-cal technique. Today, there is a substantial reawakening of interest in the practical applications of conductometry. Recent electronic developments have resulted in automated precision conductometric instrumentation and applications... 

The continuous methods combine sample collection and the measurement technique in one automated process. The measurement methods used for continuous analyzers include conductometric, colorimetric, coulometric, and amperometric techniques for the determination of SO2 collected in a liquid medium (7). Other continuous methods utilize physicochemical techniques for detection of SO2 in a gas stream. These include flame photometric detection (described earlier) and fluorescence spectroscopy (8). Instruments based on all of these principles are available which meet standard performance specifications. 

Besides, the aforesaid visual methods of assay i.e., observing the change in colour of indicators used, alternative instrumental methods such as potentiometric, amperometric, polarographic, conductometric methods are also employed in determining the end-point. 

The parameter can change in a vessel being part of the analytical instrument, for example, an ultraviolet-visible (UV-Vis) spectrophotometric cell [39,41,45,14,47, 48], an infrared (IR) cell [42, 46], or a fluorometer cell [45, 51], or a polarimetric tube [27, 49]. It can change in a reactor vessel where the analytical signal can be read in some way, for example using an optical fiber cell for spectrophotometry [52-54] or a conductometric cell [16,34,40]. Another possibility is to transport the solution from the reaction vessel to the analytical instrument by a peristaltic pump [38]. When altenative ways are not practicable, samples can be taken at suitable time intervals and analyzed apart [29,31,35,39,43,50]. 

The circuit may be easily constructed for a modest cost using inexpensive operational amplifiers and any of a number of integrated circuit oscillators that are commercially available. This basic piece of instrumentation is suitable for student laboratory experiments, conductometric monitoring of distilled water or... 

The titrimetric modifications described above exemplify either an instrumental adaptation (as in the case of the potentiometric and conductometric titrations) or a chemical manipulation (olfactory indicators). The first uses the sense of hearing, while the latter type appeals to the sense of smell. Unfortunately, instrumental adaptations that utilize one-of-a-kind homemade instruments are not readily available to the typical educator or may even be out of circulation. Also considered is the fact that at institutions where the enrollment of handicapped people is low, the justification of specialized equipment can be very difficult to obtain. On the other hand, the chemical manipulations discussed rely on readily available chemicals (onions and cloves) and represent comparable costs and laboratory preparation times as traditional titrimetric experiments. Since all students may perform these... 

This discourse tries to give an overview of the current state-of-the-art instrumentation in real-time pulse radiolysis experiments utilizing optical, conductometric and other methods. Pump-and-probe techniques for the sub-nanosecond time domain are believed to be beyond the scope of this discussion. 

Manahan and Chassaniol (Cosa Instruments and Dionex) describe an oxidative combustion followed by ion chromatographic conductometric method for the determination of a number of nonmetallic elements such as sulfur and halogens in liquid and gaseous hydrocarbons. A standard based on this technique is under development in ASTM for designation as a standard method. 

In practice, electrochemistry not only provides a means of elemental and molecular analysis, but also can be used to acquire information about equilibria, kinetics, and reaction mechanisms from research using polarography, amperometry, conductometric analysis, and potentiometry. The analytical calculation is usually based on the determination of current or voltage or on the resistance developed in a cell under conditions such that these are dependent on the concentration of the species under study. Electrochemical measurements are easy to automate because they are electrical signals. The equipment is often far less expensive than spectroscopy instrumentation. Electrochemical techniques are also commonly used as detectors for LC, as discussed in Chapter 13. 

Because of the possibility of ion association and charge transfer the use of conductometric titration is suggested. Preliminary unpublished conductometric studies by the authors on the reaction between bromophenol blue and quaternary ammonium and arsonium salts failed, within the sensitivity of the instrument used, to detect any conductivity changes. The reaction is, however, accompanied by metachromasia and can be followed spectrophotometrically, showing again the complementary nature of the physical methods available. 

Figure 14 Conductometric titration curves of various acids by sodium hydroxide. Curve 1 represents a strong acid, and curve 5 an extremely weak one, while the others are intermediate. The acids are (1) hydrochloric acid, (2) dichloroacetic acid, (3) mon-ochloroacetic acid, (4) acetic acid, and (5) boric acid. (From Ewing GW (1985) Instrumental Methods of Chemical Analysis, 5th edn., p. 337. New York McGraw-Hill.)...

Conductance measurements were carried out using a Serfass Conductance Bridge Model RCM 1581 made by Industrial Instruments Co. (Cedar Grove, N.J.), with cells containing freshly coated platinum black. Conductance measurements and conductometric titrations were carried out with NaB(/)4 in order to understand the mechanism of the disproportionation reaction. 

Kappes, T Galliker, B., Schwarz, M.A., and Hauser, P.C. (2001) Portable capillary electrophoresis instrument with amperometric, potentiometric and conductometric detection. TrAC, Trends Anal. Chem., 20 (3), 133-139. 

Nearly all chemical sensors useful for liquid samples can be utiUzed to indicate titrations. Besides the preferred potentiometric, other electrochemical probes are also used, mainly amperometric and conductometric sensors. The so-called biamperometric titration works with simple wire pairs. Photometric and thermometric indication techniques are less common than electrochemical methods. Miniaturization does not play an important role for titration probes. Classical arrangements predominate to this day. Commercial titration instruments are only slowly starting to make use of the achievements of modern sensor technology. As an example, optodes have achieved a certain popularity in recent years for titration applications. 

Other titration designations have also been given in the literature. Some are named after the instrumental method used to detect the equivalence point. We can list, for example, conductometric, potentiometric, amperometric, spectrophotometric, and thermometric titrations. Others are named after the titrant nature. We speak then of iodometric, complexometric, and acidimetric titrations. 

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