Oxidation of Methanol by Nitric Acid Solutions of Quadrivalent Cerium D. A. SKOOG
AND SISTER MONICA (3IARIE BUDDE)' Stanford University, Stanford, Calif.
an investigation of the reactions of quadrivalent D cerium with aliphatic alcohols in nitric acid solutions it x a s noted that oxidation of the alcohols occurred particularly a t
concentration was increased. Thus, in 4 S acid solutions, 15 t o 30 minutes were required t o oxidize methanol t o formic acid; in 2 K acid the oxidation was complete in 15 minutes; in 1 t o 1.5 Y acid the oxidation was finished in 7 t o 8 minutes. Consumption of Oxidizing Agent by Blank. A solution of a ceric salt in nitric acid is a sufficiently strong oxidizing agent t o cause a certain amount of reduction of the ceric ion by water, particularly when elevated temperatures are used. If any appreciable amount of the oxidizing agent is consumed under the conditions of analysis, a blank must be run, and it is essential that the magnitude of the blank be independent of variables that are not readily controlled during an analysis.
URISG
elevated temperatures. Further investigation revealed that methanol could he readily and quantitatively oxidized t o formic acid by this reagent and a method !vas developed for the determination of small quantities of methanol in dilute aqueous solutions. REAGENTS
Standard Ceric Nitrate in 1.4 NNitric Acid. An approximately 0.1 N solution m-as prepared by dissolving 180 grams of (NH,),C,e(N03)~in 3 liters of water containing 270 ml. of concentrated nitric acid which had been boiled until colorless. This solution was standardized regularly against a standard arsenite solution as follows: About 30 ml. of the ceric nitrate was measured into a flask, 10 ml. of 6 N sulfuric acid and 3 drops of 0.01 osmium tetroxide solution were added, and the solution was titrated with the standard arsenite until it changed to a pale yellow. One drop of o-phenanthroline indicator was added and the titration was continued to the first faint pink color. Standard Aisenite Solution, 0.1 N. This and the following solutions were prepared by standard procedures (8). Osmium Tetroxide Solutions, 0.01 M . Kitric Acid, 8 X. Sulfuric Acid, 6 A-. Standard Methanol Solutions. Anhydrous methanol was prepared by distillation of methanol from magnesium metholate (1), and standard solutions were prepared by diluting weighed quantities of this product to s ~ t a b l volumes. e
Table I.
Table 11. Effect of Reagent Excess on Methanol Oxidation (0.552 millimole of alcohol taken for each analysis) Recovery of Excess of 0.1004 N Methanol, % Ceric Iiitrate, M I . 2.24 98.9 99.3 7.91 99.3 99.7 99.8 12.90 99.3 99.7 22.79 100 2 99.8
PROCEDURE
An aliquot of the sample containing between 10 and 40 mg. of methanol is transferred to a 150-mi. flask equipped with a ground-glass neck. One milliliter of 8 N nitric acid is added for each 5 ml. of sample taken, 50 ml. of 0.1 N ceric nitrate is added, and the flask is fitted with a 24-inch air condenser. The solution is heated in a boiling water bath for about 10. minutes and cooled under a water tap, and the condenser is rinsed and removed. The excess ceric ion is determined by titration with the arsenite solution after addition of 10 ml. of 6 N sulfuric acid, 3 drops of the osmium tetroxidesolution, and 1 drop of the o-phenanthroline indicator.
