In practice it is convenient t o reduce raw intensity data to K-values for a given accelerating potential before the corrections are calculated for the sample. Thus:
Table V. Calculated and Measured a-Factors for Selected Elements A.R.L. microprobe, 52.5 take-off angle, 15-kV acceleratingpotential Evaluated Evaluated at 1 : l at end molecular member oxide composition Measureda aE03
a$* aM.0 AI
%io*
G
O
aEgO aAi20J CaO
ago a%0 @io GP*03
a&* @EO*
1.03 1.09 1.67 1.04 1.13 1.40 1.44 1.05 1.20 1.39 0.90 1.07 1.08 1.14 1.20
1.03 1.09 1.62 1.04 1.14 1.39 1.43 1.05 1.23 1.45 0.92 1.06 1.08 1.15 1.20
(7) and:
1.023 1.102 1.563 1.036 1.172 1.320 1.346 1.029 1.19 1.33 0.924 1.123 1.188 1.16 1.27
(8) The correction program then only needs to contain a single matrix of &-factors and elements measured at various accelerating potentials can be handled together.
ACKNOWLEDGMENT The authors are indebted t o A. A. Chodos for the microprobe analyses and perceptive criticism.
aEo* a Measured using following simple phases, mostly synthetic crystalline phases: A1203, SOz, SiOz glass, AlzSiOj, Mg~SizOs, MgzSi04, MgAlzOa, Mg3A12(SiO+, 5 glasses between MgzSizOs and Mg3Alz(Si04)3,CaSiO3, CaS103 glass, Ca3A12(SiO4)3,Ca(Al2Si208),CaAI(AlSiO,), Fel-,O, FeZSiO4,FezSiz06, ZnO, Znz SO4, ZrSiOa.
RECEIVED for review May 18, 1970. Accepted July 10, 1970. This work was supported by NSF grant (GA-12867) and NASA contract (NAS-9-8074). Division of Geological Sciences Contribution number 1852.
Determination of Unsaturation by Analytical Hydrogenation and Null Point Pressuremetry D. J. Curran and James L. Driscolll Department of Chemistry, Uniwrsity of Massachusetts, Amherst, Mass. 01002 A multirange differential capacitive-type pressure transducer system has been used to follow pressure changes in a closed system during analytical hydrogenation of unsaturates and subsequent in situ regeneration of hydrogen gas by hydrolysis of sodium borohydride. A graphical treatment of the recorded data yields the volume of NaBH, needed to generate hydrogen equivalent to that consumed in the hydrogenation reaction. The method uses an experimentally determined correction to the data for the free space equivalent of the volume of solution and liquid added to the system. Precision and accuracy are a few parts per thousand for sample sizes corresponding to 0.1 millimole of unsaturation in favorable cases and as little as 1.2 micromoles of octene-1 have been determined with an accuracy of about 2 per cent relative.
As PART OF OUR STUDIES of applications of pressure transducers in chemical analysis, we have investigated the determination of unsaturation by following pressure changes in a close reactor. A number of procedures for low pressure analytical hydrogenation at atmospheric pressure are well known ( I ) . However, they often suffer from lack of high Present address, Rhode Island Hospital, Providence, R. I. (1) S. Siggia, “Quantitative Organic Analysis via Functional Groups,” 3rd ed., John Wiley and Sons, New York, N. Y., 1963, pp 318-341. 1414
precision and accuracy and are frequently time-consuming. A notable exception is the recent work of H. C. Brown, C. A. Brown, and coworkers (2-12). Their apparatus and procedures feature in situ preparation of the catalyst and generation of hydrogen gas, and automatic restoration of the internal pressure of the system to the value prevailing prior t o the hydrogenation reaction. The former involve sodium borohydride reactions and the latter involves a self-sealing mercury valve. Precision and accuracy were in the range 0.5 to 1 . 5 % relative in most cases for a number of types of unsaturation for sample sizes ranging from 2 mmoles to 50 pmoles of (2) H. C. Brown and C. A. Brown, J. Amer. Chem. SOC.,84, 1493 (1962). (3) Ibid., p 1494. (4) Ibid., p 1495. (5) Ibid., p 2827. (6) H. C. Brown and K. Sivasankaran, ibid., p 2828. (7) C. A. Brown and H. C. Brown, ibid., p 2829. ( 8 ) H. C. Brown, K. Sivasankaran, and C. A. Brown, J . Urg. Chem., 28, 214 (1963). (9) .~ H. C. Brown and C. A. Brown, Tetrahedron.. Suool.. _ _ 8, . . Part I. 149 (1966). (10) C. A. Brown and H. C. Brown. J . Ora. Chem.,. 31,. 3989 ‘ (’1966). (11) C. A. Brown. S. C. Sethi. and H. C. Brown. ANAL.CHEM.. 39, 823 (1967). ’ (12) C. A. Brown, ibid., p 1882. I
