Oct., 1961
ISi7
sEP.4R.4TIOV OF N I T R O G E V \ YD OXYGEY ISOTOPES
comparing thcse results with those in Table I, we see that a t the low polysoap concentrations the adsorption is substantially enhanced by the presence of the hydrocarbon layer, and, if the results for a are to be interpreted literally, that only a few hydrocarbon groups per adsorbed polysoap molecule can be accommodated in the interface. While it has been shown that solubilization enhances the surface activity of polysoaps, this effect plays only a minor role in the interfacial tension depression. There are two pieces of evidence for this, First, while benzene is solubilized to a much
greater cui,cnt than heptane,7 ?” 3 l ilic interfacial tension depressions with both hydrocarbons were about the sani(’. Sccond, it is known that the ((9.5%’’polysoap does not solubilize heptane, while the “35.0%” polysoap does.’ Yet, under corresponding conditions, the “9.5y0”polysoap n’aq never the less effective of the two in depressing the interfacial tension in the heptane syqtems. It seems therefore more likely that the enhancement of the depression is due to the free energy of mixing of the liquid hydrocarbon with the parafin side chains of the polysoap in the interface.
SEPARATIOX OF NITROGEN AND OXYGEN ISOTOPES BY EXCHANGE O F NITltIC OXIDE WITH NON-AQUEOUS SOLUTIONS OF NITRIC OXIDE COMPLEXES1 BY A. NARTEN AND T. I. TAYLOR Department of Chemistry, Columbia University, New York, N . Y . Receiaed Mag 24, 1961
The distribution of the nitrogen and oxygen isotopes between gaseous NO and the CuClrromplex in liquid methanol has been studied. Thij separation factor (r(N14/N16) has been determined by single stage equilibration: a(N14/N16)vanes from 1.017 a t -20” t o 1.012 a t + l o o . The separation obtained in a 200 cm. column packed with 2 X 2 mm. silver mesh rings corresponds to swge heights of 2.2 and 3.0 cm. a t interstage flows of 1.0 and 1.3 m M NO/cm.Z min. and temperatures + 1 4 O and -20”. From column experiments the separation factor for the oxygen isotopes h a been estimated a8 1.01 > u(0l6/O1*) > 1.005 a t room t emperaturr. The system can be refluxed thermally (exchange-distillation) and is attractive for N15-enrichment. It is not feasible for 018-enrichment due to aide reactions in the column.
Introduction Nitric oxide reacts with cupric halides in nonaqueous solutions forming a loose addition compound according to the reversible reaction CU&(S) + NO(g) )JCuXzNO(s) (1) X being the anion C1 or Br, s being a great number of organic solvents, such as alcohols, ethers, esters, ketones, nitriles and cbarbonic acids. A maximum amount of 1 mole S O per mole CuX2 can be absorbed by thcse solutions.2 The addition compounds CuXzKO can be obtained only in solution. They are intensely colored, ranging from deep blue t o black violet. The color of the complex solutions is reported to vanish if an inert gas is bubbled through the liquid. The absorbed YO is also desorbed upon dilution with water.3 Aqueous solutions of cupric halides will absorb only traces of XO, and the nitric oxide complexes in non-aqueous Cu.Xz-solutions are highly sensitive w water.4 Methanol as it solvent for XO-complex forming cupric halides is commercially available a t low cost.; it rttii be dehydrated easily and dissolves the largest amc-unt of CuX2. It is for these reasons t>hat systems oi’ CUXZin methanol with X O as the gas phase weye investigated by us as a possihlc iiic’ans i’or the separation of the isotopes cti niwogen nnci oxygen. I :?e deep I ! . u ( ~ color of the CuSaNO-complexes *> ,.
(1) Supnor‘v3 h s n grant from the U. 3. Atomic Energy Commission. ( 2 ) \V. Manchot. Chem. Ber., 47 ltifli (1914). ( 3 ) ?‘. Xohinchutter nnd M. Kutscherof?. ibid., 37, 31344 (1904). (4) W.Xlarichut a n d E. Linkh, i b i d . , 69, 406 (1926).
