Cerimetric Determination of Molybdenum in High Chloride Media Using the Molybdenum Blue Reaction CLARENCE M. CALLAHAN, STEPHEN C. FOTI, and JOHN R. LA1 U. S. Naval Radiological Defense Iaborafory, Sun Francisco 24, Calif.
b A new volumetric method for determining molybdenum in a high chloride medium is described. After passage through a Jones reductor the reduced molybdenum is caught under a receiver solution of sodium molybdate whose p H is so adjusted that an equivalent amount of molybdenum blue is formed. The molybdenum blue is then titrated to a colorless equivalence point with standard ceric sulfate solution. The formation of molybdenum blue serves two purposes: The oxidation-reduction reaction between molybdenum(lll) and molybdenum(V1) forms an intermediate oxidation state that is stable against air oxidation and the intermediate oxidation state, molybdenum blue, serves as its own indicator in the titration. The results are comparable to those obtained by the use of the permanganate method in sulfuric acid.
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titration of a molybdenum-sulfuric acid solution, reduced to molybdenum(II1) by passage through a Jones reductor and caught under excess ferric sulfate, is accurate (5) except in the presence of an appreciable amount of chloride ion despite the addition of Zimmermann-Reinhardt solution. I n this case a ceric titration of a cooled solution of molybdenum(V) in 2N hydrochloric acid that has been reduced by passage through a silver reductor maintained a t 60" to 80" C. is employed. Ferrous o-phenanthroline complex is used as indicator (2). Hiskey, Springer, and Meloche (6) have shown that the normality of the hydrochloric acid must be closely controlled around 2N to prevent the reduction from falling short or passing beyond the pentapositive state. During recent investigations of the oxidation states of molybdenum in sea water media it was necessary to produce the several reduced states of the element and to measure their stability against air oxidation. Molybdenum(V) was prepared by using the silver reductor, and the subsequent slow air oxidation was followed by titrating aliquots of the reduced solution with standard ceric sulfate solution a t suitable time intervals. Molybdenum(1V) is unstable in aqueous solution, disproportionating rapidly to molybdeHE PERNAPI'GANATE
num(V) and (111) (4). Molybdenum (111) is readily obtained by use of the Jones reductor, but the air oxidation could not be followed because the permanganate titration is not applicable in sea water. Attempts to adapt the ceric titration using ferrous o-phenanthroline indicator, after catching the reduced molybdenum(II1) under excess ferric sulfate (1) gave a n indefinite end point. Excess sodium molybdate was then substituted for ferric sulfate as the receiver solution according to the procedure used by Hiskey, Springer, and Meloche (6). This also mas unsatisfactory, but it was observed that the intensely colored molybdenum blue was formed under certain conditions when reduced molybdenum and molybdate solutions were mixed. The reasonable stability of molybdenum blue against air oxidation ( 7 ) coupled with the possibility of using its intense color as a n indicator suggested that this system could be used for determining molybdenum in a chloride medium. The optimum conditions for forming molybdenum blue and the results that may be obtained in determining molybdenum in a high chloride medium by titrating molybdenum blue to a colorless end point with ceric sulfate were investigated. APPARATUS AND REAGENTS
The amalgamated zinc for the Jones reductor was prepared from analytical reagent grade 20-mesh zinc according to Kolthoff and Sandell (8). The reductor column was made from a 250ml. French-type separatory funnel by fusing an extension into the stem so that the over-all dimensions of the stem were 27 cm. long by 2 cm. in diameter. A plug of glass wool was placed a t the bottom and the entire length of the stem was then packed with amalgamated zinc. Tygon tubing that had been drawn out to a tip by heating over a n open flame was slipped over the exit end of the column so that the reduced molybdenum could be introduced directly under the surface of the receiver solution, thus dispensing with the use of an inert atmosphere. Three molybdenum stock solutions were prepared: stock solution A by dissolving 12.100 grams of Baker & Adamson Na2MoO~.2Hz0in 1 liter of distilled water; stock solution B by dissolving 10.000 grams of hferck