~
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
unsaturation. Five pmoles of octene-1 were determined with a n accuracy of - 4 z relative and a relative standard deviation of 3 ~ 2 % . Miwa and coworkers (13, 14) reported a n extensive investigation of the application of this method t o the determination of unsaturation in oils. Trutnovsky (15) has developed a mathematical formula relating reactor vessel dimensions to the precision of volume measurement for a null-type hydrogenation system where the null is restored by volume compensation. An apparatus was developed using a differential manometer and a micrometer driven microburet. An accuracy corresponding to k l 111 of hydrogen was claimed. Little work has been reported on the use of pressure transducers in conjunction with analytical hydrogenation studies. Reuter (16) used a n apparatus based on a differential pressure transducer system. The technique involved a direct measurement of hydrogen uptake by plotting the transducer system output voltage, calibrated in microliters of hydrogen uptake, cs. time on a strip chart recorder. Samples which consumed 100-300 p1 of hydrogen could be determined with a n accuracy of 1 p l . Of interest is the work of Rohwedder (17) who reported a hydrogenator for kinetic studies. The apparatus measured the internal volume change of the system due to hydrogen uptake at constant pressure. A differential pressure transducer system compared the system pressure t o a reference and the output signal from the transducer system controlled a piston t o maintain constant system pressure. An electrical signal, proportional to piston displacement (and therefore t o hydrogen uptake), was plotted against time o n a strip chart recorder. In our work, hydrogen gas is initially supplied t o the reactor from a tank source. The sample and sodium borohydride, which is used t o regenerate the hydrogen following the hydrogenation reaction, are added by micrometer-driven microburets. The reaction media is a n acidic water-isopropyl alcohol mixture containing 5 palladium on charcoal as catalyst. The output of the transducer system is recorded and the null pressure determined from a graphical treatment of the strip chart readout. Most sample sizes corresponded t o approximately 0.1 mmole of unsaturation. Octene-1 determinations covered a sample size range from 0.5 mmole t o 1 pmole. Additional compounds studied were: oleic acid, dl-pinene, 1,3-cyclooctadiene, 1,4-butyndiol, styrene, and methyl methacrylate. Precision was always at least 1 relative and as good as two parts per thousand in several instances. Accuracy was also found as high as two parts per thousand in some cases. All hydrogenation techniques which require the addition of sample and/or reagents t o a closed system must involve a correction for the change i n system volume or pressure produced by these additions. Usually this is done by measuring the volume or weight of material delivered and converting this t o a n equivalent number of moles of hydrogen gas, taking into account the vapor pressure of the solvents involved and assuming that the solutions are ideal. We have demonstrated that a n alternative to this procedure is to obtain a correction factor based on experimental measurement of the pressure change of the system upon addition of solutions but i n the absence of any chemical reactions.
z
z
(13) T. K. Miwa, W. F. Kwolek, and I. A. Wolff, Lipids, 1, 152 (1966). (14) I. A. Wolff and T. K. Miwa, J . Amer. Oil Clzem. Soc., 42, 208 (1965). (15) H. Trutnovsky, Mikroclzim. Actu, 1963,685. (16) A. Reuter, 2.Anal. Clzem., 231, 256 (1967). (17) W. K. Rohwedder, J. Catul., 10,47 (1968).
