Equivalent molar conductivity

This table gives the equivalent (molar) conductivity A at 25 °C for some common electrolytes in aqueous solution at concentrations up to 0.1 mol/L. The units of A are 10 m S moh. ... 

Another approach toward examining the variation of conductivity with NiBr2 concentration is that of calculating the equivalent molar conductivity. 

The terms specifie conductivity and equivalent conductivity were previously used. However, these terms are not recommended for use as the SI units. They should be replaced by molar conductivity according to the SI recommendation, which states as follows, When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. Thus, when we previously used equivalent conductivity, we should now use molar conductivity, where we define the molar unit so that it is equal to the equivalent unit previously used. For example, we define (l/2)Ca, (l/3)La, (l/2)CO and Alj/jF as molar units. 

In aqueous solutions, concentrations are sometimes expressed in terms of normality (gram equivalents per liter), so that if C is concentration, then V = 103/C and a = 103 K/C. To calculate C, it is necessary to know the formula of the solute in solution. For example, a one molar solution of Fe2(S04)3 would contain 6 1CT3 equivalents cm-3. It is now clear as to why A is preferred. The derivation provided herein clearly brings out the fact that A is the measure of the electrolytic conductance of the ions which make up 1 g-equiv. of electrolyte of a particular concentration - thereby setting conductance measurements on a common basis. Sometimes the molar conductance am is preferred to the equivalent conductance this is the conductance of that volume of the electrolyte which contains one gram molecule (mole) of the ions taking part in the electrolysis and which is held between parallel electrodes 1 cm apart. 

As in the case of solutions, the specific conductance, K, the equivalent conductance, a, and the molar conductance, am, are also distinguished for molten electrolytes. These are defined in the same manner as done for the case of solutions of electrolytes. It may, however, be pointed out that molten salts generally have much higher conductivities than equivalent aqueous systems. 

When reporting the molar conductivity data, the species whose amount is given in moles should be indicated. Often, a fractional molar conductivity corresponding to one mole of chemical equivalents (called a val) is reported. For example, for sulphuric acid, the concentration c can be expressed as the normality , i.e. the species H2S04 is considered. Obviously, A(H2S04) = 2A( H2S04). Consequently, the concept of the equivalent conductivity is often used, defined by the relationship... 

Ccen designates the cell constant (/fcen = d/A). Its value cannot be obtained by direct measurement, but is determined using a standard solution for which the conductivity k is known. The equivalent ionic conductance (S m 2 mol 1) refers to the conductivity of an ion with a valence of z in a solution at 25 °C when the molar concentration C (mol I 1) tends towards 0 (Table 4.1). 

The conductivity of electrolyte solutions depends on the concentration and the charge number of the ions in the solution. It is expressed as the molar or equivalent conductivity or molar conductivity, which is given by ... 

Apart from the equivalent conductance another quantity is used, namely the molar conductance p under this we understand the conductance of that volume of solution which contains one mole of dissolved substance and is placed between two parallel electrodes of sufficient size and set 1 cm apart. If one gram-molecule of a substance corresponds to n — vz gram-equivalents of the ions (z being the number of charges of an ion and v the number anions or cations formed byjhe dissociation of the molecule) then the following relation can be... 

It will be seen from the above equation that in the case of electrolytes composed of two univalent ions there is no difference between molar and equivalent conductances. In case of electrolytes with bivalent ions the molar conductance is double of the equivalent conductance, with triva-lent ions treble and so on. 

Markov et al. [60,61] proposed an equation for the equivalent electrical conductivity of simple binary molten salt mixtures. In binary systems (MjX + M2X or MXj + MX2) there is the possibility of the following ionic arrangements MjX — MjX M2X — M2X MjX — M2X. The probabilities of forming the combinations MjX - MjX M2X - M2X and MjX - M2X are proportional to X, x2 and 2xxx2, respectively, where Xt and x2 are the molar fractions of the two salts. For monovalent molten salts, the equivalent electrical conductivity of a mixture of these salts, Am, can be written as... 

Dividing the molar conductivity by t/ z gives the so-called equivalent conductivity Aeq = NAeo(u+ + ). It is discouraged to use that quantity. 

Second, the specific conductivity can easily be related to the molar and equivalent A conductivities. Take the case of a z z-valent electrolyte. With Eqs. (4.161), (4.136), and (4.138), it is found that... 

Since the mode of ionization of the salt is not known, it is not possible to determine the equivalent weight and hence the equivalent conductance cannot be calculated it is necessary, therefore, to make use of the molar conductance, as defined on p. 31. In the simple case of a series of salts all of which have one univalent ion, either the cation or anion, whereas the other ion has a valence of z, the gram molecule contains z gram equivalents the molar conductance is thus z-timcs the equivalent conductance. If the mean equivalent conductance of all ions is taken as 60, the equivalent conductance of any salt is 120 ohms cm., and the molar conductance is 120 z ohms" cm. The approximate results for a number of salts of different valence types with one univalent ion at 25 are given in Table XXL The observed molar conductances of the platinosammine... 

When the molar conductivity is related to the unity of the amount of positive or negative charge we get equivalent conductivity. This enables direct comparison of the mobility of ions of different valence. The equivalent conductivity is the ratio k/cz. 

In the older literature, the limiting conductance is usually given per equivalent of the ion rather than per mole as cited here. The equivalent conductance is equal to the molar conductance divided by the number of charges on the ion. Thus, the equivalent conductance for the divalent cations listed above is equal to the molar conductance divided by two. 

Molar and Equivalent Conductances. A quantity much used in computations and in tables of constants is the molar conductance, Am. It can be computed from the specific conductance, l, and the concentration, C, of the solute, which is usually stated in mols per liter of solution, by means of the formula... 

One gram mol of a salt contains vs equivalents, so that the relation between the molar conductance, A, and equivalent conductance, A, is... 

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