cause major shifts in thi! slope or shape of the curve, or they can compensate for one another. For example, if the data of Table IT' are examined closely for the T-arious batch parameters, it is seen that for Batches D 126,'151 t>hroughD 133,158 a good straight line is obtained o for thc plot of ( X C O ) L ~ ' ( S C O )2s. castability n-ith different slopes when different cabalysts are used; when the pai,ticle size, distribution of the solid ingrrdient~is varied, the slope remains nearly the s m e ; however, different intercrpts arc observed. In Figure 1 (lata deinonst,rating this latter effcct are -h(iivii. For the same degree of polymerizatioii, the castability increased in the following oydcr of mnionium perchloyate dist'ribution 51.6%, 59.7 and 72.6Yc ungroiind, and has the ma mum ra5tability whe:i the oxidizer 1)article size distribution n-as that which gave thv maximum hulh density.
time, t, the sum of the isocyanate concentration ( S C O ) , and carbonyl COILcentration (CO),, equals the initial concentration of isocyanate (KCO),,
(SCO),
=
(SCO),
+ (CO),
(6)
Substituting Lambert-Beer-Bouger law definitions and experimental parameters in Equation 8 the follon-ing results
where 771 is the initial concentration of isoeyanat,e, meq. per gram; the rrst of the terms have been defined earlier. From laboratory experiments or on actual quantitative extractions of the propellant mix for a series of different extraction times, st'raight line curves fitting the expression of Equation 9 result when -Irco, zr.bsco is plottrd as a function of -Icn zcbco. The molar absorptivities of the isocyanate and carbonyl gIoups are calculated from the slope, s, and intercept, I , of the straightline ciir1.e as follows: I
DETERMINATION OF MOLAR ABSORPTIVITIES
For the d[+rminatio i of the various ratios described abow, an accurate i ~ i l u cfor the molar a bsorpbivities of isocyanate, carbonyl, and hydroxyl are required-the values rqresentative of the grouping in the exact medium. Throughorit this work a necesary as3iiniption \\-hose validity has been pro\.eii clscwhere ( I ) is t,hat a t any
and e[,, =
- esco -~
1\11)
The molar absorptivity of the hydroxyl group is obtained from knonn diluted concentrations of polypropylenc glycol in a manner similar to that ieported previously ( 2 ) . The valueb of €\LO, ~ C O , and used in Tables I, 11, and 111 (sodium chloride optic-) ale 1165, 376, and 47.6 ml.:meq.-cm., I C spectirely. The xalues of E \ C O and ecO used in Table IV (prism-grating interchange) are 1289 and 450 nil. meq -em. ACKNOWLEDGMENT
The author acknowledges the efforts of E. ;i.Lawler and -1.Longwell who asaided 17-ith some of the determination-. LITERATURE CITED
(1) Burns, E;. A . , Lawler, E. A , , J . ( ' h e u i .
Ena. Datn. in mess. ( 2 ) B"urns, k,d., Muraca, R. F., A%\ CHEM.31, 397 (1959).
%I,.
(3) Heigl, J. J., Bell, 11.F., White, J. I-., Ibzd., 19, 293 (194i). ( 4 ) Smith, D. C . , Miller, F. C J . Opt. S O C . -4m.34, 130 (1944).
RECEIXED for review December 31, 1962. Accepted ?.lay 10, 1963. Presented in part at the Division of Analytical Cheniistry, 144th Meeting, ACS, Los hgeles, Calif., April 1963. Work supported by TJnited Technology Corporation.
Gravimet r'ic Deter minutio n of Germanium in Si Iico n-Germa nium AI I oys K. L. CHENG and B. L. GOYDISH RCA laboratories, Princeton, N. 1. b When silicon-germanium alloys are dissolved in a mixture o f nitric and hydrofluoric acids, the silicon i s completely volatilized as its fluoride. nf o rt una t e I y , a pp ro;< ima t e1y 5 0 of the germanium present may also be lost due to the formatilm of a volatile g e r ma nium fluor ide (:ompI ex. Citric acid can effectively eliminate this loss, thus making it possible to determine germanium gravimetrically as GeOt. This has resulted in an accurate method for determining the composition of silicon-gernianium alloys, wherein, after dissoluticn of the sample and volatilization o f tlie silicon as its fluoride, the citric acid i s removed b y ignition, leaving a reijidue of determinable GeOl. Complexants other than citric acid have been studied; however, only citric ticid has been found to be completely effective.
u
70
I
s- A DLTAILLD study ( 2 >6 ) of the intrinsic optical absorption y e c t r a and the electronic propertie- of the germanium-silicon alloy -y'tem, it became necessary to analyze these alloys quaiititatively. -illthough qpectrographic and polarographic methods for analyzing the germanium-ilicon alloys have heen reported, their accuracy does not meet the requirement for the work currently carried out in our laboratories. The ition of the alloy, must t h an accuracy of better than i1% relative error over the range of 20 to 100% gernianium. The standard deviation of the spectrographic method (4) is only 3=8 and the accuracy of the polarographic methcd is not better than 1 2 to 3% relative error. The accuracy of the polarographic method to better than +lye relative error was probably mistakenly claimed (3, 5 ) .
