METHODS FOR QUANTITATIVE DETERMINATIONS. 261 



potassium dichr ornate, both of which salts can be obtained in a pure 

 crystallized condition. 



Potassium permanganate, KMnO 4 = 157.67, acts generally in the 

 presence of free acids, upon deoxidizing substances, by losing 5 atoms 

 of oxygen of the 8 atoms contained in two molecules, as is shown in 

 the following equations : 



2KMn0 4 + 5H 2 C 2 4 + 3H 2 SO 4 = K 2 SO 4 -f 2MnSO 4 + 10CO 2 + 8H 2 O. 

 2KMn0 4 + 10FeS0 4 + 8H 2 SO 4 = K 2 SO 4 + 2MnSO 4 + 5Fe 2 3SO 4 + 8H 2 O. 



It follows, that two-fifths of the molecular weight of potassium 

 permanganate, or 63.068 grammes, are the equivalent of 1 oxygen 

 atom. But'as oxygen is diatomic and the volumetric normal is cal- 

 culated for monatomic values, this number must be divided by 2, 

 and 31.534 grammes of pure crystallized potassium permanganate is 

 therefore the amount to furnish 1 liter of normal solution, but as this 

 is too concentrated for most determinations, a deci-normal solution 

 containing 3.1534 grammes to the liter is generally employed. 



Permanganate solution, when recently made, without observing 

 certain precautions, will deteriorate for a certain length of time, i. e., 

 until all traces of organic and other deoxidizing matters have become 

 oxidized by the permanganate. 



In order to prepare permanent volumetric solutions of perman- 

 ganate it is advisable to make two solutions, one too concentrated and 

 the other too dilute for standard. These solutions are boiled and set 

 aside in well-closed bottles for two days, in order to allow any pre- 

 cipitated matter to settle. By mixing the two solutions in the proper 

 proportions a solution of the desired strength can be obtained, and 

 as there is no longer any matter in the solution which can act 

 decomposingly upon the permanganate the solution retains its stand- 

 ard for many months. 



To prepare the two solutions necessary for deci-normal potassium 

 permanganate solution, dissolve 3.5 grammes of potassium perman- 

 ganate and 3 grammes in one liter of water each. Boil the solutions 

 for a few minutes, set them aside for two days, and pour off the clear 

 portions of each solution into separate vessels provided with glass 

 stoppers. 



To find the proportions in which these solutions have to be mixed 

 in order to obtain a deci normal solution the strength of each one has 

 to be determined ; this is done as follows : To a mixture of 10 c.c. 

 of deci-normal oxalic acid solution and 1 c.c. of concentrated sulphuric 

 acid, while yet hot, is added from a burette the weaker permanganate 



