Molybdenum Blue Reaction DONALD F. CL.iUSEN1 AND JOHN H. SHROYER, Bradley University, Peoria, 111.
Areas of decreased density producing dips in the density-concentration curves made with molybdenum blue may be found and are reproducible about 50% of the time. Such dips take place at all points of the molybdenum blue spectra between 400 and 800 mp; they are not associated with color changes. Under the conditions of these experiments the dips appear above 0.14 mg. of phosphorus (in 100.0 ml. of solution), but are not associated with specific changes in redox potential. The dip phenomenon represents a possible source of error when the molybdenum blue reaction is used for the determination of phosphorus with chlorostannous acid as the reducing reagent.
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them exhibited the dip phenomenon; these results were independent of the worker or the laboratory. In every case the dips took place a t the same place on the density-concentration curve. Simultaneously 1% ith the density determinations on some of the curves, the p I l and redox potentials of the solutions were obtained to ascertain whether or not the dips were associated with changes in redox potential. The pH determinations were made in the event pH variations might require that the redox potentials be corrected by the amount of such variation. No such corrections were necessary. Figure 1 presents a set of typical data. The dip in the density-concentration curve over the range 0.14 to 0.18 mg. of phosphorus is clearly seen. The concomitant changes in the pH and redox potential curves are considered meaningless because they varied widely in shape from experiment to experiment. The pH and redox potential determinations n-ere done on a laboratory model Beckman pH meter. The density data were obtained for the most part from a Coleman Universal spectrophotometer and partly from a Leitz photoelectric colorimeter.
HE molybdenum blue reaction has been used for routine
colorimetric determinations of phosphorus, molybdenum, silicon, and other constituents in a ~ i d evariety of materials. However, in spite of its importance, the mechanism of the reaction and the structure of molybdenum blue are not well understood The equilibrium characteristics of the reaction (2) have been studied and the colloidal character of the molybdenum blue (6) has been established. The average valence of molybdenum in molybdenum blue appears to be about 5.67 and if reduction is carried further the density of the color decreases ( 8 ) and may change from blue to green or brown (4). Lesser color variations can be produced by varying other conditions ( 5 ) . Woods and Mellon (9),using sodium sulfite as a reducing reagent, reported that the blue color of molybdenum blue when used for the determination of phosphorus follows Beer's law up to 1.0 p.p.m. of phosphorus, above which a negative deviation takes place. Clausen ( I ) , using chlorostannous acid as a reducing reagent, reported that the density-concentration curve obtained when the molybdenum blue reaction is used for the determination of phosphorus is not a straight line but a curve and that a t a point above 0.14 mg. of phosphorus (in 100.0 nil. of solution) an area of decreased density may take place, producing a dip in the curve. This phenomenon is almost without parallel, although Lundberg (9) has observed a similar behavior when 2,6-dichlorobenxenoneindophenol is reduced with various reducing agents. The present investigation was undertaken to ascertain whether or not the dip phenomenon is reproducible and is associated with a change in color or a decrease in density of a fixed color, and to attempt to correlate the dip phenomenon with changes in redm potential and pH.
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EXPERIMENTAL
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In the first series of experiments a number of density-conceritration curves were made under varying conditions in an attempt to demonstrate that the dip phenomenon actually existed.
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The method used was that of Todd and Sanford (7) with chlorostannous acid as the reducing agent for the determination of phosphorus in blood filtrates. Amounts of from 0.02 to 0.22 mg. of phosphorus, as potassium dihydrogen phosphate, in 6.0 ml. of 97? aqueous trichloroacetic acid solution, were used for the curves. To each of these were added 2.0 ml. of a reagent composed of equal amounts of 7.5% sodium molybdate and 10 4 sulfuric acid and 2.0 mi. of a reagent made by dilution of a 60% solution of stannous chloride in concentrated hydrochloric acid to 400 volumes with water. The blue color produced is stable after about 5 minutes, at which time the solution is diluted to 100.0 ml. with water.
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0
.
0.04 0.08 0.12 0.16 0.20 MILLIGRAMS PHOSPHORUS
0.24
Figure 1. Inflection Point or Dip in Molybdenum Blue Color of Density-Concentration Curve from Phosphorus Determination Changes in pH and redox potential are not significant. All points represent volume of 100 m l .
T o ascertain whether or not the dip phenomenon existed only a t the absorption maximum of molybdenum blue, absorption curves were obtained from a series of solutions of molybdenum blue made from varying amounts of phosphorus (Figure 2). The color was allowed to stand for 5.0 niinutes before the density determinations were niade. The proximity of the curve r e p resenting 0.14 mg. of phosphorus to that representing 0.18 mg. of phosphorus clearly indicates that the dip phenomenon exists over all parts of the absorption curve. The points at 710
A large number of curves were obtained by three workers in two widely separated laboratories, using several batches of fresh reagents. All curves produced 'IT ere nonlinear and about half of Preaent address. Department of Surgery, University of hllnnesota. Mmneapolia, Minn.
