units is obvious in the polymer containing only 12.5 wt% propylene. Examples of the application of the computer program to the 900-1000 cm-* region of the spectrum are given in Table V. Because the bands were badly overlapped in this region the band position parameter, v, was fixed in most cases. The 964 cm-l band for the atactic propylene was a well-defined shoulder and, rather than fixing V , the computer program defined its value. Band areas and other calculations were similar to that described for the methylene rocking region. Plots of the contributions for the different microstructures against composition are presented in Figure 7. From these curves it is obvious that the behavior in the methyl rocking region is rather analogous to the behavior in the methylene rocking region of the spectrum. Spectral Stripping Using the Computer. An attempt was made to subtract the spectrum of a n-alkane from the spectrum of a copolymer in the 700-800 cm-' region. In effect, this is simulating the differential technique of Veerkamp and Veer-
mans (15). For the spectral stripping, the amount of the n-alkane spectrum to be subtracted was determined from the computer-resolved band parameters in a preliminary application of the computer program described herein. The results, which were included in Table IV are nearly the same as results when the computer program was applied alone. More significantly, the stripping technique eliminated the weak absorption at 770 cm-' which was also present in the n-alkane, making it unnecessary to include this band in the second application of the program following the stripping operation. ACKNOWLEDGMENT
The authors thank Esso Research Laboratories for permission to publish this material. We also wish to thank H. Stone of Shell Development Company for supplying information on the computer program. RECEIVED for review July 27, 1967. Accepted November 27, 1967. Presented at the Eight Annual Eastern Analytical Symposium in New York, N. Y . November 16-18,1966.
Indirect Ultraviolet Spectrophotometric and Atomic Absorption Spectrometric Methods for Determination of Phosphorus and Silicon by Heteropoly Chemistry of Molybdate Thomas R . Hurfordl and D. F. Boltz Department of Chemistry, Wayne State Unicersity, Detroit, Mich.
The feasibility of effecting a selective extraction separation of molybdophosphoric and molybdosilicic acid followed by quantitation either by ultraviolet spectrophotometry and/or atomic absorption spectrometry has been investigated. The molybdophosphoric and molybdosilicic acids are formed in acidic solution by t h e addition of an excess of molybdate. Molybdophosphoric acid i s extracted with diethyl ether f r o m an aqueous solution which is approximately 1M i n hydrochloric acid. After adjusting the hydrochToric acid concentration of the aqueous phase to approximately 2M, the molybdosilicic acid i s extracted with a 5: 1 diethyl ether-pentanol solution. The extracts of molybdophosphoric acid and molybdosilicic acid are subjected t o acidic washings to remove excess molybdate. Each extract i s then contacted with a basic buffer solution to strip the heteropoly acid f r o m the organic phase. The molybdate resulting f r o m the decomposition of the heteropoly acid in the basic solution i s then determined either by measurement of the absorbance a t 230 mccusing an ultraviolet spectrophotometer or by measurement of the absorbance a t 313.3-mp resonance line of molybdenum using a n atomic absorption spectrometer. The optimum concentration ranges are approximately 0.1-0.4 p p m of phosphorus or silicon for the indirect ultraviolet spectrophotometric method and 0.1-1.2 ppm for t h e indirect atomic absorption spectrometric method.
BOTHPHOSPHATE and silicate ions react with molybdate ions in acidic solution to form heteropoly adds. Therefore, methods based on the formation of a heteropoly acid report l Present address, E. I. du Pont de Nemours & Co., Inc., Plastic Dept., Polyolefins Division, Orange, Texas.
