Accuracy of reproduction is limited by the accuracy of the
X-Y recorder slidewire and the accuracy with which the various controls can be set, and is affected by the sweep speed used. A sweep speed of 50 or 20 inches/sec is about right for typical spectral curves, with lower speeds for sharp fine-structure-type peaks and greater speeds for flattish curves. With the Moseley components described, the accuracy is such that if unit scaling factors are used, the reproduced curve will be in register with the curve being reproduced within ca. 0.5 mm at worst. As a further test of accuracy, experimental visible absorption spectral curves of green-blue Cr(en)(OHz)zBrz+ and magenta Cr(en)(OH&Br*f ( 5 ) were converted by the curve expander to plots of molar absorbance index a,,, US. wavelength on a common scale and superimposed to locate the isosbestic points to be expected in the conversion of the dibromo to the monobromo complex. These predicted values, namely, 430 (24.5), 492 (24.5), and 580 mp (32.0 M-' cm-l), agreed well with the experimental values found (6) by scanning the changing spectra of a solution of the dibromo complex during its aquation to the monobromo complex, ( 5 ) R. G . Hughes and C. S. Garner, Inorg. Chem., 6,1519 (1967).
namely, 432 (25.0), 492 (25.0), and 585 mp (31.5 M-l cm-1). The values in parentheses are the molar absorbance indices, and the average estimated probable errors in predicted and found values are & 2 mp and f 0.5 M-1cm-l. Figure 2 presents an example of reproduction of an esr curve (a), with ordinate expansion by a factor of 1.75 and abscissa scale unchanged in (b), reproduced at 20 sec/inch, and with ordinate expansion by a factor of 2.8 and abscissa contraction by a factor of 3.8 (c), reproduced at 50 sec/inch. The original curve has narrow peaks and presents a challenge to the curve expander. The difficulty of obtaining curve ( c ) by ordinary hand-plotting methods is evident; the curve expander reproduces such a curve with ease, with either expansion or contraction. RECEIVED for review October 16, 1967. Accepted November 24, 1967. Work partly supported by the U. S. Atomic Energy Commission under Contract AT(11-1)-34, Project 12; this paper is Report No. UCLA-34P12-68 to the AEC. (6) R. G. Hughes and C. S. Garner, Department of Chemistry, University of California, Los Angeles, Calif., 90024, unpublished research, 1967.
Indirect UItraviolet Spectrophotometric Determination of Vanadium Utilizing Molybdovanadophosphoric Acid Robert Jakubiec and D. F. Boltz Department of Chemistry, Wayne State University, Detroit, Mich.
INA STUDY of the analytical applications of mixed heteropoly acids the composition and extractability of molybdovanadophosphoric acid were investigated. As a result of this work it was found feasible to separate the mixed heteropoly complex from excess molybdate and molybdophosphoric acid. On the basis of this quantitative separation a new sensitive indirect ultraviolet spectrophotometric method for the determination of vanadium was developed. Vanadium(V) is capable of replacing one of the 12 molybdenum(V1) atoms in 12-molybdophosphoric acid to give a mixed molybdovanadophosphoric acid complex in which the phosphorus to vanadium to molybdenum ratio is 1:l:ll. This composition has been verified experimentally. Kitson and Mellon (I) have studied the molybdovanadophosphoric acid method for the determination of phosphorus and measured the absorbance of the yellow complex to minimize the effect of certain interfering ions at 460 mp. They established that a 1:1 ratio for phosphorus to vanadium existed for the complex but could not delineate the ratio of molybdenum to phosphorus because of the excess molybdate required for formation of complex. Gee and Deitz (2) used differential spectrophotometry with absorbance measurements at 390 mp to increase the sensitivity of the method, confirmed the 1:l ratio for phosphorus to vanadium, and suggested a (1) R. E. Kitson and M. G. Mellon, IND. ENG.CHEM., ANAL.ED., 16, 379 (1944). (2) A. Gee and V. R. Dietz, ANAL.CHEM., 25, 1320 (1953).
