Conductance of aqueous solutions

Arrhenius, insofar as his profession could be defined at all, began as a physicist. He worked with a physics professor in Stockholm and presented a thesis on the electrical conductivities of aqueous solutions of salts. A recent biography (Crawford 1996) presents in detail the humiliating treatment of Arrhenius by his sceptical examiners in 1884, which nearly put an end to his scientific career he was not adjudged fit for a university career. He was not the last innovator to have trouble with examiners. Yet, a bare 19 years later, in 1903, he received the Nobel Prize for Chemistry. It shows the unusual attitude of this founder of physical chemistry that he was distinctly surprised not to receive the Physics Prize, because he thought of himself as a physicist. 

The discussion of molecules and molecular ions will be continued in Sec. 29. Here we shall begin the detailed examination of solutes that are completely dissociated into ions. The conductivity of aqueous solutions of such solutes has been accurately measured at concentrations as low as 0.00003 mole per liter. Even at these concentrations the motions of the positive and negative ions are not quite independent of each other. Owing to the electrostatic forces between the ions, the mobility of each ion is slightly less than it would be in a still more dilute solution. For example, an aqueous solution of KC1 at 25°, at a concentration of 3.2576 X 10 6 mole per liter, was found to have an equivalent con-... 

The conductivity of sodium dodecyl sulfate in aqueous solution and in sodium chloride solutions was studied by Williams et al. [98] to determine the CMC. Goddard and Benson [146] studied the electrical conductivity of aqueous solutions of sodium octyl, decyl, and dodecyl sulfates over concentration ranges about the respective CMC and at temperatures from 10°C to 55°C. Figure 14 shows the results obtained by Goddard and Benson for the specific conductivity of sodium dodecyl sulfate and Table 25 shows the coefficients a and p of the linear equation of the specific conductivity, in mho/cm, vs. the molality of the solution at 25°C. Micellization parameters have been studied in detail from conductivity data in a recent work of Shanks and Franses [147]. 

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. 

Ordinary water behaves very differently under high temperature and high pressure. Early studies of aqueous solutions under high pressure showed a unique anomaly that was not observed with any other solvent.11 The electrolytic conductance of aqueous solutions increases with an increase in pressure. The effect is more pronounced at lower... 

Figure 20.2. Equivalent conductance of aqueous solutions of acetic acid at 25°C. Based on data from D. A. Macinnes and T. Shedlovsky, J. Am. Chem. Soc. 54, 1429 (1932).

The separation medium inside the capillary has to be electrically conductive. It is easy to manipulate the conductivity of aqueous solutions as there are numerous salts soluble in that medium. With organic solvents, the choice of suitable salts/electrolytes is limited. One of the most widely used... 

Electrical Conductivity of Aqueous Solutions generally high low or zero... 

W. Ostwald, Zeit. phys. Chem., 1. 83, 1887 E. Franke, ib., 16. 463, 1895 P. Walden, ib., 2. 49, 1888 H. C. Jones, The Electrical Conductivity, Dissociation, and Temperature Coefficients of Conductivity of Aqueous Solutions of a Number of Sails and Organic Acids, Washington,... 

Electrical Conductance of Aqueous Solutions of Ammonia and Metal Hydroxides. Check the electrical conductance of 1 W solutions of sodium hydroxide, potassium hydroxide, and ammonia. Record the ammeter readings. Arrange the studied alkalies in a series according to their activity. Acquaint yourself with the degree of dissociation and the dissociation constants of acids and bases (see Appendix 1, Tables 9 and 10). Why is the term apparent degree of dissociation used to characterize the dissociation of strong electrolytes ... 

Molecular Weight and Constitution.—The electrical conductivity of aqueous solutions of potassium perdisulphate supplies distinct evidence of the dibasicity of the corresponding acid.12 This is confirmed by molecular weight determinations made by the cryoscopie method... 

Other physical phenomena that may be associated, at least partially, with complex formation are the effect of a salt on the viscosity of aqueous solutions of a sugar and the effect of carbohydrates on the electrical conductivity of aqueous solutions of electrolytes. Measurements have been made of the increase in viscosity of aqueous sucrose solutions caused by the presence of potassium acetate, potassium chloride, potassium oxalate, and the potassium and calcium salt of 5-oxo-2-pyrrolidinecarboxylic acid.81 Potassium acetate has a greater effect than potassium chloride, and calcium ion is more effective than potassium ion. Conductivities of 0.01-0.05 N aqueous solutions of potassium chloride, sodium chloride, potassium sulfate, sodium sulfate, sodium carbonate, potassium bicarbonate, potassium hydroxide, and sodium hydroxide, ammonium hydroxide, and calcium sulfate, in both the presence and absence of sucrose, have been determined by Selix.88 At a sucrose concentration of 15° Brix (15.9 g. of sucrose/100 ml. of solution), an increase of 1° Brix in sucrose causes a 4% decrease in conductivity. Landt and Bodea88 studied dilute aqueous solutions of potassium chloride, sodium chloride, barium chloride, and tetra-... 