EXPERIMENTAL
Reagents. Sources of organics were: octene-1 (Phillips, minimum 99 mole %), methyl methacrylate (Eastman White Label), dl-pinene (Eastman Yellow Label), oleic acid (Mallinckrodt), 1,3-cyclooctadiene (Columbia Carbon), 1,4butyndiol (General Aniline), and styrene (Matheson, Coleman and Bell). Sodium borohydride was Fisher, minimum 98%. Tank hydrogen was obtained from Air Reduction Co. All other chemicals were reagent grade. Solvents. Isopropyl alcohol was refluxed in the presence of sodium borohydride for 1 hour and then distilled. Dimethylformamide was prepared by drying 100-ml portions over 25 grams of magnesium sulfate for 24 hours. Two successive treatments were used. Laboratory distilled water was redistilled from alkaline permanganate. Solutions. Solutions of sodium borohydride were prepared in dimethylformamide. All except the lowest concentration were standardized by adding aliquots to aqueous hydrochloric acid solutions and backtitrating the excess acid with NaOH. Potentiometric end-point detection was used and end-point volumes were obtained from plots of the first derivative of the titration curve. No blank correction was found when the procedure was repeated on D M F alone. A 0.4920M NaBH4 solution was stable for more than a month, in agreement with thq data published by VentronMetal Hydrides Division (18). A tenfold dilution of this solution with dimethylformamide was taken as 0.04920M. Hydrogenation Medium. Hydrogenation media were prepared from 30 ml of isopropyl alcohol, 10 ml of aqueous acid solution (10 volume %’ H2S04or HOAc) and 50 to 500 mg of catalyst (5 weight % palladium on charcoal, Matheson, Coleman and Bell). Densities. Densities of the organics were obtained from the weight of 25.00-ml volumes at 23.0 “C, the bath temperature. Check Analyses Methods. The check method for determination of the mole per cent unsaturation of octene-l , cyclooctadiene, oleic acid, and dl-pinene was the Hanus iodine bromide method as given by Siggia (19). 1,4-Butyndiol was analyzed on the basis of hydroxyl content according t o the acetylation procedure given by Siggia (20) using potentiometric end-point detection. Apparatus. Pressure measurements were made with a n apparatus consisting of five units: reactor, reference vessel, pressure transducer system, potentiometric recorder, and constant temperature bath. A drawing of the reactor is shown in Figure 1. The glass stems of the microburets (RGI, Vineland, N. J., Type S-3100, 0.25-ml capacity) were ring sealed to the male section of a 14/35 T joint fitted with a n “0” ring seal. Side arm, E, permitted hydrogen t o be introduced from a n external source. The reactor, along with a reference vessel similar t o our reactors for pressuremetric titrations (21), was mounted in a Lucite platform (Figure 2). The transducer system consisted of a Type 700 Power Supply, a Series 511 Barocel Transducer, and a Type 1015 Signal Conditioner (Datametrics Div., C.G.S. Scientific, Waltham, Mass.). The Barocel, a 0 t o 10 psi model, is a differential capacitive type transducer arranged in a bridge circuit. Details of this pressure transducer system and its operation have been described by Curran and Driscoll (21), and by Curran (22). A range switch selects one of seven multiplication factors so that full scale output of 5.000 V corre(18) Ventron-Metal Hydrides Division, Beverly, Mass., “Sodium Borohydride,” p 8. (19) S. Siggia, “Quantitative Organic Analysis ciu Functional Groups,” 3rd ed., John Wiley and Sons, New York, N. Y., 1963, pp 316-318. (20) Ibid., pp 12-14. (21) D. J. Curran and J. L. Driscoll, ANAL.CHEM.,38, 1746 (1966). (22) D. J. Curran, J . Cllem. Educ., 41, A465 (1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1415
E
Figure 2. Figure 1. Hydrogenation reactor A , Microburet for NaBHI reagent. B, Sidearm with 12/9 female glass socket joint for connection to transducer system. C, microburet for sample. D , Male section of 14/35 standard taper joint. E, Side arm with 2 mm stopcock. F, Female section, 14/35 standard taper joint. G , Vessel for the hydrogenation medium. H , Teflon (Du Pont) coated magnetic stirring bar
