Elimination of Nitrite Interference in lodometric Sugar Determinations JULIAN CORMAN Northern Regional Research Laboratory, Peoria, 111.
IS frequently desired to determine reducing sugars by ITiodometric methods (ferricyanide or copper reduction followed
by iodometry) in bacteriological culture media containing nitrites. However, erroneous sugar values are obtained when such a medium is assayed by any of these methods, as for instance, the Somogyi procedure ( 2 ) . The cause of the error lies in the fact that potassium iodide, in an acidified sodium nitrite solution, produces free iodine. It is obvious, therefore, that nitrites must be eliminated before one proceeds with the sugar determination. In the Groak ( 1 ) iodine microdetermination excess sodium nitrite is eliminated by addition of urea to the hot acid solution. H2KCONH2
+ 2HOSO +COz + 2Nz + 3HzO
Adaptation of this procedure for the elimination of nitrite interference prior to iodometric sugar determinations was therefore investigated. Ten milliliters of 1 . O q sodium nitrite solution and 10 ml. of 10.Oa% urea were mixed in a 100-ml. volumetric flask. Five milliliters of 1N sulfuric acid were added. The flask was shaken occasionallv and after 30 minutes 2 drops of phenolphathalein were added. The solution was neutralized with sodium hydroxide and made up to volume with distilled water. Assay of a 5-ml. aliquot resulted in a titration of 24.50 ml. of thiosulfate, equal t o the blank titration. Thus in the absence of sugar, nitrite interference had been eliminated completely.
The efficacy of this procedure was tested with different sugars. Ten milliliters of a glucose solution were pipetted into each of three 100-ml. volumetric flasks. The solution in one flask was diluted to volume and assayed in the regular manner as a control. Ten milliliters of 1.Oy0sodium nitrite solution were added t o each of the other two flasks. One of these was diluted to the mark and assayed to show the extent of nitrite interference. The remaining flask was treated with an excess of urea and sulfuric acid
Table I. Elimination of Nitrite Interference in lodometrir Sugar Determinations 10-JI1. Aliquot in 100-Ml. Volumetric Flask
0.005
.v
Thiojulfate ritration Treatment Titration Difference0 Mi. Jfl. 0 . 3 % glucose None (control) 10.30 14.20 Negative Nitrite 47.10 urea HzSOI 14.20 10.30 Nitrite 0.53% maltose S o n e (control) 13.60 10.90 Segative Sitrite 45.00 urea H~SOI 10.90 Nitrite 13.60 1 .O% sucrose Kone (control) 24.45 0 05 Seaativr 45.50 Nitrite 0.25 24.25 Nitrite f urea f HzSOi 5 Titration resulting from presence of sugar minus reagent blank. Reagent blank 24.50 ml.
+ +
+ +
-
as indicated above, to show removal of nitrite intcrference. Similar experiments were performed with maltose and sucrose solutions. Results given in Table I show that interference by nitrite, io the presence of sugars, was completely eliminated. Sucrose was apparently hydrolyzed to a neglisible extent by the mild treatment with sulfuric acid while maltose vas not affected. Not only are nitrites commonlv added as such t o culture media, but they are sometimes formed bv the action oi microorganisms. as from the oxidation of ammonia or the reduction of nitrates. Regardless of the source of nitrite interference, this interference can be eliminated effectively as described above. LITERATURE CITED
(1) Groak, B., Biochem. Z . . 270, 291-6 (1934). (2) Somogyi, M..J . B i d . Chpm., 160. 6l--F: (19451
Removal of Oxygen from Gas Streams LOUIS MEITES' AND THELT1.i IIEITESl Frick Chemical Laboratory, Princeton University, Princeton, nl. J .
HE most popular agents for removing oxygen from gases Tare hot finely divided copper and a train of chromous sulfate solutions. Although either absorbs oxygen substantially completely, both have disadvantages that render their use inconvenient. Not only is a copper heater somewhat tedious to construct and place in operation, but much time is required to attain operating temperature: these are serious drawbacks when the apparatus is to be used only infrequently. Chromous sulfate solutions, generally prepared by the reaction of metallic zinc and acidified chromic sulfate, must be allowed to stand for many hours before they are ready for use. Further, because the hydrous chromic oxide formed on aging is dissolved and reduced only slowly on addition of acid, the maintenance of these solutions in operating condition is a source of constant annoyance. The authors have found that vanadous sulfate solutions present none of these difficulties. A solution initially 0.1 ;If in vanadyl sulfate and containing some free sulfuric acid is ready for use within a minute or two after amalgamated zinc is added, and 1 Preaent address, Department of Chemietry Haven, Conn.
regeneration by addition of sulfuric acid (conveniently througk a small separatory funnel n-hose stem projects through the stopper of the wash bottle) proceeds so rapidly that the solutions are again ready for use almost instantly. Unlike alkaline suspensions of hydrous chromic oxide, alkaline suspensions of hydrous vanadic oxide are highly effvctive in oxygen removal, being oxidized to vanadite, vThich i t v l f strongly absorbs oxygen and is converted to vanadate. Thtswfore the absorptive capacity of a vanadous sulfate solution is much greater than that of an equiconcentrated chromous sulfate solution, and regeneration is necessary much less frequently. The authors have used a train of three wash bottles, the first two of which were initially charged with 100 g r a m of lightly amalgamated zinc and 100 ml. of 0.1 M vanadyl sulfate, and the third contains 100 nil. of water to ensure absence of vanadium from the emergent gas stream. This assembly has noTv been in operation for over 9 months, and has been regenerated only twice. Its performance in nearly continuous service has been eminently'satisfactory. Tank nitrogen passed through this train was used to remove dissolved air from 50 nil. of air-saturated 0.1 S potassium chloride, 0.005 M sodium hydroxide, and 0.01'-; gelatin contained in a polarographic H-cell ( 2 ) . and the oxygen cliff usion current at
Yale University, New
984
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
