would develop. The six-coordinate tungsten and molybdenum atoms would be distorted toward five-coordination, and silicon toward three-coordination. However, the surface of hydrated silica is apparently screened by hydroxyl rather than oxide groups and it reacts as a polysilicic acid. The smaller size and higher charge of the tungsten and molybdenum cations presumably enhance retention of the more polarizable oxide surface. iit discontinuities of the crystal lattice-e.g., grain boundaries or dislocations-the surface layer of bly bonded” oxygen atoms would supposedly be interrupted and Ill-0-Ill bonds would be exposed to hydrolysis. Ignition a t high temperatures eliminates crystalline defects by recrystallization,
while grinding can introduce defects by plastic deformation. The layer lattice structure of molybdic oxide would prevent the complete surface shielding which W M postulated for tungstic oxide. Each lattice layer of molybdic oxide might be considered as a single crystallite separated from its neighbors by pseudo grain boundaries. ACKNOWLEDGMENT
The author thanks Sam Leber of the Refractory 1Ietals Laboratory for obtaining the x-ray diffraction patterns.
(2) Andersson, G., Magneli, A., Zbid., 4, (lg5O). (3)793 Freedman, hl. L., J. Am. Chem. SOC. 81, 3834 (1959).
(4) Greenberg, S. .4.,Price, E. Phys. Chem. 61, 1539 (1957). ( 5 ~ ~ ’ $ $ ’ ~ ~ ~ ; 5 ~ ~ 1‘lark, ey~ (6) RIakrides, A, c,, ~ Ibid., 6 3 , 594 (1959).
W.,J .
G.j
~
x.,
( 7 ) Sears, G. W., Jr., ANAL.CHEM.28,
1981 { 1956).
(8) Wells, A. F., “Structural Inorganic Chemistry,” 2nd ed., p. 359, Oxford
University Press, London, 1950. (9) Weyl, W.A., “Structure and Properties of Solid Surfaces,” R. Gomer and C. S. Smith, eds., Chap. IV, p. 155, University of Chicago Press, Chicago, 1953.
LITERATURE CITED
(1) Andersson, G., Acta Chem. Scand. 7, 154 (1953).
RECEIVEDfor review August 24, 1959. Accepted February 4, 1960.
RefIecta nce Spectrophotometric Deter mination of Soluble Iron in DeIustered Acrylic Fibers M. E. GIBSON, Jr., D. A. HOES,’ J. T. CHESNUTT, and R.
H. HEIDNER
Research Center, The Chemstrand Corp., Decatur, Ala.
b A new application of reflectance spectrophotometry permits colored complexes to be developed and measured in essentially opaque solutions. The principle has been applied to the determination of trace amounts of soluble iron in delustered acrylic fibers. Tedious and time-consumingdecomposition procedures are eliminated by reducing the iron and developing the 1,lO-phenanthroline complex directly in o dimethylformamide sobtion of the fiber. The method will detect about 1 p . p m of iron in fiber samples.
T
spectrophotometric determination of traces of iron by formation and measurement of the iron(I1)-1,lOphenanthroline complex is widely used (10, 14, 16). Conventional methods of applying this technique to organic materials, however, are tedious, timeconsuming, and subject to substantial errors because of the decomposition step required. Numerous investigators have studied various phases of this determination, including ashing techniques, reducing and buffering agents, and the order of reagent additions (1, 5, 5 ) . Dry-ashing procedures are not only lengthy but pose problems in the formation of insoluble residues or in fusion of the ash into the crucible. As HE
1
Present address, United States Patent
Office, Washington 25, D. C.
