The color intensity was independent of the perchloric acid concentration in the range 1.0 to 4.OM. Contrary to reports, an increase in sensitivity using 2M hydrochloric acid ( I ! ) was not observed. When higher concentrations of hydrochloric acid are used, the apparent increase in sensitivity is due to the formation of chlorine from the hydrochloric acid. N o significant difference in the spectra of the titanium-peroxy complexes in 2M hydrochloric, perchloric, or sulfuric acid media was observed in the visible region. The advantage of high acidity and lack of interferences by common metals in the peroxide method for titanium are attractive attributes. However, other methods that are much more sensitive are recommended for amounts of titanium less than lOOpg(11-14). In Table I are listed the results for the analysis of a titanium standard, a simulated alloy, and a series of four U-Pu-Ti metallurgical samples. Alloys A through C were subjected to arc melting followed by induction melting in a MgO crucible. All had separated skulls of approximately 5 of the charged weight. Accuracy data are given only for alloy D , which was prepared by arc melting only. The ratio CIA is the reciprocal of the slope obtained from a Beer's law plot. N o difference was observed between the recovery data for the direct analysis of a titanium standard as compared to one carried through the ion exchange column. In the presence of plutonium and (12) J. V. Griel and R. J. Robinson, ANAL.CHEM., 23, 1871 (1951). (13) B. J. Kenna and F. J. Conrad, [hid.,35, 1255 (1963). (14) J. P. Young and J. C. White, Ibid., 31, 393 (1959).
uranium the recovery shows a bias of - 0 . 4 x in this ratio as compared to titanium-only samples. This is probably due to uranium or plutonium interferences even though the separation procedure is 9 9 . 9 9 z effective. An increase in the elution volume does not improve recovery. The relative standard deviation was 0 . 3 z over the range studied, which compares favorably with the range for the high titanium alloys. The principal interferences for the present application arise from alloy constituents or from those impurities that result from the metallurgical process. Most of the interferences in the peroxide method have been adequately described elsewhere (3, 11, 15). Uranium interference was tested over the range 0.3 to 7.5 mg and was found to have an absorbance of 0.010 at the maximum concentration. Microgram amounts of uranium broke through the column as indicated by spot tests. Plutonium forms a pink peroxy complex which has negligible absorption if the concentration is less than 8 X lOWM. The absorbances for 0.7,1.5, and 5 mg of plutonium under the conditions given for titanium were 0.007, 0.009, and 0.031, respectively. The plutonium breakthrough was less than 5 pg. Americium was tested up t o 15 pg without interference. Fluoride in the dissolver solution can be tolerated up to 0.05M without interference. RECEIVED for review January 16, 1967. , Accepted March 15, 1967. Based on work performed under the auspices of the U. S. Atomic Energy Commission. (15) R. A. Pappucci, ANAL.CHEM., 27, 1175 (1955).
