68 ELECTRICAL CONDUCTIVITIES, ETC. 



Mackay* "that compounds of this type first break down into the constituent sul- 

 phates, especially in dilute solution, and these then dissociate as if they alone were 

 present in the solution." 



Potassium permanganate underwent slight decomposition, especially at more ele- 

 vated temperatures. The high conductivity of potassium ferrocyanide is explained 

 by the large number of ions into which it breaks down. 



Ammonium salts with the ordinary mineral acids crystallize with little or no water. 

 This means that the ammonium ion is very slightly hydrated in aqueous solution. 

 Ammonium salts, in general, conduct to just about the same extent as potassium 

 salts. Tetraethylammonium iodide decomposes slowly around 50 degrees. 



Turning to the bivalent metals, let us consider, first, salts of calcium, strontium, 

 barium, and magnesium. Most of the salts of these metals with the ordinary min- 

 eral acids crystallize with six molecules of water; calcium nitrate, which crystallizes 

 with four molecules of water ; strontium nitrate, which crystallizes anhydrous ; barium 

 chloride and bromide, which crystallize with two molecules of water each, and barium 

 nitrate, which crystallizes without water, are exceptions. 



Earlier work in this laboratory on the approximate composition of the hydrates 

 formed by various substancesf has shown that salts of calcium, strontium, barium, 

 and magnesium hydrate to approximately the same extent, and that all four of these 

 elements have very great hyclrating power. While the masses of the atoms of these 

 four elements vary from magnesium = 24.36, calcium = 40.1, strontium = 87.6, to 

 barium = 137.4, yet the amounts of water with which these substances in solution 

 are combined are so large that the total masses of the four ions when hydrated as 

 they are, especially in dilute solution, are not very different. Further, the atomic 

 volumes of these four substances are not very different, magnesium being somewhat 

 less than the other three. Ionic velocity is a function of the ionic volume and ionic 

 mass of the hydrated ions. We should, therefore, expect the velocities of these four 

 ions to be just about the same, and such is the fact. The velocities are: Mg = 58, 

 Ca = 62, Sr = 63, and Ba = 64. 



Conductivity is a function of the number and velocities of the ions taking part in 

 the conduction of the current. Since salts of the above four elements are dissociated 

 to just about the same extent, it follows that salts of calcium, strontium, barium, and 

 magnesium should give conductivities of the same order of magnitude. An exam- 

 ination of the results will show this to be the case. The salts of these elements with 

 the organic acids formic and acetic are probably somewhat hydrolyzed, especially 

 the salts of acetic acid. The formate showed a short hydrolysis time factor, while 

 the acetate precipitated a small amount of barium hydroxide on the platinum plates. 



Zinc nitrate, like magnesium nitrate, crystallizes with six molecules of water and 

 the two have very nearly the same conductivity. There was evidence that zinc 

 nitrate underwent slight hydrolysis. Zinc sulphate and magnesium sulphate crys- 

 tallize with the same amount of water each with seven molecules and they have 

 very nearly the same conductivities. Zinc acetate was undoubtedly strongly hydro- 

 lyzed, especially at the high dilutions and high temperatures. There was an 

 appreciable odor of acetic acid in these solutions. 



*Amer. Chem. Journ., 19, 83 (1897). fCarnegie Institution Publication No. 60. 