Fia. 31.—Specific Elee-trical Conductivity of Aqueous Solutions of Ammonia. 

Table 4. Equivalent conductances of aqueous solutions of electrolytes... 

Noyes, The Electrical Conductivities of Aqueous Solutions, Washington, 1907 Noyes, Meloher, Cooper and Eastman, Zeitsch. physikal. Chem., 1910, 70, 335. See also Kohlrauseh and Holborn, [Pg.162]

The electrical conductivities of aqueous solutions of pyrophosphoric acid are as follows —4... 

Fig. A.5. Electrical conductivity versus mass% H2SO4 in sulfuric acid. Source Roughton, J.E. (1951) The electrical conductivity of aqueous solutions of sulphuric acid from 25°C to 155°C, J. Appl. Chem.,1, Supplementary Issue, No. 2., 141 144.

Kohlrausch discovered, in the last century, that the molar conductivity of aqueous solutions of electrolytes increases with dilution, and reaches a limiting value at very great dilutions. The increase of molar conductivity, in line with the Arrhenius theory, results from the increasing degree of dissociation the limiting value corresponds to complete dissociation. This limiting value of the molar conductivity is denoted here by A0 (the notation A C is also used), while its value at a concentration c will be denoted by Ac. The degree of dissociation can be expressed as the ratio of these two molar conductivities... 

A study of the electric conductivities of aqueous solutions of the salt indicates that the hydrolysis proceeds in two stages, embodying (1) a rapid change unaccompanied by precipitation, and (2) a slower change, progressing at a measurable rate, and accompanied by the production of a so-called basic salt.8 Colloidal ferric hydroxide does not appear to be formed during hydrolysis,9 the salt thus differing from ferric chloride and nitrate. 

The results obtained for the osmotic pressures and electric conductivities of aqueous solutions of calcium and strontium ferroeyanides (see pp. 208 and 220) indicate that these molecules possess the double formula, M4[Fe(CN)6]2. On the other hand, tetra-ethyl ferrocyanide is known to have the single formula, (C2H5)4Fe(CN)6.s... 

Thus, because of the lower dielectric constant values, the effect of an increase of electrolyte concentration on lowering the equivalent conductance is much greater in nonaqueous than in aqueous solutions. The result is that the specific conductivity of nonaqueous solutions containing practical electrolyte concentrations is far less than the specific conductivity of aqueous solutions at the same electrolyte concentration (Table4.26 andFig. 4.107). 

Electrical conductivity of aqueous solutions. The circuit will be completed and will allow current to flow only when there are charge carriers (ions) in the solution. Note Water molecules are present but not shown in these pictures, (a) A hydrochloric acid solution, which is a strong electrolyte, contains ions that re adily conduct the current and give a brightly lit bulb, (b) An acetic acid solution, which is a weak electrolyte, contains only a few ions and does not conduct as much current as a strong electrolyte. The bulb is only dimly lit. (c) A sucrose solution, which is a nonelectrolyte, contains no ions and does not conduct a current. The bulb remains unlit. 

The Electrolytic Dissociation Theory. —From his studies of the conductances of aqueous solutions of acids and their chemical activity, Arrhenius (1883) concluded that an electrolytic solution contained two kinds of solute molecules these were supposed to be active molecules, responsible for electrical conduction and chemical action, and inactive molecules, respectively. It was believed that when an acid, base or salt was dissolved in water a considerable portion, consisting of the so-called active molecules, was spontaneously split up, or dissociated, into positive and negative ions it was suggested that these ions are free to move independently and are directed towards the appropriate electrodes under the influence of an electric field. The proportion of active, or dissociated, molecules to the total number of molecules, later called the degree of dissociation, was considered to vary with the concentration of the electrolyte, and to be equal to unity in dilute solutions. 

Less accurate measurements of the conductances of aqueous solutions of various electrolytes have been made, and in general the results bear out the validity of the Onsager equation. A number of values of the experimental slopes are compared in Table XXIV with those calculated... 

Table XII.—Molecular Electrical Conductivities of Aqueous Solutions of...

Werner realized that he could test his hypothesis by measuring the electrical conductivity of aqueous solutions of the salts of these complex ions. Ions are the electrical conductors in aqueous solutions, and the conductivity is proportional to the ion concentration. If Werner s proposal was correct, then an aqueous solution of Compound 1, for example, should have a molar conductivity close to that of an aqueous solution of A1(N03)3, which also forms four ions per formula unit on complete dissociation in water (one 3+ ion and three 1— ions). His experiments confirmed that the conductivities of these two solutions were, indeed, similar. Furthermore the conductivity of aqueous solutions of compound 2 was close to those of Mg(N03)i, and solutions of compound 3 conducted electricity about as well as those containing NaN03. Compound 4, in contrast, did not dissociate into ions when dissolved in water, producing a solution of very low electrical conductivity. 

State at room temperature Melting point Conductivity in liquid state Water solubility Conductivity of aqueous solution... 

The Interionic Attraction Theory of Conductance of Aqueous Solutions of Electrolytes... 

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