in methanol is due to the covalent CuXzNO (probably solvated). The degree of association, p = moles NOjmoles Cu in solution, of the complex according to k‘l
+
C U X ~ ( C H ~ O I I )NO ~ CuXt(CH3OH)NO
+ CHIOH
(2)
can be calculated from the equilibrium constant K 1 5 ; a t room temperature and atmospheric pressure the values are 0 = 1 for X = Br, and /3 < 1 for X = C1. The CuXzNO-complex in methanol dissociated into colorless ions & CuXZNO
CuXz-
+ 90+
(3)
(IC2 1.005
(7)
a t room temperature and atmospheric pressure. No attempt was made to determine for the CuBrzNO/n'O-system (see next section). The separat'ion factors discussed in this sect'ion are summarized in Table I. (Y
SUMMARY OF DATA GASEOUSY O -
ON
TABLE I EXCHAXGE SYSTEMSBETWEEN
CUC~~SO-COMPLEXES I N METHASOLAT ATMOSPHERIC PRESSURE
AND
'.1.&0
+lti'
e'
Moles Cu/l
B Moles NO/l. a(N14/N") a( 0 ' 6 / 0 ' 8 )
1.45 0.40 0.58 1,012 > 1 ,00.5 4.5"
- 20 J
3.50 0.75 .SG 1.88 1.012 1.017 > 1 ,005 > 1 ,005 2.42 0.40
H.E.T.P. (cm.) a t 2 3h 3.0'' Optimum flow ( m M NO/cm.* min.) 2.8" 1 .Oh 1.3* Height of packed section 200 cm., 1 ctn. i,d.; column 1.6 mm. i.d. p1a.r~lwliccs; 6 2 X 2 mni. d v r r packed with: mesh rings, 1500 mesh/cm.*.
Column Experiments
8
Fig. 1.-Column
used for the determination of separation parameters.
separated by ,standard techniques and the11 reduced to nitrogen.6 Isotopic analysis \vas carried out it1 a CEC 21-201 dual collector mass spectrometer using thc 29/28 ratio. At least, three independcnt equilibrations each a t temperatures het,ween I O aiid -20' and atmospheric pressure were c a r r i d out to obtain the equation
+
log (5'4/N'3) =: (4.80/T
-
1.17 X
zt
1.5 x 1 0 - 3
(5)
The separation fact'or o ( O ' ~ / O ~for ~ ) the distrigaseous bution of the oxygen isotopes I~et~vvecii NO and the CuCl2S0-complex in met,hanol according to CIIC~~N (1)O-t~ ~~ 0 1 8 ( g j CuC12pI:O'8 (1)
+ XO'Yg)
(6)
was not drtcrmined. l o r the evaluation of single stage equilibration expcrirneiit.s it, is necessary t.o assay the oxygen from the nitric oxide in hot'h (or a t lcast, oiic of t,he) phases. With our present' techniques UT ~vercuriahlc to separate thc NOoxygen from the oxygen origirtatiiig from reaction (6) T. I. Taylor a n d W. Spindel. in "Proowdings of t h e 1nt.ernationul Symposium on Isotope Frl,urution," N o r t h Holland Publishing Co., Arwtcrdml. 1G8, p. 158.
I n order t o utilize the exchanyc of nit,roxrn and isotopes bct,ween the gas and liquid phases of the Cu N O systems according t,o (4) or ( 6 ) in an isotope sep process, the liquid and gas strrams must tw passed countercurrently in an exchange colllmn wit.h coiivenient rrflris mechanisms on both ends. Since bot,ti SI5 nnd 0:8concentrate in the liquid phases of t,he system it is possible to produce the two heavy nuclides in thtl sxme separ:ition unit. Figure I shows the column which was used in the experiments. It consisted essentially of 3 scct,ions: the exchange column (200 cm. long, 1 cm. i d . ) was packed with two different kinds of packing P2 deserikjed bclow. Bot'h t.he decomposcr (40 em. long, 2 cm. i d . ) and the recombiner (30 cm. long, 2 cm. i d . ) were packed with 3.3 mm. i.d. glass helices (Scientific. Glass Co.). All thrpe sections were kept at constant temperature by means of a circulnt,ing coolant C. Feed NO was introduced a t it point Iwtwec~tl t,he column and the recombiner; in the cxperimrnts tiescribed, mixing of t h r feed gas with the upstreaming, tiepleted N O from the exchange column was :tllowed. The feed flow mas adjusted so that the top concentration a t 7 was near the natural isotopic abundance, and was controlled by the flow meter 5 . T h e decomposer consisted of a stillpot, a rect,ifying section 2 and e condenwr 11. The downstreaming liquid equilibrium mixture of C u X 2 S 0 , CuS? and CH,OH was warmed to some BO" t).v at1 upstre:ini of methanol vapor which a t the sanic time kept t h e SOpartial pressure in the rectifying section low. 13y t!ie time the liquid reached the still-pot 12, the C u X 2 N 0complex was completely dissociat,ed, and the pure CUXZ-CH.;OI€ solution was recycled to the top of the recomk)iner ljy Ineiins of a finger pump 6. The liquid flow was controlled hy :I flow meter 10. Methanol vapor from the dwomposer was refluxed in the condenser 11. Top and bottoni samples %-err' taken from the gas phase through the thrre-way stopcor 7 and 8, and were processed for m:tss spectrometer and. as described before. Reflux of the two countercurrent phases was accomplished bv absorbing the upstreaming KO in the re-cycled liquid CuSy-CH,OH solution in t,he r c combiner. The CuX2NO-complex in the downstrc.xmin: liquid was converted to gaseous K O in the ttiermal decomposer. T o determine the reflux efficiency in the rcw):nhiner, samples of the liquid were t,aken a t a point !I (Fig. I ) :tnd analyzed for nitric oxide. S o 1'0 could be detect,d iri the liquid, even at very low boil-up rates in t,he pot of the dtscomposer. The only mechanical part in the whole systcio
S E P ~ R I T I OO FYSITWGEV ~ Y OXYGEN D ISOTOPES
Oct., 1961
-0-0-0-1/16 I.D. QLA-
Fig. 2. -Flow
+ 18c
- 20. C M.Y. SILVER MESH, + 14. C
0 0 0
2 X 2 M.M. SILVER MESH,
0 0 0
2X2
4 5 6 7 8 9 m M (NO)/cm.2 min. dependence of the stage height (H.E.T.P.) for the NO-CuC12NO-CH30H system in a 300-cm. column, 1 cm. i.d.