sponds to input ranges of X1, X0.3, XO.l, X0.03, XO.01, X0.003, and XO.001. The recorder was a Model 43 Linear/ Log Varicord (Photovolt Corp.), and the constant temperature bath was a Model TV 70 (P. M. Tamson, Zoetemer, Holland) with temperature control to *0.002 "C. The transducer housing supplied by the manufacturer cannot be immersed so a Lucite waterproof box was constructed in which the transducer is mounted. The lid of the box was secured by eight bolts and the seal maintained by a rubber gasket. Figure 2 is a drawing of the complete assembly which is submerged in the bath. The box is held in place by fitting it between two platforms attached to the stand. The side connection to the brass elbow, I , (Weatherhead, Co.) was made through an 'i8-inch Imperial fitting and the second connection shown was made by cementing (2-Ton Epoxy, Devcon Co., Danvers, Mass.) the copper tubing into a hole drilled in the top of the elbow. The transducer is connected to the pressure system by a reducing union (Weatherhead) and an Imperial fitting (the combination represented by K ) joined to the copper tubing and cemented in brass bushing, J. The submersible magnetic stirring motor was a Tri-R Instruments Model MS-7 (Jamaica, Long Island, New York). All platforms were constructed of Lucite and secured to a length of aluminum rod. Aluminum clamps were used. An aluminum platform built into the bottom of the bath permitted the stand to be lowered or raised. In the lowered position, the entire assembly was submerged except the stopcocks and upper sec1416
Stand assembly and housing for pressure transducer
A , Microstopcock epoxied to '/&ch copper tubing. B, '/&ch copper tubing epoxied to 12/9 male ball joint for connection to the reaction vessels. C, Lucite platform for reactor and reference vessel. D,Magnetic stirring motor. E , Lucite box housing transducer. F, Transducer. G, 1/2-inchbrass pipe threaded in box and epoxied to polyethylene tubing for electrical connections. H , Stand. I , 'isinch brass elbow. J , '/?-inch brass bushing threaded through wall of box. K,Connection to transducer
tions of the microburets. In the raised position, only the lower section of the reactor is submerged. Procedure for Null Point Pressuremetry. The reaction medium is placed in the reactor, the components are assembled on the stand, and the stand is placed in the bath, which was controlled at 23.000 i 0.002 "C. With the transducer connections secured, the stand is lowered to the bottom of the bath and the transducer system is mechanically, electrically, and pressure zeroed. The system is purged with hydrogen from the tank at a rate of 30 to 35 cm3 per minute with rapid stirring. With the transducer range setting on X1, a small positive pressure of hydrogen, 3 to 5 psi, was applied to the reactor for 5 minutes. The pressure is then released and 1O.OO-pl volumes of sample and sodium borohydride are delivered to establish an initial setting on the burets which is free of possible error due to diffusion from the buret tips. The stopcocks are closed and the system is allowed to attain a steady state as shown by the base line drawn on the recorder. Sample is added from microburet, C, of Figure 1. After the hydrogenation reaction is complete as indicated by the recorder readout, hydrogen gas is regenerated by incremental additions of sodium borohydride from microburet A . RESULTS AND DISCUSSION
The recorder readout for the analysis of 80.00 p1 of octene-1 is shown in Figure 3. Full scale readout was adjusted to be
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
~~~
Table 1.
A ' %
~~~
Ratios for Conversion of Sample or Reagent Volume to Pressure Signal
Materials Ratio, mm/jd* Octene-1 2.00 Oleic Acid 1.80 dl-Pinene 1.88 Butyndiolc 1.78 Styrene 1.70 Methyl methacrylate 1.70 1.80 1,3-Cyclooctadiene Heptane solution of octene-ld 1.80 Dimethylformamide 1.90 Measurements made on 20.00-fi1 volumes under experimental conditions but in the absence of a hydrogen atmosphere. Values measured and expressed on the XO.01 range. To convert to other ranges, multiply by the ratio obtained from division of 0.01 by the range setting used. Pressures are expressed in millimeters of chart paper. Butanol solution. Tenfold dilution of octene-1.
B
0
+
I
I
- 4 - 2
Figure 3.