phosphate and silicate as mutual interferences. This paper is concerned with the development of a method in which molybdophosphoric acid and molybdosilicic acid are formed, individually separated, and the amount of nonmetal determined indirectly by measuring the amount of molybdenum present in each heteropoly complex. Indirect ultraviolet spectrophotometric methods for phosphorus and silicon have been reported (1,2), but these methods did not eliminate the interference due to the other constituents presence. DeSesa and Rogers ( 3 ) determined phosphate in the presence of silicate by measuring the absorbance at 330 mp of the molybdophosphoric acid extracted with isoamyl acetate. The silicon was determined indirectly by correcting the absorbance at 332 mp of the combined molybdophosphoric acid and molybdosilicic acid in aqueous solution (prior to extraction) by using the previously determined absorptivity of molybdophosphoric acid in aqueous solution. They observed the trend of higher results for increasing amounts of phosphorus and a corresponding decrease in silicon and indicated that this separation needed further study. An indirect atomic absorption spectrometric method for phosphorus has been reported while this work was in progress. Zaugg and Knox ( 4 ) extracted molybdophosphoric acid in 2octanol and used citrate to suppress the effects of the
(1) C. H. Lueck and D. F. Boltz, ANAL.CHEM., 30,183 (1958). (2) L. Trudell and D. F. Boltz, Ibid., 35, 2122 (1963). (3) M. A. DeSesa and L. B. Rogers, Ibid., 26, 1381 (1954). (4) W. S. Zaugg and R. J. Knox, Ibid., 38, 1759 (1966). VOL 40, NO. 2, FEBRUARY 1968
379
excess molybdate. Silicate and arsenate are interfering ions. They aspirated the 2-octanol extract of molybdophosphoric acid directly into the burner of the atomic absorption spectrometer. In our work the heteropoly acid in the extractant was decomposed by contacting with a basic buffer solution which results in the transfer of equivalent amount of molybdate to the aqueous phase. The aqueous molybdate solution was aspirated into the burner. The atomic absorption spectrometric determination of molybdate has been reported previously (5-7). Since the completion of this investigation indirect atomic absorption spectrometric methods for the determination of phosphorus (8) and for the sequential determination of phosphorus and silicon have been published (9). EXPERIMENTAL
Apparatus. The ultraviolet spectrophotometric measurements were made in 1.000-cm. silica cells using a Cary 14 recording spectrophotometer. The atomic absorption spectrometric measurements were made using a Beckman Model 1301 atomic absorption accessory, a Beckman Model DB prism spectrophotometer equipped with a Beckman potentiometric recorder, and a Techtron burner assembly. The hollow cathode tube was neon filled and supplied by Beckman. A Thomas shaking apparatus was used for extractions. Reagents. STANDARD PHOSPHATE SOLUTION. Dissolve 2.200 grams of potassium dihydrogen phosphate, KH2P04, in distilled water and dilute to 1 liter. Dilute a 20.00-ml aliquot of this solution to 1 liter with distilled water. This solution contains 0.010 mg of phosphorus per ml. STANDARD SILICATESOLUTION.Dissolve 5.40 grams of sodium silicate, Na2SiO3.9H20,in distilled water and dilute to 1 liter. Standardize this solution gravimetrically. After standardization, use a microburet to transfer sufficient silicate solution to a 500-ml volumetric flask so that on dilution to the mark the solution contains 0.010 mg of silicon per ml . MOLYBDATE SOLUTION.Dissolve 25.0 grams of ammonium molybdate, (NH4)6M07024-4H20, in distilled water and dilute to 250 ml. BUFFERSOLUTION,Dissolve 53.5 grams of ammonium chloride and 70 ml of concentrated ammonium hydroxide in distilled water and dilute to 1 liter. PENTANOL-ETHER SOLUTION.Mix 50 ml of 1-pentanol with 250 ml of diethyl ether. This solution should be prepared fresh daily. All chemicals were reagent grade and all aqueous solutions were stored in polyethylene bottles. Recommended General Procedure. SAMPLE.Weigh, or measure by volume, a sample containing up to 0.05 mg of silicon and 0.05 mg of phosphorus and treat it so that it is present as soluble orthophosphate and silicate. The sample may contain up to 0.13 mg of silicon and 0.13 mg of phosphorus when the atomic absorption spectrometric method is used. SEPARATION OF MOLYBDOPHOSPHORIC ACID. Transfer sample to a 125-ml separatory funnel. Add 1.0 ml of 1 : 2 hydrochloric acid and dilute to approximately 40 ml with distilled water. Add 4 ml of the 10 ammonium molybdate solution, swirl to mix, and allow to stand for 10 minutes. The pH of this solution should be approximately 1.3. Add 5.0 ml of concentrated hydrochloric acid, swirl to mix, and allow to stand for 5 minutes. ( 5 ) D. J. David, Nature, 187, 1109 (1961). (6) D. J. David, Analyst, 86, 730 (1961). (7) R. A. Mostyn and A. F. Cunningharn, ANAL.CHEM.,38, 121 (1966). (8) G . F. Kirkbright, A. M. Smith, and T. S. West, Analyst, 92, 411 (1967). (9) T. Kumarnatu, Y. Otaui, and Y. Yarnarnoto, Bull. Chem. Soc. Japan, 40 (2), 429 (1967).