a consequence, most published procedures recommend low, closely controlled ashing temperatures in the general range of 450’ to 600’ C. (7, IS). Insoluble ash residues, if present, are removed by filtration and treated separately. Despite precautions, Hoffman, Schweitzer, and Dalby (6) and Jackson (8) report that dry-ashing techniques resulted in low and variable results, Pringle (IS), however, found that dry ashing of grain and cereal products gave as satisfactory results as did other techniques. Brown and Hayes (2) were successful in applying dry-ashing methods to delustered rayon fibers, They concluded that hydroquinone was unsuitable as a reducing agent, because it formed a brown complex with titanium(1V) in the presence of sulfuric acid. Hydroxylamine hydrochloride, however, proved a satisfactory reductant. l17et-ashing techniques minimize certain of these problems but create others. Reagents must be carefully purified to remove traces of iron ; otherwise blanks may be so high and variable as t o obscure significant differences between sample and blank. Other investigators have sought methods which eliminate the need for ashing. Petersen (11) treated blood serum and plasma directly with a reagent of trichloroacetic and thioglycolic acids prior t o developing the iron complex with various 1,IOphenanthroline derivatives. Maute, Owens, and Slate (9) successfully
applied 1,lO-phenanthroline to the direct determination of iron in acrylonitrile. EXPERIMENTAL
Investigations carried out in this laboratory have for some time sought ways of eliminating the problems previously enumerated. Research studies concerned with the preparation and spinning of acrylic fibers frequently required analyses for traces of iron in the resulting fibers. Because, in our experience, acrylic polymers and fibers are more difficult to ash than many other types of organic materials, the problem was further accentuated. The low temperature dry-ashing technique was found impractical from the standpoint of time required. Elevated temperatures, on the other hand, produced very poor recoveries of iron. Consequently, attempts were made to develop a compromise technique where samples were charred at comparatively low temperatures and the carbonaceous residues extracted R-ith hot hydrochloric acid. This procedure held some promise, but the acid extracts were frequently colored and the results highly variable. Wet-ashing techniques likewise did not provide the answer to the problem. Because of their resistant chemical properties, acrylic fibers require substantial quantities of sulfuric and oxidizing acids to obtain a clear digest. Consequently, even with analytical VOL. 32, NO. 6, MAY 1960
639
~
grade reagents, it was not unusual for the reagents to contribute more iron than the sample. Studies along the lines of Maute, Owens, and Slate (9), however, demonstrated that with bright fibers ashing was unnecessary. Solutions of the fibers could be prepared in an appropriate solvent such as N,N-dimethylformamide, the iron reduced, and the 1,lOphenanthroline complex developed in situ. As long as the fiber was completely soluble, such solutions were as satisfactory as aqueous solutions for the spectrophotometric absorption measurement of the iron complex. The solutions show a maximum absorbance a t about 510 mp, the same as aqueous solutions. The presence of insoluble titanium dioxide in the delustered fibers, however, rendered these solutions opaque or highly translucent, Because the solutions would not transmit light, the above absorption methods were valueless. Preliminary experiments with spectral-reflectance measurements, on the other hand, indicated that these techniques might be suitable.
-1solution of the fiber in dimethylformamide was treated with hydroxylamine hydrochloride and the complex formed by addition of 1,lO-phenanthroline. The solution was then scanned over the range 400 to 700 mp on a General Electric recording Spectrophotometer, using a magnesium oxide standard as reference. As a blank, a second portion of the same solution to which no phenanthroline had been added was similarly scanned. The area of maximum difference between the two curves occurred between 510 and 520 mp, indicating that in all probability the iron(1I)-1,lO-phenanthroline complex was being measured. Reagents. N,N-Dimethylformamide ( D M F ) . D u Pont, Technical Grade, was used as received. This solvent is low in iron and water, having typical analyses of 0.02 p.p.m. iron and 0.06% water. I t s basicity is also low with a tentative specification of 0.010% (max.) total alkalinity (calculated as dimethylamine). Hydroxylamine Hydrochloride Solution. Dissolve 8-00 grams of reagent grade hydroxylamine hydrochloride in 100 ml. of dimethylformamide. Slight warming may be necessary to dissolve the crystals. 1,lO-Phenanthroline Solution. Dissolve 0.40 gram of o-phenanthroline monohydrate (1,lO-phenanthroline) in 100 ml. of dimethylformamide. Iron Solution. Dissolve 1.000 gram of pure iron wire in 50 ml. of 10% sulfuric acid. Use gentle $eat to effect solution. When the iron is completely dissolved, allow the solution to cool to room temperature; transfer to a 1000ml. volumetric flask and dilute to volume with dimethylformamide. Carefully dilute an aliquot of this solution 100-fold with dimethylformamide for a working standard containing 10 -y of iron per ml. 640
ANALYTICAL CHEMISTRY
01
I
400
Figure 1.
,
''
I I I ' I 1 5OO2O 4 0 sa 6002' O C WAVE LENGTH IN MILLIMICRONS
1
'O
I
1
I
I
1
I
I
700
Typical reflectance spectrophotometric curves
of the iron(l1)-1,1 0-phenanthroline complex A.
E. C.
D.
Apparatus.
Delurtered acrylic fiber blank Same fiber with complex developed (tlber contains 2.5 p.p.m. of soluble iron) Same as B with 1.3 pap.m. of a d d e d iron Same as B with 4.1 p.p.m. of a d d e d iron
Spectrophotometers.
