the same Rf value as an authentic sample of chloroacetic acid contained only 25% of the total amount of radioactivity on the plate. A thin-layer chromatogram (TLC) of the 2,4-dinitrophenylhydrazone derivative of low specific activity formaldehyde-14C exhibited a single yellow spot which migrated at the correct Rf value in System B. However, this spot when scanned was almost devoid of radioactivity. The radioactivity had remained a t the origin as shown in Figure 2. Apparently the supplier had added unlabeled formaldehyde to reduce the specific activity. A re-synthesis produced an equally unsatisfactory sample. A sample of benz 0 i c - 7 - ~ ~acid C which exhibited the correct melting point and total radioactivity, demonstrated on TLC in System C that only 65% of the radioactivity migrated at the correct Rf value while the rest of the radioactivity migrated as a nonacidic area at a higher Rf value. The radioscan is shown in Figure 3. Several commercial samples of lysineJ4C were examined (15) and found to contain as much as 18.5% of radiochemical impurities. Braddock (16) was unable to repeat published work using sodium thiopental-35s. Using both 14C- and 35S-labeled drug, he confirmed his results and concluded that the earlier work was done with sodium
(15) W. S. Chow, L . (1970). (16) L . I . Braddock
Kesner, and H . Ghadimi, Anal. Eiochem.. 37, 276
and H L. Price, "Organic Scintillatlon and Liquid Scintillation Counting," D . L . Horrocks and C . T. Peng, E d . . Academic Press, New York, N . Y . . 1971, p 599.
t h i o ~ e n t a l - ~containing ~s a radiochemical impurity. The chemical and radiochemical impurities found in a number of hydrocarbons labeled with I4C have been reported (17 ) . Radioisotopic Purity. Routinely, the spectra of betaemitting radioisotopes are examined to establish their radioisotopic purity. This is an unusual test to run, but is considered necessary after receiving a mislabeled commerical sample of cyclopentanol-l-14C. This sample gave an unusually large number of counts in the window used to count tritium. The high count rate could not be explained since only 14C should have been present. This problem remained unresolved until examination of the beta-ray spectrum of this sample demonstrated the presence of both tritium and 14C,as illustrated in Figures 4 and 5. The emphasis that individual commercial suppliers have placed on quality control has varied over the years. It would seem that these suppliers are stressing better quality control by the appearance of literature to this effect by all of the major suppliers over the last several years. The excellence of any quality control program must be a continuous effort, and cannot be allowed to deteriorate if customer confidence is to be maintained. Received for review February 23, 1973. Resubmitted ,January 31, 1974. Accepted January 31, 1974. This manuscript was condensed from a paper presented before the Medicinal Chemistry Section, Metrochem '71, San Juan, Puerto Rico, April 30, 1971. (17) M A lsotop
Muhs, E L Basttn and B E Gordon l n f J Appi Radiat 16, 537 (1965)
Studies on Several Uranyl Organophosphorus Compounds in a Poly(Viny1 Chloride)(PVC)Matrix as Ion Sensors for Uranium D. L. Manning, J. R. Stokely, and D. W. Magouyrk Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
The field of ion selective electrodes is probably one of the most active and flourishing branches of electrochemistry. The preponderance of material covered in the more recent reviews (1-5) certainly confirms this interest. Notably absent, however, are ion electrodes which are responsive to the uranyl ion. To our knowledge, the work of Dietrich (6), who utilized poly(viny1 chloride) (PVC) membranes of uranium in 2-ethylhexyl phosphate, is among the first in this area. Uranyl ion sensors should find wide applications in many areas. The electrode measurements are rapid and nondestructive. Such an electrode senses ionic activity rather than total concentration; therefore, it would be useful for analytical determinations of free ions by emf measurements as well as total concentration through such methods as titrations, standard additions, etc. Sensors which are responsive to uranyl ion activity should also prove useful for investigating equilibra and kinetics of various reactions. In view of the increasing demands for uranium coupled with expanding uranium (1) G . J MoodyandJ D . R.Thomas, Taianfa, 19,623 (1971). ( 2 ) R . P. Buck, Anal. Chem., 44, 270R (1972). (3) J Koryta. A m i . Chim. Acta, 61, 329 (1972). ( 4 ) E. Pungor and K . Toth, Analyst, (London). 95, 625 (1970). ( 5 ) E . Pungor and K. Toth, Pure Appl. Chem., 34. 105 (1973). (6) W . C Dietrich, Tech. Prog. Repf., No. Y1174D, Y-12 opment Division, Aug.-Oct. 1971
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A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 8 , J U L Y 1974
purification programs, it is evident that uranyl ion sensitive electrodes could play a useful role as uranium monitors under favorable analytical conditions. In this paper, we present the results of a survey of several uranyl organophosphorus complexes incorporated in a poly(viny1 chloride) matrix as possible uranyl ion sensors. We chose the more favorable membranes for additional studies such as Nernstian response, pH effects. useful lifetimes, and evaluation of selectivity coefficients for some interferences.