380
ANALYTICAL CHEMISTRY
Extract the molybdophosphoric acid by adding 45 ml of diethyl ether and shaking vigorously for 3 minutes on a mechanical shaker. Remove the separatory funnel stopper and rinse it with 1-2 ml diethyl ether using a pipet. Transfer the lower aqueous phase containing molybdosilicic acid to another 125-ml separatory funnel and rinse the tip of the original separatory funnel with a small stream of water from a wash bottle. Save this aqueous solution for the determination of silicon. DETERMINATION OF PHOSPHORUS. Wash the diethyl ether extract containing molybdophosphoric acid with 10 ml of a 1:lO hydrochloric acid by shaking for 10 seconds. Swirl the funnel to collect all of the water and discard lower aqueous layer. Wash the tip of the separatory, funnel with a stream of distilled water to remove any remaining traces of excess molybdate. Add 30 ml of the ammonium hydroxideammonium chloride buffer solution and shake for 15 to 30 seconds. Drain the lower aqueous phase into a 100-ml volumetric flask. Add another 15 ml of the buffer solution and shake for 15 to 30 seconds. Drain the aqueous phase into the 100-ml volumetric flask and dilute to the mark with distilled water. Ultraviolet Spectrophotometric Method. Measure the absorbance at 230 mp in a 1.000-cm cell using a reagent blank solution in the matched reference cell. Refer absorbance reading to a standard calibration graph obtained using standard phosphate solutions. Atomic Absorption Spectrometric Method. Adjust current applied to hollow cathode at 20 mA. A reducing acetylene-air flame is produced by using a support gas pressure of 21 Ib and acetylene pressure of 5 lb and opening the burner fuel restrictor until a highly luminous flame is observed. Use a slit width of 0.20 mm and adjust the monochromator setting to obtain maximum absorbance reading at 313 mp. A solution containing 26.0 ppm of molybdenum is asrirated so that therestrictor can beadjusted to give maximum abso ,bance. Aspirate known and unknown samples using the pe' cent transmittance readout of recorder. Aspirate water bet veen all samples to clean the premix chamber and check base line. Prepare a calibration graph after conversion of trar smittince to absorbance. SEPARATION OF MOLYBDOSILICIC ACID. Add 5 ml of concentrated hydrochloric acid to the aqueous molybdosilicic acid solution which remained after the initial extraction of molybdophosphoric acid. Swirl to mix and allow to stand for 5 minutes. Add 20 ml of the 5 :1 diethyl ether-pentanol extractant and shake vigorously for 3 minutes on the mechanical shaker. Remove the stopper from the separatory funnel and rinse with 1-2 ml of diethyl ether using a pipet. Remove and discard the lower aqueous phase. Wash the ether-pentanol extract twice with 25 ml portions of 1 :10 hydrochloric acid. Shake for 10 to 15 seconds each time and remove and discard the lower aqueous phase. Wash the tip of the separatory funnel with a stream of distilled water to remove any remaining traces of excess molybdate. Add 30 ml of the ammonia buffer and shake for 15 to 30 seconds, Drain into a 100-ml volumetric flask. Add 15 ml of the buffer solution and shake again. Drain this aqueous phase into the same volumetric flask and dilute to the mark with distilled water. DETERMINATION OF SILICON. Ultraviolet Spectrophotometric Method. Measure the absorbance at 230 mk in a 1.000-cm. cell using a reagent blank solution in the matched reference cell. Refer absorbance reading to a calibration graph obtained using standard silicate solutions. Atomic Absorption Spectrometric Method. Adjust the atomic absorption spectrometer (AAS) settings as previously outlined for indirect AAS determination of phosphorus. Aspirate standard and unknown solutions. Prepare a calibration graph and determine concentration of unknown samples.
