With increase in HF concentration, the Eliz becomes slightly more positive (-10 mv.). The direction of the change-opposite to that expected if complexation occurs-and its small magnitude may possibly indicate that complexation between P b +2 and F- is very limited or does not occur. Bottari and Ciavatta (I) obtained results that also indicate only limited complexation between Pb+* and F-. Change in P b f 2 concentration apparently does not affect the Elizvalues. Over the ranges of P b + 2 and HF concentrations studied, the relation of i d , and also of (di/dt)maz, to Pb+2 concentration is linear. The id values were determined from the difference between the height of the polarogram for the supporting medium and that of the polarogram for the supporting medium which contained Pb+*in known concentration; measurements were made a t a potential within the range over which
the limiting-current region and the polarogram of the supporting medium are parallel. With increase in HF concentration, i d decreases somewhat. ACKNOWLEDGMENT
The infrared absorption spectrophotometry was done by Lucy E. Scroggie and M. M. Murray. F. L. Layton made the independent polarographic analyses by which the Pb+2concentrations of the standard solutions of Pb+2 in aqueous HF were established. During the work, many helpful discussions were held with P. F. Thomason and with members of the Analytical Instrumentation Group. This assistance is gratefully acknowledged. LITERATURE CITED
U.S. At. Energy Comm. U-1711 (June 11, 1951),
(3) Fisher, D. J., Belew, W. L., Kelley, M. T., ‘Recent Developments in D. C. Polarography,’ in “Polarography 1964,” G. J. Hills, ed., Macmillan, London, in press. (4) Headridge, J. B., Hamza, A. G., Hubbard. D. P.. Tavlor. M. S.. ‘Polarographic ’Investigations ’in Acidic Fluoride Solutions,’ in “Polarography 1964,” G. J. Hills, ed., Macmillan, London, in nww r-
( 5 ) Headridge, J. B., Hubbard, D. P., Analyst 90, 173 (1965). (6) Kelley, 31. T., Jones, H. C., Fisher, D. J.. ANAL.CHEM.31. 1475 11959).
( 7 ) hleaaric, S. S., Hum;, D. N.,Znorg. Chem. 2, 788 (1963). ( 8 ) Raaen, H. P., ANAL.CHEM.34, 1714 (1962). (9) Zbid., 36, 2420 (1964). (10) Zbid., 37, 677 (1965). (11) West, P. W., Dean, J., Breda, E. J., Colleclzon Czech. Chem. Commun. 13, 1 (1948).
RECEIVEDfor review May 10, 1965. Accepted July 26, 1963. Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corp.
Reversed-Phase Chromatographic Separation of Zirconium and Hafnium JAMES S. FRITZ and RUSSELL T. FRAZEE Institute for Afornic Research and Department o f Chemistry, Iowa State University, Ames, Iowa
A quantitative separation of zirconium(lV) and hafnium(lV) is obtained b y reversed-phase chromatography using Teflon-6 as the solid support. The stationary phase is methyl isobutyl ketone equilibrated with aqueous 3M thiocyanic acid. Zirconium(1V) is eluted from the column with an aqueous ammonium thiocyanate-ammonium SUIfate solution, then hafnium(IV) is stripped from the column with aqueous ammonium sulfate or sulfuric acid.