The j ~ r o p o m lnicthod involve3 separation of silicon as silicon fluoride a i d deterniination of germanium as Ge02, citric acid being uied t o prevent the loss of germanium as a volatile fluoride. Silicon is determined by difference. Ah x-ray fluoresence method for analyzing the silicon-germanium alloys has been developed ( 1 ) . EXPERIMENTAL
K i t h gentle hrating, disqolre thc .ample containing 10 t o 100 mg. of germanium in a covered, tared 50-nil. platinum crucible, using 2 nil. of concentrated nitric acid, 1 nil. of 4b7G hydrofluoric acid, and 10 ml. of 1M citric acid. *ifter diisolving the cample, wash t h e cover with a minimum amount of water and evaporate on a hot plate a t approsimately 300" C. to dryness. Then ignite Procedure.
VOL. 35, NO. 9 , AUGUST 1963
1273
Table 1. Ignition of Germanium Dioxide Precipitate a t Different Temperatures
Germanium determined, wt. %" Difference, 07 850" C. 1060" C. /c 48.3 48.2 0.1 49.5 49.4 0.1 a The samples were decomposed by the acid mixture with a cover on the crucible.
in a muffle furnace at 850" to 900' C. for 20 minutes, or until free of organic matter. Cool and weigh the precipitate as GeOz, the gravimetric factor being 0.6941. RESULTS
Effect of Temperature. For complete destruction of citric acid, i t was necessary to learn t h e optimum temperature at which t o ignite the GeOz precipitate. Two 100-mg. samples (approximately 70 mole % Si and 30 mole % Ge) were treated according t o t h e above procedure. The residues were ignited first a t 850" and later at 1060" C. Incomplete decomposition of citric acid resulted a t temperatures below 800' C.; a t temperatures higher than 1100' C., appreciable loss of platinum occured. The results shown in Table I suggest that the optimum ignition temperature lies bet\veen 850' and 1060" C.
Table II. Effect of Complexing Agents on Masking Formation of Germanium Fluoride
28 mmole HF present Germanium Amount PresReadded, ent, covered, Compound mmole mg. 7c Citric acid 0 . 5 72.3 65.18* 1 . 0 72.9 76.45" 1 . 5 73.7 84.95'& 2 69.5 88.80 2 . 5 75.2 99.38a 3 68.8 99.40 3 56.4 99.70° 63.0 99.25 5 8 75.3 99.80 10 69.5 99.45 10 71.8 99.81a 10 29.1 99.55. 90.0 99.420 10 Tartaric acid 10 53.3 9 7 . 0 10 58.1 9 8 . 0 Oxalic acid 10 46.0 48.5 10 59.1 7 7 . 8 Mannitol 10 3 8 . i 90.0 Glycerol Sulfuric acid 3 . 6 76.7 96.0 3 . 6 60.0 90.2 None . . . 53.1 49.8 . . . 64.7 4 9 . 2
Effect of Hydrofluoric Acid. The amount of hydrofluoric acid added must be sufficient t o volatilize all silicon present; however, a n unnecessary excess of acid may compete with citric acid. For a 100-mg. sample (mole yo: 70y0 Si, 30% Gel, a minimum amount of 15 nimoles of fluoride is required to dissolve the sample and volatilize all the silicon One milliliter of hydrofluoric acid (equivalent to 28 mmoles) is general1)sufficient for the 100-mg sample. Effect of Citric Acid. The stabilit? constants of both germanium fluoride and germanium citrate complexe; are not known; however, i t seems that germanium forms a more stable coniplex a i t h citric acid than with fluoride as evidenced by the fact t h a t citric acid can prevent formation of volatile germanium fluoride. When the ratio of citric acid t o fluoride is small, formation of germanium fluoride will not be effectively prevented because of competition. .in experiment was conducted by adding various amounts of citric acid to the mixture of 28 mmoles of hydrofluoric acid and approximately 70 mg. of pure germanium according to the procedure previously described. For approximately 1 mmole of germanium, 2 5 mmoles of citric acid or more are required, and the excess of citric acid had no effect on recovery of germanium. The results shown in Table I1 and Figure 1 suggest that the germanium citrate complex may consist of 1 germanium and 2 citrate ions. When free fluoride is present, it is necessary to have an excess of citric acid The stoichiometric ratio aids in calculating the amount of citric acid required. Other Complexing Agents. Germanium is knoivn to form complexes ivith glycols and oxy acids; therefore other organic compounds containing polyhydroxy groups a c r e tested. Thc results in Table I1 also shorn that these organic compounds could c o n pete with fluoride-but not as effectively as citric acid-in preventing germanium from forming a fluoride complex. Sulfuric acid complexes germanium but does not give sufficiently accurate results. Furthermore, evaporation of sulfuric acid is time consuming
