Measurements of electrolytic conductance

Measurement of conductivity The measurement of electrolyte conductivity — the reciprocal of the resistivity —is a fairly simple matter, being calculated from the resistivity as measured by some of the methods described above. 

ELECTROLYTIC CONDUCTIVITY AND RESISTIVITY MEASUREMENTS. Industrial interest in the measurement of electrolytic conductivity (of which electrolytic resistivity is the reciprocal) arises chiefly from its usefulness as a measure of ion concentrations in water solutions. Also, by comparison with other analytical methods, this is relatively simple and inexpensive. 

In this chapter we take a careful look at the phenomenon of electrical conductivity of materials, particularly electrolytic solutions. In the first section, the nature of electrical conductivity and its relation to the electrolyte composition and temperature is developed. The first section and the second (which deals with the direct-current contact methods for measuring conductance) introduce the basic considerations and techniques of conductance measurement. This introduction to conductance measurements is useful to the scientist, not only for electrolytic conductance, but also for understanding the applications of common resistive indicator devices such as thermistors for temperature, photoconductors for light, and strain gauges for mechanical distortion. The third section of this chapter describes the special techniques that are used to minimize the effects of electrode phenomena on the measurement of electrolytic conductance. In that section you will encounter the most recent solutions to the problems of conductometric measurements, the solutions that have sparked the resurgent interest in analytical conductometry. 

Figure 8.7 Four-contact measurement of electrolytic conductance.

Providing traceability for electrolytic conductivity measurements is a new activity of PTB. It is a consequence of the growing demand for reliable calibrations of electrolytic conductivity measuring cells. The measurement of electrolytic conductivity is a useful analytical tool often applied in various fields of science and technology, in particular in the case of aqueous media, for which electrolytic conductivity is a measure of the concentration of ionized substances. Although it is a non-specific sum parameter, it can, under given conditions, be used as an easily accessible quantitative measure of the water quality, replacing cumbersome and expensive chemical analyses. 

Cells for Conductimetry. Reliable and precise measurements of electrolytic conductance require attention to the design of cells, electrodes, and measuring circuitry. Extraction of an ohmic resistance from AC bridge measurements is not a trivial task, particularly in solutions with high resistance (such as organic solvents) or low resistance (molten salts). Expositions of the principles are provided in monographs that emphasize aqueous solution,54,55 and in a review of conductimetry and high-frequency oscillometry that emphasizes analytical applications.56... 

Two main types of cell have been devised for the accurate measurement of electrolytic conductance there is the pipette type, used by Washburn (1916), and the flask type, introduced by Hartley and Barrett (1913). In the course of a careful study of cells of the pipette form, Parker (1923) found that with solutions of high resistance, for which the polarization error is negligible, there was an apparent decrease of the cell constant with increasing resistance. This phenomenon, which became known as the Parker effect, was confirmed by other workers it was at... 

The Wien Effect. If, instead of using potentials of the order of one volt per centimeter in the measurement of electrolytic conductance, voltages of several hundred thousand times this are employed, the conductances of solutions of electrolytes are no longer constant but tend to increase with the potential. Under these conditions Ohm s law is evidently no longer valid. This increase of conductance at high potentials is called the Wien effect. The passage of high potentials... 

Some additional selective detectors have been described, but their use in biochemical GC has been minimal thus far. Among them, most notably, belong various optical spectroscopic detectors as well as various element-specific detectors based on the solute combustion and measurement of electrolytic conductivity [128]. While little has happened during the last decade with further development of the latter detector types, various gas-phase optical devices remain among the most interesting detectors for future studies. Element-specific plasma devices [129], UV absorption... 

Emulsions can be found as two basic types, i.e., 0/W and W/O, but in some particular cases, multiple or double emulsions labeled Wj/0/W2 and Oj/W/02 also occur. The emulsion type may be determined by different methods. In most applications the aqueous phase contains one or various electrolytes, and thus conducts electricity somehow, whereas the oil or organic phase does not. Consequently, the measurement of electrolytic conductivity is a handy way to ascertain the emulsion type. Moreover, the continuous monitoring of the electrolytic conductivity allows the determination of the change in emulsion type which is referred to as emulsion inversion. 

