A Modified Persulfate-Arsenite Method for Manganese With Special Reference to Steel Analysis E. B. SANDELL, I. M. KOLTHOFF, AND J. J. LINGANE School of Chemistry, University of Minnesota, Minneapolis, Minn.
T
HE persulfate-arsenite A method for the determination of manExperimental method, in which the ganese in steel is proposed in which the TITRATION OF POTASBIUM permanganic acid formed permanganic acid formed by the oxidation PERMANGANATE WITH SODIUM by the oxidation of manganous of the sample with ammonium persulfate, ARSENITE-SODIUM N I T R I T E ion in acid solution with ammoKnown amounts of potassium in the presence of phosphoric acid with silnium p e r s u l f a t e i s t i t r a t e d permanganate solution standwith sodium arsenite using silver ver as catalyst, is titrated with a reducing ardized against Bureau of Standsalt as catalyst, has long been solution containing equivalent amounts of a r d s sodium oxalate were a favorite routine method for sodium arsenite and sodium nitrite instead titrated under various conditions the determination of m a n g a of with the customary sodium arsenite w i t h t h e mixed titrating renese in iron and steel. Bright agent sodium arsenite-sodium alone. With the mixed reducing solution, and Larrabee (1) have improved nitrite. In all the experiments the method to the extent that heptavalent manganese is reduced to the except those of (1) below, a the results obtained by it comdivalent condition, and the solution therestandardized reducing solution pare very favorably with those fore becomes colorless at the end point inwas used in which the ratio of obtained by the standard bisstead of yellow or brown as when arsenite arsenite to nitrite was 1 to 1 muthate method. These (based on reducing normality). alone is used for the titration. Small authors oxidize manganese with persulfate in t h e p r e s e n c e of amounts of chromium, vanadium, nickel, Equal volumes ( a c c u r a t e l y phosphoric acid, which permits and molybdenum do not interfere. Silver measured) of sodium arsenite and the complete oxidation of much sodium nitrite solutions, both apmust be precipitated as the chloride before more manganese than was proximately 0.05 N , were mixed the titration can be made. formerly possible, and titrate to give a reducing solution which was'thus 0.05 N. T h e e x a c t t h e D e r m a n g a n i c acid with normality of the mixed reducing solution was calculated from sodium arsenite. Permanganic acid is reduced by arsenite the normalities of the component solutions. The arsenite to a more or less indefinite valence state corresponding apsolution was prepared by dissolving pure arsenious oxide in a proximately to Mn+3.3. Manganese in this state of oxidaslight excess of sodium hydroxide and neutralizing the resulting solution with sulfuric acid; it was standardized by titration with tion has a yellowish or brownish color and it is therefore somestandard permanganate in acid solution, using a trace of potastimes difficult to detect the exact end point when relatively sium iodate as catalyst according to the method of Lange (5). large amounts of manganese are present (the end point may The sodium nitrite solution (prepared by dissolving c. P. sodium be found potentiometrically in such cases) (9, 6 ) , especially nitrite in water) was standardized by a modification of the method of Hoeg and Klemenc (3, 4 ) as follows: A measured volume of if the operator has not had experience with the titration. the nitrite solution was allowed to flow into a partially evacuated It would be desirable to titrate the permanganic acid formed flask containing a measured excess of standard potassium perin the persulfate oxidation with a reagent that would reduce manganate solution acidified with sulfuric acid. The excess of manganese to the manganous condition-i. e., to a colorless permanganate was determined iodometrically after the mixture had stood for 5 minutes at room temperature, followed by warmform. Such a reagent must not react appreciably with the ing t o 