In order t o test the reproducibility of the blank, several runs were made by heating varying quantities of ceric reagent in 1.5 -Ir nitric acid for different periods of time. The results from these experiments, shown in Table I, indicate that if the reagent excess is kept betlveen 0.175 and 1.75 meq. of ceric ion and the heating time is controlled to within 5 minutes, the blank can be expected to be constant within 0.003 meq. of ceric ion, which is sufficient for accurate results. Effect of Reagent Excess. Several evperiments were undertaken to determine hon large an excess of oxidizing agent was
DISCUSSION
At elevated temperatures, methanol was rapidly and quantitatively oxidized by nitric acid solutions of quadrivalent cerium. The oxidation required 4 equivalents of ceric ion per mole of alcohol, which suggested that formic acid was the oxidation product. This was confirmed by separation of the oxidation product by distillation and determination of the equivalent weight of the , sodium salt of the acid. Rate of Reaction. A study of the rate of the oxidation of various methanol solutions showed that in every case oxidation was complete after 8 minutes of heating a t 95" to 100' C. Further heating, however, does not cause appreciable consumption of ceric ion; after 1 hour a t 100" C., 4.04 meq. of ceric ion were consumed per mole of methanol present. Effect of Nitric Acid Concentration. A study of the oxidation of methanol a t various nitric acid concentrations showed that the reaction was sometvhat dependent upon the amount of acid, but close control of this factor was unnecessary for satisfactory results. The solution must be greater than 1 lVin nitric acid or a white precipitate forms as the solution is heated, which causes an increase in the apparent consumption of ceric ion. The precipitate is presumably a hydrolysis product of ceric ion. The rate of oxidation was found to decrease as the nitric acid 1
Consumption of Ceric Ion by Blank
(25 ml. of 1.5 A' HIiOu solution heated with ceric nitrate a t 95' t o 100' C.) 0 0825 7V Ceric Ceric Ion Heating Time, Pr-itrate Ceric Xitrate, Consumed, 111. hlin. Consumed, RI!. Meq. 2.00 10 0.030 0.025 10.00 10 0.030 0.025 20.00 10 0.037 0.031 10.00 5 0.030 0.025 10.00 15 0.039 0.032
Table 111. Analysis of Methanol Solutions Methanol Taken, M g . 7.10 10.63 14.18 17.68 18.23
21.13
Methanol Found, big. 7 . OF 10.63 14.2% 17.58 17.63 17.63 18.11 18.28 18.02 18.17 21.10 21.12
Error, % -0.6 +0.2 +0.3 -0.7 -0.2 -0.3 -0.6 +o. 2 -1.1
Av.
Present address, College of Iiotre Dame High School, Belmont. Calif.
822
-0.3 -0.1 -0.1 0 .4
V O L U M E 25, NO. 5, M A Y 1 9 5 3 needed to complete the oxidation of methanol and whether large excesses of reagent would cause further oxidation of the formic acid produced during the reaction. Data from these experiments are given in Table 11. It is apparent that an appreciable excess of the reagent is desirable for the analysis and that an excess of 20 ml. of 0.1 iY ceric reagent does not adversely affect the results. Interferences. Most easily oxidized organic compounds such as aldehydes and ketones might be expected to interfere with this procedure. The higher aliphatic alcohols are also oxidized under the conditions used and further investigations of these reactions are now in progress. Formic and acetic acids are not oxidized under the conditions used. Results. Table I11 shows typical recoveries of methanol obtained by the proposed procedure. The methanol used for
823 these experiments was dried by distillation from a magnesium metholate solution, and considerable care was taken in making up the solutions for analysis to avoid losses of the alcohol by volatilization. The average per cent error of these results m s 0.47,. LITERATURE CITED
(1) Blott, A. H., and Gilman, Henry, “Organic Syntheses,” Collective Yol. 1, 2nd ed., p. 220, New York, John TViley 8i Sons, Inc., 1941. ( 2 ) Kolthoff, I. M,,and Sandell, E. B., ”Textbook of Quantitative Inorganic Analysis,” 3rd ed., pp. 593, 583, 475, New York,
3Iacmillan Co., 1952. RECEIVED for review April 15, 1952.
Accepted January 13, 1953.
