METHODS FOR QUANTITATIVE DETERMINATIONS. 265 



Deci-normal iodine solution is the one generally used, and is made 

 by dissolving 12.653 grammes of pure iodine in a solution of 18 

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



Many substances, such as sulphurous acid and its salts, hydrogen sulphide, 

 arsenous oxide, etc., are acted upon by iodine in such a manner that this 

 element enters into combination with constituents of the compounds named. 

 The quantity of iodine thus taken up forms the basis for calculating the quan- 

 tity of the substance acted upon. 



In the case of arsenous oxide the titration is made in alkaline solution. 

 Arsenous oxide and sodium bicarbonate are dissolved in water, and this solu- 

 tion, containing sodium met-arsenite, is titrated with iodine solution, when 

 sodium met-arsenate and sodium iodide are formed : 



NaAsO 2 4 21 4 2NaHCO 3 = NaAsO 3 + 2NaI + H 2 O + 2CO 2 . 



When sulphurous acid, sulphites, or acid sulphites are titrated with iodine the 

 addition of an alkali is unnecessary ; the action is this : 



H 2 S 4- 21 = 2HI + S. 

 H 2 SO 3 4- 21 4- H 2 O = H 2 S0 4 -f 2HI. 

 Na^SOg 4 21 4- H 2 O = Na. 2 SO 4 -f 2HL 



In the titration of antimony and potassium tartrate by iodine an alkaline solu- 

 tion is required, and for this reason sodium bicarbonate is added to the solution. 

 The reaction which takes place is somewhat doubtful, but the following equa- 

 tion, even if not absolutely correct, corresponds to the quantities of the substances 

 acting upon one another : 



2KSbOC 4 H 4 O 6 + H 2 O + 41 + 4NaHCO 3 = 

 2HSbO 3 + 2KHC 4 H 4 6 4 4NaI + 4CO 2 -f H 2 O. 



One c.c. of deci-normal iodine solution, containing of iodine 

 0.012653 gramme, is the equivalent of: 



Gramme. 



Antimony and potassium tartrate, 2KSbOC 4 H 4 O 6 .H 2 O . . 0.016560 



Arsenous oxide, As 2 O 3 0.004942 



Hydrogen sulphide, H 2 8 0.001699 



Potassium sulphite, crystallized, K 2 SO 3 .2H 2 O .... 0.009692 



Sodium bisulphite, NaHSO 3 0.005193 



Sodium hyposulphite (thiosulphate), Na 2 S 2 O 3 .5H 2 O . . . 0.024764 



Sodium sulphite, crystallized, Na 2 SO 3 .7H 2 O . . . . 0.012579 



Sulphur dioxide, SO 2 0.003195 



Sodium thiosulphate (Hyposulphite). The crystallized salt, 

 Na 2 S 2 O 3 .5H 2 O = 247.64, is used for making the deci-normal solution 

 by dissolving 24.764 grammes of the pure crystallized salt in water 

 to make 1000 c.c. If the salt should not be absolutely pure, a some- 