EXPERIMENTAL All chemicals were reagent grade. The uranyl organophosphorus complexes were prepared by dissolving 1.00 gram of UOz(N03)2* 6Hz0 in 2 ml of t h e organophosphorus acid. T h e aqueous phase was then removed from the yellow viscous exchanger by centrifuging and then the exchanger was dried with two 100-mg portions of anhydrous NaZS04. The exchanger was next separated by again centrifuging and stored in a dry stoppered tube. Preparation of the uranyl ion exchanger-PVC membrane was accomplished by weighing into a clean, dry, 50-ml beaker, 45 mg of the uranyl organophosphorus complex and 450 mg of the organophosphorus solvent in the optimum weight ratio of 1 : l O (7l. TO this solution was then added 6 ml of a solution of poly(viny1 chloride) which was prepared by dissolving 2.75 grams of PVC in 60 ml of tetrahydrofuran. T h e beaker was covered with two to three sheets of filter paper held in place by rubber bands and set aside
Table I. Survey of Uranyl Organophosphorus Complexes as Ion Sensors for U r a n i u m Membrane NO.
Slope Exchanger, uranyl complex of:
1 2 3
di(2-ethylhexyl)phosphoric acid
4
di (2-ethyl-4-methylpentyl)phosphoric acid mono-n-butyl phosphoric acid
5
6 7 8 9 10 11
di(2-ethylhexyl)phosphoric acid
12 13
14 15 16 17 18 19
20 21
mono-n-butyl phosphoric acid di(2-ethyl-4-methylpentyl) phosphoric acid di- (2-methylpentyl)phosphoric acid di(tridecy1)phosphoric acid di(n-butyl)phosphoric acid
Diluent
=t2 mV
diamyl amyl phosphonate di(2-ethylhexyl)ethyl phosphonate tri(2-ethylbutyl) phosphate diamyl amyl phosphonate diamyl amyl phosphonate di(2-ethylhexy1)ethyl phosphonate tributyl phosphate di-sec-butyl phenyl phosphonate carbon tetrachloride di(2-ethylhexyl)2-ethylhexyl phosphonate di(2-octyl)phenyl phosphonate tri-(2-ethylhexyl) phosphine oxide di-n-decyl phthalate di(2-ethylhexyl)-2-ethylhexyl phosphonate di(2-ethylhexyl)ethyl phosphonate diamyl amyl phosphonate di(2-ethylhexyl)ethyl phosphonate diamyl amyl phosphonate di(2-ethylhexyl)ethyl phosphonate diamyl amyl phosphonate tributyl phosphate
a a. Favorable response; b. slow response; c. high resistance; d. near Nemstian slope; e. far below Nernstian slope; f. erratic response; g. h. not responsive at 10-6M;i. not favorable; j. too erratic for slope determination.
to allow the tetrahydrofuran to slowly evaporate. Twenty-four to 48 hours were generally required for the membranes to set up. Membrane electrodes were prepared by cutting out a 8-mm diameter PVC disk with a cork borer and affixing it to a 1.5-inch length of Tygon tube as described by Moody et al. (7, 8). The elec-
trodes were generally soaked in 10-2M UO2C12 overnight prior to use and when not in use were stored in this solution. Changes in the membrane potential were made by making electrical contact to the inner solution contained in the Tygon-PVC tube compartment with a Coleman 3-511 reference electrode (-7-mm 0.d. tip) to which the tube was attached. The sample solution was then contacted with the membrane electrode and a second reference electrode, in this case a Corning fibre junction calomel reference electrode. A Keithley electrometer model 610 C and a Beckman Research pH Meter were used for all potential and resistance measurements which were made at room temperature with slow stirring. Electrode immersion depth was -1 cm. A stock solution of 0.10M uranyl chloride solution was prepared by dissolving 5.02 grams of UOz(N0&.6HzO in 50 ml of concentrated HC1, then evaporating to dryness. The salt was dissolved in 100 ml of HC1, pH 3; and dilutions to 10-sM were then made. pH adjustments were made using 1N HC1 and 1N NaOH.