T
separation of zirconium(1V) and hafnium(1V) is a difficult analytical problem. The ion exchange method of Hague and Xachlan ( 6 ) appears to be the best available, although this method has not been checked in our laboratory. N a n y workers have attempted separations based on solvent extraction ( I , 5, 8-15). ’4single batch extraction will not effect a complete separation. Counter-current extraction can be used, but it is rather cumbersome on a n analytical scale. Reversed-phase chromatography is a n attractive possibility, although Crawley ( 2 ) obtained only a partial separation of zirconium and hafnium using tributylphosphate on a Kieselguhr support with a n aqueous nitrate system. QUAKTITATIVE
HE
1358
0
ANALYTICAL CHEMISTRY
The success of reversed-phase chromatography for quantitative separation of uranium ( 7 ) ,iron (S),and molybdenum (4) led us to attempt a zirconiumhafnium separation by this technique. The solvent extraction behavior of zirconium and hafnium from acidic thiocyanate solutions has been well studied (8, I O , la),and this system was selected for our chromatographic investigation. EXPERIMENTAL
Apparatus. A Nuclear - Chicago scintillation counter Model DS5 with a sodium iodide crystal was used as t h e detector. A Nuclear-Chicago recording spectrometer Model 1820 was used to isolate the 0.480 m.e.v. gamma ray of Hfl*l. A decade scaler was used t o count t h e pulses received from the spectrometer. Reagents. Practical grade methyl isobutyl ketone ( M I B K ) was redistilled before use. Zirconium(1V) solutions were prepared by dissolving zirconium oxychloride octahydrate in 2M hydrochloric acid. The salt was stated to contain less than 100 p.p.m. of hafnium. Hafnium(1V) solutions were prepared by dissolving hafnium oxychloride octahydrate in 1M hydrochloric acid. This salt was stated to contain less than 100 p.p.m. of zirconium.
A hafnium(1V) solution containing Hfl*l was prepared by adding 20 pc. of Hfl*I in 6;M hydrochloric acid to 1.0 ml. of 0.018M hafnium(1V) stock solution and diluting to 100 ml. with sufficient water and hydrochloric acid to make the final solution 1.5M in hvdrochloric acid. I n e r t Supports. Teflon-6, a hard variety of polytetrafluoroethylene, was obtained as a pre-sieved 70/80 mesh powder from Analytical Engineering Laboratories, Hamden, Conn. Before using, the powder was saturated with MIBK, washed with 6M hydrochloric acid, acetone, and ether, and air dried. Kel-F low-density molding powder was obtained from Minnesota Mining and Mfg. Corp. The 80/100 mesh fraction was used. Microporous polyethylene was obtained as a 40/60 mesh powder from U. S. Industrial Chemicals Corp. A portion of the powder was converted to 60/80 mesh by grinding in dry ice and diethyl ether, followed by air drying and sieving. Eluents. T h e eluents used for the separation of zirconium(1V) and hafnium(1V) were equilibrated before using with M I B K , which had been equilibrated with a n equal volume of aqueous, 3 M thiocyanic acid. (Titration showed t h a t t h e equilibrated M I B K is 2.OM in thiocyanic acid.)
i
IlOk
loot
'd
80
I0 0
I 0
L3
4
A
MOLARITY OF HCL IN EFFLUENT
Figure 1 . Volume for complete elution of zirconium and breakthrough volume for hafnium as funciion of hydrochloric acid concentration of eluent 1.4 X 50 cm. column used
T h e following eluents were used for selective elution of zirconium: Eluent A: 3.051 ammonium thiocyanate, 0.2M ammonium sulfate. Eluent B: 4.0,U ammonium thiocyanate, 0.251 ammonium sulfate. Hafnium was stripped from the column n-ith 1.231 ammonium sulfate or with 1 . O J 1 sulfuric acid. Column Preparation. amount of Teflon-6 supported sufficient to pack three 50-em. columns (about 180 grams) was stirred with a glass rod while 40 to 50 ml. of N I B K was added in 10-ml. increments. T h e mixture was brought t o a uniform powdery consistency a n d slurried with equilibrated eluent. T h e slurry was added t o t h e column and gently packed down with 11flattened stirring rod. S o suction was applied unless it was needed to remove excess liquid trapped between the glass frit and stopcock of the column. (Liquid here causes hold-up of solute and thus increases the amount of eluent required). The Teflon-6 became yellow vvith use, owing to the decomposition products of thiocyanic acid. Although this yellox stain ca.uses no difficulty, it can be removed by digestion with 1 to 1 nitric acid. Separation Procedure. Because of t h e adverse effect of high acidity on t h e separation, the acid concentration of the sample must be kept low; however t h e acidity must be sufficient t o prevent hydrolysis of the zirconium or hafnium. For low column loading (Zr/Hf ratio of I ) , 1 ml. of 0.05.11 hafnium(1V) and zirconium(1V) solutions were mixed, and 3 ml. of eluent' A was added to the sample. This sample is rinsed onto the column. For higher column loading (Zr/Hf ratios of 5 , 25, and 500), an equal volume of concentrated hydrochloric acid was added to the zirconium-hafnium mixture, and the sample was evaporated
