DETECTION AND DETERMINATION OF ALCOHOLS, ETC. 7 



As magnesium filings seem to give the best results the reaction is 

 carried out as follows: 



Strongly acidify the solution with phosphoric acid and distill about 

 one-third of it. To the distillate add dilute sulphuric acid and magne- 

 sium filings in sufficient quantities to cause a vigorous but not a 

 violent evolution of hydrogen. (In case quite a large quantity of 

 acid is present in the distillate it is not necessary to add any sulphuric 

 acid.) If the amount of formic acid is small (about 0.1 per cent) Con- 

 tinue the action for one hour. If larger quantities are present, the test 

 for formaldehyde will usually be obtained in from two to five minutes. 

 Of course the absence of formaldehyde in the original solution must 

 be determined. 



The common methods for the determination of formic acid are 

 based on the following general reactions : (1) The reduction of a mer- 

 curic salt to a mercurous salt, the insoluble mercurous salt being 

 weighed; (2) the reduction of potassium permanganate in alkaline 

 solution; (3) the action of concentrated sulphuric acid, the evolved 

 carbon monoxid being measured. With regard to methods 1 and 2, 

 aldehydes, certain alcohols, and many other volatile substances which 

 may be present in fruits and vegetables effect the same reduction as 

 formic acid. With regard to method 3, a very large number of 

 organic acids evolve carbon monoxid when treated with concentrated 

 sulphuric acid. 



While platinic salts are readily reduced to metal by many aldehydes 

 and other reducing agents in alkaline solution, it was found that of a 

 great number of substances tried only formic acid accomplished this 

 reduction in neutral or weak acid solutions. The following method is 

 therefore proposed: Distill the solution containing the formic acid 

 with a small quantity of phosphoric acid until the distillate is no 

 longer acid. If the volume of this distillate is too large to be con- 

 veniently handled, neutralize it with sodium hydroxid and evaporate 

 to a convenient volume. Add an excess of platinic chlorid and suffi- 

 cient acetic acid to make the solution strongly acid (usually about 1 

 or 2 cc of glacial acetic acid for less than 1 gram of formic acid), and 

 boil the solution for one hour, using a reflux condenser. Collect the 

 reduced platinum in the usual manner and weigh. The weight of the 

 platinum multiplied by 0.472 equals the formic acid present. 



The following are examples of results obtained by this method : 



(1) A 100 cc solution containing 0.0782 gram of formic acid; 

 platinum reduced, 0.160 gram = formic acid 0.0755 gram; 97 per 

 cent of theoretical amount. 



(2) A 100 cc solution containing 0.243 gram of formic acid; plati- 

 num reduced, 0.504 gram = 0.238 gram of formic acid, 98 per cent 

 recovery. 



