390 Hale — Initiative Action of Iodine and other 



hydrogen dioxide were poured, brought to a boil and boiled 

 five minutes. The solution was cooled and iodine was added. 

 A few drops brought out a fine blue with perhaps the barest 

 trace of purple. 



II. Into an Erlenmeyer flask 100 cm3 of water, 10 cm3 of acid 

 potassium carbonate, 4 cm3 of the starch solution and l cm3 of 

 potassium permanganate were poured, brought to a boil and 

 boiled five minutes. The permanganate faded to a light 

 brownish yellow. The solution was made acid and the excess 

 of permanganate was faded with a few drops of very dilute 

 sulphurous acid. The solution was then cooled, made alkaline, 

 and iodine was added. A few drops produced a blue. There 

 was not a trace of red. 



These facts substantiate the statement that pure starch causes 

 no red color, nor loss of iodine, in alkaline titration of arsenite 

 solution or of tartar emetic. If any purplish tinge occasion- 

 ally occurs it is no hindrance to the reading and it causes no 

 appreciable loss of iodine, since under most advantageous con- 

 ditions the loss is but slight, even with an impure starch. 



With an impure starch the reading from the first permanent 

 color, whether red or blue, is nearest to the correct value. 

 The readings with impure starch may be compared with plain 

 iodine readings and a correction applied, since the loss for a 

 constant quantity of starch is constant in the titration of 20-50 cm3 

 of arsenite solution. It is better to titrate with considerable 

 dilution, e. g., 150-200 cm3 , and to add a crystal of potassium 

 iodide if necessary, since the production of red is at a mini- 

 mum and less troublesome, and the loss of iodine is hardly 

 appreciable. With impure starch fresh solutions should be 

 frequently made. 



Experiments showing the Cause of the Trouble. 



The foregoing experiments have shown that pure starch is 

 not attacked by iodine, hydrogen dioxide, or potassium per- 

 manganate in the presence of acid potassium carbonate, and 

 that impure starch is attacked. There were found in the 

 impure starch employed two impurities, one of which colors 

 red with iodine, and the other under the influence of nascent 

 oxygen and acid potassium carbonate is changed to a body 

 which colors red with iodine. These two impurities were sepa- 

 rated from the starch in the following manner. A solution 

 was made of the impure starch. To two separate portions 

 iodine was added. 



I. Iodine was run in to excess, and the resulting starch blue 

 was precipitated by dilute sulphuric acid and filtered. The 

 excess of iodine was shown by its action on the filter paper 