to near dryness. Approximately 2 ml. of water and 3 ml. of eluent B were added, and the sample was rinsed onto the column. The zirconium was eluted from the column with the required volume of eluent a t a flow rate of approximately 1.0 ml. per minute. Then hafnium was stripped from the column with the required amount of ammonium sulfate or sulfuric acid solution a t the same flow rate. Titration Procedure. T h e large amount of thiocyanate in the column effiuent interferes with analysis for zirconium. The following procedure was used to destroy the thiocyanate and prepare the zirconium effluent for analysis : Divide the effluent into two parts; dilute each to 200 ml. with water, and add 20 to 25 ml. of concentrated nitric acid to each. Evaporate carefully t o approximately 10 ml., then combine and evaporate to 5 to 10 nil. Add approximately 100 ml. of water and 25 ml. of 2-11 sulfuric acid. Adjust the p H to 1.0 with ammonia, add exactly 10 ml. of standard 0.05JI EDTA and heat to boiling form the zirconiumE D T A complex. Cool, adjust the pH to 2.0 with ammonia and backtitrate with standard 0.01J1 bismuth nitrate using Xylenol Orange indicator. The procedure used for hafnium in column effluents is essentially the same except that it is not necessary to divide the effluent into two portions. Also, the evaporation with nitric acid can be carried out more rapidly. CONDITIONS FOR SEPARATION
The use of several organic supports was investigated for the separation of zirconium and hafnium by reversedphase chromatography. illicroporous polyethylene has a high solvent capacity, but its low density makes it difficult to pack a column because the particles stick together. Kel-F 3031 is dense and has a high solvent capacity, but commercial production of low-density Kel-F has now stopped entirely. Teflon-6 proved to be a good support because of its porosity, hardness, a n d relatively dense particles. Preliminary experimentation indicated that a very good column separation of zirconium(1V) and hafnium(1V) could be obtained using eluent A (3M ammonium thiocyanate and 0 . 2 V ammonium sulfate) and a 50-em. column. The variables in the eluting system are acidity, and thiocyanate and sulfate concentration. T o ascertain the effects of variables and to find the optimum eluent, the composition of the eluent was studied systematically. Acid Concentration. Eluent A contains thiocyanic acid from equilibration of the eluent with M I B K , which is 2.0JI in thiocyanic acid. Thiocyanic acid is a strong acid and is apparently completely ionized in aqueous solution. Titration showed t h a t the equilibrated eluent A con-
,220
,
l
l
l
0.8
1.0
2ml 1 \
l
l
I80
l6OL 140
-
0
0
0.2 0.4 0.6
1.2 1.4
MOLARiTY OF (NHq)pS04 IN EFFLUENT
Figure 2. Volume for complete elution of zirconium and breakthrough volume for hafnium as function of ammonium sulfate concentration of eluent 1.4 X 50 cm. column used
tains 0.29M thiocyanic acid. T h e concentrations of ammonium thiocyanate and ammonium sulfate in eluent A were held constant at 3.0-11 and 0.2JI, respectively, and the acid concentration was varied b y the addition of hydrochloric acid. T h e volume of eluent necessary to cause breakthrough of hafnium(1V) and the volume required to elute zirconium (IV) completely from a 1.4 X 50 cm. column was plotted against the concentration of hydrochloric acid added to the eluent. The results, shown in Figure 1, show that higher eluent acidity cause5 hafnium to breakthrough too soon and thus ruins the separation. Sulfate Concentration. T h e concentration of thiocyanic acid was held constant a t approximately 0.25M and the ammonium thiocyanate concentration maintained a t 3.031 while the concentration of ammonium sulfate was varied. T h e breakthrough volume of hafnium(1V) and t h e volume for complete elution of zirconium(1V) from a 1.4 X 50 em. column are plotted in Figure 2 as a function of ammonium sulfate concentration in the eluent. It is evident from Figure 2 that some sulfate is necessary to ensure the rapid breakthrough of zirconium. Apparently sulfate reduces the extraction of zirconium thiocyanate by M I B K by forming a competing zirconium sulfate or a mixed sulfate-thiocyanate complex t h a t is not extracted. Too much sulfate ruins the separation by causing early breakthrough of hafnium(1V). The optimum concentration of sulfate is 0.1 to 0 . 2 x . VOL. 37, NO. 11, OCTOBER 1965
1359
140
?
