Mechanism and Reaction Rate of the Karl-fischer Titration Reaction Part I. Potentiometric Measurements

The react ion rate of the cou lomet r ic variant of the Karl-Fischer t i t ra t ion reac t ion (in which electrolyt ical ly generated t r i iodide is used as ox idan t instead of iodine) has been measured in methanol . The react ion is first order in water , sulfur d ioxide and t r i iodide, respectively. For pH < 5 the react ion rate cons tan t decreases logari thmical ly wi th decreasing pH. Addi t ion of pyr id ine solely influences the pH (by fixing it to a value o f about 6) and has no direct inf luence on the react ion rate. A linear relat ion exists be tween the react ion rate cons tant and the reciprocal value of the iodide concent ra t ion , f rom which we can calculate the individual react ion rates for the ox ida t ion by iodine and tr i iodide, respectively. While the react ion rate cons tant for t r i iodide is relatively small (k 3 ~ 350 12 tool 2 s--l) , the react ion rate cons tant for iodine is much larger (k 3 ~ 1.5 x 107 12 tool 2 s-~). I N T R O D U C T I O N Among the many methods for the determination of water that are available [1] the titrimetric method as introduced by Karl Fischer [2] in 1935 has surpassed all others in simplicity and applicability. Although the t i tration according to Karl Fischer is used nowadays as a routine determination, one still has not succeeded to unravel completely the reaction mechanism [3--7]. The mechanism that Fischer originally proposed stems from the benzene experiments, for in his publication he assumes that one mole of iodine is equivalent to two moles of water: I2 + 2 H 2 0 + PY2 " SO2 + 2 Py -~ (PYH)2SO4 + 2 PyHI (1)