-
RESULTS AND DISCUSSION
In our initial experiments, uranyl ion sensitive electrodes were prepared utilizing the uranyl complex of di(2ethylhexy1)phosphoric acid (U02-DPEHPA) in the form of poly(viny1 chloride) membranes (6). In general, the electrodes exhibited near Nernstian (29 mV/decade) behavior; the responses of some were better than others. The most ,serious problem we encountered was maintaining reproducible and reliable potential measurements for a given electrode as a function of time. The resistance of the membrane increased from a nominal lo7 ohms to 109-1010 ohms in 1 to 3 days (sometimes less). This was accompanied by erratic and nonreproducible emf values. The PVC membrane changed from a clear yellowish hue to a white opaque appearance. It was thought that this may be caused by leaching of the uranium from the membrane material. To check this further, small portions of the U02DZEHPA compound were exposed to aqueous solutions consisting of distilled water, pH 3.5, 3.0, 2.0, 1.0, and 1M Griffiths, G . J. Moody, and J. D. R . Thomas, Analyst, (London),97, 420 (1972). ( 8 ) G . J. Moody, R . 6 . Oke. and J. D . R . Thomas, Analyst, (London).95, 910 (1970) (7) G . H
Comments"
a,d a,d a,d a,d a,d a,d 1
c,f,i b,e,i i i i b,e,h,i b,e,h,i i i i
i 1
-
1
three weeks lifetime
HC1 solutions. In all cases, a white coagulated solid material formed a t the aqueous, U02-DZEHPA interface within a few minutes and increased in quantity in the U02DPEHPA phase with time. Baes et al. (9) indicated that polymer chains are formed a t fairly high concentrations of uranium in D2EHPA which may, in part, be responsible for the formation of the solid material we observed and subsequent erratic behavior of the electrodes. A similar observation was also made by Tomazic ( I O ) who reported the formation of gelatinous phase when uranium in 0.2M "03 was contacted with 0.1M DBEHPA in toluene. At this point, we selected an alternate approach. With a proper diluent, it was thought possible to minimize the formation of the solid material through matrix isolation and yet provide sufficient UO22+ exchange sites for satisfactory performance. In the Orion calcium sensitive electode ( I l ) , the diluent is dioctylphenylphosphonate. We tested several uranyl mono and dialkyl phosphate complexes in combination with various organic diluents and incorporated in a PVC matrix as potential uranyl ion sensors. The results are tabulated in Table I. Of the membranes tested, only the first six exhibited favorable response characteristics and approached a near Nernstian response. However, in Table I again, it is seen that most of the membranes were in the range of lo7 ohms which is desirable because, as the resistance increases above -lo7 ohms, the emf values become erratic and nonreproducible. The success of the electrode also depends on how the exchanger is formulated. If the uranium is loaded into a mixture of the organophosphoric acid and diluent, the resulting membrane is not responsive to UOz2+. If, on the other hand, the uranium is loaded into the organophosphoric acid first, then the uranyl complex mixed with diluent, the membrane becomes responsive. It has been reported (12) that certain organophosphorus compounds such as the trialkyl phosphates or the dialkyl alkylphos(9) C. F. Baes, R . A . Zingaro, and C . F. Coleman, J . Phy. Chem.,62, 129 (1958). (10) B. Tomazic, Anal. Chim. Acta, 49, 57 (1970). (11) J. W. Ross, Science, N . Y . , 156, 1378 (1967). (12) J. E. Grindler, "The Radio-Chemistry of Uranium." National Academy of Sciences, National Research Council, Nuclear Science Series, NAS-NS 3050, 1962, p 160. A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
1117
"!
I
B
I
-60L
I
I
$ 0 -5
I
10-2
(0.4
I
10.'