I
I
I
I
,
0 2 4 6 8 TIME MINUTES
-
Recorded data for determination of octene-1
A , Base line. B, Curve for hydrogen uptake. C, Curve for hydrogen evolution following addition of an increment of reagent. D, Intercept of the extrapolated base line to the regenerated signal
5 0 z (2.500 V) of full scale readout of the pressure transducer system (5.000 V equivalent to 10 psi in this case). However, pressure readings were usually measured in millimeters of chart paper. Consumption of hydrogen in the reactor produced a negative voltage signal from the transducer system and the recorder was zeroed accordingly. Base line A of Figure 3 is not parallel to the time axis and indicates a continuous pressure decrease in the reactor but at a constant rate. This is in contrast to our earlier observations of a positive drift in background for pressuremetric titrations ( 2 1 ) and is attributed to uptake of hydrogen by the catalyst. The slope of this line is slight at high range settings and a steady rate of decrease was attained in about fiveminutesafter final adjustments of the apparatus. At higher sensitivities, the slope is correspondingly higher and smaller quantities of catalyst must be used to obtain a suitable background signal. Usually 20 minutes were necessary to reach a steady state under these conditions. At high sensitivities, the background signal also shows noise due to temperature differences between reactor and reference vessel and to stirring. The magnitude of these effects on the X0.003 range setting was about 50 mV, peak to peak. However, it was still possible to obtain an average pressure reading at any instant in time. Curve B records the observed pressure change for hydrogen uptake, APo. This value is smaller than the true pressure change for hydrogen uptake, AP,, by an amount equal to the pressure change due to the addition of sample to the system,
AP,. Thus, APu = APo AP,. The data in Table I were obtained to calculate AP, from the volume of sample added to the system. It is clear that the correction depends on the nature of the sample. Curve C shows the pressure response for a 10.00-p1 increment of reagent. Two features are important: the pressure is proportional to the volume of NaBHd added, and only a part of the total pressure change, AP1, is due to hydrogen evolution. The latter may be calculated as APE^ = A p t - APT where APTis the pressure change due to addition of NaBH4 solution and is calculated from the data in Table I for dimethylformamide. Extrapolation of base line A to its intersection with the curve for hydrogen evolution gives an apparent null point which can be read as a volume of NaBH4. If n increments of NaBH4 solution are required to pass the null point, then the volume of reagent added at the apparent null point is given by: the sum of (n - 1) increments plus the volume of one increment times (AP,JAPt) where APz is the total pressure change for the last increment of reagent added, and APn is that part of A p t required to reach the apparent null point. Multiplication of this null point volume by the ratio, AP,/ AP,, will correct the data for compression of the gas phase due to the addition of sample. The volume of NaBH4 solution which must be added to the system to produce a change in system pressure which is equal to the true change in system pressure due to hydrogen uptake has now been established. However, only a fraction, APH21APt,of the former change is due to hydrogen evolution. Multiplication by AP,/AP,, will yield the volume of NaBH4 solution necessary to produce a pressure change due only to hydrogen evolution equal to that due to hydrogen uptake. These corrections can conveniently be combined to yield a single correction factor for the calculation: Volume of NaBH4 req'd =
Vol of NaBH4 at D(AP,/AP,)(AP,/AP,,)
=
Vol of NaBH4 at D(Correction Factor)
(1)
All pressure changes are conveniently expressed in terms of millimeters of chart paper. Our experience with base-line drift indicates that its rate is constant over the time span of an experiment. An assumption in using the data of Table I is that the ratios are independent of the total volume of material
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1417
Table 11. Results of Hydrogenation of Octene-la Corr. Moles HzX lo5 Range Corr. P1 Graphic No. of setting trials n.p., PI n.p.9 P1 Uptake Theory* Taken factor 7 80.00 76.00 =t 0.20 1.013 76.99~ 51.18 51.38 x1 63.40 =t 0.41 1.028 65. 20d 12.83 XO. 3 9 20.00 12.85 1.364 9 20.00f 47.61 & 0.55 64.94e 1.278 X0.03 1,285 X0.003 6 2.00J 4.89 =t0.05 1.363 6.67. 0.1312 0.1285 a Per cent recovery by independent assay, 99.8. * Based on density listed in Table 111. 1.662M NaBH4 reagent. 0.4920M NaBH4 reagent. e 0.04920M NaBH4 reagent. f Tenfold dilution of octene-1 with heptane.