A General Electric recording spectro-
photometer was used for the reflectance measurements. Transmittance measurements were made with the Cary Model 14 and Beckman Model B spectrophotometers. Absorption Cells. Rectangular cells (40 x 45 mm.) having a 10-mm. light path were used for the reflectance procedure. These were obtained from the American Instrument Co. (Catalog No. 11-508). Shaker. A Burrell wrist-action shaker was used for preparing the fiber solutions. Procedure. For each determination, weigh out duplicate (sample and blank) 5-gram samples of the dry fiber and place in separate 8-ounce, narrowmouthed bottles. (Bottle caps should be lined with polyethylene or Teflon to prevent attack by the solvent.) Treat both bottles in a n identical manner throughout the procedure, b u t add no phenanthroline reagent t o the blank. Add 60 ml. of dimethylformamide and allow the samples to dissolve by shaking for about 1hour on a wrist-action shaker. Heating is generally unnecessary and may result in an undesirable increase in the solution color. Add 5 ml. of hydroxylamine hydrochloride solution and shake for 15 minutes. At this point, add 5 ml. of 1,lO-phenenthroline solution to the sample for development of the colored complex. Add 5 ml. of dimethylform-
amide to the blank. Return the solutions to the shaker and agitate for 30 minutes. Pour the solutions into 10mm. absorption cells and place on the sample port of the spectrophotometer. Measure the absorbance by reflectance a t 510 and 610 mp, using a magnesium oxide reference. Make a correction for the iron content of the reagents. This can be done by conventional transmittance-type analysis. The ferrous-1,lO-phenanthrolinecomplex in D M F is stable for several weeks, Because the fiber solutions may develop a yellowish color on prolonged standing, however, it is desirable to measure the absorbance of the samples within 24 to 48 hours after development of the complex. Calibration Curve. T o a series of bottles, each of which contains 5 grams of the same, essentially ironfree, fiber, add 0.0, 0.50, 1.0, 2.0, 3.0, 4.0, and 5.0 ml. of the standard dimethylformamide solution containing 10 y of iron per ml. Then add, respectively, 60.0, 59.5, 59.0, 58.0, 57.0, 56.0, and 55.0 ml. of dimethylformamide. Follow the analytical procedure above for the preparation and measurement of these solutions. This calibration curve will cover the range from 1 t o 10 y of iron per gram of fiber, Calculations. Determine the absorbance due to the iron-1,lO-phenanthroline complex in a sample by subtracting the absorbance a t 610 mp from that a t 510 mu. Follow this same
procedure for determining the blank correction. Obtain the net absorbance due to the colored complex in the sample by subtracting the blank value from the sample value. DISCUSSION
Typical spectrophotometric reflectance curves of a sample and blank are shown in Figure 1. The portion of the sample in which the iron(I1)-1,lO-phenanthroline complex has been developed (curve B ) can be seen to reflect less light than the undeveloped portion of the sample (curve A ) over the region 400 to 600 mp. The maximum difference between the two curves occurs a t about 510 mp. This is the same as the wave length of maximum absorption as established from transmittance measurements on transparent solutions. Above 600 mp the reflectance curves for sample and blank essentially coincide. The base point a t 610 mp is used to correct for any instrument instability or minor differences in background characteristics between sample and blank. Such a correction is obviously superfluous if the curves in fact coincide. Occasionally, however, some departure from coincidence a t 610 mM is encountered. Furthermore, the preferred procedure has been to take readings in “per cent reflectance” directly from the instrument reflectance counter. This eliminates the necessity for estimating values from the spectrophotometric curves. Because, in these instances, the curves were not utilized, the reading at 610 mp provided the necessary base point. The method of setting up a calibration curve is illustrated in curves C and D.Figure 1. As additional amounts of iron are added, the per cent reflectance at 510 mp decreases proportionately. The calibration curve follows an essentially linear relationship for concentrations up to about 10 p.p.m. of iron. For higher concentrations, the curve deviates from linear behavior. Consequently, to analyze the higher concentrations, the necessary dilutions were generally made so that only the linear portion of the curve was employed. Use of the upper portion of the calibration curve without dilution of the samples may, however, be satisfactory in some instances. Because experimentally produced fibers may vary substantially in both initial color and the absolute level of titanium dioxide present, the effects of hoth factors on the reflectance method were studied. Samples of bright fiber were dissolved in dimethylforniamide and varying amounts of titanium dioxide added covering the range 0.5 to 1.0% on a fiber basis. Upon development of the complex in the usual manner, it was found that although the absolute level of the reflectance curves was affected by variations in delusterant content,