reagent RIoOa (assay 99.5%) in ammonium hydroxide followed by dilution to 1 liter with distilled water; and stock solution C from 10.000 grams of J. T. Baker A1003 (assay 99.9%) in the same way as solution B. The permanganate and ceric sulfate solutions were prepared and standardized against sodium oxalate (Baker's analyzed reagent, primary standard) according to the directions given by Willard and Furman (10). The receiver solution was prepared by weighing out sufficient sodium molybdate so that the ratio of molybdenum in the molybdate receiver solution to that in the aliquot was a t least 16 to 1. This was dissolved in 100 ml. of 1N hydrochloric acid just prior to the commencement of the reduction. For determinations containing up to 4.5 meq. of molybdenum 5.5 to 6 grams of sodium molybdate are sufficient. All other chemicals used were of reagent grade. A Beckman p H meter, Model G, was used for all pH measurements, Calibrated glassware was employed in all operations involving measurements or volume transfers. RECOMMENDED PROCEDURE
To a neutralized molybdenum aliquot add an equal volume of 2 5 hydrochloric acid and pass through a Jones reductor that has just been washed with 1N hydrochloric acid. Adjust the flow rate of the reduced molybdenum solution to 20 to 30 drops per minute and catch in a 500-ml. Erlenmeyer flask under a receiver solution of sodium molybdate by means of the Tygon tip attached to the reductor column or, alternatively, drop through an atmosphere of nitrogen. A dense opaque solution of molybdenum blue is immediately formed when the two molybdenum solutions come into contact, provided the pH is correct. Swirl the contents of the flask occasionally as the reduction proceeds, to ensure If particles of thorough mixing. molybdenum blue begin to coagulate, add about 5 ml. of concentrated sulfuric acid. When the level of the aliquot approaches the top of the zinc column, begin washing the last traces of molybdenum through the column by adding small portions of 1N hydrochloric acid. Continue the washing until about 150 ml. of the acid has passed through the column. When the washing is completed, remove the Tygon tip and wash adhering droplets of solution into the flask. Titrate the VOL. 32, NO. 6, M A Y 1960
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reduced molybdenum slowly with standard ceric solution until about 1 ml. from the equivalence point. At this point the solution is just beginning t o change from a deep opaque blue to a clear blue or blue-green. Add 5 ml. more of concentrated sulfuric acid (add 10 ml. if none has been added during the reduction). The solution should turn sky blue. If there are particles present or a green color persists, warm slightly until the particles disappear and the solution turns blue. Carefully continue the titration to a colorless equivalence point. One or two drops in excess turn the solution yellow. There is no indicator correction. RESULTS A N D DISCUSSION
Reliability of Method. T h e results obtained by the permanganate and the ceric-molybdenum blue methods in determining the purity of three molybdenum compounds are given in Table I. When available, the manufacturer’s assay is included. I n each determination 20-ml. aliquots of
Table 1.
the stock solutions were employed. Using the standard deviation of a single determination as a measure of precision, a comparison of the results shows t h a t the permanganate method is slightly more precise. On the other hand, in the two cases where the manufacturer’s assays were availa.ble (stock solutions B and C) the molybdenum blue method showed the better agreement. T h e differences in the averages obtained by the t x o independent methods for solutions A, B, and C are: 0.04, 0.11, and 0.15’jZo. This spread of values is about the same as reported in t h e assays of Bureau of Standards molybdenum samples. Molybdenum Blue. The composition of molybdenum blue has engendered considerable controversy since the substance was first described by Berzelius in 1826. T h e oxidation state of this material is between molybdenum(V) and (VI), the average being about 5.67 (a). Schirmer
Comparison of Molybdenum Blue
VJ.