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molybdenum to phosphorus ratio of about 14:l. Quinlan and DeSesa ( 3 ) determined by factorial experiment the optimum concentrations of vanadate, molybdate, and acid for the formation of molybdovanadophosphoric acid. Michelsen (4) using very dilute solutions recommended measurements at 315 mp for improved sensitivity. In utilizing the formation of molybdovanadophosphoric acid for the determination of vanadium, a mixed reagent containing phosphate and molybdate is used. After formation of the molybdovanadophosphoric acid, the excess molybdophosphoric acid is extracted with diethyl ether. The molybdovanadophosphoric acid is extracted with a 1:4 pentanol-diethyl ether extractant and the excess molybdate is removed by washing the extract with an acidic aqueous solution. The purified molybdovanadophosphoric acid is backextracted with an ammoniacal buffer solution and the absorbance due to the molybdate is measured at 228 mp. EXPERIMENTAL Apparatus. Absorbance measurements were made in 1.OOcm matched cells with a Cary Model 14 spectrophotometer. A reagent blank was used in the reference cell. Reagents. STANDARD VANADIUM SOLUTION(3.01 pg of vanadium per ml). Dissolve 0.3460 gram of reagent grade ammonium metavanadate in 500 ml of distilled water, add (3) K. P. Quinlan and A. M. DeSesa, Ibid.,27, 1626 (1955). (4) 0.B. Michelsen, Ibid.,29,60 (1957).
220 260 300 WAVELENGTH (fry)
Figure 1. Ultraviolet absorption spectra for buffer extract of molybdovanadophosphoric acid 1. O.24ppmV 2. 0 . 4 8 p p m V 3. 0.72 ppm V
thoroughly. Add 20 ml of diethyl ether and 40 ml of the saturated acidic wash solution, and shake for 30 seconds with an automatic shaker. Allow layers to separate for 5 to 10 minutes and drain aqueous layer into a second separatory funnel. Rinse thoroughly the tip of the first separatory funnel with the acidic wash solution. Use wash bottle to add 2 ml of the acidic wash solution to the first funnel. Swirl the funnel to collect aqueous layer and add aqueous solution to second separatory funnel. Discard ether layer in first funnel. Add 10.0 ml of the extractant reagent to the aqueous solution in the separatory funnel and shake 30 seconds with an automatic shaker. Wait 5 to 10 minutes for two layers to separate. The aqueous layer may be cloudy after separation. Insert a cotton pledget into stem of separatory funnel and drain aqueous layer at a steady rate. Wash tip of funnel with water. Add an additional 40 ml of the saturated acidic wash solution to the funnel and shake 30 seconds on an automatic shaker. After allowing 5 to 10 minutes for separation of the two layers, drain aqueous layer slowly and wash tip of funnel thoroughly with water. Add 10.0 ml of the ammoniacal buffer solution to the separatory funnel and shake for 30 seconds with automatic shaker. After the two layers have separated, drain the aqueous extract through original cotton pledget into a 25 ml volumetric flask. Add another 10.0 ml of the ammoniacal buffer solution to separatory funnel and shake for 15 seconds. After separation add aqueous solution to volumetric flask. Wash tip of separatory funnel with small amount of distilled water and collect washings in the volumetric flask. Dilute to the mark with the buffer solution. Measure absorbance at 228 mp against a reference solution prepared in a similar manner except for addition of sample solution. RESULTS AND DISCUSSION
28 ml of concentrated HCl, and dilute to 1 liter with distilled water. Transfer a 20-mI aliquot of this solution to 1-liter volumetric flask and dilute to mark with distilled water. Store in a polyethylene bottle. STOCK PHOSPHATE SOLUTION (0.66 mg of phosphorus per ml). Dissolve 0.2891 gram of reagent grade potassium dihydrogen phosphate in distilled water and dilute to 1 liter. Store in a polyethylene bottle. MOLYBDATE SOLUTION(2.6 Dissolve 2.60 grams of reagent grade ammonium molybdate in 100 ml of distilled water. Store in a polyethylene bottle. STOCK HYDROCHLORIC ACIDSOLUTION (2.5N). Store in a polyethylene bottle. MIXEDREAGENTS.Combine 