V O L U M E 23, NO. 5, M A Y 1 9 5 1
793
In thtlse very dilute solutions, the solute behavior is expressed by Heni,y’s law: P, = S,k where k is a constant chaiacteristic of the solvent-solute system. The Henry’s law constant, in thermodynamic terms, is an expression of the escaping tendency, or fugacity, of the solute. This may b ( aLilormally ~ high for a substance like ethyl alcohol, because the assoc,iation effect from hj-droxyl bonding is eliminated in very dilute solution We are then dealing with a low molecular weight species, : l ~ ~ the d solvent-solute interactiim twcomes one common w to pol~r-nonpolar systems gena r~raliy. Even a t.uhstance like n-butyl a alcohol tends to escam from cara bon tetrachloride, although the R effect is rather slight. I t is interesting fcs note the difference of x X’ 3
The determination of water in nonpolar solvents has been found to be subject to all the effects mentioned, to an extreme degree. It might have been more to the point to illustrate this discussion with a water-containing system. But the kind of experiment performed for the systems chosen would offer formidable difficulties when water is involved. Under ordinary conditions the experimenter would be operating in an atmosphere containing amounts of solute (water) significant with respect to the solution concentrations of interest. Evaporating carbon tetrachloride containing small amounts of water leads to either a decrease or an increase in water content. An equilibrium is reached with atmospheric water vapor concentration depending on the humidity. If these observations are accepted, several rather obvious precautions must be taken in handling solutions of this kind for analysis. Determinations on hundreds of samples analyzed independently by both chemical and spectroscopic means gave very erratic results until the sources of error were successfully traced back to handling procedures ordinarily considered entirely adequate. Samples must be kept hermetically sealed. Transferring a sample from one vessel to another must be done with considerable care. Pouring a carbon tetrachloride solution containing a few parts per million of water, in such a way as to expose a considerable surface of solution to the atmosphere, can greatly affect the concentration of water. Simple calculation may show that the amount of vapor space above the solution in a container may be significant with respect to the solution volume itself. The investigator must be prepared to ask himself if the determination of water content has any meaning. According to the history of the sample, water content may be more a function of atmospheric conditions coupled with sample handling than anything else. Only when the sample container is handled with very special care will water concentration be anything more than an expression of solvent-atmospheric water vapor equilibrium.
rg +
99.93 1007. behavior tretneen carbon disulfide CCI, and ~ L - ~ J U I ,alcohol. ~] \vliich h a w very n e a r l y e q u a l n i v l e c u l a r Figure 2 weights, but are v e different ~ with respect to polar rharacter Acetic acid was also iiivestigated to some estent. At 0.1% concentration very little change in concentration is noted upon partially evaporating the solution. But a t lower concentrations, about O.Olyo,there if h marked l o ~ e r i n gof acetic acid content with evaporation. This i.q probably explained by the fact that a t ~ is present to a high degree as the the higher concentration r l i acid dinirir. However, a t 0.01% and loner, there is a noteworthy incr(mc of monomer to dinier ratio as shown by the infrared spectrum, P O t h a t a t very low concentrations a low molecular weight species i k prrwnt (60 for the monomer as against 120 for the dimer). .is a i.ule. the authors concluded that the loss of solute is proportionately greater the loner the concentration for the conipounds investigated. If this latter observation is correct, the neces.4tj- for emphasis on extreme care in sample handling is increased at minute concentrations for solutions such as those considercd lirrr
LITERATURE CITED
(1) Wright,
s..ISD. ESG.C H E Y . , A4N.%L.E D . .
13, 1 (1941).
RECEIVED June 5 , 1950. From a paper presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Joint Meeting, Division of Analytical Chemistry, Pittsburgh Section. ~karERIcaNCHEMICAL SOCIETY,and Spectroscopy Society of Pittsburgh, February 13 t o 17, 1950.