Pi
is
240
210
I80
Figure 4. Separation of approximately 250 pmoles of zirconium and 50 ,moles of hafnium on 1.4 X 50 cm. column Eluent B contains 4M ammonium thiocyanate and 0.2M ammonium sulfate. Eluent E is aqueous 1.2M ammonium sulfate
120
w
0
I
I
I
2
3
I
I
20
40
I
I
1
I
I
I
I20 100 I-
z
80
W
s W -I
60
W IL
0 v)
Y0 3; a
40
20 0
I
r
0
4
60 80 DO 120 ML OF EFFLUENT
140
160
180 200
MOLARITY OF N h S C N d N EFFLUENT
Figure 3. Volume for complete elution of zirconium and breakthrough volume for hafnium as function of ammonium thiocyanate concentration of eluent 1.4 X 50 cm. column used
Thiocyanate Concentration. With the concentration of thiocyanic acid maintained a t approximately 0.25M and the concentration of ammonium sulfate constant a t 0 . 2 J 1 , the ammonium thiocyanate concentration was varied. T h e results are plotted in Figure 3. The elution of zirconium(1V)
Table I. Effect of Column Length
1.4 X 25 cm. column
1.4 X 50 cm. column
Zr
elution, ml .
Hf breakthrough, ml .
45
165
90
330
Table II. Effect of Loading on Elution of Zirconium(1V) and Hafnium(lV) 1.4 X 50 cm. column, eluent B 811. for 311. for
pmoles Zr
44 132 220 308 440
complete pmoles elution of Hf 52 50-60 104 50-60 50-60 1.56 260 60-70 364 70-80
breakthrough 160-180 140-160 80-90
50-60 40-50
is hardly affected by thiocyanate concentration] but the retention of hafnium (IV) by the column is much greater at high concentrations of thiocyanate. Column Length. The volumes required for complete elution of zirconium (IV) and the breakthrough volumes for hafnium(1V) mere compared on columns 25 and 50 cm. in length using a n eluent containing 431 ammonium thiocyanate and 0.1M ammonium sulfate (see Table I). The results are in accord with predicted column behavior. The results of the above experiments indicate that for the optimum column separation of zirconium and hafnium, the eluent should contain no added hydrochloric acid, should be 0.1 to 0.2M in ammonium sulfate and 4-11 in ammonium thiocyanate. An eluent containing 4 J 1 ammonium thiocyanate and 0.2M ammonium sulfate was found to elute zirconium more rapidly and to give a better separation than one containing only 0.1M ammonium sulfate. RESULTS
A typical elution curve for separation of a mixture of zirconium and hafnium is shown in Figure 4. The slight tailing of the zirconium curve is almost missing in some of the elution curves t h a t were plotted. The effect of column loading on the elution of zirconium and hafnium using eluent B (4M ammonium thiocyanate,
Table 111.