& URANYL i ACTIVITY
Figure 1. slope - 2 7 DH
Nernstian plot for membrane No. 1 at pH -3, mV decade. Fill solution: 10-3M U02C12, same
phonates when combined with dialkylphosphoric acids provided a synergistic enhancement of the uranium partition coefficient. The reason for the enhancement is explained on the basis of the addition of an extraneous reagent (probably acid or water molecules) to the uranyl dialkylphosphate through hydrogen bonding. Apparently, this added reagent within the exchanger renders it nonselective for uranium, as exchange sites for both uranium and other ions are then provided. Our procedure then is to form the uranyl exchanger first, then combine with the diluent prior to casting the PVC membranes. For an ion selective electrode to be useful, it must demonstrate Nernstian behavior (1).That is:
E
= constant
f 2.303 RT log la,
zf
+ K,,(a,f
(1)
where i = specific ion, of activity a,, and charge z, in the presence of an interfering ion, j , of activity a, and charge y, and K,, = selectivity coefficient, the second term on the right side being positive for cations and negative for anions. When z is a constant charge, a convenient method of evaluation is to plot concentration or activity us. emf. If z = 2, this should then be 29 mV per decade change of activity. A typical plot of emf us. activity (13) of uranyl ion is shown in Figure 1. The relation is linear from about 10-1 to 10- 4M U02C12 with near Nernstian slope. Some tailing off is observed a t the 10-5M concentration which is typical of the favorable membranes examined. The dynamic response of membrane No. 1 to changes in the concentration of UOzC12 is shown in Figure 2. The concentration of uranium was increased by adding aliquots of a standard solution of uranyl chloride to 50.0 ml of stirred test solution with a micropipet. Emf us. time was recorded with an X-Y recorder connected to the recorder output of a Keithley electrometer. Steady state potentials were generally realized within a few minutes; however, it is also worth noting that approximately 95% response times were achieved much quicker. Response times were more rapid when proceeding from dilute to concentrated solutions than conversely. It is intended, however, that the response typified in Figure 2 serve only as a guide because some variations were observed among the different membranes and occasionally the same electrode would be temperamental with regard to response time. Useful lifetimes for the membranes are encouraging. The calibration slope revealed no significant change with time over the period studied (4 to 8 weeks). The resistance remained a t a nominal lo7 ohms. The "constant" (13) "Analytical Chemistry of Uranium," Academy of Sciences of the U.S.S.R., translated by N. Kanar, Israel Program for Scientific Translations, Jerusalem, 1963, Daniel Davey & Co., New York, N.Y.. p 25.
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
0
1
2 3 4 T l M E , rnin
5
Dynamic response curve for membrane No. 1 in pure uranyl chloride solutions. A . to 10-4M B. Figure 2.
to 1 0 - 3 ~
term, however, became somewhat more negative, (for example, membrane No. 1, -10 mV shift in -1 month). Selectivity coefficients were determined using a reduced form of the Eisenman equation (14)
log K,, =
( E , - E,)nF 2.303 RT
- log
a
1111
:]-I)
(2)
where E l = emf of cell assembly with activity a of ion to be determined with charge n; E2 = emf of cell assembly with activity a of interfering ion with charge 2. The quantity K,, is defined in such a way as to be less than unity relative to the primary uranyl ion set a t unity. The selectivity coefficients are tabulated in Table 11. These coefficients seem to indicate that these membranes experience no serious interference from the cations tested. The higher numerical value a t a lower interference ion activity probably relates to a slower approach to equilibrium a t the lower activity levels (15). As previously noted (15) however, selectivity coefficients are not necessarily high precision quantities; they primarily serve as an index to indicate the approximate conditions for achieving negligible level of interference. Membranes exposed to copper were slow in responding to uranyl ion, but normal response could usually be restored by soaking in 10-2M U02C12 overnight. Iron(II1) poisoned the electrodes, and this interferant should be absent. Membranes exposed to iron(In) were probably converted to an iron form which could not be regenerated. The effect of sodium chloride on the response of the six membranes to uranyl ion was similar to that observed by Shatkay (16) on various calcium electrodes. The effect appears to be primarily that of lowering the activity or the uranyl ion through an increase in ionic strength until a point is reached (high NaCl concn) where the membrane is no longer specific. Nitrate and perchlorate anions cause an abnormal lowering of the cell potential when the concentration exceeds -10-3M. We believe this to be due, in part, to the greater solubility of nitrate and perchlorate species in the membrane phase which is further enhanced by the synergistic action of the uranium (12). A similar effect from C104- and I- was observed by Rechnitz and Lin (28) for a calcium electrde. Sulfate and phosphate anions decrease the activity of the uranium through complex fofmation (19) the effect of which is also reflected in a lowered cell potential.
(In,
(14) R. J. Levins,Anal. Chem., 44, 1544 (1972). (15) R. P. Buck in "Physical Methods of Chemistry," A. Weissberger and B. W. Rossiter, Ed, Part 11-A, Electrochemical Methods, Wiley-lnterscience, New York. N . Y . , (1971) p 84 (16) A. Shatkay, Anal. Chem., 39, 1056 (1967). (17) Reference 12, pp 136, 156. (18) G. A. Rechnitz and 2. F. Lin, Anal. Chem., 40, 696 (1968). (19) Reference 13, p 14.