Reference Electrode for Anhydrous Dimethylformamide Leland W. Marple Department of Chemistry, Iowa State University, Ames, Iowa
A NUMBER OF WORKERS have recently examined the reduction of organic compounds in N,N-dimethylformamide solvent ( I ) . I n nearly all cases, the reference electrode employed was a mercury pool electrode which also served as the second working electrode. Lambert ( 2 ) has found that the potential of the mercury pool is relatively constant (even with passage of several microamperes current) under the conditions employed in polarographic measurements, but in general, the mercury pool is a n unpoised electrode. The silver-silver perchlorate electrode has been used as a reference electrode for controlled potential reductions in D M F ( 3 ) ,but no indication of the stability of the electrode system was given. Our experience with the silver-silver nitrate electrode in D M F showed that a chemical change occurred over a period of time, with a concurrent change in potential. It appeared that a stable, well-poised reference electrode for D M F solvent was not available. [The use of a N a ( ~ ~ ) / N a C 1 0 ~reference (.) electrode has been reported by McMasters et al. (4). However, the authors give no evidence that the electrode system (1) D. J. Pietrzyk, ANAL.CHEM.,38, 278R (1966). (2) F. L. Lambert, Occidental College, Los Angeles 41, Calif.,
private communication, 1967. (3) P. H. Rieger, I. Bernal, W. H. Reinmuth, and G . K. Frankel, J . Am. Chem. Soc., 85, 683 (1963). (4) D. L. McMasters, R. B. Dunlap, J. R. Kuempel, L. W. Kreider, and T.R. Shearer, ANAL.CHEM.,39, 103 (1967). 844
0
ANALYTICAL CHEMISTRY
50010
is poised or that separate electrodes (of same construction) have the same potential. In addition, it would appear that oxygen would have t o be rigorously excluded from the cell compartment.] The development of such an electrode appeared worthwhile because it would lead to standardization of potential measurements in D M F , as well as obviate the use of three electrode systems for some types of electrochemical measurements. A number of likely cell systems were investigated. However, the best operating characteristics were afforded by the following cell types: A. Cd(,.Hg)]CdCla(,),CdCl,. HzO(,), NaCl(,), DMF//CdCly(,),CdC1,. H20(,),NaCIm, DMF//DMF, NaCI(.), CdC12(,), CdCh. H?O(,)/Cd(s,Hg) B. Cd(.,H,)/CdCl,(,), CdClz. HzO(s), NaCh,), DMF//Nac104(.), DMF//DMF, NaCh.), CdCI?(,),CdClz . H~O(d/Cd(a.Hg) C. Cd(,.Eg)/CdC12c,),NaCIw , NaCIO4(.),DMF//NaClO,(.), DMF//DMF, NaC104(,),NaCI(,), CdClzc,)/Cd(..Hg) While long term measurements have not been carried out, it is felt that the data presently at hand bear reporting at this time. EXPERIMENTAL
All reagents used in this work were reagent grade. Anhydrous CdClz was prepared by heating CdC1z.21/zHz0 to 110" C. The cadmium amalgam was prepared by heating
I 000
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0.1
2 g
o
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s 0
5
IO
15
20
25
30 0.1
TOTAL. OVERVOLTAGE, m V
Figure 1. Current-overvoltage characteristics of type A, B, and C electrochemical cells based on the Cd(s.Hg)/ CdC12, CdC12. HzO, NaCI half cell stick cadmium (Mallin1:krodt) with triply distilled mercury for several minutes, cooling to room temperature, and adding sufficient pure mercury to give a slurry. The amalgam was warmed slightly, filtered through a pinhole in filter paper into the cell compartmlmt, and covered with dimethylformamide. The dimethylformamide (Baker) was presaturated with reagents before addition to the cell compartment. Several cells of each type were constructed using the H cell design and ultrafine sintered glass frits to separate the cell compartments. The potential was measured to the nearest 0.1 mV using a breadboard potentiometer of k e d s & Northrup components. Cell resistance was measured with an Industrial Instruments conductivity bridge R C 16B2. Current flow through a cell was determined by measurement of the potential drop across a standard resistor. The overvoltage was determined by subtraction of the IR drop from the applied potential. RESULTS AND DISCUSSION
