V O L U M E 20, NO. 10, O C T O B E R 1 9 4 8
985
Ed.#. = - 1.5 volt us. the saturated calomel electrode was measured, after 15 minutes’ bubbling, with a polarographic assembly in which a Type K potentiometer was used t o measure the potential drop across a 10,000-ohm standard resistance in series with the dropping electrode. As shown by the constancy of the measured currents, this length of time suffibed for the establishment of equilibrium in every case. Residual currents were then secured after addition of 5 ml. of 0.4 M sodium sulfite to remove oxygen completely ( 1 ) .
of the polarographic measurements. When nitrogen intentionally contaminated with air from the laboratory compressor was used, passage through the vanadous sulfate train reduced the oxygen diffusion current a t equilibrium from0.051 to 0.0003 microampere. These data prove the efficiency of the proposed method for oxygen removal. LITERATURE CITED
With unpurified tank nitrogen, the equilibrium oxygen diffusion current was 0.0027 microampere; when the nitrogen was passed through the vanadous sulfate train, this current was only 0.0007 microampere, whichisequal tozero within the probable uncertainty
(1) Kolthoff, I. M., and Laitinen, H. A.,Science, 92, 152 (1940). (2) Lingane, J. J., and Laitinen, H. A,. IND.ENG.CHEM.,ANAL ED.,11, 504 (1939). RECEIVED March 12, 1948.
S CIENTl FIC C 0 MM UNI CAT10 N S. Equilibrium and Kinetic Studies on the Formation and Dissociation of Ferroin and Ferriin H E reaction of o-phenanthroline with ferrous iron to form Tferroin is of great analytical importance in connection with the colorimetric determination of iron. We have found that phenanthroline behaves strictly as a monoacidic base in aqueous solutions and that the acid dissociation constant of the phenanthrolium ion is 1.1 X 1 0 - 6 a t 25.0’ C. The equilibrium between ferrous iron and phenanthroline in acid solution was found to be represented by either of the following expressions, in which P h and PhH denote phenanthroline and phenanthrolium ion, respectively. +
+
Fe++ 3Ph FePh8++ Fe++ 3PhII+ eFePha++ 3H+
+
+
Consequently, the half-life period of ferroin in acid solutions is about 2.5 hours a t 25 ’C. The decomposition of ferriin is a firstorder reaction with a rate constant of the same order of magnitude as that of ferroin. The rate of formation of ferroin is given by the expression: v = k form. ferroin (Fe++)(PhI3;k form. ferroin =
1.3 X 1019min.-’ at 25’
(7)
In acid solutions the rate can also be represented by: v = k’form. ferroin
(1)
(Fe++)(PhH+)3 (H+)3 ; k’form. ferroin = 2 X lo4 min.-l a t 25’
(8)
(2)
I. 11.KOLTHOFF T. S. LEE
The equilibrium constants were found to be:
D. L. LEESSISG School of Chemistry University of Minnesota
Minneapolis, Minn.
It is evident from Equation 4 that the quantitative conversion of ferrous iron to ferroin in acid solutions is dependent on the ratio of excess phenanthroline to acid. In order that the reaction be 99% complete, the ratio of excess (with reference to ferrous iron) phenanthrolium to hydrogen ions must be 0.035 or greater in the equilibrium mixture. A consideration of the effect of excess phenanthroline is also of importance in the colorimetric determination of ferrous iron in the presence of other metal ions which form complexes with phenanthroline. This is being studied. Assuming that the dissociation of ferriin is represented by an expression similar to Equation 1:
Replica Studies of Dyed Nylon -Correction article on Replica Studies of Dyed Nylon [ ~ A L CHEM., . 20,861 (1948)],the following errors occurred. The next to the
hi THE
I-
last sentence in the abstract (page 861) should read: “The ability to fingerprint a dyestuff on a fiber using only a few milligrams of cloth and a few micrograms of dyestuff makes this type of specimen preparation very interesting.” The fourth sentence in the fourth paragraph of the section entitled “Electron Diffraction” (page 870) should read: “A few milligrams of each of the three nylon cloths dyed mith a few micrograms of dyestuff adequately served as a sample for the identification of the three dyes under = 8 X 10-’6 (in 0.05 ill H2S0,at 25”) (5) discussion.” In the section entitled “Conclusions” the first senK ’ferriin = ‘Fe’+f)(Ph)3 (FePh,+++) tence in the fourth paragraph should read: “These two observaThe rates of formation and of dissociation of ferroin are of tions can be used to account for the three modifications in the importance in connection with the colorimetric determination of qualities of dyeings described above.” The second sentence in the iron (and of phenanthroline), while the rates of dissociation of last paragraph should read: “The ability to identify a dyestuff OD ferroin and ferriin are of practical consequence in connection with a fiber, utilizing a few micrograms of dyestuff on a few milligrams the use of ferroin as an oxidation-reduction indicator. The of cloth, precludes the usual rather tedious procedure of extractdissociation of ferroin is a first-order reaction, the rate of which ing relatively large amounts of the dyestuff which might then be is independent of the acidity of the solution. identified by other chemical or physical analyses.” F. A. HAMM k dis:. ferroin (FePhl++); k diss. ferroin = J. J. COMER 0.0045 min.-1 at 25” (6) 9