Analytical Chemistry of Niobium and Tantalum Separation of Iron and Manganese f r o m the Earth Acids C . F. HISKEY AND A. L. BATIK1 Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn 2, S. Y. REvIous
reports (1, 4 ) have described a chlorination procedure
Pfor the earth acids and other analytically related oxides using
octachloropropane as the chlorinating reagent and working a t atmospheric pressures. Quantitative distillation of titanium( IV) and tin(IS-) chlorides may be made from their mixtures with niobium(V) and tantalum(T’) chlorides. Before attempting chlorination, hon-ever, it is most important to separate iron. Failure to do so results in a catalytic decomposition of the reagent by the iron( 111) chloride formed before the remaining oxides can be chlorinated. hnother reason for eliminating the iron a t an early stage is its interference with the spectrophotometric determination of tantalum. I n 100% sulfuric acid a t a wave length of 285 mw, iron as the sulfate complex has an absorptivity 22 times greater than the peroxytantalate complex which peaks there (6). BIODIFIED SCHOELLER SCHEME
The Schoeller ( 7 ) scheme of analysis provides for iron and manganese renioval as the divalent sulfides from ammoniacal tartrate solution. This separation occurs, following precipitation of the major fraction of the earth oxides by acid hydrolysis from tartrate solution and prior to the subsequent precipitation with tannin of the minor fraction from a n ammoniacal tartrate medium. ThuP, two separate steps are required to gather the earth oxides for chlorination. I n an effort to eliminate one of these steps it was first thought that it might be possible to remove the iron and manganese as sulfides from ammoniacal tartrate. T o do this the pyrosulfate fusions of niobites or tantalites were leached into acid tartrate media and the iron was reduced by passing hydrogen sulfide into the solutions. .ifter the yellow color disappeared, the solutions were made ammoniacal and either hydrogen sulfide was bubbled in or ammonium sulfide, was added. The recovery of iron was satisfactory, but the amount of manganese that precipitated varied widely. T o illustrate the recovery of combined iron and manganese osides obtained in this way, data have been assembled in Table I for a group of synthetic oxide mixtures having the following approsimate conipoqition: iron plus manganese oxide, 20%; titania, 57,; zirconia, 1%; stannic oxide, 5%. The remaining i O % v a s divided between niobia and tantala, their ratios varying from 10 to 0.1. The results reported are totals for the iron and manganwe osides, which were weighed as Fe,Oa and lInaO,. 1 Present address, Repearch Analytical Laboratories, Nathieson Chemical Co.. Xianara Fall*. S . T.
Recovery of the combined oxides is good. Vnfortunately, however, this is due to the fact that the iron-manganese ratios in these samples were large. I n all cases they were in excess of 15 and sometimes as high as 30. Although no systematic quantitative analyses of these precipitates were made, it was established that when the percentages were low most of the loss was due to incomplete manganese precipitation.
Table I. Recovery of Iron and RIanganese by Sulfide Precipitation from Ammoniacal Tartrate Solution % Taken
% Found
Diff.
%Taken
%Found
20.92 21.57 20.49 20.65 20.50
20.48 21.01 19.70 20.73 20.38
-0.44 -0.56 -0.78 tO.08 -0.12
20.82 25.29 20.61 21.28
21.06 25.23 19.86 21.32
Diff.
+o.
12 -0.06
-0.76
$0.04
It is concluded that moderate amounts of iron may be quantitatively removed in niobite and tantalite mineral analyqis, permitting the subsequent chlorination of the remaining earth oxides. Manganese recovery in such a case is likely to be very faulty and its determination had best be ignored if this separation is used. The remaining manganese will be recovered for the moqt part in the filtrate from the earth oxide precipitation step. A trace ail1 be found with the earth oxides. It represents only a very small fraction of 1% of the earth oxides and so will not interfere either in the chlorination step or in subqequent colorimetric reactions contemplated for the niobium and tantalum determinations. It cannot, of course, follow the titanium because this element is distilled out as a volatile chloride. Procedure. A half gram of the fincly powdered minrral(80- to 100-mesh) is fused with 20 times its weight of potassium pyrosulfate, first a t Ion. heat and then st high heat until a clwr melt results. After cooling, the cakr is tapped out of the crucible into about 200 ml. of a 10% solution of ammoniacal tartrate. The remaining melt is leached out by immersing it in the same solution. The tartrate is then heated to boiling to speed the dissolution of the melt. After the crucible has tieen rinsed and removed, the solution is made ahout 0.1 JT with hydrochloric acid-Le., 10 ml. of 2 -11acid are added. Hydrogen sulfide is now bubbled in to reduce the iron to the ferrous state. If the ferric sulfide or hrdrouide is precipitated in the ammoniacal solution, large amounts of the earth oxides (Loprecipitate. Stannous sulfide precipitating at this point is now filtered out. T o the filtrate remaining, excess of ammonium hydroxide is added along 1% ith ammonium sulfide to precipitnte the iron and the manganese