0
1
HELICOS,
1879
2
3
was the finger pump which recycled the YO-free liquid phase. Thus, the operational stability of the entire unit w m found t o be excellent for CuC12, (with Br2 see below). The column was operated for several weeks without continuous attent ion. Considerable difficulties had to be of the corrosiveness of the materials tern. Glass was used where this was iger pump, latex tubing was very satisfactory. Only glass and silver conlcl be used as packing materials in the wchange column. All metals which are less noble than copper will reduce the CU*+,giving C u X ,
comparable stage heights in a distillation column have been reported* as A = 1 cm. From an equation given by K ~ h n , ~ which relates the stage height with the various possible resistances t o diffusion and interface mass transfer in a separation column, it can be shown' that for exchange processes with a liquid carrier phase (if the concentrations of the substance to be separated in the liquid (CL) and gas phase (C,) is CL > lOC,) carried out in two different columns (1 and 2), or for a column operated a t two differmt temperntures or (and) pressures (1 and 2 )
D arc the diffusion coefficients, p thc apecific wciglits, 7 the viscosities, T the kelvin temperatures, and C the c:onrcmtrations in the liquid phase at the opwating conditions 1 :tiid 2; ( 9 ) gives a good approximation if tht l i q r t i t l niut qns //O/JM in thc column uvuler the conditions 1 m r i 2 ai-+ simtlor. i . c . , if the iritcrstugc flow is the same. The, +t:tge f i t ~ i q l i ti n N I I c.schange c:olumri is necessarily higher thaii the oiic 01)taincd in tlistillatiun columns under compar;tbl(~coiiclitioiis brcause of the fact that the concentrthm uf thi, substance t o be separated iii the liquid phase is lowc.r, twn if the ctirmical exchange reaction as such is very fitst. L being t,hr .:irlmn ;erigth. T h e r e ~ i i l tare ~ presented in Fig. 2. T h s b~:,vz.ii+ c l = 2 cni. at room temperature clearly indicates that tk,> :':ite '-if exchange is very faet,. It is in fact determined L:- tht. w t i x cif rli3iision in the liqiiid phase?;
... ..
.___
3 ;B. Stuke, 2. Eleklrochem., 67, 655 (1953). M. Thiierkauf. -4. Narten and W. K u h n , Hela. China. Acta, I S , 9P-J \!QFO). :9) W . Kuhn, ibid., 37, 1416 (1954). equation 1 7 .