40' C. persulfate left after the oxidation, nor with chromate, vanadate, and other substances possibly present in an oxidized The influence of the following factors on the titration of solution of a steel sample. Alkali nitrites when added to an permanganate by the mixed reagent was investigated. acid permanganate solution reduce heptavalent manganese 1. The composition of the standard solution (the ratio of t o the divalent state without giving a visible trace of the sodium arsenite to sodium nitrite). yellow or brown intermediate oxides. However, the reaction Without giving the numerical results of this set of experiments, is slow, and it is necessary t o warm the solution to about it may be stated that the standard reducing solution in which the arsenite-nitrite ratio was 1 to 1, prepared as just described, gave 40' C. to obtain fairly satisfactory results. Without modificathe most satisfactory results. When the ratio of arsenite to tion the reaction is not suitable for the determination of nitrite in the reducing solution was 1 t o 2, the reaction with permanganese in ferrous products. It was found in the present manganate was so slow that the end point could not be satisfacwork that a suitable reagent for the reduction of permanganic torily determined at room temperature, whereas when the ratio was 2 to I the ap earance of a brownish tinge in the solution tiacid to colorless manganous salt could be obtained by mixing trated indicated tRe formation of small amounts of the intermedisodium arsenite and sodium nitrite in equivalent amounts. ate oxides of manganese. The arsenite and nitrite exert a peculiar reciprocal influence 2. The influence.of nitric and phosphoric acids, iron, etc. upon each other: On the one hand, the nitrite prevents the To test the effect of substances that would be present after the oxidation of a steel sample by the method of Bright and Larformation of the intermediate oxides of manganese in the rabee (f), 30 ml. of sulfuric-phosphoric-nitric acid mixture (see reduction of permanganic acid by the arsenite, and the arRecommended Procedure) were boiled for 1 minute to remove any senite, on the other hand, greatly increases the speed of oxides of nitrogen present, cooled to 20" to 25" C.,and diluted interaction of nitrite and permanganic acid, so that the titrawith water plus the indicated volume of standard permanganate together with the indicated added substance to R volume of 100 tion with the mixed reagent can be made a t room temperature.
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256
JULY 15, 1935
ANALYTICAL EDITION
257
ml. Before the titration was made, 10 ml. of 12 N sulfuric acid were added to bring the acidity of the solution well above 3 N. Table I contains the results of this series of experiments.
Silver when present with persulfate must be precipitated as the chloride by the addition of a slight excess of sodium chloride or hydrochloric acid before permanganate can be titrated with the mixed reagent. Otherwise permanganate TABLEI. VALUEOF ARSENITE-NITRITEIPERMANGANATE RATIO is not reduced to manganous salt and a brown solution is IN PEESENCE OF SULFURIC-NITRIC-PHOSPHORIC ACID MIXTURE obtained. AND ADDEDSUBSTANCES When small amounts of permanganate are titrated, slightly (Theoretical value of arsenite-nitrite/permanganate ratio = 1.164) more arsenite-nitrite is required than is demanded by the Volume preceding equations, especially a t low acidities, whereas the of ca. Ratio 0.05 N Arsenitetheoretical value is rather closely approached when the perKMnO4 Nitrite/ No. Titrated Addition Permanganate manganate concentration is larger than about 0.005 N . AlM1. though it has been found that the ratio of arsenite-nitrite to 1.164 1 5 1.156 permanganate depends upon the amount of permanganate 1,164 titrated, the ratio is sufficiently constant to permit the use of 1.160 2 10 ~. 1.153 the mixed reducing solution in the determination of manga3 10 1 g. (NH4)zSiOs 1.163 nese in steel (Table 11). 