925
ANALYTICAL CHEMISTRY
926 mp (the absorption maximum) from each curve were plotted as a density-concentration curve on the same graph, using the concentration abscissas a t the top. The resulting dotted line again shows the dip phenomenon. MILLIGRAMS
PHOSPHORUS
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TIME I N MINUTES I
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.
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Figure 3. Development of 5Iolybdenum Blue Color 550
600
650
700
750
800
Relation between density and redox potential as color develops
WAVELENGTH IN MMU
Figure 2. Absorption Curves of RIolybdenum Blue Color from Phosphorus Determination Showing Dip Phenomenon Dotted line represents points at 710 m+ from each curve plotted against concentration abscissas at top
Because it is necessary to allow solutions of molybdenum blue to stand for a few minutes for color development, it was of interest to ascertain the nature of this development and to trace the path followed by the redox potential during this period. Accordingly, samples containing 0.12 mg. of phosphorus each in 6.0 ml. of trichloroacetic acid were treated with reagents and allowed to stand varying lengths of time from 0 to 190 hours. They were then diluted to 100.0 ml. n-ith distilled water as usual and pH, redox potential, and density were obtained. Typical data show the density dropping from 0.45 (the figure after the first 5 minutes) to 0.36, the redox potential rising from -90 to -67 mv., and the p H remaining approximately constant after 190 hours. S o explanation for the rise in E is apparent. Figure 3 shows curves of this type extended from 0 to 16 minutes. As the redox potential becomes more negative-i.e., the molybdenum becomes more reduced-the density drops until, after 3 minutes, both curves level out. These factors suggest that the molybdenum is reduced by degrees and that in early stages the molybdenum blue molecule contains more vibrational energy than in subsequent stages. This explanation receives additional support
from the fact that the phosphomolybdic complex before reduction to molybdenum blue absorbs strongly in the near ultraviolet region. ACKNOWLEDGMENT
Thanks are due E. J. Meehan, professor of analytical chemistry, University of Minnesota, for valuable suggestions and to Theresa Somers and Minnie Finn for their excellent technical assistance. LITERATURE CITED
(1) Clausen, D. F., paper delivered before Conference of Medical Technologist,s, Tri-Stat,e Hospital Assembly, Chicago, May 6, 1947. (2) Hein, F., Burawoy, I . , and Schwedler, H., Kolloid Z . , 74, 35 (1936). (3) Lundberg, W. O., private correspondence. (4) Munro, L. A, Proc. Trans. .\rova Scotian Inst. Sci., 16, 9 (1928). (5) Rinne, K., Z . anal. Chem., 113, 241 (1938). (6) Shirmer, F. B., Audrieth, L. F., Gross, S. T., McClellan, D. S., and Seppi, L. J., J . Am. Chem. Soc., 64, 2543 (1942). (7) Todd, J. C., and Sanford, A . H., "Clinical Diagnosis by Laboratory Methods," 5'01. 9, Philadelphia, V. B . Saunders Co., 1940. (8) Treadwell, W.D., and Schaeppi, Y., H e h . Chim. Acta., 29, 771 (1946). (9) Woods, J. T., and Mellon, M . G., IND. ENG.CHEM.,ASAL. ED., 13, 760 (1941). RECEIWDMarch 24. 1948.
Chemical Analysis of Refinery C, Hydrocarbon Fractions RICHARD F. ROBEY AND HERBERT K. WIESE Standard Oil Deselopment Company, Elizabeth, N. J .
P
ETROLEUM refinery distillate boiling between 4' and 51 ' C. (39 ' and 12.1" F.) a t normal pressure is generally designated as the Cg fraction. Hydrocarbons of most common occurrence in this range are listed in Table I. T4e number of these which make up a Cs fraction varies widely, from only two, isopentane and n-pentane, in certain natural gasolines, to essentially all in naphtha from cracking operations conducted at 870" C. (1600" F.) Recent developments have made the petroleum industry increasingly aware of the value of Cj hydrocarbons as chemical raw materials. 1- and 2-pentenes comprise the starting materials
in the production of sec-amyl alcohol, and thence methyl npropyl ketone and sec-amyl acetate, all important as solvents. Tertiary Cb mono-olefins are employed in the synthesis of tertamylphenol, from which important chemicals and resins are manufactured. Isoprene is consumed in the formation of Butyl rubber and other polymeric substances. Cyclopentadiene and piperylenes also have valuable chemical and plastic derivatives. During the war a welcome supplement of high-grade motor fuel was obtained by feeding isopentane and Cg mono-olefins to alkylation units. All these processes have required adequate hydrocarbon analytical methods in development and control.