Spectrographic Determination of Silicon in Uranyl Nitrate Solutions F. T. BIRKS .4tomic Energy Research Establishment, Harwell, Didcot, Berkshire, England
I L I C O S has been determined in uranous uranic oxide (LT308) and in uranium metal and compounds after conversion to U308, according t o the direct current arc carrier distillation method ( 1 ) . and in uranium metal by Kalsh (b),who used graphite counter e!ectrodw in the controlled alternating current arc. A number of uranyl nitrate solutions, each containing about 300 mg. of uranium per nil. of dilute nitric acid solution, were to be examined for silicon. The carrier distillation method was considered too slow for this purpose, as it would involve the conversion of the whole or most of the sample t o U308 and the subsequent operations of mixing with the carrier are time-consuming. Because the only impurity element sought was silicon and there were sensitive silicon lines located in a region free from an excessive number of uranium lines, it was decided to apply a direct hurn procedure and to use uranium itself as an internal standard PREPARATION OF STANDARDS
A pure sample of U308was prepared by extraction of strong uranyl nitrate solution with ether, removal of the ether under reduced pressure, and final ignition of the residue to u308. Pure precipitated silica was diluted with this base by thorough grinding in an agate mortar to obtain the standards: 5000, 1000, 500,
100, 50, and 10 p.p.m. of silicon relative to UsOs. A blank estimation of silicon was carried out on the pure U308 used for the dilution. PRELIMINARY INVESTIGATION
The electrodes were shaped from National Carbon Co. pure graphite rod 0.5 inch (1.25 cm.) in diameter to fit on a graphite support 0.125 inch in diameter, and had a crater 6 / 3 2 inch in diameter X 1/20 inch deep. They xere preburned for 30 seconds a t 10 amperes before loading, to remove surface contamination and reduce the silicon in the electrode material to a uniform low level. A moving plate exposure was first made on a 6-mg. charge in a 10-ampere direct current arc to study the relative emission rates of silicon and uranium. It was found that the silicon intensities followed the uranium fairly closely and fell off rapidly after 20 seconds becoming zero a t 30 seconds. The exposure conditions adopted were 30 seconds a t 10 amperes with the sample as anode. To investigate the effect of charge weight on the result, weights of 4,5,6,and 7 mg. of the 500 p.p.m. standard were exposed; the intensity ratio was found to remain reasonably constant. It had been shown in connection with other work t h a t in general the sensitivity of trace elements added in the form of an aqueous solution was higher if the solution were prevented from soaking into the porous graphite electrode by the presence of a thin grease film. Furthermore, nonpenetration of the electrode
794
ANALYTICAL CHEMISTRY
by the samples was desirable, as it v a s intended to add the standards in the form of solid U308. The grease film was prepared by evaporating one drop of a solution of Apiezon M grease in analytical reagent 60" to 80" C. petroleum ether (0.5 gram in 10 ml.) on the surface. The film was, however, attacked and penetrated with consequent loss in sensitivity for silicon, when solutions containing much nitric acid were evaporated on it. This difficulty was overcome by allowing the electrode with its drop of solution to stand for a few minutes in an atmosphere of ammonia until the yellow ammonium diuranate commenced to form. The ammonia atmosphere was simply obtained by placing a small beaker of 0.880 specific gravity ammonia solution under the crystallizing dish which was used aa a dust cover for the electrodes. To check the sample preparation, several milliliters of a sample were evaporated in a platinum crucible and ignited to U?Os. Quadruplicate exposures were made on Bmg. aliquots of this residue and compared with the results obtained by treatment of the solution on the electrodes as described above a-ith the following results: 1. Ignition in platinum, 93, 100, 98, 100; mean 98 p.p.m. silicon 2. Ignition on thr rlectrodes, 115, 107, 114, 105; mean 110 p.p in silicon The somewhat higher figure arrived at by method 2 \vas not due to the blank on the grease, as this w a ~ negligible, but might have been due to slight penetration of thr graphite by the nitric acid resulting in the introduction into the arc stream of a small amount of silicate from the electrode. However, a blank run on nitric acid alone showed that no silicon g a s introduced in this way. The difference is probably due to small losses of silicon on the walls of the platinum crucible in method 1; 'no such loss can occur when the solution is processed directly on the electrode. PROCEDURE
