METHODS FOR QUANTITATIVE DETERMINATIONS. 421 



chiefly used, the end of the reaction is determined by the method of spotting, 

 i. e., by taking out a drop of the solution and testing it on a white porcelain 

 plate with a drop of freshly prepared potassium ferricyanide solution ; when this 

 no longer gives a blue color the reaction is at an end. 



In all determinations by this solution dilute sulphuric acid has to be added, 

 because both the potassium and the chromium require an acid to combine with, 

 as shown in the above equation. 



The titration equivalents of this solution for ferrous salts are the same as 

 those of deci-normal potassium permanganate solution. 



lodimetry. Solutions of iodine and of sodium thiosulphate (hypo- 

 sulphite) act upon each other with the formation of sodium iodide 

 and sodium tetrathionate : 



21 + 2Na 2 S 2 O 3 = 2NaI + Na 2 S 4 O 6 . 



A normal solution of one can be standardized by a normal solution 

 of the other. As indicator, is used starch solution, which is colored 

 blue by minute portions of free iodine. 



Starch solution is made by mixing 1 gramme of starch with 10 c.c. of cold 

 water, and then adding enough boiling water, with constant stirring, to make 

 about 200 c.c. of a transparent jelly. If the solution is to be preserved for any 

 length of time, 10 grammes of zinc chloride should be added. 



Many other substances, such as sulphurous acid^ hydrogen sulphide, 

 arsenous oxide, act upon iodine with the formation of colorless com- 

 pounds, and may, therefore, be estimated by normal solution of iodine, 

 while the iodine may be standardized by the thiosulphate solution. 

 In many cases the latter solution is also used for the determination of 

 chlorine, which is caused to act upon potassium iodide, the liberated 

 iodine being titrated. 



Deci-normal iodine solution. Iodine being a univalent element, 

 the weight of its atom, 125.90, in grammes, is used to make one 

 liter of normal solution. Deci-normal solution is generally employed, 

 and is made by dissolving 12.590 grammes of pure iodine in a solu- 

 tion of 18 grammes of potassium iodide in about 300 c.c of water, 

 and diluting the solution to 1000 c.c. 



To the article to be estimated by this solution is added a little starch 

 solution, and then the iodine solution until, on stirring, the blue color 

 ceases to be discharged. 



Iodine of sufficient purity to permit of weighing an exact amount for a stand- 

 ard solution does not occur in the market. It can be purified, but as this is 

 somewhat tedious, a simpler plan of making a solution, which is given in the 

 U. S. P., is generally followed. It consists in making a liter of solution, some- 

 what stronger than decinormal, by dissolving about 14 grammes of iodine 