4 10 1 g. (NHdzSzOs + 1 ml. 0.5 M AgNOa No end point; brown color I n determining manganese in a ferrous material by titrating 5 10 1 ml. 0 5 M AgNOa + 2 ml. 0.5 M NaCl 1.160 1 g. (NHa)zSzOs, 1 ml. 0.5 M AgNOa + 2ml. 6 10 the permanganic acid formed in the oxidation of the solution 0.5 M NaCl 1.163 1.154 with ammonium persulfate, the arsenite-nitrite solution 7 10 As in (5) + 1 g. Fe as ferric alum 8 10 ABin (5) + 5 ml. 0.1 N KxCrzO + 5 ml. should be standardized under the same conditions that ob0.05 N NHiVOs 1.160 1 .155 _ .-9 20 tain in the titration of the oxidized steel sample-i. e., a 1.151 25 10 measured amount of standard potassium permanganate solution should be reduced and then reoxidized with ammonium persulfate in the presence of iron. A slight difference is TITRATION OF REDUCED POTASSIUM PERMANGANATE SOLUfound in the titer of the arsenite-nitrite solution if iron is TIONB OXIDIZEDWITH AMMONIUM PERSULFATE. Measured omitted in the standardization (columns 1 and 2, Table 11). amounts of standard potassium permanganate solution were Iron should be added as the pure metal containing minimal added to 30 ml. of the sulfuric-phosphoric-nitric acid mixture, amounts of manganese (electrolytic iron), and not as ferrous sometimes together with 1 gram of iron as electrolytic iron ammonium sulfate or ferric alum, because the ammonium or as ferrous ammonium sulfate. When no iron was added, salt introduced in the latter case affects the results slightly the permanganate was reduced by adding a slight excess of sodium bisulfite. The reduced manganese solution was treated with silver nitrate and ammonium persulfate, and oxidixed exactly as described below; the titration was also TABLE11. EFFECTOF IRON IN STANDARDIZATION OF SODIUM ARSENITE~ODIUM NITRITE AGAINST STANDARD POTASSIUM made as there described. These data are of importance in PERMANGANATE connection with the standardization of sodium arsenite(Corrections have been applied for manganese in the added iron.) sodium nitrite solutions, especially as regards the dependManganese Value of 1 M1. of Sodium Arsenite-Soence of the titer upon the concentration of permanganate dium Nitrite Solution 1 g. Fe as electro- 1 g. Fe as ("4)Zpresent. The results are given in Table 11. Manganese lytic iron SOcFeSOa,6HzO DETERMINATION OF MANGANESE IN STEELS BY PROPOSED Taken No Fe added added added PROCEDURE. The sodium arsenite-sodium nitrite solution was MU. Mg . Mg. M Q. prepared and standardized as described in the Recommended 0.51 1.20 0.49 Procedure. Bureau of Standards steels were used to test 0.514 0 : 507 0.510 3.01 0.518 0.503 the plsrformance of the method, and the results are reported 0.517 6.02 0.511 0:5i5 0.516 0.512 0.514 in Table 111. 0.516 9.03 0.513 0.513 ~
Discussion of Experimental Results
12.04
The following important conclusions may be drawn from the experimental work: The ratio of sodium arsenite to sodium nitrite in the standard reducing solution should be approximately 1 to 1 (normality ratio); the ratio may be varied between 0.8 to 1 and 1.2 to 1 without ill effect. The titration of permanganic acid with the mixed reducing solution must not be made too rapidly. The reaction between nitrite and permanganate in the presence of arsenite is not instantaneous. If the standard reducing solution is added too rapidly, free nitrous acid is formed and may be partially lost by volatilization, and in any event irregular results are then likely to be obtained, owing to the fact that the true end point may be overstepped. It is especially important to add the standard solution slowly near the end point. With a sufficiently high acidity (greater than 3 N ) and proper speed of titration, it is found that the reaction between the mixed reagent and permanganate is nearly stoichiometric according to the equations:
15.05
+ 5-4s++++ 16H++ 2Mn++ + 5As++++f+ 8Hz0 2MnOla- + 5N02- + 6H+ + 2Mn++ + 5N03- + 3Hz0 2MnOir-
0.513 0.516 0.517 0.519 0.518
0.514 0.519 0.517 0.519 0.519
0.517 0.518 0.517 0.523 0.520
TABLE111. DETERMINATION OF MANGANESE IN BUREAU STANDARDS STEELS BY PROPOSED METHOD" Standard CertifiSteel cate No. Value Mn Found Difference