A suitable volume of the solution for anal\&, in this case 0.02 ml. which contained 6 mg. of UYOg, was transferred by means of a graduated pipet with plunger control to the prepared electrode, the free acidity was neutralized as described above, and the solution was evaporated to drl-ness under radiant heat lamps. The samples could not be arced in this condition, because copious vapor evolution would have expelled the charge bodily from the electrode cup. They were therefore first ignited for a short time in a Bunsen flame to convert the uranium into UaOs, the base of the electrode being held in platinum-tipped tongs. A little care was necessary in the first stage of the ignition; the surface of the charge had to be heated just outside the flame until the ovide crust became sufficiently porous to allow the passage of vapor
Table I. SaullJlc
Reproducibility of Analysis Silicon P.P.lf. 2 3
1
-
4
Mean
from ivithin the bulk of thcl charge. Aftcr the first two or t,hree ignitions this procedure gave no difficulty. At least two exposures n'ere made on each solution together nith the synthetic standards (6-nig. charge) on the same photographic plate under the following conditions. Haird, grating spectrograph (&meter), 15,000 l i n e per inch. First ordrr. Range 2.5 (2165 to 3580 .A,). Slit 10 microns. Gmting 3-em. aperture, image of source focused on grating. Plate Ilford Ordinary. Exposure 30 seconds a t 10 amperes, sample as anode. Plate calibration l.5-st,ep sector at secondary focus, range 8-second order. Hilgcr Littrow spect,rograph. Range 2220 to 2900 h. Slit 10 microns. Plate Ilford Ordinary. Exposure 30 seconds a t 10 amperes, sample as anode. A suitable line pair.for use w t h 1 ~ 1 t hspectrographs was Si 2516.1 and U 2519.0 A , , measurement being made with the Hilger nonrecording microphotometer. .2 calibration curve n-as drawn up of log intensity ratio against log of parts per million of silicon in U308. RESULTS
R.esults were expressed as parts per niillion of silicon in utos and, assuming that the conc,entration of uranium in the original solution was known, could be converted into micrograms per milliliter of solution. Typical results on samples run in quadruplicate are shown in Table I. The standard deviation CAIculated on sixt,y duplicate sample results wm 9.42%. ACKNOWLEDGMENT
Ackno~vledgment is made to the Director, Atormc Energy Research Establishment, €Tarwell, England, for permission to publish this paper. LITERATURE CITED
(1) Sciibner, A. F., and Mullin, H. R., J . Research Natl. Bur. Statidarda, 37, 379-89 (1946); Research Paper 1753. (2) TTalsh, A , , Spectrochim. Acta, 4 , S o . 1. 47-56 (1950). RECEIVED Jiine 21, 1950
Tiselius-Claesson Interferometric Adsorption Analysis Apparatus Improvements in Design and Use HALF11 T. HOLMAN AND LENNART HAGDAHL' Texas Agriculture Experinen t Station, Texas Agricultural and AMechnicalCollege System, College Station, Tex.
HE adsorption analysis apparatus developed by Tiselius in the
has proved to bc a Chesson ( I , chromatographic separation of sugars (9,121, amino acids and peptides ('O), fatty acids ( 4 , maCromolecules (', 8)' '), Experience gained through building two instruments for this laboratory has led to some modifications in design that have increased the versatility of the apparatus and altered and simplified its manipulation. The ~ k ~ l iapparatus ~ ~ coIlsists - ~ ofl tu-o ~ main ~ ~ parts, ~ the vertical chromatographic column and the horizontal interferometerby of Tvhich observations are made upon the effluent. The model described is shown in Figure 1. 1 Permanent address, Biocheniical Institute, University of Gppsala, Uppsala. Sweden.
COUPLED FILTER SYSTEM
The chromatographic colunln, A , is segmented, consecutive segments decreasing in internal diameter from the top to the l,ottom of the column. The filters (segments) are joined by couplings having capillary bores through which the effluent passes. This arrangement, first used by Hagdahl (S'), thoroughly mixes the effluent from one filter and passes it on to a fresh filter of diameter. The inevitable irregular front! are thus ~smaller ~ "ironed out," and, as the fronts pass to successively smaller filters, these irregularities are expressed in smaller and smaller volumes of effluent. Frequently fronts are observed enconipassing only 1 or 2 ml. Without this coupled filter system and the mixer, observation of the separated zones is difficult.