the blanks changed comparably, SO that the corrected absorbance remained constant for a given iron content. Theoretical considerations indicate that the absorptivity should probably remain constant, within limits, once a threshold concentration of delusterant has been passed. In order to analyze samples of high iron contents where dilutions are required to come within the scope of the above procedure, it has been found desirable to add additional delusterant to bring the titanium dioxide concentration in the final solution back into the range previously investigated. The effect of original fiber color was investigated by adding a constant amount of iron to a series of fiber samples which varied widely in initial color characteristics. Again it was found that the principal effect of this variation ww to change the background or absolute level of the reflectance curves. The corrected absorbance values remained substantially constant for a given iron level. Consequently, it appears that the blank serves &s a correction for changes in initial fiber color and differences in delusterant content. The blank, however, will not correct for iron in the reagents. Any soluble iron in the solvent or reagent solutions will be determined along with the iron in the sample and must be taken into consideration. The res,gent correction is determined by conventional transmittance procedures. A study of the absolute level of this correction showed that the reagents normally used with a 5-gram sample contribute about 0.0008 to 0.0010 mg. of iron. The correction therefore amounted to about 0.2 p.p.m, The order of reagent additions and agitation times specified are necessary for effecting homogeneous solutions and optimum, reproducible color development. Studies by Pflaum and Popov (la) using 2,2’-bipyridine as the complexing agent have shown that, in DMF solution, no hydroxylamine hydrochloride or other added reducing agent is necessary for formation of the ferrous complex. Their report also claims similar results with 1,IO-phenanthroline aa the complexing agent. I n the systems studied, however, using both transmittance and reflectance techniques, it has been found that with soluble ferric salts in a DMF solution, essentially none of the ferrous -1,lO-phenanthroline complex is formed in the absence of the hydroxylamine hydrochloride reducing agent. Reproducible results are dependent upon carrying out the reduction step ahead of the complexing reaction. Precision and accuracy, on the average, are both about =t5 to 10% of the amount present for iron concentrations of 1 to 5 p.p,m. These limits were
considered satisfactory for the objectives a t hand when the method waa developed. Accuracy was determined by adding known amounts of iron to a series of fiber solutions. The data are
Table 1. Accuracy of Reflectance Method for Iron in Delustered Acrylic Fibers Iron __ __
Sample
Added, P.P.M.
A
1.oo
Iron Found, P.P.M. 1.16 0.90 0.96 1.08 Av. 1.03
B
2.24 2.30 Av. 2.22
C
4.00
D
6.00
Av
3.90 4.54 4.16 3.96 3.90 3.78 3.90 3.72 3.98
5.88 5.65 6.07 5.56 6.20 5.65 5.56 6.14 Av. 5.84
Table II. Comparison of Transmittance and Reflectance Methods for Iron in Acrylic Fibers
Sample
Iron, P.P.M. Transmittance4 Reflectance* method method
E F G H
2,45 2.45 1.65 0.60
2.40 2.45 1.80 0.7’5
Bri ht fiber samples used. TiOp, baeed on fiber wei ht added to solutions for measurement eby reflectance method. a
* 0.7b
Table
111.
Detn. No.
Precision of Reflectance Method for Iron
Iron, P.P.M. 1.90 2.30 2.50 2.15 2.15
6
7 8
2.00 - .~
2.05 2.35 2.18
Av. Std. dev. k0.20
VOL. 32, NO, 6, MAY 1960
641
presented in Table I. Accuracy was also checked indirectly by a comparison with a second procedure. Four samples of bright fiber were analyzed for soluble iron by the previously devcloped transmittance procedure. The same solutions were then delustered by addition of titanium dioxide and measured by the reflectance procedure. The results were in substantial agreement, as shown in Table 11. Sample H,which shows the largest percentage error, is actually outside the scope of the reflectance method, because of its low iron content. Precision was determined by making eight replicate determinations on a particular sample of fiber. The values given in Table 111 show that a t an iron concentration of 2 p.p.m., the standard deviation found was 1 0 . 2 p.p.m. Interference in the 1,lO-phenanthroline method for fcrrous iron has been extensively studied by previous investigators (2, 6, 9). Bccause interfering ions were essentially absent in the samples used for the present investigation, no additional work along these lines was undertaken. It is to be expected that those ions which are themselves colored or which form colored complexes with 1,lO-phenanthroline will
interfere. Some interferences can undoubtedly be corrected by use of properly compensated blanks, as described above for fiber samples of differing initial color. Certain interferences will manifest themselves by a change in shape of the spectrophotometric curves. I n such cases it may be possible to employ a technique similar to that described by Diehl and Smith (4) for the simultaneous determination of iron and copper, utilizing measurements a t two wave lengths. The technique described can probably be improved by various modifications and refinements. However, the basic concept of reflectance spectrophotometry, as presented, is believed to offer a new approach to the analysis of opaque or translucent solutions which may have numerous applications beyond the scope of the one discussed. LITERATURE CITED
(1) Bandemer, S. L., Pchaible, P. J., IND.ESG. CHEX, A s . 4 ~ .ED. 16, 317 11944’1.
