Page:Text-book of Electrochemistry.djvu/162

 Calculation of X„ for Slightly Dissociated Electro- lytes. — The equivalent conductivity at infinite dilution of slightly dissociated substances, such as ammonia or acetic acid, cannot be estimated by extrapolation from the results at higher concentrations. If we know the values /„ for the mobilities of the ions, however, we can easily calculate the value of X^. From what has been said, it follows that X^ is made up of the sum of the mobilities of the two ions i^oo *^^ ^Aoo ^f *^® ^^^ ^^ > thxxs, at 18° for potassium chloride, X„ = /«„ + kv. (130-11 = 6467 + 65-44); for acetic acid, X^ = 347*7 ; and for ammonia, X„ = 236*2.

In order to determine the value of the conductivity at infinite dilution of a weak electrolyte, the corresponding value for one of its salts is estimated, and from the value so obtained the number sought may be calculated by substitution. Thus, to find the value of X^ for benzoic acid, X„ is determined for potassium benzoate, and from the result the value of /k„ is subtracted, and that of L added.

Absolute Velocity of the Ions. — As we have seen, the

galvanic conducting power of a solution depends on the

number of ions present, and on their mobilities, i.e. on

their capability of wandering through the solution. The

ions exert a kind of friction against the liquid, which can

be calculated from 'the conductivity of the solution. This

" electrolytic friction " is measured by the force required to

+ -

impel a gram-ion (1 gram of H or 35*45 grams of Gl) at a

speed of 1 cm. per second. Imagine a

column of a normal solution of hydro- ^

chloric acid at 18°, PPi in Fig. 36, of 1 sq. f^^^

��P,

��cm. cross section. Suppose two planes, A _^

and J5, laid through this column perpen- A B

dicularly, at a distance of 1 cm. apart, and Fro- 36.

a current flowing in the direction AB,

driven by a potential difference between the two planes of V

volts. The current strength is then —

/= Vx K

�� �