50 ml of stock phosphate solution with 12.5 ml of stock HC1 solution. Add 20 ml of stock molybdate solution and dilute to 1 liter with distilled water. Store in a polyethylene bottle. (Prepare new solution after about 2 weeks.) SATURATED ACIDICWASHSOLUTION.Add 115 ml of concentrated HCl to 500 ml of distilled water and after cooling, dilute to 1 liter with distilled water. Transfer to glass bottle and add 100 ml of reagent grade ethyl ether; shake until ether is dissolved. EXTRACTANT REAGENT(1:4). Mix 1 volume of l-pentanol with 4 volumes of diethyl ether and store in a glass bottle. AMMONIACAL BUFFERSOLUTION.Dissolve 53.5 grams of reagent grade ammonium chloride in distilled water in 1 liter volumetric flask, add 70 ml of concentrated ammonia and dilute to mark with distilled water. Recommended General Procedure. Weigh or measure by volume an amount of sample containing not more than 50 pg of vanadium(V). The resulting solution should be adjusted to pH 1.5 to 2 and diluted to 50 ml. Transfer a 25-ml aliquot of this sample solution to a 125-ml separatory funnel, add 10 ml of the mixed reagent and mix
z).
Vanadium Concentration. Figure 1 shows the ultraviolet absorption spectrum of the solution containing the molybdate resulting from the decomposition of molybdovanadophosphoric acid. Conformity to Beer’s law was observed for up to 1.5 ppm of vanadium. The optimum concentration range is approximately 0.2 to 1 ppm of vanadium, corresponding to the 0.2-0.9 absorbance range. It is important to prepare a reagent blank solution to be used in the reference cell. Each set of reagents gives a characteristic blank. For a series of different reagents this blank was in the 0.02 to 0.03 absorbance range. However, the absorbance blanks for each specific combination of reagents used on a given day were very reproducible. Acidity Prior to Initial Extraction. A pH of 1.5 to 2 was satisfactory for the formation of molybdovanadophosphoric acid. Preliminary Extraction. The preliminary extraction with diethyl ether extracts the excess molybdophosphoric acid and leaves the molybdovanadophosphoric acid in the aqueous phase. Extraction of Molybdovanadophosphoric Acid. A mixture of 1-pentanol and diethyl ether (1 :4) was superior to l-pentanol. 1-Butanol and diethyl ether (5) and 3-methyl-1-butanol and dietyl ether (6) have also been reported as extractants for molybdovanadophosphoric acid. Traces of excess molybdate are removed by washing organic phase with a hydrochloric acid solution saturated with ether. ( 5 ) J. Kinnunen and B. Wennerstrand, Chemist Analyst, 40, 33 (1951).
( 6 ) N. V. Maksimova and M. T. Kozlovekii, Zh. Anal. Khim, 2, 353 (1947). VOL 40, NO. 2, FEBRUARY 1968
447
I I I
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5
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0
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Figure 2. Determination of vanadate to phosphate ratio in molybdovanadophosphoric acid Concentration of Mixed Reagent. The concentrations of the reagents chosen for the molybdate-phosphate reagent were based on a study of the effect of one solution variable while the other three variables were held constant. Molybdate, vanadate, phosphate, and acidity were the variables investigated. Composition of Heteropoly Complex. The mole ratio of vanadate to phosphate was determined by plotting mole ratios GS. absorbance as the phosphate, molybdate, and acid concentrations were fixed and the vanadate concentration was varied. Figure 2 shows that a 1 :1 ratio was obtained for the vanadate to phosphate ratio based on the spectral data cited in Table I. When an attempt was made to determine the molybdate to phosphate ratio by fixing the phosphate, vanadate, and acid concentrations and varying the molybdate concentration, a ratio of 18:l was obtained. This high ratio is attributed to excess molybdate being required for complete formation of the mixed heteropoly acid in aqueous solution and indicated that it was not feasible to correlate this 18:l ratio for molybdate to phosphate with the 1:l ratio for vanadate to phosphate.