Figure 2 Types of contacting conductivity cells (A) Jones and Bollinger (B) Roseveare (C) Shedlovsky (D) flask type (Shed-lovsky) (E) dipping cell. (From Light TS and Ewing GW (1990) Measurement of electrolytic conductance. In Ewing GW (ed.) Analytical Instrumentation Handbook, pp. 641-658. New York Dekker.)...

After direct current became available, attempts were made to use it in electrolytic conductance measurements. In 18 7, Eben Horsford (1818-1893) tried to allow for the effects of electrode polarization (J 0). He varied the length of the liquid path, then adjusted a calibrated series-connected resistor to restore the current to its original value. However, precise measurements of electrolytic conductance by d.c. methods had to await the perfection of the -electrode system by Gordon and his co-workers in the 19 0 s. 

Light, T.S. Ewing, G.W. The Measurement of Electrolytic Conductance in Handbook of Analytical Instrumentation Ewing, Ed. Marcel Dekker New York, (in press). 

Instruments in this category are used for the measurement of electrolyte resistivity, resistance, and insulation (i.e. protective-wrap) conductivity. 

Numerous measurements of the conductivity of aqueous solutions performed by the school of Friedrich Kohhansch (1840-1910) and the investigations of Jacobns van t Hoff (1852-1911 Nobel prize, 1901) on the osmotic pressure of solutions led the young Swedish physicist Svante August Arrhenius (1859-1927 Nobel prize, 1903) to establish in 1884 in his thesis the main ideas of his famous theory of electrolytic dissociation of acids, alkalis, and salts in solutions. Despite the sceptitism of some chemists, this theory was generally accepted toward the end of the centnry. 

Measurement of the conductance of an electrolyte solution using an ac source. Rate of change of conductance as a function of added titrant used to determine the equivalence point. 

The safety sensor, however, gives only qualitative information. For a quantitative determination of the concentration of HF in a solution, it is necessary to determine JpS, which can be done by scanning the anodic potential from about 3 V to 0 V and measuring the relative current maximum in a unstirred solution. If JPS and the temperature T are determined, the electrolyte concentration c can be calculated using Eq. (4.9). This method of determining the concentration of HF is superior to simple measurements of the conductivity of the solution, because it is insensitive to dissolution products of Si or Si02, or to other ionic species in the analyte. 

Kohlrausch method phys chem A method of measuring the electrolytic conductance of a solution using a Wheatstone bridge. kol,raush, meth-3d ... 

Sensitive to handling Reference electrode is not filled/Top up with electrolyte solution, free of air bubbles. Reference electrodes tilled with the wrong solution/Empty and refill the reference electrolyte. Diaphragm clogged/Clean diaphragm. Measurement of poorly conductive solutions/Measure with different amplifier or add supporting electrolyte. 

Of perhaps greater interest to electrochemists who work with organic solvents are reviews that survey measurements of the conductance of electrolytes in a number of solvents, discussions of experimental methods, cell design, and... 

As can be seen, the measurement of the conductivity of an electrolyte solution is not species selective. Individual ionic conductivities can be calculated only if the conductivity (or mobility) of one ion is known this in the case of a simple salt solution containing one cation and one anion. If various ions are present, calculation is correspondingly more difficult. Additionally, individual ionic conductivities can vary with solution composition and concentration. 

On the other hand the equivalent conductance of weak electrolytes rises much steeper on dilution yet it doesn t nearly attain its limit value A° at concentrations mentioned in the previous case. As the measurement of the conductance at still higher dilution is extremely inaccurate due to high resistances of the solution, the same method of extrapolation as used with the strong electrolytes is unsuitable for determination of A0 of weak electrolytes. In such cases we resort to the Kohlrausch law of independent migration of ions, to l e discussed further on. 

Analogous results have been reported from the systematic measurements of electrical conductivity and transference numbers of ions (// and tf) in black foam films [336] and parallel measurements of these quantities in highly concentrated surfactant/water system [337], Furthermore, it has been found that while the electrical conductivity of CBF depends on the electrolyte concentration in the initial solution, that of NBF does not. The transference numbers of the ions measured for films and a gel obtained from NaDoS-NaCl-HCl system are given below... 

Fig. 39. Electrode configuration for the measurement of the conductivity in a thin layer of electrolyte in contact with the electrode surface.

This empirical relationship between the equivalent conductivity and the square root of concentration is a law named after Kohlrausch. His extremely careful measurements of the conductance of electrolytic solutions can be considered to have played a leading role in the initiation of ionics, the physical chemistry of ionic solutions. 

---