Quantitative Separation and Analysis of Zirconium-Hafnium Mixture Conditions: 1.4 x 50 em. column; eluent A for first four samples and eluent B for last two
Ratio 1:l 1:l 1:l 1:l 25: 1 25: 1
1360
Zr taken, pmoles
52.9 52,9 52.9 52.9 258.4 2%. 4
Zr found, pmoles 52.9 52.7 52.9 53.3 259.9 258. 9
ANALYTICAL CHEMISTRY
Diff., pmoles 0.0
-0.2
0.0
+0.4
+l.5
+0.5
Hf taken, Hf found, pmoles pmoles 49.2 49.3 49.3 49.0 49.5 49.3 48.9 49.3 7.8 7.4 8.0 7.8
Diff., pmoles -0.1
-0.3
+0.2
-0.4
-0.4
+0.2
0.2.1.1 ammonium sulfate) was investigated. The results are given in Table 11. Breakthrough of zirconium always occurred in the 10- to 20-ml. fraction. The results show that the elution of zirconium is hardly affected by loading (at least in the range investigated)] but that hafnium breakthrough occurs much earlier in solutions containing a higher concentration of hafnium. Column lengths can be adjusted to allow for some variations in hafnium loading (see Table I). These data suggest that the column separation method might be useful for the separation of small amounts of hafnium from large amounts of zirconium, as well as for the separation of approximately equal amounts of zirconium and hafnium. Using radioactive Hf181as a tracer, a n elution curve is shown in Figure 5 for a sample containing zirconium and hafnium in a mole ratio of 500 to 1. The total quantity of hafnium on the column was 0.5 pmole. Approximately 0.5 to 1.0% of the activity was found in the zirconium(1V) fraction. This activity is either due to radioactive zirconium(1V) impurities or to hydrolysis and/or polymerization of the radioactive hafnium which might prevent it from being retained by the column. A gamma ray energy spectrum of the radioactive sample showed no energy peaks corresponding to gamma rays of zirconium; therefore, the activity was attributed to hafnium. The original zirconium sample contained 0.2y0 hafnium; since 99.0 to 99.5y0 of the hafnium was separated, the zirconium from the column contained only 0.001 to 0.002y0 hafnium. Quantitative results for the separation of other samples containing zirconium and hafnium in various ratios are given in Table 111. hlost of these separations show quite good recovery of both elements. As a further check on the completeness of the separations, the zirconium and hafnium fractions from the column separations for 5 to 1, 25 to 1 and 500 to 1 Zr/Hf were precipitated
140
QS
=
ML OF EFFLUENT
Figure 5. Separaition of approximately 250 pmole,s of zirconium and 0.5 pmole of hafnium on 1.4 X 50 cm. column
some iron breaks through in the 20to 30-ml. fraction when eluted with eluent B. Behavior of several other elements can be predicted from the distribution ratios between M I B K and eluent B or aqueous 4M ammonium thiocyanate (see Table IV). The elements not extracted or only slightly extracted should accompany zirconium in the column separation. Cobalt(I1) is strongly extracted and should stay on the column with hafnium(1V). Titanium(IV), which is approximately 3573 extracted, may well appear partly in the zirconium and partly in the hafnium fraction.
