in Titrations by Sodium Thiosulphate. 



289 



stand 5, 10, and 15 minutes. Sulphur was thrown down in 

 nearly every case. 



Table II. 



Volume of the 

 liquid at 



beginning of 



titration. 



cm 3 . 



200 



a 



a 



X 



HCl 



lNa202W3 



approximately 



Volume of — iodine used 

 10 



sp gr.(112) 

 present. 



N 



To 



taken. 



in titration after standing. 

 5 min. 10 min. 15 min. 



cm 3 . 





cm 3 . 



cm 3 . cm 3 . cm 3 . 



10 





30 



30-80 31*30 32-32 



a 





25 



25-50 26-00 26-30 



u 





20 



20-30 20-70 20-68 



The results of the table emphasize sufficiently the necessity of 

 proceeding as rapidly as possible with the titration of sodium 

 thiosulphate by iodine in presence of hydrochloric acid, when 

 the thiosulphate is present in considerable amount. As would 

 be expected, the effect of temperature upon the reaction is also 

 marked. In the following experiments the sodium thiosul- 

 phate was run into the acidified water, previously heated to 

 the temperature indicated, and then titrated with iodine. 







Table III. 









Volume of 

 liquid at 

 beginning 

 of titration. 



HCl. 



sp. gr. (112) 

 taken. 



Temp. 



Centigrade. 



Na 2 S 2 3 

 approximately. 



— taken. 

 10 



N 

 Volume of — 

 10 

 iodine used in ti- 

 trations at differ- 

 ent temperatures 



cm 3 . 



cm 3 . 



C. 





cm 3 . 



cm 3 . 



400 



10 



6° 





25 



23-52 





a 



22° 





« 



23-73 





a 



34° 





c< 



24-35 





a 



42° 





(< 



24-5 





(i 



54° 





a 



25 





a 



64° 





u 



26-1 



From these results it is plain that the conditions under which 

 considerable amounts of sodium thiosulphate are titrated in 

 presence of hydrochloric acid must be carefully guarded when 

 accuracy is a consideration. It is also apparent that in all 

 cases the temperature should be reduced as nearly to 0° C. as 

 possible and rapidity of titration by the iodine is an essential. 

 So long as the thiosulphate present does not exceed 20 cm3 of 



the — solution, rapid titration in cold solution proceeds with 



fair regularity in presence of hydrochloric acid up to 10 cm3 of 

 the acid of sp.gr. 112. When, however, the amount of thio- 



