V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6
243
The results obtained for four different compounds are given in Table XIII. The chemical analyses were done by potassium fusion of the organic compound, distillation of fluosilicic acid, and titration with thorium nitrate. Quantities of fluorine bettveen 0.5 and 25.0 mg. can be determined by this procedure. The uncertainty of results ranged between &20% for low amounts of fluorine to &3.570 for higher amounts. When run successively, the method takes 5 to 10 minutes per sample. INTERFERESCES. No attempt was made in this investigation to work out methods of avoiding interferences. Chlorine] bromine, and iodine all interfere in the determination of any one of the three elements. Chlorine could be determined in the presence of the other two by using a ?-ray spectrometer to measure the activity of the 2.1- and 1.6-m.e.v. -prays from chlorine-38. The use of this technique with bromine and iodine however would result in loss in sensitivity since the percentages of disintegrations resulting in -pray production from both bromine-80 and iodine-128 are low. Because of the short half life, fluorine-20 does not interfere in the determination of the other three halogens. Fluorine determinations can be made in the presence of chlorine by applying a baekpound correction for the chlorine-38 activity. This does not work in the cases of bromine-80 and
iodine-128, because the activity levels from these isotopes are too high. The use of suitable absorbers or a scintillation counter with pulse height discrimination offers possibilities for eliminating interference from these elements. Sodium interferes in the determination of fluorine because of the nuclear reaction Na23(n,a)F20. If the sodium content of the sample is known a suitable correction can be applied. Determinations with C.P. sodium benzoate gave a value of 37 counts in 50 seconds per milligram of sodium for the conditions of irradiation and counting used for the fluorine method. Other reactions which might produce a short-lived activity and thus interfere in the fluorine determination were checked. These reactions, all of which produced negligible activity under the irradiation conditions for the fluorine method, were N15(n,y)N15 Ole(n,p)S15,and C137(n,~)P34. LITERATURE CITED
(1) Atchison, G. J., and Beamer, W. H., AXAL.CHEM.24, 812 (1952) (2) Boyd, G. E., Ibid., 21, 335 (1949). (3) Leddicotte, G. W.,and Reynolds, S. A . , Sucleonics 8 (So. 3 ) , I
6 (1951). (4)
Senftle, F. E., and Leavitt. IV. Z . , Ibid., 6 (No. 5 ) 54 (1950).
RECEIVED for review July 21, 19.55, Accepted November 21, 1955.
Amperometric Titrations of Micromolar Solutions JOHN G. NIKELLY and W. DONALD COOKE Baker Laboratory, Cornel1 University, Ithaca,
N. Y.
Amperometric titrations of dilute solutions of metal ions with disodium ethylenediamine tetraacetate have been carried out using a mercury pool indicator electrode. Some of the problems of mercury pool polarography, such as the reproducibility of the electrode area and the constancy of stirring, have been eliminated by this technique. The area of the electrode used was 1 to 2 sq. cm. and stirring was accomplished by bubbling nitrogen through the solution. Cadmium, copper, lead, nickel, and zinc have been titrated at concentrations well below the range of conventional amperometric methods which employ the dropping mercury electrode. In favorable cases, titrations were feasible For example, soluat dilutions approaching l O - 7 M . tions containing 0.027 of cadmium per ml. can be titrated with an average error of 0.0001 y. It was also found possible to titrate solutions of ions which could be polarographically determined with a mercury pool cathode.
T
HE advantages of amperometric titrations over conventional polarographic procedures have been discussed by various authors ( 2 , s ) .Such titrations are more precise, and have a somewhat greater sensitivity than polarographic methods, but these advantages are offset by the necessity of using burets and standard solutions. The choice as to which of the two procedures to apply is determined by the requirements of the particular problem confronting the analyst. The increased precision arises from the fact that the electrode is not used to translate current directly into concentration, as is the case in polarographic procedures. The error in the measurement of the titration volume is much smaller than the error in the translation of current into concentration. Thus, many of the factors that limit the precision in polarography are no longer
critical. The electrode is used only to indicate the appearance or the disappearance of a particular species during the course of a titration. Both dropping mercury and rotating platinum electrodes have been used to indicate the end point of various titrations, but because of the high overvoltage of hydrogen on mercury, the former electrode covers an important range of potentials and has been extensively applied to amperometric methods. The purpose of this research was to evaluate a mercury pool electrode in such titrations. This electrode offers a greater sensitivity than the dropping electrode in some polarographic applications ( 3 ) and it was hoped that the titration of more dilute solutions could be accomplished. I n amperometric titrations of dilute solutions involving precipitation reactions] the limit of sensitivity is often determined by the solubility of the product formed. The titration curves in such cases deviate from straight lines over a large portion of the titration and precision, as well as sensitivity, is decreased. Even when the solubility of the salt formed is sufficiently small, the slow rate of formation of the precipitate becomes a problem with dilute solutions (