Huent F is aaueous 1 .OM sulfuric acid
as the hydroxides, ignited, and analyzed by our spectrographic laboratory. These analyses showed that the zirconium oxide contained less than 0.01% hafnium and that the hafnium oxide contained less than 0.01% zirconium. Behavior of Other Cations. Experiments with single elements showed t h a t molybdenum(VI), tin(IV), and zinc(I1) are extracted into M I B K ; these elements fail to breakthrough in 300 ml. when added to a 1.4 X 50 cm. column and eluted with eluent B. Although iron(II1) is partly extracted,
LITERATURE CITED
(1) Cox, R. P., Beyer, G. H., U . S . At. Energy Comm. Rept. ISC-682 (1955). (2) Crawley, R. H. A., Xature 197, 377 (19631. (3j-F&z, J. S., Hedrick, C. E,, ANAL. CHEM.34, 1411 (1962). (4) Ibid., 36, 1324 (1964). ( 5 ) Glemser. 0.. British Patent 874.510. . Aug. 10, 1960: (6) Hague, J. L., RIachlan, L. A., J. Res. Natl. Bur. Std. 65A, 75 (1961). (7) Hamlin, A. G., Roberts, B. J., Loughlin, W., Walker, S. G., ASAL. CHEM. 33, 1547 (1961). (8) Hoshino, Y., Nippon Kagaku Zasshi 81, 1574 (1960). ( 9 ) Iowa State University, U . S . At. Energy Comm. Rept. ISC-506 (1954). (10) Leaders, W. M., U . S . At. Energy Comm. Rept. Y-553 (1955). I
,
Table IV. Distribution Coefficients for Batch Extraction of Various Elements from Aqueous Thiocyanate Solution into MIBK
Distribution ratio 4M
NH4SCNElement Al(II1
~
-~
C&II) Cd(I1) Ce(IV) coiIn' cu(11 j Mg(I1) ,1In(II) Pb(I1) Nd(II1) Ni(I1) Srn(II1) Th(1V) Ti(1V) V(1V)
0.2M
4M
(NH4)2SOa NHaSCN -n -n N O -0 0.02 N O 22.6 0.06
-0
N O
-0 18.7 0 05
N O
-0
...
N O 0.02
-0 -0
0.01
N O
-0
-0 0.03 0.54 0.03
...
0.51
-0
(11) Millard, W. R., Cox, R. P., U . S. At. Energy Comm. Rept. 1%-234 (1952). (12) Overholser, L. G., Barton, C. J., Grimes, W. R., U . S. At. Energy Comm. Repts. Y431, Y-477 (1949), Y-560, Y-611 (1950). (13) Schultz, K., Larsen, J., J. A m . Chem. SOC.72, 3610 (1950).
RECEIVEDfor review April 15, 1965. Accepted May 12, 1965. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
Gas Chromatographic Separat on and Determination of Pentaerythritol System by Tr methyIsilyl Ether Derivatives RICHARD R. SUCHANEC Research Center, Hercules Powder Co., Wilmington, Del.
b A new gas chromatographic method for analyzing the complete pentaerythritol system i s presented. The method i s based on the trimethylsilyl ether derivatives of these polyhydroxy compounds. This procedure i s not only shorter and simpler than the best previous method but also makes possible a more detailed analysis of commercial grades of pentaerythritol. Using this method with an internal standard, mono-, di-, tri-, tetra-, and pentapentaerythritol can be detected under easily obtaihable conditions with a conventional instrument equipped with a thermal conductivity detector. Other components that have been definitively detected are pentaerythritol dicyclic diformal, penta-
19899
erythritol cyclic monoformal, and pentaerythrose. Additional peaks, which were detected, were tentatively assigned to the following derivatives: bis(pentaerythrito1) monoformal, dipentaerythrose hemiacetal, pentaerythritol-dipentaerythritol monoformal, tris(pentaerythrito1) diformal, and bis(dipentaerythritol) monoformal.
E
BACKGROUND for the synthesis and analysis of pentaerythritol (PE) is provided in the ACS Monograph of Berlow, Barth, and Snow ( 2 ) . Accepted chemical methods, such as the benzal method for P E and the acetylation method for hydroxyl groups, have long been known to be
XCELLENT
nonspecific as applied to PE analysis. A selective technique such as gas chromatography could provide this specificity, but because of inherent thermal instability above the melting points, the PE system cannot be chromatographed directly. Volatile derivatives such as the acetate esters, on the other hand, have been chromatographed (11). The general applicability of this time-consuming acetate method is somewhat limited, however, by the greatly reduced sensitivity of even the tripentaerythritol (triPE) peak and the failure to detect any components with a higher retention time than t r i P E in spite of the use of extreme instrument conditions. Trimethylsilyl (TMS) ethers are advantageous derivatives for VOL. 37, NO. 1 1 , OCTOBER 1965
1361