RESULTS AND DISCUSSION Phosphorus Concentration. In the indirect ultraviolet spectrophotometric method, conformity to Beer’s law was observed from 0.05 to 0.5 ppm of phosphorus. The optimum concentration range is approximately 0.1 to 0.4 pprn of phosphorus. A calibration graph of slight curvature was obtained for 0.05 to 1.3 ppm of phosphorus by the indirect atomic absorption spectrometric method. Silicon Concentration. In the indirect ultraviolet spectrophotometric method, conformity to Beer’s law was observed from 0.1 to 0.5 ppm of silicon. The optimum concentration range is 0.1 to 0.35 pprn of silicon. For the indirect atomic absorption spectrometric method a virtually linear calibration graph was obtained in the 0.05 to 1.1 ppm of silicon range. Molybdate Concentration. A relatively high excess of molybdate is required to ensure the complete formation of molybdophosphoric and molybdosilicic acid. The effect of molybdate concentration was studied using 0.40 ppm of phosphorus as orthophosphate and 0.35 ppm of silicon as silicate. It was found that 4.0 ml of the 10% ammonium molybdate was sufficient. Acidity for Formation of Heteropoly Acids. Molybdophosphoric acid and molybdosilicic acid form in an acidic solution of about pH 1. Boltz and Mellon (IO) recommend a pH of 0.9 to 1.25 for the formation of molybdophosphoric acid while Jean (11) recommends 0.7 to 1.25 as the optimum pH range. Milton (12) indicated molybdosilicic acid formed in a pH range of 1 to 5, Jean (11) recommended a pH range of 0.8 to 3.8, and Trudell and Boltz ( 2 ) recommended a pH of 1.4. Strickland (13) has shown that molybdosilicic acid formed in slightly acidic solutions is stable in strongly acidified solutions. A pH of 1.3 was selected as the optimum pH for the formation of both heteropoly acids for this work. Heating was found unnecessary provided a period of 10 minutes was allowed for complete formation. Extraction of Molybdophosphoric Acid. In general, oxygenated solvents are the best extractants for heteropoly acids. Wadelin and Mellon (14) investigated the liquid-liquid extraction of heteropoly acids and found that a 20 % by volume solution of I-butanol in chloroform selectively extracted molybdophosphoric acid in the presence of arsenate, silicate, and germanate ions. DeSesa and Rogers (3) used isoamyl acetate, and Zaugg and Knox ( 4 ) used 2-octanol. Lueck and Boltz (15) used isobutyl alcohol to extract molybdophosphoric acid from a 1.2M perchloric acid solution. Diethyl ether was selected as an extractant because of its low wavelength cutoff in the ultraviolet region and the preferential extractability of molybdophosphoric acid from moderately acidic solutions (16). Forty-five milliliters was finally selected as the optimum volume of diethyl ether inasmuch as there is considerable loss of solvent because of its solubility in the 1.2M hydrochloric acid solution from which the initial extraction of molybdophosphoric acid is made and in the acidic wash solution used in removing the traces of excess molybdate. One extraction was found to be sufficient. If cloudiness occurs in the ether extract, it can be cleared either by vigorous shaking or by allowing to stand for approximately 45 minutes, (10) D. F. Boltz and M. G. Mellon, Bull. Chem. SOC.Japan, 20, 749 (1948). (11) M. Jean, Chim. Anal.,44, 195(1962). (12) R. F. Milton, Analyst, 76, 431 (1951). (13) J. D. H. Strickland, J. Am. Chem. SOC.,74, 872 (1952). 25, 1668 (1953). (14) C. Wadelin and M. G . Mellon, ANAL.CHEM., (15) C. H. h e c k and D. F. Boltz, Ibid., 28, 1168 (1956). (16) W. S. Clabaugh and A. Jackson, J . Res. Natl. Bur. Std., 62, 201 (1954).
