Conductometric method instruments

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. 

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. 

We must also keep in mind that equivalence points of EDTA titrations can be detected by using several instrumental methods. A first method, potentiometry, was just mentioned. There are also other potentiometric methods, based on other principles than the previous one, that may be used. Amperometric and conductometric methods have been proposed equally (see electrochemical methods of analysis). Finally, we ll mention photometric and spectrophotometric indications. 

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. 

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. 

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 Preliminary unpublished conductometric studies by the authors on the reaction between bromophenol blue and quaternary ammonium and arsonium salts failed, within the sensitivity of the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.)...

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. 

Conductometry is a very attractive method for alpha acids analysis, because of the low cost of the necessary instrumentation. One of the major causes of trouble is that other compounds can be present which also react with lead(ll) acetate. Iso-alpha acids for example consume half the amount of lead(ll) acetate as the same weight of alpha acids would do. Some oxidation products of the alpha and beta acids can also consume lead(ll) acetate. Whether or not they do so depends on the titration medium. At the 1970 EBC Interlaken meeting it was shown that addition of water, even in traces, changes the conductometric titration curve drastically (5). It is even possible to derive two analysis figures from one titration curve, one for iso-alpha acids and oxidation products, and one for alpha acids. The results are however not sufficiently reliable for... 

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