1907-8.] Electromotive Force of Iodine Concentration Cells. 385 
giving us the E.M.F. between *001 and '0001 solution ’0305 volts. Now, this 
value, with so much potassium iodide present, as will be shown later, should 
have approximated very closely to *0291 volts if ’0001 of iodine was present. 
If we calculate the amount of iodine present in the solution, on the 
assumption that the correct reading is ‘0291 volts, from this E.M.F. reading, 
the result is to show that the percentage of iodine present is *00009 instead 
of *0001. 
It is, however, quite possible that at such extreme dilution some of the 
iodine is lost owing to slight traces of ammonia, the alkalinity of the glass, 
and what not, although the usual precautions were taken, both by boiling 
out the glass and preparing with great care the distilled water. Moreover, 
it was found that, although boiled distilled water had been used to make 
the dilution of the hyposulphite used in this titration, yet, after keeping a 
few days, the solution had become completely oxidised at this extreme 
dilution, and therefore might be slightly oxidised in the process of diluting 
and using. 
It is evident, then, that the titration of such a solution is likely to give 
too high a value, owing to slight oxidation of the thiosulphate, while it is 
probable that calculation from the E.M.F. reading is too low, as there is a 
very small correction for the mass equation, and that its real value lies 
between *0001 and *00009. It is, however, evident that even at these 
extreme dilutions the E.M.F. readings and the titration results show a close 
agreement, so that we may safely accept the solutions made up for *001 
normal iodine and upwards as containing that amount of iodine. 
The following readings were taken with two separately prepared sets of 
solutions of iodine and potassium iodide : — 
Volts. 
Difference, 
Volts. 
1st Solution. 
2nd Soluiion. 
•115 'potassium iodide. 
1 
(1) *1 : *01 normal iodine 
E = *0445 
•0442 
•0003 
*01 : -001 „ 
E = *0304 
•0303 
•0001 
*23 potassium iodide. 
(2) -1 : *01 normal iodine 
E = *0350 
•0354 
•0004 
*01 : *001 „ 
E = *0298 
•0296 
•0002 
•344 potassium iodide. 
(3) ’1 : *01 normal iodine 
E = *0327 
•0324 
•0003 
*01 : *001 „ . 
E = -0294 
•0296 
•0002 
VOL. XXVIII. 
25 