%
%
l5a
0.372
14b
0,493 0.668 1.23b 1.26b 1.50b 0.244
0.374 0.373 0.498 0.665 1.24 1.27 1.49 0,242
0.662
0.659
90
90
32a 111
0,665 30a
0.805
0.82 0.81
OF
Type of Steel
% +0.002
B. 0. H., 1 per cent carbon
f0.01
B. 0. H., 0.8 per cent carbon Bessemer, 0.2 per cent carbon Bessemer, 0.2 per cent carbon with additional manganese
t0.001 f0.005 -0.003 +0.01 -0.01 -0.002
-0.003 t O ,003 +0.015 +0.005
a Except for No. 90 with added amounts of manganese, the titer of the arsenite-nitrite solution was obtained by titrating 10 ml. of approximately 0.05 N permanganate. The titer used in calculating the results of the titrations of 90 with 1.23, 1.26,and 1.50 per cent of manganese was obtained by standardization against 25 ml. of 0.05 N permanganate. All standardizations were made in the presence of 1 gram of eleotfolytlc iron. b Calculated from the known amount of potasslum permanganate solution added.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(columns 2 and 3, Table 111). When the most accurate results are not required, the standardization may be made in the absence of iron. Since the titer of the reducing solution depends upon the amount of permanganate titrated, the amount of the latter taken in the standardization should correspond approximately to the quantity of manganese present in the sample when the best results are desired, especially when the percentage of manganese in the sample is larger than 0.8 or 0.9 per cent. In most cases it will be satisfactory to take 10 ml. of 0.05 N potassium permanganate solution (corresponding to 0.55 per cent manganese in a 1-gram iron or steel sample) for the standardization. The titer of the reducing solution thus obtained can be applied in the calculations when 1-gram samples containing from 0.3 to 0.8 per cent of manganese are taken, without introducing an error exceeding =t0.005 per cent of manganese from the lack of proportionality (column 2, Table 11). The maximum amount of manganese that may be present in the solution titrated is 15 mg. per 100 ml. when iron is present. Above this concentration the reduction of permanganic acid to manganous salt no longer proceeds smoothly, but becomes very slow. In the absence of iron more manganese may be present. Alloying elements such as chromium, vanadium, nickel, and molybdenum do not interfere when present in the usual amounts, except that the end point is less sharp when the solution is colored. The reproducibility of the method is moderately good, as Tables I1 and I11 indicate, and the accuracy is satisfactory for most purposes. The arsenite-nitrite solution is fairly stable. A 0.05 N solution having a p H of 6.8 decreased in strength by 1.5 per cent after standing in the dark for one year. The method of Bright and Larrabee for the oxidation of manganese with ammonium persulfate has been found to be highly satisfactory and has been incorporated in the procedure given below.
Recommended Procedure for Manganese in Steels REAGENTS.(1) Acid mixture (recommended by Bright and Larrabee): Water, 525 ml.; sulfuric acid, concentrated (sp. gr. = 1.84),100 ml.; phosphoric acid, 85 per cent, 125 ml.; and nitric acid, concentration (sp. gr. = 1.42), 250 ml. First add the sulfuric acid to the water, cool, and then add the nitric and phosphoric acids. (2) Silver nitrate solution, 0.1 M . (3) Ammonium ersulfate solution, 25 grams (of 95 per cent salt) in 80 ml. o? water. Ammonium persulfate slowly decomposes in solution and therefore this solution should not be kept for more than 2 or 3 days. (4) Sodium chloride solution, 0.2 M . (5) Sulfuric acid, 12 N . STANDARD SOLUTIONS.(1) Sodium arsenite-sodium nitrite, 0.05 N . Dissolve 2.5 grams of pure arsenic trioxide in 25 ml. of 4 N sodium hydroxide solution, dilute to 200 ml., add slightly more than enough dilute sulfuric acid to neutralize the hydroxide, and then a slight excess of sodium bicarbonate to neutralize the acid (the solution should finally be neutrpl to litmus paper). Dissolve 0.85 gram of sodium nitrite in the solution thus obtained and dilute to 1 liter. (2) 'Potassium permanganate, 0.05 N . Prepare and standardize against pure dry sodium oxalate in the usual manner. STANDARDIZATION OF SODIUMARSENITE-SODIUM NITRITE SOLUTIOX.Pipet 10 ml. of standardized 0.05 N potassium permanganate solution (for the best results the amount of permanganate taken should correspond approximntely to the amount of manganese in the material under test) into a 250-ml. Erlenmeyer flask, and add 1.0 gram of electrolytic iron of known man-