DISCUSSION
The gravimetric method is still lireferred for the accurate determination of germanium. Since the cheniical behavior of germanium is so similar to that of silicon, the chemical methods for germanium require prior separation of silicon. Hvdrofluoric acid is comnion!i.~ used to &move silicon; homver, germanium fluoride is also partially volatile, as evidenced by a pur1)Iish residue upon heating. -It the beginning, ~~
The in a platinum covered by the acid mixture crucible. for other samples were obThe a
tained without using- a cover. 1274
ANALYTICAL CHEMISTRY
~
io0
0
2
3
[C I T R 4 T E ]
/ [ G e4+]
I
Figure 1. of citrate
4
5
Effect of various amounts
One mmole of germanium and 28 mmoles of hydrofluoric acid present
low results for germanium were invariably obtained ivhen hydrofluoric acid was used to decompose the alloy and to remove silicon. The loss of germanium caused by formation of a volatile germanium complex n as apparent; it n-as thought that addition of a stronger complexing agent to mask the reaction of germanium and fluoride might prevent such loss. The experimental results proved this to be true. I t Yhould be emphasized that citric acid in the present case is actually a masking agent which protects germanium by preventing formation of the interfering germanium fluoride complex, although citric acid reacts with neither silicon nor fluoride. As applied here. "masking" differs from the more common cases !There the masking agent reacts with the interfering substances. llthough the silicon--germanium alloys are not attacked by common acids, it was found that they are easilj dissolved in a mixture of concentrated nitric acid and hydrofluoric acid ACCURACY A N D PRECISION
The accuracy (better than 11 % relative error) of the method is demonstrated by the results obtained for 20 to 100 mg. of germanium. Precision of the method was tested by running 10 determinations with 100 mg. of germanium (plus 30 mg. of silicon) with a standard deviation of about 10.125 The initial results, although satisfactory, were obtained by dissolving the sample 15 ith concentrated nitric acid and hydrofluoric acid without a cover on the crucible. Slightly low result? for the .ynthetic mixtures vere occasionally obtained: later, in the adopted final procedure. the crucible n as covered during dissolution and the accuracy was imy Iiroved. .ipparently the relatively rapid reaction between the alloy and the xcitl mixture ma>- have enured a slight
splattering. which may have been overlooked since the reaction occurred in a small platinum crucible. 9 n average 99.59i., rwovery was always obtained with pure germanium (semiconductor grade) hy the proposed procedure. For further improvement of the accuracy, there might be no objection to an empirical correction -Le., using an empirical gravimetric
factor of 0.6976 instead of the theoretical one, 0.6941. LITERATURE CITED (1) Bertin, E, p,, ~ ~carp. d of ~i ~ ~ Harrison, S . J., unpublished work,
Feb. 1963.
o., H e h . PhyS. A C t U 33,437 (1960). (3) Gardels, Marvin C., Lincoln Laboratory, Massachusetts Institute of Tech-
( 2 ) Busch, G.,
nology, Lexington, Mass., private communication, 1962. (4) Gardels, Marvin C., Whitaker, Hubert H., ANAL. CHEW 30, 1496 (1958). (5) Johnson, ~ ~ E. R.1 i Chistian, ~ s. 1~T . j Phys. Rev. 9 5 , 560 (1954). RECEIVEDfor review March 21, 1963. hccepted >lay 1, 1963. The research reported was supported by the Department of Savy, Bureau of Ships under contract Sobs-84660.
