264 ANALYTICAL CHEMISTRY. 



Potassium dichromate, K 2 Cr 2 O 7 = 293.78. Whenever this salt 

 acts in the presence of free acid, as an oxidizing agent, it transfers 3 

 atoms of oxygen upon the deoxidizing agent, thus : 



K 2 Cr 2 7 + GFeSO, + 7H 2 SO 4 = K 2 SO 4 + Cr 2 (SO 4 ) 3 + 7H 2 O + 3[Fe 2 (SO 4 ) 3 ]. 



A normal solution should, therefore, contain one-sixth of the molec- 

 ular weight, or 48.963 grammes per liter. For most purposes a 

 deci-normal solution is preferred, and this is made by dissolving 

 4.8963 grammes of pure potassium dichromate in a sufficient quantity 

 of water to make 1000 c.c. 



The disadvantage of this solution is, that the final point of titra- 

 tion cannot be well seen, for which reason, in the determination of 

 iron, for which it is 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 pre- 

 pared 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 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 one another 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, under 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. 