Table I. Spectrophotometric Data for Determination of Composition of Heteropoly Complex Concn of vanadium Ratio = Absorbance Concn of phosphorus
Concn of molybdenum Ratio = Concn of phosphorus 0.0 3.82 6.12 9.66 11.48 15.30 19.12 22.95
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ANALYTICAL CHEMISTRY
i
0
GWP,
0.0 0.165 0.495 0.824 1.15 1.32 1.48
I
0.560 0.615 0.720 0.880 0.965 0.975 0.975
Absorbance O.OO0 0.188 0.458 0.909 0.984 0.995 1.021 1.011
4
8
12
16
20
24
c PlmPMTE Figure 3. Determination of molybdate to phosphate ratio in molybdovanadophosphoricacid Therefore, the following modified method was devised to determine the molybdate to phosphate ratio by employing a liquid-liquid extraction step to partition the molybdovanadophosphoric acid from the excess molybdate. Thus, the phosphate, vanadate, and acid concentrations were fixed and the molybdate concentration was varied as in the previous experiment. However, 45 minutes were allowed for the complete formation of molybdovanadophosphoric acid. The mixed heteropoly acid forms more slowly with limited concentrations of molybdate. The molybdovanadophosphoric acid formed in each case was then extracted with the (1:4) l-pentanolTable 11. Tolerance of Diverse Ions Added as AI(ClOa)3 Ca(C104)? Cd(C1Oa)z COCI, Cr(C10d3 CU(C104)Z Fe(C104)3 Fe(C104)? KC104 Pllg(ClOa)? hln(C104)2 NHdCI NaC104 Ni(C104)z Pb(C104)z SnC14 H?Ti(S033 Zn(C104h Na3As03 Na3As04 NaC2H302 NHdCI KC104 KzCr04 Na2hlo04 KNOB Na3P04 K~SOI
Na2Si03 NalW04
PPf" permitted 500 500 100 500 250 500 5 0 500 500 500 500 500 500 500
50 10 500 500 50 100 500 500 100 500 250 500 250 10 10
diethyl ether solution, the excess molybdate remaining in the aqueous phase. After a thorough washing of the extract with the hydrochloric acid wash solution, the absorbance of the molybdovanadophosphoric acid in the organic phase was measured at 308 mp. These absorbances were then plotted us. the corresponding mole ratios of molybdate to phosphate. As the spectrophotometric data listed in Table I and plotted in Figure 3 show the molybdate-phosphate ratio was 11:l. Therefore, if the vanadate to phosphate ratio was 1 :1 and the molybdate to phosphate ratio was 11 :1, it was concluded that the molybdate to vanadate ratio must also be 11 to 1. Diverse Ions. A study was made to determine the permissible amounts of various ions that may be present without interfering with the determination of parts per million of vanadium. N o attempt was made to determine the effects of ion concentration larger than 500 ppm, since this concentration is rather large compared to the concentration of vanadium that was being determined. Errors twice the relative standard deviation were considered negligible.
Table I1 summarizes the results of this study. As expected silicate and tungstate which also readily form various heteropoly complexes interfere. Iron(II1) gives a negative error presumably because of complexation with the phosphate while iron(I1) acts as a reductant with the mixed heteropoly complex. The observed interference with the Ti(1V) is attributed to the high concentration of sulfate resulting in preparation of the titanium(1V) solution. Reproducibility. An estimate of the precision of this method was ascertained from the results of eight samples each containing 0.48 ppm of vanadium. These samples gave a mean absorbance value of 0.422 at 228 mp. The standard deviation was 0.0045 absorbance unit, or a relative standard deviation of 1.06z. In a series of determinations, about 40 minutes is required for each determination, RECEIVED for review August 28, 1967. Accepted November 27, 1967. Presented at the 15th Anachem Conference, Detroit, Mich., October 1967.
