422 



ANALYTICAL CHEMISTRY. 



instead of 12.59 grammes as described above. 10 c.c. of this solution are titrated, 

 while stirring constantly, with deci-normalthiosulphate solution until the yellow 

 color of iodine just vanishes. The iodine solution is then properly diluted so 

 that it is exactly equivalent to the thiosulphate solution. 



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, or 

 iodine acts as an oxidizing agent through the medium of water. The quantity 

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

 stance 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 : 



NaAs0 2 + 21 + 2NaHC0 3 = NaAsO 3 + 2NaI + H 2 O + 2CO 2 . 



The essential change in the above reaction may be shown thus : 

 As 2 3 + 41 + 2H 2 O = As 2 O 5 + 4HL 



That is, the arsenous oxide which may be considered present in the metarsenite 

 is oxidized to arsenic oxide in the metarsenates. Hence it is seen that 1 liter 



As 2 O 3 

 of one-tenth normal solution is equivalent to 4 y in in grammes. 



When hydrogen sulphide, sulphurous acid, sulphites, or acid sulphites are titrated 

 with iodine the addition of an alkali is unnecessary ; but in titrating these sub- 

 stances they must be added to a measured excess of iodine solution, and the 

 excess after the reaction is complete determined by back -titration with thio- 

 sulphate solution ; the action is this : 



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

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

 Na^SOg + 21 + H 2 O = Na. 2 SO 4 + 2HI. 



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

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



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

 0.01259 gramme, is the equivalent of: 



Gramme. 



Antimony and potassium tartrate, 2KSbOC 4 H 4 O 6 .H a O . . 0.016495 



Arsenous oxide, As 2 O 3 . 0.004911 



Hydrogen sulphide, H 2 S . 0.001691 



Potassium sulphite, crystallized, K 2 SO 3 .2H 2 O . . 0.009648 



Sodium bisulphite, NaHS0 3 ... . 0.005168 



Sodium hyposulphite (thiosulphate), Na2S 2 O 3 .5H 2 O . . . 0.024646 



Sodium sulphite, crystallized, Na. 2 SO 3 .7H 2 O 0.012520 



Sulphur dioxide, SO 2 . 0.003180 