Permanganate Methods for
Determining Molybdenum
Stock Solution A
Xone’
Permanganate % Found Dev.0 99.62 0.02 99.62 0.02 99.69 0.05 99.64 0 041C 99.57 0.06 99.69 0.06 99.64 0.01 I
B
99,5%b C
99.63 0.060C 99.49 99.42 99.45
0.04
0.03 0.00
99.45 0. 035c Individual deviations from average. Manufacturer’s assay. c Single measurement. n-1 QQ.O%’gh
Molybdenum Blue Dw.4 0.01 99.59 0.08 99.52 0.09 99.69 99.60 0 08S8 0.02 99 64 0.02 99.54 0.10 99.42 99.57 0.05 99.52 0. 067c 99.52 0.08 99 57 0.03 99.64 0.04 0.09 99.69 99.59 0.01 99.54 0.06 99.64 0.04 99.60 0 . 061e
% Found
I
I
I
Table II. Advantages and Disadvantages of Permanganate and Molybdenum Blue Methods for Determining Molybdenum
Disadvantages Permanganate Serves as own indicator Permanganate solution less stable than ceric solution Higher precision than ceric method Indicator blank required Shorter titration time Cannot be used in high chloride media Difficult to read meniscus Advantages
Serves as own indicator Ceric solution very stable No indicator blank necessary Used in high chloride media Meniscus easily read
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ANALYTICAL CHEMISTRY
Molybdenum Blue Lower precision than permanganate method Longer titration time
arid Loworkers (9) proved by x-ray and electron microscopy studies t h a t rnolyhdcnuni blue is of a colloidal nature. They determined the approximate empirical formula from samples prepared by four different , where z methods t o be i l l o ~ O ~zHPO varied from 6 t o 14, depending upon how long the samples had dried. Although molybdenum blue is relatively stable against air oxidation, the titration of the reduced solution should not be unduly delayed, as experiments on reduced solutions gave low results when allowed t o stand overnight. Occasionally i t was necessary t o heat the solutions near t h e end point of the titration t o expedite the dissolution of particles of molybdenum blue. No deleterious effect was observed if the temperature did not go much higher than 50’ C. Prolonged heating a t this or higher temperatures should be avoided, however. Molybdenuni blue is readily formed when a reduced molybdenum solution of molvbdenum(V) -or (111) is mixed with ah excess of molybdate solution, solution is if the pH of the between 0 and 1. This condition is usually obtained if the receiver and wash solutions are mepared from 1N hydrochloric acid a n d the acid concentration of the aliquot is between 0.5N and 2 N . When the acidity of the receiver solution is too high, a dark brown solution is formed instead of molybdenum blue. On the other hand, molybdenum(II1) forms a green precipitate with the receiver solution if the acidity is too low. I n the former case molybdenum blue may be obtained b y the dropwise addition of a sodium hydroxide solution, but conversely, the green precipitate is not easily dissolved t o form molybdenum blue by adding acid. An aliquot with a n acidity of 0.2.1’ or less will precipitate molybdenum (111) on the zinc column if introduced into a Jones reductor. The re-solution of this precipitate with strong acid washes is difficult. I n such cases repacking the column with fresh amalgamated zinc is advised. Sulfuric acid may be substituted throughout the procedure for hydrochloric acid. Receiver Solution. T h e receiver solution should contain at least a sixteenfold excess of sodium molybdate. Experiments with solutions containing only a n eightfold excess showed t h a t the reduced solutions were more sensitive t o air oxidation. Receiver solutions should be freshly prepared just prior t o use, as results are not reliable with bulk receiver solutions t h a t have been stored for several days. Experience showed, that on standing for several days, acidified
receiver solutions became cloudy and complex molybdenum compounds prccipitated while neutral or basic solutions turned yellow and pave low results in the procedure. This latter effect was surprising and was presumed to be due t o a slight air oyidation of the molybdate to the pero.;yniolybdate. Occasionally the colloidal niolybdenum blue coagulates into lumps t h a t persist despite the addition of concentrated sulfuric acid. I n such cases the volume of 1.V hydrochloric ncid used in preparing the receiver solution should be increased in subsequent determinations. The p H of n receiTer solution consisting of 5.5 grams of sodium niolybdate in 100 ml. of 1.V hydrochloric acid is about 9.5 and an additional 50 or IO0 mi. of 1 S acid will not materially change this value. as this amount of molybdate everts a considerable buffcring capacity. Cnder the conditions specified in the procedure the receiver solution, by forming molybdenum blue with the reduced molybdenum, performs the dual function of protecting the reduced molybdenum from air ovidation and serving as the
indicator for the equivalence point in t h e titration. When Tygon tubing is used t o introduce the reduced molybdenum solutJion under t h e surface of the receiver solution, care should be exercised t h a t hydrogen gas generated in the reductor column does not bubble through the receiver solution. This usually occurs when the drop rate is too fast. T h e level of the solution in the reductor should never fall below the top of the zinc during the reduction. Evaluation of Method. Under ordinary circumstances the permanganate method is probably t o be preferred to the molybdenum blue method. However, in cases where the use of permanganate is inadmissible, the molybdenum blue method offers definite advantages over other ceric methods for determining molybdenum. Table I1 compares the more obvious advantages and disadvantages of the permanganate and the molybdenum blue methods for estimating molybdenum. The procedure may also be adapted to the silver reductor, b u t without advantage. The titration of
molybdenum blue x i t h potassium dichromate does not proceed to a stoichiometric equivalence point, LITERATURE CITED
(1) Banks, C. V., Diehl, H., ANAL. CHEM.19,222 (1947). (2) Birnbaum, Tu’., Walden, G. H., J . Am. Chem. SOC.60,64 (1938). (3) Clausen, D. F., Shroyer, J. H., ANAL.
