DIAPHRAGM-FREE CELL FOR TRACE DETERMINATION OF WATER-BASED ON THE KARL-FISCHER-REACTION USING CONTINUOUS COULOMETRIC TITRATION

A new type of diaphragm-free coulometric cell for continuous coulometr ic Karl Fischer titrations of water in the range 0.1-1000 mu g is desc ribed, The relative standard deviation obtained for titrations of 1 mu g amounts of water was typically 1%. The background due to diffusion of water from the air was normally in the range 0.3-0.9 mu g of water min(-1) depending on environmental humidity, The variation in the back ground was normally +/-0.01 mu g min(-1). The construction makes it po ssible, at any time in a sequence of titrations, to renew the catholyt e by means of a Tenon plunger inside the cathode compartment, in this way, the interference effects caused by oxidizable reduction products of methyl sulfite which are formed at the cathode can be controlled in a very simple way, These products are rapidly eliminated by means of a normal titration before a new titration starts, The need for this dr aining step differs depending on the type of reagent used, The coulome tric titration system makes use of true potentiometric end-point detec tion, and this principle makes it possible to control the iodine level at the end-point at much lower levels as compared with commercial ins trumentation. The analytical advantages gained by this option are demo nstrated for the determination of water in ethylenediamine, a task whi ch was found to be impossible when using end-point concentrations in t he range (3-7) x 10(-5) M, which is typical for the bipotentiometric i ndicating system used in commercial instruments, Recovery rates in the range 100-102% were obtained and are shown to be dependent on the typ e of reagent used, The most accurate results were obtained for an imid azole-buffered methanolic reagent in which the concentration of sulfur dioxide was kept relatively low (0.10 M), The diaphragm-free cell des cribed was shown to be compatible with all of the commercial reagents (designed for coulometry) investigated, including the well-known Hydra nal products Coulomat A, AK, AG, AG-H, and AD.