VOL. 7, NO. 4
ganese content and 30 ml. of the sulfuric-phosphoric-nitric acid mixture. Warm to hasten solution, and finally boil for 2 to 3 minutes to expel oxides of nitrogen. Then add 50 ml. of cold water, 5 ml. of silver nitrate solution, and 10 ml. of ammonium persulfate solution. Heat to boiling, and boil for 30 to 45 seconds. Immediately cool the flask and its contents to 20" to 25" C. by placing in water. Add 5 ml. of sodium chloride solution and 10 m]. of 12 N sulfuric acid, Then titrate with arsenitenitrite solution. Run in the reducing solution at a uniform rate not exceeding 5 or 6 ml. per minute, swirling the liquid in the flask continuously until the solution has become pale pink, and then continue the addition more slowly, allowing about 5 seconds between drops. When very near the end point as indicated by the very pale pink color, allow ap roximately 10 seconds between drops or fractions of a drop. Tiis slow addition is required only for the last few drops. The end point is reached when the color changes abruptly to white (suspended silver chloride). If the titration has been correctly performed, a drop of 0.05 N permanganate solution added after the end point has been reached will give a coloration persisting for at least 3 minutes. DETERMINATION. Dissolve a 0.9- to 1.1-gram sample of steel in 30 ml. of the acid mixture and then proceed as directed in the standardization (beginning with the second sentence).
Notes If an accuracy greater than *0.01 per cent of manganese is not required, the addition of iron in the standardization may be omitted. Then standardize as follows: Transfer 30 ml. of acid mixture to a 250-ml. Erlenmeyer flask and add 50 ml. of water, 5 ml. of silver nitrate solution, and 10 ml. of ammonium persulfate solution. Roil for one minute, cool to room temperature, add 10 ml. of 0.05 N standard potassium permanganate, precipitate the silver, and titrate with the arsenitenitrite solution a9 described above. An equivalent amount of pure ferrous or ferric sulfate (but not ferrous ammonium sulfate or ferric alum) as free as possible from manganese, may be used instead of electrolytic iron in the standardization. I n any case the manganese content of the iron or iron salt added must be determined colorimetrically, preferably with periodate as reagent (7, 8), and a correction applied if necessary. When the manganese content of the sample falls outside the range 0.3 to 0.8 per cent (using a 1-gram sample) the arsenite-nitrite solution should be standardized against a volume of standard potassium permanganate that contains roughly the same quantity of manganese as the sample. Not more than 15 mg. of manganese may be present in the solution when diluted to 100 ml. previous to titration. I n other words, using a 1-gram sample, not more than 1.5 per cent of manganese may be present in the material analyzed if the above directions are to be followed. Cast irons may be analyzed by the above procedure if the graphite, left after dissolving the sample, is filtered off before the oxidation.
Literature Cited (3) (4)
(5) (6)
Bright and Larrabee, Bur. Standards J. Research, 3, 573 (1929). Hall and Carlson, J . Am. Chem. SOC.,45, 1615 (1923). Hoeg, 2. anal. Chem., 71, 102 (1927). Klemenc, Zbid., 61, 448 (1922). Lange, 2. anorg. allgem. Chem., 152, 203 (1926). Lundell, Hoffman, and Bright, "Chemical Analysis of Iron and Steel," p. 197, New York, John Wiley & Sons, 1931.
(7) Zbid., p. 199. (8) Willard and Greathouse, J . Am. Chem. SOC.,39, 2366 (1917). RECFOIVFOD April 1, 1935.