Table 11. Selectivity Coefficients Membrane number Ion
Concn ( M )
Ca2+
10 - 2
Cdl+
10 - 4 10 - 2 10 - 4
Mgz+
10 - 2 10 - 4
ZnZ+
10 - 2
cut+
10 -2 10 - 4 10 - 2 10 - 4 10 - 4 10 - 2 10 - 4
10-4 ~
1+ 3
Fe3 + NiZ +
1
0.0027 0.24 0.0027 0.0044 0.0025 0.16 0.0064 0.39 0,0093 0.38 0,0029 0.18 0.42 0.006 0.40
0.0014 0.083 0.0013 0.065 0.0004 0.044 0.0007 0.032 0.0005 0.026 0.0038 0.057 0.48 0.003 0.032
0.0009 0.017 0.0034 0.022 0.0010 0.0066 0.0017 0.060 0,0019 0,066 0.0055 0.076 0.15 0.0017 0.061
A typical graph illustrating the effect of pH is shown in Figure 3. The optimum pH for reproducible emf measurements appears to be about 3. Above approximately pH 4, hydrolysis of the uranyl ion occurs and, below pH 2, the graph resembles that obtained by Griffiths et al. (7) in their work on the Ca2+ electrode; that is, the hydrogen ion contributes to the charge transport process across the membrane, and the slope becomes too steep to obtain reproducible measurements. In conclusion, electrodes prepared from the six membranes listed in Table I exhibit near Nernstian response to uranyl ion activities from -lo-’ to W 4 M . No major differences were observed among the more favorable membranes; however, our preference would be the first three. Electrodes incorporating these membranes exhibited the best all around stability and response characteristics as a function of time. The preferred media is chloride although small amounts ( 510- 3 M ) of NOa- and c104- can be tolerated. By combining the uranyl exchanger with a suitable solvent prior t o membrane casting, the useful lifetimes of the electrodes can be extended from days to weeks.
ACKNOWLEDGMENT The authors acknowledge helpful discussions with Gleb Mamantov of the University of Tennessee. The authors
0.0039 0.21 0,0031 0.14 0.0020 0.11 0.0014 0.041 0.0019 0.65 0,0026 0.060 0.67 0.007 0.033
+20
-
5
6
0.0023 0,096 0,0019 0.065 0.0014 0.065 0.0037 0.065 0.018 0.21 0.0014 0.14 0.92 0,001 0.042
0,0018 0.075 0.0015 0.048 0.0009 0,005 0.0025 0.045 0,0036 0.043 0.0034 0.050 0.38 0.003 0.061
4
3
2
t\
M E M B R A N E NO.
3
I
1
MEMBRAVE NO. 3
u 1
2
3
4
DH
Figure 3. Effect of pH on potential response of membrane No. 1 and 3.UOzCIz 10-3M.Fill solution, same at pH - 3
also thank Boyd Weaver of the Chemical Technology Division, Oak Ridge National Laboratory, for supplying many of the organophosphorus compounds. Received for review December 31, 1973. Accepted March 11, 1974. Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. D.W.M. was an ORAU Summer Employee from Arkansas College, 1973.
Determination of Diphenylhydantoin by Phosphorescence Spectrometry Lee D. Morrison and C. M. O’Donnell’ Department
of Chemistry. Colorado State University. Fort Collins. Colo. 8052 7
Wallace et al. ( I ) introduced an assay for 5,5-diphenylhydantoin (DPH, Dilantin, Parke-Davis) which was later modified by Wallace and others (2-4). The latest modifiAuthor to whom reprint requests should be sent. (1) (2) (3) (4)
J. Wallace, J. Biggs. and E.V . Dahl. Anal. Chem. 37, 410 (1965). J. Wallace, Ana/ Chem.. 40, 978 (1968) P L. Morselli, Ciin Chim. A c t a . 28, 37 (1970). W A . Dill, L. Chucot, T. Chang, and A . J. Glazko. Clin Chem. 17, 1200 (1971)
cation of Dill et al. makes the procedure suitable for use in the clinical laboratory. Dill’s procedure involves an extraction of DPH from plasma into 1,2-dichloroethane, a subsequent extraction into alkaline medium followed by oxidation of the DPH with basic KMn04. The oxidation product (benzophenone) is extracted into a ,hydrocarbon solvent (i.e., isooctane) and analyzed by ultraviolet absorption. The blanks obtained from control plasma showed little absorption, ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
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