CHEM.20.925 (1948). (4) Crouthamel, ‘C. E., Johnson, C. E., Ibid., 26,1284 (1954). (5) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A. Hoffman, J. I., “Applied Inorganic Analysis,” pp. 307-10, Wiley, Xew York. 1953. (6) Hiskey, ’ C. F., Springer, V. F., Sleloche, V. W., J . Am. Chem. SOC. 61,3125 (1939). (7) Killeffer, D. H., Linz, A,, *ling, L., “Molybdenum Compounds, p. 44, Interscience, New York, 1952. (8) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inor anic Analysis,” p. 598, Macmillan, %ew York, 1949. (9) Schirmer, F. B., Audrieth, L. F., Gross, S. T., McClellan, D. S., Seppi, L. J., J . Am. Chem. SOC. 64, 2543 (1942). (10) U’illard, H. H., Furman, N. H., “Elementary Quantitative Analysis,” , 223-4, 256-7, Van Nostrand, New ork, 1940. RECEIVEDfor review Sovember 13, 1959. Accepted February 17, 1960.
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The Surface Acidity of Tungsten(V1) and Molybdenum(V1) Oxides MEYER L. FREEDMAN Refractory Metals laboratory, General Electric Co., Cleveland 17, Ohio
b The surface acidity of a series of tungstic and molybdic oxides was investigated by potentiometric titration with sodium hydroxide a t high ionic strength. The surface acidity and dissolution of tungstic oxide were found to depend on crystalline imperfections. Molybdic oxide did not exhibit surface acidity, and the limiting pH for its continuous dissolution depended only on particle size.
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HE OBJECT of this study was the determination of the surface acidity of tungstic and molybdic oxides. The surface of silicon dioxide reacts with hydroxyl ions a t pH values below those required for dissolution (7). The extent of this reaction, defined as surface acidity, varies somewhat with the degree of crystallinity and hydration of the silica (6). I n contrast to the 4 to 2 crystal coordination of silica, both tungstic and molybdic oxides have 6 to 2 coordinate structwe8 (q). While tungstic
oxide, like silica, forms an infinite three-dimensional complex (I), molybdic oxide forms a layer lattice (8). It is thus of interest to compare the surface properties of these oxides. Although a number of quantitative methods have been developed for measuring surface acidity, these apply to the strong acid areas of catalytic solids (6). Potentiometric titration a t high ionic strength is the most general qualitative method and can be made quantitative if the p H range of surface neutralization is sufficiently lower than the p H range of dissolution. EXPERIMENTAL
Materials. Analytical reagent grade chemicals were used throughout. Tungstic oxide was prepared by adding sodium tungstate solution t o a large excess of boiling 6N hydrochloric acid and washing the precipitated WOS H20 with 0.1N hydrochloric acid until the sodium chloride content was less than O.Ol%, as shown
by analysis. Portions were then ignited for 1 hour a t 500°, 700°, and 900’ C., respectively. Residual water, determined by ignition a t 700’ C., amounted to 0.57% a t 300’ C. and 0.23% a t 500’ C. Particle size of the 300’ C. ‘oxide, as estimated by optical microscopy, was submicron, A small increase in particle size was observed for the sample ignited a t 700’ C.,, while the bulk of the tun stic oxide ignited a t 9000 C. was in tBe 2- to 3-micron range. A portion of the ignited oxide was reduced to 1- to 2-micron size by ball milling. Hydrous MOO* was precipitated by adding 50 ml. of saturated sodium molybdate solution to 1 liter of boiling 1N nitric acid. The precipitate was washed free of sodium nitrate and dried a t room temperature to a water content of 2.90%, as determined by ignition a t 500’ C. One portion was ignited for 1 hour a t 500’ C. and a second portion was sublimed a t 850’ C. Particle size of the precipitated oxide was less than 0.5 micron, while that of the ignited material was in the 5- to 10-micron range. The sublimed oxide was macroVOL. 32, NO. 6, M A Y 1960
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