A Rapid Potentiometric Method for Determination of Sulfate B. E. CHRISTENSEN, HERBERT WYMORE, AND VERYON H. CHELDElLIN Oregon State College, Corvallis, Ore.
ITH the recent applications of vacuum-tube technic to potentiometric work, a number of new devices have been developed for analytical work. These setups are particularly suited for routine control, in that they are rapid, compact, and easy to operate. Such instruments as the electron beam spectrometer can be operated under conditions in which the change in potential may be as small as 50 millivolts. The use of this technic, being limited to those reactions which can be determined electrometrically, has stimulated considerable research for new indicator electrodes and potentiometric methods. It is apparent that any means by m-hich such common ions as sulfate, nitrate, sulfite, etc., could be estimated electrometrically would greatly enhance the usefulness of these instruments. Because of the great need (especially in control work) for a rapid method of determining sulfate ion, the electrometric possibilities of solving this problem were studied. As far as the authors n-ere able to ascertain from the literature, there were no satisfactory methods for such a determination ( 1 ) . hluller and TVertheim (3) describe an indirect method using the ferri-ferrocyanide system, but this, according to Kolthoff, is not convenient for practical use ( 2 ) . Preliminary experiments on the precipitation of the sulfate ion in the presence of such ions as persulfate had indicated that there was a slight change in the electrode potential a t the equivalence point. This striking behavior seemed worthy of further investigation. Since barium persulfate is fairly soluble, attempts were made to adapt the persulfate ion to an electrometric determination of the sulfate ion.
Experimental
employed when using a ferri-ferrocyanide electrode. These preliminary experiments indicated that the freak a t the end point of the reaction, though definite, was neither large nor very sharp. I n Figure 1 is shown a typical titration curve. One of the factors which appeared to affect the magnitude of the break a t the end point was the quantity of persulfate present. The best results were obtained when the amount of persulfate used was small (1 mg. or less). When larger charges of persulfate were employed, the break did not appear to be qvite 10
