Potassium Ion, Kalion

With the exception of rubidium and caesium, potassium has the greatest electroaffinity or tendency to ionization, a fact regarded by Abegg and Bodlander as according with the ready solubility of most of its salts, and the comparatively slight tendency of its ions to form complex salts or hydrates. The potassium salts are strong electrolytes, being highly dissociated in dilute aqueous solution. Only those with coloured anions yield coloured solutions, an indication that the potassium ion is colourless. The conductivity of the potassium ion is 64.5 at 18° C., and 74.8 at 25° C. The transport-number is 0.514.

Potassium can form only one kind of ion, viz. the monovalent kalion, K^•. With metallic potassium the formation takes place with very great ease and energy. The chemical properties of the metal are essentially characterised by this fact, for it reacts on other substances in such a way that it passes into kalion, i.e. it forms a salt. Since, further, the passage of a solid salt into a dissolved one is in general accompanied by only a slight change of energy, it is of no essential importance for these reactions of potassium whether a dissolved or a solid salt is produced.

Isolated examples of such reactions have already been mentioned; the method of obtaining silicon and boron from their halogen compounds may be recalled. Since in these reactions the halogen compounds of potassium, i.e. salts of the metal, are formed, they come under the rule just stated.

The amount of heat which is liberated in the formation of kalion from the metal is very great; it is found, in accordance with the principles explained on p. 205, to be 259 kj.

If this quantity of heat is added to the heat of formation of an anion, the sum gives the heat of formation of the salt in dilute solution. In order to obtain from this the heat of formation of the solid salt, it is only necessary to subtract the heat of solution, or to add its numerical value if solution takes place with absorption of heat, as is chiefly the case with the salts of potassium.

The aqueous solutions containing kalion are colourless if no other coloured compound is present, nor do they exhibit any other conspicuous property. For the higher animals, kalion in great concentration is a poison; in small quantities, however, it is an indispensable Constituent of the organism. In the vertebrate animals it is contained, more especially, in the red blood corpuscles.

The detection of kalion in solution is accomplished by the aid of some acids which can form sparingly soluble potassium salts. For example, if a solution of perchloric acid be added to a solution containing kalion, there is precipitated, if the solution is not too dilute with respect to the latter, a crystalline precipitate of sparingly soluble potassium perchlorate. The same occurs on the addition of sodium perchlorate. In order to correctly understand the processes which are here met with, we shall first put forward a few general considerations.