DifferentiaI ThermaI AnaIysis CrystaII inities and Melting Points of Ethylene-Vinylpyrrolidone Copolymers Bert H. Clampitt and Richard H. Hughes Gulf Research and Deuelopment Co., Kansas City Division, Merriam, Kan.
THEMELTING POINTS and crystallinities of various high pressure polyethylenes (HPPE) have been the subject of several recent publications (1-4). It is generally inferred in these articles that all types of hydrocarbon branching (ethyl, butyl, etc.) behave identically in their effect on crystallinity of HPPE's. In fact, articles on hydrocarbon copolymers prepared with Ziegler catalysis systems indicate that in these systems this is the case. It is the purpose of this paper to show that vinylpyrrolidone (VP) groups in ethylene copolymers affect the crystallinity and melting point of them in a manner analogous to hydrocarbon branching. Further, it is the object of this paper to propose a method for estimating hydrocarbon branching in ethylene copolymers. A previous publication from this laboratory indicated a linear relationship between crystallinity determined by differential thermal analysis (DTA) and mole per cent comonomer in ethylene copolymers (5). The polymers reported in that paper were all prepared under similar reactor conditions where the amount of hydrocarbon branching would be expected to be similar. When DTA crystallinities of ethylene copolymers prepared under widely different reactor conditions, as in the present paper, are compared, no correlation exists between crystallinity and comonomer content. It is generally realized that ethylene copolymers produced by high pressure polymerization techniques contain both hydrocarbon and comonomer branching; however, a quantitative (1) K. Casey, C. T. Elston, and M. K. Phibbs, Polymer Letters, 2, 1053 (1964). (2) I. J. Bastien, R. W. Ford, and H. D. Mok, Polymer Letters, 4, 147 (1966). (3) D. Bodily and B. Wunderlich, J. Po/ymer Sci. A , 4, 25 (1966). (4) L. Mandelkern et a/.,Polymer Letters, 3,803 (1965). ( 5 ) K . J. Bombaugh and B. H. Clampitt, J. Polymer Sci. A , 3, 803 (1965).
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measure of the hydrocarbon branching is extremely difficult. This difficulty arises because most comonomers contain methyl groups which interfere with the determination of hydrocarbon branching by infrared methods. Vinylpyrrolidone is an unusual comonomer since it contains no methyl groups, and therefore both comonomer and hydrocarbon branching can be measured by infrared techniques in ethylene-vinylpyrrolidone copolymers. These infrared measurements then allow correlations to be made between total branching and DTA crystallinity and melting point measurements. EXPERIMENTAL
The DTA measurements were made using a Perkin-Elmer differential scanning calorimeter calibrated for temperature with indium metal. Sample weights of 10 f 0.5 mg were used with the samples being fabricated from approximately 10-mil thick film. The annealing procedure and the heat rate of 20" C/minute were identical to those described by Casey et al. (I). Areas of the thermograms were measured with a planimeter, and the melting points were taken as the peak of the endotherms. Methyl groups were measured on a Perkin-Elmer Model 221 spectrophotometer using the method described by Willbourn (6) with a polymethylene wedge in the reference beam. Infrared determination of vinylpyrrolidone branchings were made by measuring the relative intensities of the 5.9-p band to the 6.8-11 CH2 band on films of about 0.5 mil in thickness. A calibration curve relating this IR parameter to VP content was obtained previously using elemental carbon analysis of the copolymers as the absolute standard. The polymers discussed in this paper were prepared in semicommercial high pressure polyethylene reactors. Widely .
(6) A. H. Willbourn, J . Polymer Sci., 39, 569 (1959). VOL 40, NO. 2, FEBRUARY 1968
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