312 PRINCIPLES OF CHEMISTRY 



oxygen, a residue consisting of the lower oxides of manganese ancT 

 potassium oxide being left. 22bi3 A mixture of permanganate of potas- 

 sium, phosphorous and sulphur takes fire when struck or rubbed, a 

 mixture of the permanganate with carbon only takes h're when heated, 

 not when struck. The instability of the salt is also seen in the fact 

 that its solution is decomposed by peroxide of hydrogen, which at the 

 same time it decomposes. A number of substances reduce potassium 

 permanganate to manganese dioxide (in which case the red solution 

 becomes colourless). 23 Many organic substances (although far from 

 all, even when boiled in a solution of permanganate) act in this manner, 

 being oxidised at the expense of a portion of its oxygen. Thus, a 

 solution of sugar decomposes a cold solution of potassium permanganate. 

 In the presence of an excess of alkali, with a small quantity of sugar, 

 the reduction leads to the formation of potassium manganate, because 

 2KMn0 4 + 2KHO=0 + 2K 2 Mn0 4 + H 2 O. With a considerable amount 

 of sugar and a more prolonged action, the solution turns brown and 

 precipitates manganese dioxide or even oxide. In the oxidation of 

 many organic bodies by an alkaline solution of KMnO 4 generally three- 

 eighths of the oxygen in the salt are utilised for oxidation : 2KMn0 4 

 =K 2 O -j-2Mn0 2 + O 3 . A portion of the alkali liberated is retained by 

 the manganese dioxide, and the other portion generally combines with 

 the substance oxidised, because the latter most frequently gives an acid 

 with an excess of alkali. A solution of potassium iodide acts in a 

 similar manner, being converted into potassium iodate at the expense of 

 the three atoms of oxygen disengaged by two molecules of potassium 

 permanganate. 



In the presence of acids, potassium permanganate acts as an oxidising 

 agent with still greater energy than in the presence of alkalis. At any 

 rate, a greater proportion of oxygen is then available for oxidation, 

 namely, not f , as in the presence of alkalis, but f , because in the first 

 instance manganese dioxide is formed, and in the second case mangan- 

 ous oxide, or rather the salt, MnX 2 , corresponding with it. Thus, for 



22 bis The solution of this salt with an. excess of impure commercial alkali generally 

 acquires a green tint. 



23 A solution of potassium permanganate gives a beautiful absorption spectrum 

 (Chapter XIII.) If the light in passing through this solution loses a portion pf its rays 

 in it (if one may so account for it), this is partially explained by the increased oxidising 

 power which the solution then acquires. We may here also remark that a dilute solution 

 of permanganate of potassium forms a colourless solution with nickel salts, because 

 the green colour of the solution of nickel salts is complementary to the red. Such a 

 decolorised solution, containing a large proportion of nickel and a small proportion of 

 manganese, decomposes after a time, throws down a precipitate, and re-acquires the 

 green colour proper to the nickel salts. The addition of a solution of a cobalt salt (rose- 

 red) to the nickel salt also destroys the colour of both salts. 



