12 GENERAL METHODS 



As the 2 permanganate molecules liberate oxygen enough for 10 

 hydrogen atoms it takes only one-fiftieth of the molecular weight 

 expressed in grams (3.156 gms.) to make 1 liter of tenth normal 

 solution. 



The calculation of what constitutes normal or equivalent solu- 

 tions of any reagent is not very difficult provided the equation 

 representing the chemical reaction involved is thoroughly clear. 



All the common mineral acids and strong alkalies contain so 

 much water that it is in practice not feasible to weigh out with suffi- 

 cient accuracy the theoretical quantity required for a standard solu- 

 tion of acid or alkali. The carbonates of sodium or calcium (or the 

 carbonates of sodium or potassium, obtained by ignition of the 

 corresponding oxalates) give exceedingly accurate results. Oxalic 

 acid is very serviceable as starting material for the preparation of 

 standardized solutions of acids and alkalis if it is pure and has 

 lost none of its water of crystallization. 



1. Preparation of .5 N Oxalic Acid (500 c.c.)- The usefulness 

 of oxalic acid as a starting point for the preparation of standard 

 acids and alkalies is due entirely to the fact that it can be obtained 

 chemically pure and in condition suitable for direct weighing. 

 Oxalic acid is, however, not a strong enough acid to titrate well 

 with all the common indicators, and it is therefore not serviceable 

 for acidimetric titrations in general. But by means of oxalic acid 

 and with phenolphthalein as indicator, standard solutions of a 

 strong alkali, like caustic soda, can be obtained, and by means of 

 the latter standard solutions of the stronger mineral acids can then 

 be prepared. 



The reason why the strong acids and alkalies give more accurate 

 and reliable results is the fact that the salts which they form when 

 neutralized are not appreciably hydrolyzed by water into acid and 

 base, as are the corresponding salts of the weaker acids and 

 bases. The zone of neutrality to different indicators is therefore 

 more sharply defined, and corresponds more nearly to the point 

 represented by the presence of exactly equivalent amounts of acid 

 and alkali. 



Weigh accurately (to the fourth decimal) a small, clean, and 

 dry beaker or large crucible. Then add to the weights on the 

 balance pan 15.7560 gms. and add oxalic acid to the vessel on the 

 other side until exact equilibrium is reached. Dissolve in dis- 

 tilled water this oxalic acid without the loss of a single crystal. 



