496 
The displacement of iodine even in very dilute Solutions of KI 
by chlorine or by broraine is very rapid and almost instantaneous 
even at 0°. The teraperature coëfficiënt is practically unity. 
The liberation of iodine from KI,KMnO< and H^SO^ is almost 
instantaneous even at 0°. The temperature coëfficiënt is practically unity. 
The liberation of iodine from KI -j- KCIO, and H^SO^ or HCl, 
KI -|- KBrOj H 3 SO 4 or HCl has a measurable velocity and terap- 
erature lias a marked influence on these reactions, but KIO, + KI 
-f- HCl or H 3 SO 4 liberates iodine at once and no etfect of temperat- 
ure could be detected. 
The reactions between iodine and sodiura thiosulphate, and iodine 
and sulphite are extremely rapid even at 0°. There is hardly any 
possibility of measurihg the velocity of the reactions concerned. 
In a foregoing paper (Jour. Chem. Soc. 1917, 111 , 707) I have 
shown . that the oxidation of oxalic acid by chromic acid has the 
temperature coëfficiënt 1.95, but in presence of NajSO^, MgSO^ and 
NaF, the temperature coëfficiënt goes down to 1.63, 1.61 and 1.59 
respectively. The explanation seems to me to be the same as in the 
Skrabal reactions. The reaction between oxalic acid and chromic 
acid is accelerated by H’ ions. Sodium sulphate, sodium fluoride etc. 
are hydrolysed to a much greater extent at a higher temperature 
than at the ordinary temperature and the OH' ions produced at 
the high temperature neutralize the H' ions existing in the solution 
and hence the actual velocity of the reaction does not rise so high 
as it should have been if there were no increased hydrolysis and 
consequent generation of OH' ions at these temperatures. In other 
words, the effect of teraperature is partly neutralized. 
Exactly similar results were obtained by Harcourt and Esson 
(Phil. Trans. 186 A, 1895, 817) in the reaction between Hl and 
HjO,. They obtained the value 1.4 for the temperature coëfficiënt 
of the above reaction in presence of NaHCO,, whilst in presence of 
HCl, HjSO^ etc., the teraperature coëfficiënt is 2.1 ; Harcourt and 
Esson could not account for this discrepancy. The explanation is 
the same as that of the Skrabal reaction. Sodium bicarbonate is 
alkaline in aqueous solution and it produces OH' ions in a greater 
quantity at higher temperatures than at ordinary temperatures and 
these OH' ions react on the iodine which is produced, forming 
iodide and iodate. Hence at the higher temperatures less iodine seems 
to be formed. In other words, the effect of temperature is partly 
neutralized and we get the small value (1.4) for the temperature 
coëfficiënt of the reaction between Hl and HjO, in presence of 
NaHCOj. 