Table 111. Density Data on Liquid Olefins Material
Wt., 2 5 4 aliquot, go
Calcd density, g/ml
Octene-1 Oleic acid dl-Pinene 1,3-Cyclooctadiene Styrene Methyl methacrylate
18.0200 21.9100 21.3350 21.6425 22.6150 23.3750
0.7208 0.8764 0.8534 0.8657 0.9046 0.9350
a
Average of two determinations carried out at 23.0 "C.
added. The gas phase volume of the reactor was about 49 cm3 so the addition of 100 pl would result in a relative change of the total system pressure of only 2 parts per thousand. For the unsaturates listed in Table I, it is also assumed that the ratios are not significantly altered by formation of the hydrogenated form. Data and results for the hydrogenation of octene-1 are shown in Table 11. The moles of sample taken were calculated from the density data in Table 111. Precision and accuracy are excellent on the two highest range settings, and are acceptable on the lower range settings for the amounts of material taken for analysis. The correction factor becomes very appreciable (36 %) a t these settings. The usual methods for calculating the factor for free space equivalent (11-13) may not yield the same result since the method used here avoids any assumptions about the vapor pressure of the solvent and the ideality of the solutions. Since the correction factor is so large, it is preferable t o use more concentrated samples where possible. Approximately
Recovery, 99.6 99.9 99.4 102.1
0.1-mmole samples were taken for determinations on other compounds. Data and results are shown in Table IV. The results for &pinene and 1,3-cyclooctadiene are in excellent agreement with the Hanus iodine monobromide results, No clear explanation is available for the 3 % discrepancy for oleic acid. The data listed for butyndiol were obtained using acetic acid rather than sulfuric acid in the hydrogenation media. While the agreement between the hydrogenation and acetylation procedures is unsatisfactory, it should be noted that the two methods measure different functionalities. However, it is likely that the hydrogenation conditions were not optimum for quantitative reaction. Experiments with butyndiol using H 2 S 0 4yielded results 80 higher than those with acetic acid. This was no doubt due to hydrogenolysis. Further experimentation with the solvent and/or catalyst might improve results with this compound. In terms of sensitivity, the limiting factor for the method appears t o be the apparent reaction rate of the sample, since the background has a drift and analysis becomes difficult when the rate of drift and the rate of reaction become comparable. Further, while it is possible to reduce the rate of drift of background by using less catalyst, this may also reduce the rate of reaction. For example, with 500 mg of catalyst, equal volumes of octene-1 and &pinene were reduced in less than 3 minutes but with 50 mg of catalyst, the reductions were complete in 3.0 and 20 minutes, respectively. Changes in procedure such as a longer flush with tank hydrogen might saturate the catalyst and reduce the drift problem but at the expense of increased analysis time. The in situ preparation of a platinum catalyst according to Brown and Brown ( 9 ) was attempted since it is reported to yield a completely saturated catalyst. We observed a continual drift in pressure for this preparation similar to that for the palladium
Table IV. Hydrogenation Results for Several Unsaturates. Corr. Graphic factor Compound n.p.9 Pl Taken 46.88 i 0.34 1.028 15.00 dl-Pinene 80.67 i: 0.23 1.024 10.00 1,3-Cyclooctadiene 65.90 =k 0.46 1.026 Styrene 15.00 1.026 15.00 71.01 zk 0.84 Methyl methacrylate 1.034 35.00 54.99 =k 0.18 Oleic acid 1.027 20.00 82.70 f 0.46 ButyndioP a 0.4920M NaBH4 solution used in all cases. * 6 Trials for the first three compounds, 9 trials for the others. c Based on the densities listed in Table 111. Based on check analyses. Butanol solution, 4.0298 g/lO.OO d. PI*
~~-
1418
0
Corr. n.p., Pl
48.22 82.63 67.61 72.83 56.85 84.92
~
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
Moles Hz X lo5 Uptake Theoryc 9.49 9.40 16.26 16.00 13.03 13.31 14.01 14.33 10.86 11.19 18.72 16.71
Recovery, Hz 101.0 101.6 102.1 102.3 103.0 89.3
Assayd 101.2 102.1
...
... 100.1 98.8
catalyst. Also, the former did not appear to be as effective a catalyst as the latter for the reduction of dl-pinene. When 500 mg of catalyst were used, all samples were hydrogenated in 3 minutes or less. The composition of the hydrogenation medium (2.5 and 22.5 volume acid and water, respectively, in isopropyl alcohol) was experimentally established as optimum for the most rapid generation of hydrogen. The total time required for generation of the gas was 4 to 5 minutes. Thus triplicate determinations can be completed in about 20 to 25 minutes following initial set-up. The number of successive determinations is apparently limited by the capacity of the burets since the 500-mg quantities of catalyst lasted a number of days. CONCLUSIONS
The accuracy, precision, sensitivity, and speed of this method for analytical hydrogenations are very satisfactory. An advantage is the voltage readout which could prove to be of value in rate studies. For routine analysis, other less expensive pressure transducer systems are available which are adequate for the task (22). An important finding of the work is the effect of nonadditive volume additions to the system. The data in Table I show a maximum variation in pressure response for the same volumes of samples added of 15 %, relative to octene-1. For our experimental conditions, this would correspond to a variation in the calculation of the free space equivalent of hydrogen gas on the order of 10V mole. Thus, for reactions carried out at room temperature with a free space volume of about 50 cm3, the error due only to sample addition is in the range of a few per cent relative for sample sizes in the micromole range.
