Conductimetric methods

An immobilized enzyme flow injection conductimetric system was used by Godinho et al. for the determination of acetylcholine [17]. Aliquots 

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Conductimetric measurements can also be used to ascertain the end-point in many titrations, but such use is limited to comparatively simple systems in which there are no excessive amounts of reagents present. Thus, many oxidation titrations which require the presence of relatively large amounts of acid are not suited to conductimetric titration. Conductimetric titrations have been largely superseded by potentiometric procedures (see Chapter 15), but there are occasions when the conductimetric method can be advantageous.14... 

The relative change of conductance of the solution during the reaction and upon the addition of an excess of reagent largely determines the accuracy of the titration under optimum conditions this is about 0.5 per cent. Large amounts of foreign electrolytes, which do not take part in the reaction, must be absent, since these have a considerable effect upon the accuracy. In consequence, the conductimetric method has much more limited application than visual, potentiometric, or amperometric procedures. 

Alcohol sulfates commonly have free alcohol and electrolytes as impurities. Other hydrophobic impurities can also be present. A method suitable for the purification of surfactants has been proposed by Rosen [120]. Consequently, commercial products have CMCs that deviate from the accepted reference values. This was demonstrated by Vijayendran [121] who studied several commercial sodium lauryl sulfates of high purity. The CMC was determined both by the conductimetric method and by the surface tension method. The values found were similar for both methods but while three samples gave CMC values of 7.9, 7.8, and 7.4 mM, close to the standard range of 8.0-8.2 mM, three other samples gave values of 4.1, 3.1, and 1.7 mM. The sample with a CMC of 7.9 mM was found to have a CMC of 8.0 mM with no detectable surface tension minima after purification and recrystallization. This procedure failed in all other cases. 

G. Bellucci, A. Lippi and F. Marioni, A simple conductimetric method as an alternative to the colorimetric p-nitrobenzylpyridine test for the measurement of the reactivity of potentially mutagenic alkylating compounds. Chemico-Biological Interactions, 1986, 59(3), 255-263. 

Roy and Prakash have developed a conductimetric method for the determination of diloxanide furoate [13]. The drug undergoes hydrolysis on heating with aqueous sodium hydroxide, and the excess sodium hydroxide is then determined conductimetrically. The method is reported to be useful for the analysis of the raw material, as well as for the analysis of tablets. 

Bahbouh et al. used a conductimetric method for the analysis of dipyridamole [21], Ground tablets containing the drug were dissolved in 100 mL of 67% ethanol, and a portion was transferred to a titration cell equipped with a conductivity cell. The sample solution was diluted to 50 mL with water and titrated with 5 mM phosphotungstic acid. After each addition of titrant, conductivity measurements were recorded after a 2 min interval and plotted against the amount of titrant added. The end point was determined at the curve break. Results obtained by this method were in good agreement with those obtained by the official pharmacopoeial methods. 

Boric acid and boric oxide and probably many other boron compounds are converted in sulfuric acid to boron tri (hydrogensulfate), B(HS04)3, which behaves as a strong acid, HBfHSO ). The evidence for the formation of this acid and its salts provides an interesting example of the application of the cryoscopic and conductimetric methods for determining modes of ionization ( 4). 

Rabinovich, F. M. Conductimetric Methods of Dispersion Analysis Khimya Leningrad Otd. Leningrad, U.S.S.R., 1970. 

Figure 18.6 Direct and membrane conductimetric detection methods for measuring TOC. Radiation of a high output UV lamp (185 and 254 nm) produce, in water, by different pathways, hydroxyl free radicals (OH), which oxidize the organic compounds to carbon dioxide and water from oxygen dissolved in the water. The difference between types (a) and (b) detectors is that the direct detector is subjected to interference from ionic contamination, acids, bases, and halogenated organics. In the membrane based conductimetric method, the membrane is a protective barrier to interfering ions, enabling the analysis of COj only (reproduced courtesy of GE Analytical Instruments for illustrations).

As discussed earlier, it is important that corrosion inhibitors are transportable through the resin matrix and therefore this transport was investigated using radiochemical, spectroscopic and conductimetric methods on dry and water immersed resin, different detection techniques being applicable to different inhibitor species. In all cases it was found that diffusion was negligible in dry samples but that diffusing water could "pick up inhibitor species and transport them sufficiently well to compensate for any small amount of depletion of inhibitor at the chip surface. 

In some commercial equipment, a conductimetric method is employed to measure the so-called eutectic temperature. In practice, where supersaturated solutions are the rule, eutectic temperatures are rare, and where they do occur, they are unrelated to the physical state of the drug substance. Such measurements are of little value other than indicating a change in conductance with temperature. In this role, they might give a warning of abnormal conditions. 

Measurements of the dissociation constants of m-nitrobenzoic acid and p-nitrobenzoic acid began with Ostwald in 1889" . However, the first reasonably precise values were obtained by Dippy and coworkers in the nineteen-thirties, as part of an extensive study of the ionization of carboxylic acids by conductimetric methods. The pA"a values in water at 25 °C of m-nitrobenzoic acid and p-nitrobenzoic acid were found to be 3.493 and 3.425, respectively, compared with 4.203 for benzoic acid itself. On the basis of these values Hammett proposed a values of 0.710 for m-N02 and 0.778 for P-NO2 (see Section n.A). These a values have commonly been used thereafter, often rounded to 0.71 and 0.78, respectively. 

Conductivity measurements are much less chemically specific and so are not considered in detail in this volume, although Chapter 2.11 describes some biosensors that operate on conductimetric principles. It is also important to note, in passing, the use of conductimetric methods in analysis [3], particularly for capillary electrophoresis [4, 5] and in ion chromatography [6]. 

Conductimetric method - Between 0.2 and 0.3 g lignin were suspended in 10 mL ethanol and submitted to magnetic agitation. Then, 5 mL acetone and 15 mL distilled water were added, producing a fine lignin suspension. The suspension was bubbled... 

However, it has been imderlined [27] that this technique, in which the amoimt of matter is followed by a conductimetric method, cannot be used for concentrated solutions and when several species are implied in the extraction reaction. 

F.M. Rabinovitch, Conductimetric Method of Dispersion Analysis, Chemical Publishing House Khimiya, Leningrad, 1970. 

The solubility method for determining pKg is rather laborious but proves useful in those rare cases where (a) the substance is too insoluble in water for the potentiometric or conductimetric methods, and (/ ) the molecular and ionic ultraviolet spectra are either too similar or totally lacking. If the spectra are identical, this forms no barrier to their use for analysing the saturated solutions, which otherwise are examined by... 

For analytical purposes, conductance measurements are most useful for very dilute solutions, or where the electrolyte of interest is a minor constituent, accompanied perhaps by larger amounts of nonelectrolyte. The conductimetric method is illustrated in figure C. 14(a), which shows the conductance change when HCl is added to NaOH solution. Each addition reacts completely ... 

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