(2j Brokn, E. G., Hayes, T. J., Anal. Chim. Acta 7,324 (1952). (3) Davis, N. F., Osborne, C. E., Jr., Nash, H. A,, ASAL. CHEX. 30, 2035 (1958).
(4) Diehl, Harvey, Smith, G. F., “The
Copper Reagents: Cuproine, Neocuproine, Bathocuproine,” pp. 45-8, G. Frederick Smith Chemical Co., Columbus, Ohio, 1958. (5) Fortune, W. B., RIellon, M. G., IND. ESG. CHEM.,ASAL. ED. 10, 60 (1938). ( 6 ) Hoffman, C., Schweitzer, T. R., Dalby, G., Ibzd., 12, 454 (1940). (7) Hummel, F. C., Willard, R. H., Ibid., 10, 13 (1938). (8) Jackson, S.H., Ibid., 10,302 (1938). (9) Maute, R. L., Owens, 11. L., Jr., Slate, J. L., ASAL. CHEX 27, 1614-16 (1955). (10) Sloss, 11,L., llellon, 11,G., Smith, G. F., ISD. ESG.CHEX, ASAL. ED. 14,931 (1942). (11) Petersen, R. E., ASAL. CHEW 25, 1337 (1953). (12) Pflaum, R. I., Popov, A. I., Anal. Chitit. Acta 13, 165 (1955). (13) Pringle, n’. J. S , .4nalyst 71, 491 (1946). ( 4) Sandell, E. G., “Colorimetric Determination of Traces of lletals,” 2nd ed., Interecience, Kevv York, 1950. ( 5) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” Vol. IIA, Van Sostrand, Princeton, X, J., 1959. RECEIVED for review September 11, 1959. Accepted December 31, 1959. Seventh Annual Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., Februar 1956. Contribution 60, Research Zenter, Chemstrand Corp.
Color Complexes of Catechol with Molybdate G. P. HAIGHT, Jr., and VASKEN PARAGAMIAN’ Chemistry Deparfmenf, Swarfhmore College, Swarfhmore, Pa.
A procedure for determining molybdate developed by Seifter and Novic, involving formation of a complex with catechol, did not work when sodium sulfite was substituted for sodium bisulfite. Conditions for formation of the complex have been restudied and the need for careful control of p H has been revealed. The dependence on p H suggests a reaction mechanism and structure for molybdate ion in the region of pH 7. A new complex containing equimolar molybdate and cotechol forms in acid solution. The spectra and formation constants for the two complexes have been studied a t acidities in which only one or the other complex is formed.
C
complex compounds of molybdate and catechol (0-dihydroxybenzene) were prepared by Weinland and coworkers from 1919 to 1926 OLORED
1 Present address, Massachusetts Institute of Technology, Cambridge, Mass.
642
ANALYTICAL CHEMISTRY
(4-6). Later workers have applied the color interaction in solution t o the detection and determination of molybdate. This paper presents a study of the equilibria involved in aqueous systems involving catechol and molybdate. McGowan and Brian (2) indicate that the color results from a complex containing two catechol molecules per molybdate ion. Seifter and Kovic (3) found conditions for quantitative color development but made no attempt t o elucidate the formula of the complex. They reported working in basic solutions stabilized by addition of sodium pyrosulfite to prevent air Oxidation of catechol. Studies in this laboratory shorn that sodium sulfite yields entirely different results, virtually no comples being formed under conditions which were otherwise the same as those of Seifter and Novic (3). It is now apparent that the role of the pyrosulfite is not only t o prevent oxidation of catechol (3) but t o neutralize the base, forming a
sulfite-bisulfite buffer which stabilizes the p H near the neutral point. Complex formation has been found t o be most complete in neutral solutions. It drops off very rapidly in both acid and base. X o color is observed in 0.1M sodium hydroxide. The nature of the complex formed changes from 2: 1 t o 1:1 catechol-molybdate when the pH is changed from 6 to 2 or less. Equilibrium constants have been determined for the neutral and acid complexes. The equilibrium between the two compleses was not studied. EXPERIMENTAL
Reagent grade chemicals were used without further purification. Sodium pyrosulfite (Nai3iOs) is also called sodium metabisulfite. Baker and Adamson sodium metabisulfite and Llallinckrodt sodium bisulfite (NaHSOa) were used. Measurements were made with a Beckman D U spectrophotometer with a thermostated cell compartment maintained a t 26’ =k 0.5” C.