Table I. Separation and Ultraviolet Spectrophotometric Determination of Known Quantities of Phosphorus and Silicon Phosphorus, ppm Silicon, ppm Added Recovered Added Recovered 0.00
0.50 0.00 0.10 0.20 0.25 0.30 0.40 0.45 0.50
0.00 0.50 0.00 0 . IO 0.20 0.25 0.30 0.40 0.45 0.50
0.00 0.00 0.50 0.10 0.20 0.25 0.30 0.40 0.45 0.50
0.00 0.00
0.50 0.12 0.19 0.26 0.31 0.40 0.45 0.49
Table 11. Separation and Atomic Absorption Spectrometric Determination of Known Quantities of Phosphorus and Silicon Phosphorus, ppm Silicon, ppm ~Added Recovered Added Recovered 0.00 0.05 0.10 0.25 0.30 0.40 0.50 0.00 0.50 0.70 0.90 1.00 1 .oo 1.10 1.30
0.00 0.05 0.11 0.26 0.29 0.40 0.50 0.01 0.50 0.70 0.89 0.92 0.99 1.12 1.30
0.00 0.06 0.10 0.25 0.30 0.40 0.00 0.50 0.50 0.70 0.90 1 .OO 0.70 1.10 1.30
0.01 0.06 0.10 0.25 0.29 0.40 0.01 0.49 0.48 0.74 0.88 0.99 0.71 1.10 1.30
Washing of the Ether-Molybdophosphoric Acid Extract. It is necessary to contact the ether extract of molybdophosphoric acid with 10 ml of a 1 :IO hydrochloric acid wash solution to remove traces of excess molybdate. Exact duplication of the washing step is essential. Extraction of Molybdosilicic Acid. 1-Pentanol has been used extensively as an extractant for molybdophosphoric acid and molybdosilicic acid (9). Boltz and Trudell ( 2 ) used a 1 :5 pentanol-diethyl ether solution for extraction of molybdosilicic acid from a 0.5 to 1M hydrochloric acid solution. Investigation of higher acidities on the efficiency of the extraction showed that a minimum amount of extractant and only one extraction were required if the acidity prior to extraction was approximately 2.2M in hydrochloric acid. Hence, one extraction with 20 ml of the mixed pentanokther extractant is recommended. It was also determined experimentally that washing the extract two times with 25 ml portions of 1 :10 hydrochloric acid solution was sufficient to remove the excess molybdate. Decomposition of Heteropoly Acids by Basic Buffer Solution. Both molybdophosphoric acid and molybdosilicic acid are decomposed if the nonaqueous extract is contacted with a basic aqueous buffer solution; the molybdate of the heteropoly acids is transferred to the aqueous phase. The absorption spectrum of molybdate is somewhat pH dependent so that when the indirect ultraviolet spectrophotometric method is employed the use of a basic buffer solution is necessary. The absorbance measurements were unchanged for 10 minutes to 24 hours from the time of equilibration with the aqueous buffer solution. VOL. 40, NO. 2, FEBRUARY 1968
e
381
Effect of Instrumental Parameters on Atomic Absorption Spectrometric Method. The most sensitive absorption wavelength for molybdenum is at 313.3 mp. This resonance line was located by scanning in this wavelength region and recording relative intensity versus wavelength. The signal is then maximized for the 313.3 mp line. The flame profile for molybdenum in both fuel rich and fuel lean conditions has been reported (17). Because of small profile it is necessary to use care in making burner adjustments needed to obtain the maximum absorption. Proper alignment of the flame was accomplished by careful adjustment of vertical and horizontal adjustments on the burner. Synthetic Mixtures. A number of solutions containing known amounts of phosphorus as phosphate and silicon as silicate were prepared. These solutions were analyzed by the general recommended procedure with both the ultraviolet spectrophotometric and atomic absorption spectrometric methods of quantitation being utilized. The results of these analyses are given in Tables I and 11. These data demonstrate satisfactory results for the separation and determination of phosphorus and silicon. The indirect ultraviolet spectrophotometric method is more suitable for determining less than 0.4 ppm of either silicon or phosphorus, whereas the atomic absorption spectrometric method is more applicable to the 0.4 to 1.2 ppm range for each element. Un(17) C. S. Rann and A. N. Harnbly, ANAL.CHEM., 37,879 (1965).