A. NARTEN AND T. I. TAYLOR
1880
Vol. 65
In t l i c s L:MC of isotopically double-lal)c~lletfsiilmtancos such decreasing the operational stability of the coliimn grr:itlv S O , ~ I i estagy hcigtit a t infinite rt.fliiu, xu, is thi, mine The over-all sepamtion S u ( W ’ / V 5 ) for this system u tb for both nuclidcs. I t is tlirrvfore 1)ossit)Ic to determine found to he lower than with the CuCIrNO system nt c u n a(016/018) from ttie measiired owr-all separation S,- parable flow rates anti temperatiires. Therefore, no furtlictr (016/0’*) and Su(Ki4/K16), using the known values of A, attempts were made to determine the various parnmeteib and a(N14/Si;)),according to for the CuRrT-SO system. a(o’6/0,q = a(s ‘4/K 1 6 ) ,s,(ols/ols)/,s,(~i4/Nls! Discussion (IO) Systems consisting of CuX2h’O-comp1eses in I n the case of the CuC1,KO-NO system with methanol aa methanol with KO as the gas phase have the dethe carrier phasc, we found that S ( O ’ 6 / O i a ) / ~ ( ~ i 4 / S 1 K ) depended on the flow in the column (at constant t,clmpera- sirable feature of not requiring chemical reflux. trire). This ran he explained only by the assumption that This makes it possible to use an exchange-distllthe column was not operating at total reflux. On tlic other lation process for simultaneously concentrating hand, the parasitic production of the column was su small that it did not aflect the separation of the nitrogen isotopes both nitrogen-15 and oxygen-18. However unnoticeably; the measured stage height A = 2 cm. is in desirable side reactions in an exchangc-distillaaccord with ( ! I ) . The possibility of an exchange of oxygen tion column which cannot be avoided without afbr,tween KO :iId CH80FI in the decomposer had to be eliniinattd, since we found no exctimge upon equilibrating fecting other parameters in a way unfavorablc to thc separation characteristics of the systems, 170 N0’8 with CI1~OH-CuC12of natural oxygen abuiitltlnce. I n order i o ol,tain a certain sepswtion in nn exchange make the CuBr2-X0 system not feasible for isotope column, it is necessary to opcrnte the column at a rcfli!x separation a t all and the CuC12-X0 system, for the ratio Lt > H , H being the rnininirirn refluv ratio, w:iich is determined by the top :tnd 1)ottom roncentrations and the concentration of oxygen-18 (although this is po+ separation factor. l h t h t h t h top concentration and the sible in principle). However a system of CuClzNO in methanol separation factor are diferciit h,r OlS/Ol* and N14/N16. It is therefore conct~ivn1)ii:tli:tt the parasitic 1md:lction is with X O as the gas phase is attractive for the consuch that for the nitrogen isotopes H < < R ,and therefore centration of nitrogen-15. The separation factor, A s k o , while for the oxygen isotopes H g I Z and therefore A>X.,’O A small constant Nqthdrawal of nitric oxide a t the stage height, reflux efficiency, and decomposition bottom of the column thus can have quite different effects rate are favorable, and operation of the system is on the nitrogen or oxygen separation. Our measurements relatively simple and economical. As compared are consistent with the assumption of a small constant with other systems the only crit>ical element parasitic production and a separation factor 1.005 < a( 0 1 6 / 0 1 8 ) < 1.01 at room temperature. Losses of NO at the involved in the scale-up of a CuCln-XO cascade product end of the column did not occur as a consequence is the material of construction. Only glaw, of incomplete decomposition of the CuC12NO-complex in porcelain, or ceramic-lined metals appear to be the decomposer cir liquid reaction products between NO and feasible materials. Glass, ceramics, or silver will CH3OII. Therefore, a gaseous product muet be formed at the bottom end which leaves the column with the waste have to be used for the packing. It is our opinion that the simultaneous concenK O stream without exchanging its N O with the downst.reaming liquid. FrazelJ has found that the CuXJS’O-complexes tration of nitrogen-15 and oxygen-18 (oxygen-17) dissociate according t o (3); the NO+ ion thus formed by an exchange-distillation process using S O as docs not remain aa such in methanol (or other alcohols), instead, it reacts with methanol giving methyl Ditrite the gas phase arid solutions of NO-complex forming metal salts as the liquid phase would be very KO’ CH30fI CEI@SO H+ (11) attractive. Experiments to find NO-complex formwhich is removed as pnrasitic product with the w a d e NO ing metal salts which will permit the use of stninat a rate tirpending on the liipid arid gae flows in the column less steel or IIastelloy as construction and packing and on the temperature both in the column and in the dematerials in a solvent which is capable of solvating composer. The major part of t,he esperinients drscribed in this sec- the K O + ion without reacting with any component tion was carried o u t with CuClz as the NO-complex forming of the system so far have been unsuccessful. halide in methanol. The C u B r r X O system is inferior because CuBr? undergoes reduction in the presence of Acknowledgment.-The authors are indebted to CH,%OHand KOl, especially at the elevated temperature Professor W. Spindel of Rutgers University for of the decompoeler. The resulting CuBr is almoHt insoluble in methanol, and will plug the column, thereby use of his mass spectrometer in the single stage equilibration experiments, 2nd to Dr. H. Hoever :LY
+
-+
(10) W. Kuhn,
r’.
+
Bnertschi and hl. Thuerkauf, Chimia, 8, 109 K. Cohen, “The Theory of Isotope Separation. National Nuclear Energy Series.” 111-IB,McGraw-Hill Book Co., K e w York, N. Y., 1051, p. 32. equation 2.13. (19541, equations 1II. 20b, 2Oe.
for stimulating discussions. The assistance of Mr. E. Fischbach was veiy helpful in the column experiments.