It appears that methods of analytical hydrogenation which require a calculation oia the ideal gas law of the correction for the free space equivalent of solution or liquid added to the system could be subject to a systematic error. The magnitude of the error will depend on experimental conditions. The sign of the error will depend on whether nonideal mixing produces a volume expansion or contraction of the liquid phase. The method presented here is free of this systematic error since the correction factor is obtained experimentally, but the lower limit of sample size that can be analyzed with an accuracy of a few per cent relative does appear to be in the micromole region. The necessity of adding sample and reagents to a closed system is thus a practical problem in analytical hydrogenation. To minimize error, the same solvent should be used throughout, if possible. T o eliminate appreciable error, sufficient sample should be taken. To decrease the sample size limit, it is necessary to eliminate compression of the gas phase upon addition of samples and reagents. Coulometric generation of hydrogen gas is a possibility in regard to the latter. Several hydrogenation studies using this technique have been reported (23-26), and we are investigating it for use with the pressuremetric null point method. RECEIVED for review December 22, 1969. Accepted July 27, 1970. Work supported by PHS Grant No. GM 12284 from the National Institutes of Health. (23) J. W. Miller and D. D. DeFord, ANAL.CHEM.,30, 295 (1958). (24) P. S. Farrington and D. T. Sawyer, J . Amer. Chem. SOC., 78, 5536 (1956). (25) E. Manegold and F. Peters, KoNoidZ., 85, 310 (1938). (26) F. Burkhardt and A. Dirscherl, Mikrochim. Acta, 1965,353.
Spectrophotometric and RadiochemicaI Determination of ZiIconium by Selective Extraction with N-Benzoyl-#-Phenylhydroxylamine Robert Villarreal, John 0. Young, and John R. Krsul Argonne National Laboratory, Idaho Dioision, Idaho Falls, Idaho 83401 A highly selective method for the separation and determination of Zr has been developed and applied to uranium-based fuels and various other materials. The procedure is based on the extraction of Zr with Nbenzoyl-N-phenylhydroxylamine (BPHA) into benzene from an ammonium carbonate solution containing several masking agents. Zirconium is back-extracted from benzene into an HBFa-HCI solution, complexed with 8-quinolino1, and the colored complex extracted into xylene and measured at 390 nm. Of 60 metallic elements tested, only hafnium gave serious interference. Milligram quantities of citrate, tartrate, phosphate, sulfate, fluoride, cyanide, peroxide, and other common anions do not interfere; EDTA does interfere. Beer’s law is obeyed from 0-70 pg of Zr in 10 ml of xylene; the optimum range i s 8-55 pg of Zr. For the determination of Zr at the 0.05% level in uranium alloy, the relative standard deviation of the method is =t1.3%. The selectivity of the method makes it applicable to many types of samples. The selective and quantitative extraction of Zr with BPHA led to the development of a rapid procedure for the radiochemical determination of radioactive Zr isotopes.
ZIRCONIUM is important in the fabrication of nuclear fuels and other reactor components because of its favorable alloying characteristics and low neutron capture cross section. Zirconium is one of the major fission products accumulated in irradiated nuclear fuel. Reactor fuels used in Experimental Breeder Reactor I1 (EBR-11) are uranium-metal alloys which contain all the high-yield fission products along with common metallic impurities (1). The routine determination of zirconium in these fuels requires a method that is highly selective, sensitive, and preferably rapid. Spectrophotometric methods for the determination of zirconium are generally based on measuring the color formed by zirconium with various lake-forming reagents, 8-quinolinol, or several azochromogens. None of these reagents can be
(1) R. Villarreal and S . Barker, ANAL.CHEM.,41,611 (1969).
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