less a mechanical shaker was employed, difficulty was often encountered for the higher concentrations of phosphorus and silicon because of the formation of a very cloudy aqueous phase following the ether extraction. The main disadvantage of the atomic absorption spectrometric method is the additional time required to prepare calibration plots at the time of each analysis. Precision. An estimate of the precision of the indirect ultraviolet spectrophotometric and atomic absorption methods was obtained by analyzing seven solutions containing 0.4 ppm of phosphorus and 0.35 ppm of silicon. Five values obtained for the ultraviolet spectrophotometric determination of phosphorus gave a mean absorbance value of 0.770, a standard deviation of 0.005, and a relative standard deviation of 0.65%. Seven values obtained for the ultraviolet spectrophotometric determination of silicon gave a mean absorbance value of 0.694, a standard deviation of 0.009, and a relative standard deviation of 1.3%. When the atomic absorption spectrometric method was used, a relative standard deviation of 0.65% was obtained for the determination of phosphorus and a relative standard deviation of 1.45 % for the determination of silicon. RECEIVED for review September 25, 1967. Accepted November 1 , 1967. Presented at the 154th Meeting, ACS, Chicago, Ill., Sept. 11-14, 1967.
Height Equivalent to a Theoretical Plate of an Open Tubular Column Lined with a Porous Layer A Generalized Equation M. J. E. Golay The Perkin-Elmer Corporation, Norwalk, Conn.
The increased importance of open tubular columns lined with a porous layer makes the development of a suitably complete expression for the RFV of such columns desirable. The new formula, which can be considered as a generalized expression for the HETP of open tubular columns, makes allowance for the tortuosity of the porous layer and the degree of its porosity. Possibilities for experimental verification are also discussed.
IN A FORMER publication ( I ) , a formula for the HETP of an open tubular column lined with a porous layer was developed which contained no term for the resistance to mass transfer in the liquid phase, then assumed to be negligible. The purpose of this report is to complete this formula with such a term, as well as to make allowance for the tortuosity of the porous layer and the degree of its porosity. The starting point of the derivation will be Formula 31b in an earlier treatment (2) which can be written as: 1 r2u 1 6k l l k 2 rzu h = - 2Do + - + f - -k3 (1) U 24 (1 k)' Do 6(1 k)' F'K' D L
+ + +
+
(1) M. J. E. Golay, Nature, 199, 370 (1963). (2) M. J. E. Golay, "Gas Chromatography 1958," D. H. Desty, Ed., Butterworths, London, 1958, pp 36-55.
382
ANALYTICAL CHEMISTRY
where h = HETP Do and D L = diffusion coefficients of the particular component considered in the gas and liquid phases, respectively k = capacity ratio K = partition coefficient r = radius of open tubular column F = ratio of the liquid phase surface over the equivalent surface of a smooth bore column u = average gas velocity
For our purpose, Equation 1 will be rewritten: 2Do h = - + U
1
+ 6k + l l k 2 r2u 24(1 + k)' 0, -t-
kT -(
u
(2)
where T designates the diffusion time of the component considered in the oil film of a smooth bore column of thickness d f / F . The equivalence of Equations 1 and 2 can be shown as follows :
