418 ANALYTICAL CHEMISTRY. 



This shows that one molecule of oxalic acid requires one atom of oxygen 

 for oxidation, or one-half molecule of acid requires one-half atom of oxygen. 

 As one-half gram-molecule of oxalic acid is the quantity in one liter of normal 

 solution, it follows that the liter of normal oxalic acid is exactly oxidized by a 

 liter of normal permanganate solution i. e., the two solutions are equivalent, 

 since a liter of normal permanganate solution gives off one-half gram-atom of 

 oxygen. Hence, it is convenient to use normal or deci-normal oxalic acid for 

 standardizing the permanganate solution. 



Permanganate solution, when recently made, without observing certain pre- 

 cautions, will deteriorate for a certain length of time i. e., until all traces of 

 organic and other deoxidizing matters have become oxidized by the per- 

 manganate. 



To prepare a permanent ^ solution of potassium permanganate, dissolve 

 about 3.3 grammes of the pure crystals (Potassii Permanganas, U. S. P.) in 1000 

 c.c. of distilled water in a flask, and boil for 5 minutes. Close the flask with 

 absorbent cotton, and set aside for at least two days, so that suspended matters 

 may deposit. Then decant the clear liquid without stirring up the sediment, 

 or for greater precaution filter it through a layer of purified shredded asbestos 

 (paper or cotton should not be used). The water to be employed for di- 

 luting this solution should be distilled from about 1 gramme of potassium per- 

 manganate. 



To determine the strength of the solution draw off 10 c.c. of deci-normal 

 oxalic acid solution into a beaker, add 1 c.c. of pure concentrated sulphuric 

 acid, heat the mixture to about 80 C. (176 F.), then add gradually from a 

 glass-cock burette the permanganate, while stirring constantly, until a faint 

 pink color is produced, which remains permanent for one-half minute. Note 

 the number of cubic centimetres consumed and dilute the solution so that it is 

 exactly equivalent to the deci-norrnal oxalic acid. Verify the accuracy of the 

 dilution by a new titration. When properly prepared and preserved in glass- 

 stoppered bottles, permanganate solution will keep for at least six months with- 

 out changing its strength. 



A second method for preparing a deci-normal solution of permanganate through 

 the medium of a deci-normal thiosulphate solution is described in the U. S. P. 

 as follows : 



To a solution of about 1 gramme of potassium iodide in 10 c.c. of dilute sul- 

 phuric acid, 20 c.c. of the permanganate solution to be standardized are added. 

 This reaction takes place : 



2HI + O = H 2 -j- 21. 



The mixture is at once diluted with about 200 c.c. of pure water, and deci-normal 

 thiosulphate solution slowly added from a burette with constant stirring until 

 the color of the iodine is just discharged. The number of c.c. of the thiosul- 

 phate solution is noted, and the permanganate solution is diluted so that equal 

 volumes of the two solutions correspond to each other. 



Instead of using oxalic acid for standardizing permanganate solution, 

 metallic iron may be used, and the operation should be conducted as follows : 

 0.2 gramme of pure, thin iron wire is dissolved in about 20 c.c. of dilute sul- 

 phuric acid (1 acid, 5 water) by the aid of heat, and in a flask arranged as 

 in Fig. 67. The flask is provided, by means of a perforated cork, with a 