The different types of cells were characterized by monitoring the potential difference as a function of time, and by determination of the current--overpotential curves. The potential of type A cells one hour after fabrication was generally less than 0.0000 volt. A potential difference of 0.0040 volt developed in one cell after the second day, but slowly decreased to 0.0006 volt over a period of two weeks. A second cell showed the same potential variation, although the maximum potential was somewhat smaller (0.0010 volt). Part of the fluctuation may have been due to temperature variation as the cells were not tbermostated. Electrical resistance of the cells was on the order, of 3000-4000 ohms. In a very carefully controlled experiment, the variation of cell potential with water content was examined. Three electrodes were constructed with CdC12:CdC12.21/zH 2 0 ratios of 20:l (I), 20:2 (II), and 20:6 (111). Assuming maximum stated water content of the solvent, all three cells should have as solid phases CdCI? 'CdCI,. H 2 0 . The cells were arranged in the order IjII/III, anti thermostated a t 27" C. These cells reached equilibrium polential after two days, with cell I 0.0010 V to cell 11, Cell 111 4-0.0016 V to cell 11. These data definitely show that electrodes with reproducible potentials
+
+
0.2
30
PO
I
I
10
0
I
I
I
-10
-PO
-30
ELECTRODE POTENTIAL, mV
Figure 2. Single electrode polarization curve, type A electrochemical cell can be made, as long as a definite hydrate equilibrium is established. Figure 1, A-1 shows the current-overvoltage curve of one type A cell with total electrode area of approximately 22.7 cm2. The stability of the electrode was tested by passage of current for a period of time, and noting the change in overvoltage. For the cell A-1, the overvoltage changed less than 1 mV after 20 minutes a t a current level of 0.15 ma. Another cell, A-2, with 5.66 cm2 total area showed a change of 2.6 mV with passage of 0.19 ma for 25 minutes. Upon cessation of current, attempts were made to follow the potential decay, but it was always so fast that it could not be measured manually. Further measurements employing a three-electrode system, all identical in construction, were made to determine the single electrode polarization curve, Figure 2. As was anticipated, both cathodic and anodic branches were about the same up to 0.2 ma. The current-overvoltage curve for one type B cell is shown in Figure 1, B-1. Since the electrode area was the same as in cell A-2, the similarity of polarization curves was expected. The electrical resistance changed only slightly upon substitution of NaCIO4 in the salt bridge. Measurement of the current-overvoltage curves and electrical resistance showed no substantial change unless the amalgam solidified. The potential difference of cell B-1 was 0.0016 volt shortly after fabrication, but this decreased to 0.0000 volt after two weeks' time. No measurable difference in overvoltage for cell B-1 was found after passage of 0.1 7 ma for 40 minutes. The current-overvoltage curve for a type C cell with total area of 22.7 cm2is also shown in Figure 1 , C-1. This electrode system is considerably more difficult to work with, as the
sodium perchlorate changes solubility greatly with temperature, and the liquid in the cell frequently turns to a thick slurry. It is likely that this type of cell shows larger overvoltage because of dehydration of the cadmium chloride hydrate by the large excess of sodium perchlorate. Type A cells without hydrated cadmium chloride showed considerably larger overvoltage than cells containing the hydrated salt. Sufficient vapor pressure data have not been found t o determine whether NaC104will dehydrate CdClr. H 2 0 . The best reference electrodes appear to be those with just NaCI, CdCI2, and CdC12.H20 in contact with the cadmium amalgam. If chloride ion is incompatible with the system under investigation, a saturated sodium perchlorate salt
bridge can be interposed without difficulty. The Cd/CdCls electrode would seem to be directly applicable to polarographic use. Consideration of the low current level used in polarographic measurements, and the fact that the overvoltage is so low a t such a level, leads one to conclude that this electrode system would function as a nonpolarizable electrode in D M F solvent. The fact that no extra purification of D M F is generally necessary for the electrode to function properly speaks for the practicality of the system. RECEIVED for review December 16, 1966. Accepted March 15, 1967. Research supported by National Science Foundation Grant G P 5044.
Determination of Strontium-90 in Human Bones by Tributyl Phosphate Edmond J. Baratta and Esther S. Ferri Northeastern Radiological Health Laboratory, National Center for Radiological Health, Public Health Service, U . S . Department of Health, Education, and Welfare, Winchester, Mass.