The Determination of Oxygen in Lithium R. J. JAWOROWSKI, J. R. POTTS, and E. W. HOBART Pratt & Whitney Aircra1:t-CANEl,
Middletown, Conn.
b A method i s propo!,ed for the determination of oxygen in lithium and in other alkali metals involving solution of the metal in liquid ammonia followed b y filtration and titration of insoluble oxides. Quantitative iretention of the oxides b y fritted glass filters i s demonstrated b y the agreement of results obtained using coarse-, medium-, and fine-porosity filters. Nitrogen dissolved in lithium samples i s not retained b y the filter as indicated b y analysis of the residues. Titration curves almost always reveal the presence of carbonate in the solution of the residues in an amount approximately equivalent to the total carbon content of the lithium, as determined b y the combustion method reported previously. This observation, coupled with the previously observed fact that carbon in lithium ordinarily forms carbonate when samples are dissolved in water, leads to further speculztion regarding the form of carbor dissolved in lithium. Various methcds for handling lithium samples prior ‘0 analysis are discussed, as i s validation of the method b y recovery of known additions of oxygen as lithium oxide to lithium and b y cornparkon of results with the amalgamation method when applied io sodium, ipotassium, and cesium.
T
high heat capacity, and broad range of liquid temperatures of lithiuri qualify i t for use as a high-temperati re heat-transfer fluid. For the successful use of this substance in any such application. its interactions with container materials must be evaluated. It is well known t h a t traces of the e1em:nts carbon, nitrogen, and oxygen exert disproportionate effects on the corro+e behavior of hot alkali metals. T o study such effects, methods for the determination of traces of these element3 have been required. The development of such methods has been a tremendous chalHE LOW DESSITY,
lenge to the analytical chemists involved. Perhaps the most difficult of these problems has been the development of a method for the determination of oxygen in lithium, principally because of the fact that the chemical behavior of lithium is not typical of that of the alkali metals in many respects. This report deals with the development of a new approach to the solution of this problem-the use of liquid ammonia to separate oxides of lithium from lithium. Slthough i t has not been possible to validate fully the proposed method because of the lack of standard samples and because of the absence of sufficient comparison analyses, we feel that the results obtained to date are sufficiently promising to warrant announcement of the technique at this time. A number of methods have been proposed for the determination of oxygen in alkali metals. The first and by far the most frequently applied of these methods is the amalgamation technique proposed by Pepkowitz and Judd (6) This method involves solution of the alkali metal in mercury followed by separation and titration of the insoluble oxides. Williams attempted to adapt this method to lithium but found i t to be impractical because of the low solubility and the slow rate of solution of lithium in mercury (12). TVhite. Ross, and Rowan ( 2 1 ) devised an ingenious method for the determination of oxygen in sodium and in sodiumpotasyium alloy. This technique involves the reaction of the alkali metal sample with a n alkyl halide in a 11-urtz synthesis to produce an alkane and the halide of the alkali metal. The unreacted oxides could then be extracted and titrated. Kirtrhik (4) has modified the method and applicd it to the analysis of potassium. Unfortunately, attempts to adapt the method to the analysis of lithium failed both in our laboratory and a t Oak Ridge (10) because of various ill-definrd side reactions. The first method rrported for the chemical determination of oxygen in
lithium was that of Sax and Steinmetz (8). This method, a n adaptation of Eberle, Lerner, and Petretic’s technique for oxygen in calcium ( d ) , involves the solution of lithium in anhydrous methanol, neutralization of the solution with salicylic acid to form the salicylate of lithium and an amount of water equivalent to the oxide in the lithium, and titration of the resulting water with Karl Fischer reagent. The method as proposed suffered from intolerably-high blanks due to residual water in the methanol and was quite complex. Attempts to improve the method in our laboratory succeeded in reducing the blank to an acceptable value by substituting distilled and molecular sieve-dried butyl cellosolve for the anhydrous methanol originally proposed. The method was further . coulometric generation of cher reagent using a refinement of lleyer and Boyd’s technique (6). Unfortunately, the resulting method x w i o tedious and complex as to make it of little value Turovtseva and Lit! inor a (9) have done some work in Rusqia which indicates that oxygen might be determined in lithium by a vacuum fusion technique. It involves a predistillation of the alkali metal from a graphite crucible at a lower temperature than is required for the reduction of the oxides by carbon, followed b> a vacuum fusion analysis of the residue. Since vacuum fusion equipment is fairly common in this country, further investigation of this approach would seem to be in order. Quite recently, Goldberg (5) reported that oxygen can be determined in lithium using a high temperature fluorination technique. =it the time this work began, the only useful method available for the determination of oxygen in lithium was the neutron activation method proposed by Bate and Leddicotte ( 1 ) . Unfortunately, the required equipment is not generally available so that there still exists a need for simple methods for the determination of oxygen in lithium using VOL. 35,
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