THEPROCEDURE "Simplified Determination of Strontium-90" ( I ) has been adapted for the determination of strontium-90 in human bone. The yttrium-90, which is in equilibrium with the strontium-90 at time of analysis, is directly extracted from bone ash in approximately 14N H N 0 3 with 100% n-tributyl phosphate (TBP), and is stripped from the TBP into 3N "03. The Y is then precipitated as the oxalate for counting. Mercer (2) has shown that adequate extraction is obtained even in the presence of phosphate. Stripping with 3 N H N 0 3 assures that any naturally occurring radioisotopes of Th and U will be separated from the Y (3). Velten and Goldin ( I ) have stated that Zr-Nb and Pm would constitute a serious interference in the TBP extract. However, gamma analysis of human bones in this laboratory have shown no detectable amounts of activity due to Zr-Nb. Precipitation of the Y oxalate for counting further discriminates against carrying through of Zr-Nb. Pm activity has been reported to be about 1% of the strontium activity in steer bone (4). One would not expect this nuclide to be more concentrated in human bone. Because strontium-90 activity in human bone is one fifth or less than that of steer bone, any Pm contamination would be negligible. The method, which is being routinely applied to the determination of Human Bone Network samples of the USPHS (3, permits the determination of strontium-89, if this nuclide is present, by conventional purification treatment of the separated strontium fraction. It also permits, if necessary, re-
(1) R. J . Velten and A. S. Goldin, ANAL.CHEM., 33, 128 (1961).
(2) E. R. Mercer, J . D. Burton, K . B. Gunn, and A. Black, Healfh Phys., 11, 37 (1965). ( 3 ) V. R. Hunt, E. P. Radford, and A. J. Segall, Intern. J . Radiation Biol., l , 217-87 (1963). (4) M. Ekenbud and H. G . Petrow, U. S . At. Energy Comm. Repi. .4T-(30-1), 2896 (1964). (5) Rad. Health Data arid Reports, U. S. Depdrttnent of Health, Education, and Welfare, July 1965, April 1966, June 1966, S2ptember 1966, October 1966, and December 1966.
846
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ANALYTICAL CHEMISTRY
milking of Y from the strontium fraction after suitable ingrowth. The procedure is rapid requiring 2.5 man-hours for analysis of four samples. EXPERIMENTAL
Apparatus. A low-level background beta counter with a background of less than 0.50 count per minute was used. Procedure. A portion of bone ash (5-10 grams) is dissolved in hot 12N H N 0 3 and diluted t o 100 ml with 12N HNOI. A 50-1111 aliquot of this is transferred t o a 250-ml centrifuge bottle and 90 mg of S r + 2 and 20 mg of Y+3carriers are added. The strontium is precipitated by the addition of fuming (90%) "03, cooled, centrifuged, and the supernate transferred to a 250-1111 separatory funnel. The time is recorded at this point as the beginning of the decay of the separated yttrium-90. Then the solution is made 14N in H N 0 3 . The solution is extracted with 50 ml of 100% TBP, freshly for 3-4 minutes, and the equilibrated with 16N "03 aqueous phase transferred to a second separatory funnel. The aqueous phase is again extracted with TBP, the organic phases are combined and washed 3 times with 40 ml of 14N H N 0 3 , discarding the washings each time. The yttrium is back-extracted with one 50-ml HzO wash and four 25-1111 washes of 3N H N 0 3 , collecting the washes in a 250-ml centrifuge bottle. To the centrifuge bottle is added 10 ml of 2 N H2C204,and the pH is adjusted to 1.0-1.5 with concentrated N H 4 0 H . The precipitate is collected by centrifuging, transferred to a 40-ml centrifuge tube, and washed with warm H20. The precipitate is taken u p in warm HzO, transferred to a pre-weighed filter paper, washed 3 times with a water miscible solvent (acetone, methyl alcohol, ethyl alcohol), and dried in an oven a t 125" C for 20 minutes. The sample is cooled and reweighed to determine the chemical yield. The sample is mounted on a nylon support, covered with Mylar, and the beta activity of yttrium-90 is measured in a low-background beta counter. The strontium-90 activity is computed using the following equation:
