Determination of Lithium in Rocks Fluorometric Method CHARLES E. WHITE’, MARY H. FLETCHER2, AND JOE PARKS Eastern Experiment Station, Bureau of Mines, College Park, M d .
The gravimetric method in general use for the determination of lithium is tedious, and the final weighed product often contains other alkali metals. A fluorometric method was developed to shorten the time required for the analysis and to assure that the final determination is for lithium alone. This procedure is based on the complex formed between lithium and 8-hydroxyquinoline. The fluorescence is developed in a slightly alkaline solution of 95% alcohol and measurement is made on a photoelectric fluorometer. Separation from the ore is carried
T
HE difficulties inherent in the gravimetric determination of lithium in minerals have been reviewed by Kallmann (6)
and by Fletcher ( 2 ) . In nearly all methods previously reported for this determination, partial separations from t,he other alkalies are made and a correction factor is used t’o compensate for the sodium and potassium remaining with the lithium; or very careful separations are made and the final residue is weighed as a pure lithium salt. However, even after a supposedly “complete separation,” as in the sulfate gravimetric method, the final lithium sulfate residue contains traces of calcium and magnesium as well as some sodium and potassium. Seedless to say, it would be far better to be able to determine the lithium directly instead of having to depend on the complete separation of all other ions. In an effort to develop a direct method for the determination of lithium, a group of compounds was studied with the hope of finding a specific colorimetric or fluorometric reagent. Inasmuch as lithium has been frequently described as an interference in reactions involving beryllium, attention was directed to those compounds (chiefly the hydrosyanthraquinones) that had been reported (8) to give either direct color or fluorescence responses with beryllium. None of these compounds was found sensitive enough to serve for quantitative purposes; but oxine (&hydroxyquinoline) formed an intensely fluorescing chelate complex wit.h lithium and appeared to be the best reagent available. Oxine will give a quantitative response wit’h as little as 5 micrograms of lithium oxide in 25 nil. of solution and does not form a fluorescent substance with either sodium or potassium. Lithium may be determined with this reagent to an accuracy of 1part in lo00 in solutions that contain no other cations. Appreciable quantities of sodium decrease the intensity of the fluorescence of the lithium-oxine complex; therefore, a partial separation of the sodium is necessary. The advantages of using the oxine reaction for the determination of lithium are that a direct quantitative determination may be made without a complete separation of the other alkalies or the use of correction factors; and a t the same time, greater accuracy is achieved in shorter time than is possible with the sulfate gravimetric determination. REAGENTS AND APPARATUS
Alcohol. Redistilled 95% ethyl alcohol wr.as used for the preparation of all solutions in the fluorometric procedure. Commercial alcohol contains a highly fluorescent substance, but redistillation from an all-glass still gives a nonfluorescent reagent that should be stored in glass-stoppered bottles. When 95V’ 1
2
out by the wet method or by the distillation procedure. Sodium and potassium are removed by alcohol and ether, but complete separation is not necessary. Comparison of analyzed samples shows excellent agreement with spectrographic and gravimetric methods. The fluorometric method is more rapid than the gravimetric and produces more conclusive results. Another useful application is in the preparation of standard lithium solutions from reagent quality salts when a known standard is available. In this case no separations are necessary.
alcohol is mentioned in this paper, it refers to this redistilled product. Absolute redistilled alcohol Tyas used for separations of sodium and potassium. Standard Lithium Solution. LITHICX CARBOXATE.Reagentgrade lithium carbonate was recrystallized twice from hot water according to the directions of Caley and Elving ( 1 ) . This was analyzed by the sulfate method, and the value obtained v a s used as the lithium content. Spectrographic analysis showed that this sample contained less than 0.01 sodium oxide. STOCKSOLUTIOS. il quantity of the purified lithium carbonate equivalent to 250 nig. of lithium ovide was dissolved in a small amount of dilute hydrochloric acid. This solution was evaporated to dryness, and one drop of concentrated hydrochloric acid was added. The lithium chloride was dissolved in 95% alcohol and made up to 250 ml. with alcohol. One milliliter of this solution was equivalent to 1 mg. of lithium oxide. WORKIW~ SOLUTION.The stock solution was diluted 1 to 10 with 95% alcohol after adjustment of the alkalinity. .4 predetermined amount of 0.01 N potassium hydroxide in alcohol was added to 10 ml. of the stock solution and the mixture was diluted to 100 ml. with 95y0 alcohol. One milliliter of this 8 0 1 ~ tion was equivalent to 1 microgram of lithium ovide. The solution may be diluted further if desired. As indicators cannot be added to the solutions that are used for fluorometric work, the amount of potassium hydroxide required for adjustment of the alkalinity was determined by titration of a separate aliquot. For this, 10 ml. of the stock solution were mixed with 10 ml. of water and titrated to the methyl orange end point with 0.01 AT potassium hydroxide in alcohol. The amount of potassium hydroside used in this titration was the “predetermined amount” of potassium hydrovide added to the working solution. In many cases, the lithium chloride solution will show the correct acidity with methyl orange and it will not be neceswi-v d..-. -_ to- ~ - ..d. haw. . Ether, absolute diethyl ether. Reagent. A composition solution, 50 ml. of Solution A added to 25 ml. of Solution B, must be mixed fresh each day. SOLUTION.4. Oxine (8-hydroxyquinoline), 85 mg. dissolved in 250 ml. of 95% alcohol. SOLUTION B. Alkaline acetate solution, 0.315 gram of potassium hydroxide and 0.20 gram of anhydrous potassium acetate dissolved in 250 ml. of water. Potassium hydroxide, 0.01 N in alcohol, 0.28 gram of potaasium hydroxide dissolved in 500 ml. of 95V0 alcohol. Methyl Orange Indicator. Sensitive Fluorometer. The fluorometer used in this investigation has been descfibed ( 4 ) . Corning filter No. 3486 was used with the blue-sensitive phototube of the Beckman spectre photometer; however, Corning filters No. 3387 and No. 3780 also were satisfactory as secondary filters. PROCEDURE
A 1- to 3-gram sample was decomposed, and the mixed alkalies were isolated from the other constituents by either the Fletcher ( 3 ) volatilization procedure or the Kallmann (6) method. I n
Present address, University of Maryland. College Park, Md, Present address, U. 5. Geological Survey, Kashington. D. C.
470
V O L U M E 23, NO. 3, M A R C H 1951 either case, calcium was removed as the oxalate, and after excess oxalate mas destroyed with aqua regia, any zinc or traces of magnesium were separated with oxine. For this step, the mixed alkali chlorides or perchlorates were dissolved in a small amount of water, and 2 ml. of a 5% oxine solution in acetic acid were added; this solution x a s made alkaline with ammonium hydroxide, heated, and allowed to stand. At the end of 2 hours, the solution was filtered and the precipitate was washed with dilute ammonium hydroxide. The filtrate was then evaporated to dryness with aqua regia, followed by nitric and perchloric acids to destroy organic matter. If the Fletcher ( 2 ) method is used, magnesium is not distilled; therefore if zinc is absent the oxine separation is unnecessarJ-.
4'19 the magnesium ion did not constitute an interference, and the fluorometric measurements could be made directly on the diluted solutions after addition of the reagent. However, when a dilution of less than 1 to 5 was made, sodium fluoride was added to suppress the interference of magnesium as indicated below. For the final quantitative determination, 0.5, 1.0, 1.5, 2.0, and 2.5 ml. of the diluted sample solution were transferred to 25-m1. volumetric flasks; 3 ml. of oxine reagent was added to each, and the mixtures were diluted to 25 ml. with 95% alcohol. For samples requiring sodium fluoride, 1 ml. of 95% alcohol saturated with sodium fluoride was added to each flask. The fluorescence was measured, and the readings were interpolated from a standard curve. This curve was prepared from similar quantities of the standard solution and was made anew for each batch of reagent mixt ure. A typical working curve is shown in Figure 1. NOTES ON T H E PROCEDURE
Pipets should be used for measuring volumes. If burets are desired, a silicone lubricant should be used on the stopcock, aa ordinary stopcock grease is fluorescent. Each batch of distilled alcohol should be tested for fluorescence with a visual comparator of the type previously described ( 3 ) . EXPERIMENTAL RESULTS
Four rock samples, analyzed for lithium a t four different laboratories, were analyzed by the fluorometric method a t the Bureau of Mines. These results are compared in Table I. YICROQRAYS OF Li,O
Table I .
Figure 1. Standard Curve for Fluorescence of Lithium with 8-Hydroxyquinoline
After the isolation of the alkalies, the lithium was separated from the bulk of the other alkalies by the Palkin alcohol-ether method, as described by Scott ( 7 ) . The mixed salts were dissolved in a minimum of water (less than 2 ml. if possible), and 1 dro of concentrated hydrochloric acid was added. Twenty milfliters of absolute alcohol and 60 ml. of absolute ether were added with swirling, and the mixture was allowed to stand for 5 minutes with occasional mixing. The solution was then filtered by suction and the precipitate was washed well with a 1 to 5 mixture of alcohol and ether. The lithium-bearing filtrate was evaporated to dryness on a steam bath. When the sodium and potassium precipitate was extremely large-Le., when more than 2 ml. of water was required to dissolve the mixed alkalies-a second separation was made. The lithium residue was dissolved in 10 ml. of absolute alcohol, and 50 ml. of ether were added with swirling. One drop of concentrated hydrochloric acid was then added, and the mixture was set aside and swirled occasionally. At the end of 0.5 hour the solution was filtered with suction and the filtrate was evaporated to dryness. One drop of concentrated hydrochloric ac,id was added to the lithium chloride residue, which was then dissolved in 95% alcohol and made up to 100 ml. in a glass-stoppered volumetric flask. This solution was used for the fluorometric determination after adjustment of its alkalinity and proper dilution. FLbOROVETRIC DETERMINATION
The alkalinity of the alcoholic lithium solution was adjusted
t o duplicate that of the standard. Again, indicators could not
be added t o the solutions used for the fluorometric measurements, so a 5-ml. aliquot of the sample solution was diluted to 15 ml. with water and titrated to the methyl orange end point with 0.01 N potassium hydroxide in alcohol. Five times this amount of potassium hydroxide was added t o a 25-ml. aliquot of the sample aolution and the mixture was made up t o 50 ml. with 95% alcohol i n a volumetric flask. A sample solution that contained about 10 micrograms of lithium oxide per milliliter was found convenient for the fluorometric determination. Hence, the approximate concentration range of the sample was evaluated to determine whether or not further dilution was necessary. T o accomplish this, 3 ml. of the oxine reagent were added to 0.1 ml. of the adjusted sample solution in a 25-ml. glass-stoppered volumetric flask (or glass-stoppered graduated cylinders) and the mixture was made up to volume with 95% alcohol. The solution was thoroughly mixed a n d the fluorescence measured. The final sample solution was then prepared by further dilution of the adjusted solution if it proved necessary. When the dilution required was 1to 5 or more,
Determination of Lithium Oxide in Analyzed Samples Lithium Oxide, %
Sample Analyses by Four Laboratories Lithium ore 4.89 Lithium ore 1.37 1:;s 1:48 1.80 1.11 1.10 0 92 1.30 Lithium ore 1.10 0.92 1.30 1.11 Lithium ore 0.58 0.65 0.87 1.63 Lithium ore 0.13 0.15 0.32 0.38 Lithium ore a Separation using distillation method of Fletcher ( 2 ) . Separation using method of Kallmann (6).
Fluorometric 4.95a 1.46* 1.04b 1.06a 0.6W 0.17'
*
The first value for each rock was obtained by spectrographic analysis and the other three by gravimetric procedures. These data show that the fluorometric method produces results similar to those of the spectrographic method and within the range of the standard procedures. -4s a further check, synthetic samplee were prepared to contain known amounts of lithium, and were analyzed by the fluorometric method. Typical results are indicated in Table 11. Synthetic samples 1 and 2 mere duplicate8 and contained the equivalent of 0.175 gram potassium oxide, 0,144 gram sodium oxide, 0.03 gram magnesia, 0.05 gram of calcium oxide, 0.01 gram of ferric oxide, 0.01 gram of alumina, 0.001 gram of lead oxide, 0.001 gram of zinc, 0.001 gram of manganese oxide, and 0.2065 gram of lithium oxide. Sample 3 contained the above, but only 0.025 gram of lithium oxide. Sample 4 contained lithium oxide and magnesium oxide only. Aliquots of these were analyzed for the lithium oxide content.
Table 11. Determination of Lithium Oxide Sample 1 2 3 4
Liz0 in Aliquot, Y 57.7 33.0 37 5 22.0
Li20 Found, 56.7 32.0 39.0 23.0
Deviation. y
Y
1.0 1 .o 1.5 1.0
CHARACTERISTICS OF FLUORESCENCE
Khen exposed to ultraviolet light, mixtures of lithium and oxine produce a greenish fluorescence in weakly alkaline alcoholic solutions. The wave length of the band of emitted light covers the range of 4900 t o 5700 A. and the intensity of the fluorescence
480
ANALYTICAL CHEMISTRY
is a function of the concentration of the lithium. The fluorescence develops to a maximum almost instantly and remains stable for several days in stoppered flasks. The intensity of the fluorescence varies with changes in hydrogenion concentration, and, as these are alkaline solutions, large amounts of carbon dioxide or acid fumes in the laboratory will affect the fluorescence if the solutions are exposed to air for long periods. Alkalinity is controlled by adjustment of the neutrality of the sample and by the addition of definite quantities of potassium hydroxide and potassium acetate in the reagent. The effect of alkali on the fluorescence is indicated in Figure 2. The fluorescence increases sharply with an increase in alkalinity and then decreases to a rather constant value. This region of stability was selected as the best alkalinity at which to work, even though it did not represent the point of maximum fluorescence.
K 0
4
0
Effect of Alkali on Lithium-S-IIydroxg-quinoline Fluorescence
The presence of even very small amounts of water caused a marked decrease in the intensity of the fluorescence; therefore, the water content must be controlled carefully. If the aqueous content of the alcohol is disregarded, water is added only in the reagent mixture. A11 glassware must be dry and rimed with alcohol before use. The oxine concentration likewise affects the intensity of the fluorescence. This is illustrated in Figure 3. Although there is Rome latitude in the oxine concentration that will give consistent results, 2.0 ml. of the 0.034% oxine solution were chosen as the optimum quantity for the range of lithium determined. The fluorescence intensity is stable to ordinary laboratory light and to normal laboratory temperature changes. EFFECT OF OTHER IONS
Sodium ions in concentrations up to 1.0 mg. of sodium chloride in 25 ml. caused no fluorescence with the oxine reagent. As much ~ t 91.0 mg. of sodium chloride caused little change in the readings on 10 or 30 micrograms of lithium oxide. Over 0.1 mg. caused a decrease in the 50-microgram sample. This is illustrated in Figure 4. As the amount of sodium chloride left in the solution after the separation described is far less than this amount, it causes no difficulty. In the early stages of development of this procedure, sodium hydroxide was used in place of potassium hydroxide in the reagent. and the results were equally as good as when potassium hydroxide was used. Potassium ions up to a concentration of 1.0 mg. of potas&mi chloride in 25 ml. had no effect upon the fluorescence of lithium solutions. This is illustrated in Figure 5. Magnesium. Oxine is a very sensitive fluorometric reagent for magnesium, and as little as 0.3 microgram of magneqium
I
I O r Li,O
0
2 30 I
/
1
0.15
0.30
Figure 3.
Figure 2.
k
i
i
I
1
0.45
0.60 Yg. OF OXINE
i
1
0.75
I
I
0.90
Effect of 8-Hydroxyquinoline Concentration
oside in 25 ml. will causc H. positivt: e1'1'0rin a lithium determination. Experiments shotvrd that an ammonium hydroxide-. ammonium carbonate sepamtion left intolerable amounts of magnesium oxide in solution. The osine separation often leaves as much as 100 microgranis of magnesium oxide in solution: but with the dilution t,echnique recommended, this amount of inagnesiuni oxide will not interfere if the solution obtained after adjustment of neutrality is diluted at least I to .5. This technique overcomes the magnesium i~iterfrrence whenever the original saniple contains 10 mg. or more of littiiurn oside. Low-grade samples that require a final dilution of less than 1 to 5 are likely to contain amounts of nivynesium oxide greater than 0.3 microgram in the nli juot used for the fluoronietric determination of lithium oxide.. However, addition reagents may be used for these samples to overcomc' the magnesium interference. Several addition reagents such as-the fluoride, oxalate, borate, and hexametaphosphate ions were tried to test their efficiency in suppressing the magnesium fluorescence. Oxalate hitcl no effect upon the fluorescrncc of eit,her niagnesium o r lithiuni : borate caused a slight decrease in the fluorcwence of niagncsiuni but had no effect upon lithium; the hesariit,t:tpliosphate caused a decrease in the fluorescence of both magnesium and lithium: sodium fluoride greatly suppressed the fluorescence of magnesium and had very lit,tle efiect upon that of lithium, provided some magnesium was present. The fluoride, therefore, appeared to he the best suppressant. Typical rcwlts obtained through the tihe of fluoride ions are prcacnted in Table 111. I n the first four figures of Tstile I11 it \vi11 he noted that sodium fluoridca did not repress the l o w r aliquots sufficiently but repressed the upper ones too much. The readings of I'our or five aliquots arc ncrces.;:try for a good nver:tge.. The addition of alkali rclduccs the fluorescence of the, magnesium-osine complex much more than that of lithium. Therefore, when this method is usetl to determine microgram quantities
Table 111. Effect of Fluoride Ion on Fluorescence of Lithium-Oxine Cornplex in Presence of Magnesium
Liz0 in Aliquot, y 10 20 30
hIgO in Aliquot, y
Apparent Li90 Found without ofAddition NaF, y 25
35 45
Liz0 Found after Addition of
NnE'. 12
y
21
29 53 37 50.5 43 i 44 11.1 9.7 10 a MgO remaining in solution after oxine separation of magnesium a n d sinc. 40
V O L U M E 2 3 NO. 3, M A R C H 1 9 5 1
48 1
of magnesium, the pH of the solutions should be much closer t o the neutral point than is required for lithium determinations. The intensity of the fluorescence of the magnesium-oxine complex decreases rapidly on standing, in contrast to that of the lit’hium oxine complex, which is very stable. The nieasurement of the fluorescence of the magnesium-oxide complex was used to determine the amounts of magnesiuni left in solution after an oxine separation. The results so obtained checked very closely those obtained spectrographically on the same sample. The fluorometric method found 1 niicrogram of magnesium oxide per ml., as compared to 0.9 microgram per ml. found with the spectrograph. If the distillation method of separation is used, magnesium is effectively eliminated. Calcium. Calcium oxide in amounts as large as 30 micrograms in 25 ml. had little or no effect upon the fluorescence of lithium t;olutions. Greater quantities caused a steady increase in the intensity of the fluorescence, but the increases were not sufficiently great to allow for a method for the accurate determination of calcium. The quantity of calcium left in solution after the oxalate separation amounted to about 0.3 mg. of calcium oxide. K i t h the dilution technique, therefore, calcium does not constitute an interference. I n fact, as much BS one tent,h of the original sample could be wed before calcium would cause any apprrciable error in the detcwiiiiiation of lithium.
to remove the hydrochloric acid. Both amyl alcohol and nbutyl alcohol formed highly fluorescent substances on heating in the presence of the lithium chloride. If the solution w3s taken nearly to dryness, a tar was formed, and a t 70” the n-butyl alcohol turned brown. I t was found that by reducing the pressure and keeping the temperature below 50” C., the hydrochloric acid could be renioved without formation of these objection:ii)le materials. However, the process was too time-consunling for practical analysis. Ninety-five per cent ethyl alcohol as a separation solvent did not produce consistent results. The etheralcohol separation, as outlined by Scott ( 7 ) ,gave excellent result.. I n this method, it was found that if less than 2 ml. of water were required to dissolve the initial precipitate, a second precipitation was not necessary. COMPOUNDS TESTED A S REAGENTS
The compounds tested for color or fluorescence reactiwi3 with lithium were: 1,4-,1,s-, and l,Mihydroxyanthraquinones, 1,25,&tetrahydroxyanthraquinone, 1-amino-4-hydroxyanthraqui2 -naphthylamine, corhineal, none, p - nitroazoresorcinol, I-aniino-2-naphthal-4-eulfonieacid, p-nitrobenzeneazoresorcinol, morin, and quercetin. None of these proved sensitive enough for either qualitative or quantitative reagents for lithium. ACKNOWLEDGMENT
The study reported in this paper was completed under the general direction of Paul h m b r o ~ e ,chief, College Park Branch, Metallurgical Division, Bureau of Mines, and under the iinniediate supervision of Alton Gabriel and Howard Carl. The authors wish to express their appreciat,ion to M. J. Peterson for the spectrographic analysis and to H. TT’. Jaffe for the spectroscopic chrclr of residues for lithium. LITERATURE CITED
0.4
0.8
Mg. NoCl ADOEO
Figure 4. Effect of Sodium Chloride
0.4
0.8
MQ. K C I ADDED
Figure 5. Effect of Potassium Chloride
(1) Caley, E. R., and Elving, P. J., “Inorganic Synthesis,” Vu1 1, p. 1, New York, McGraw-Hill Book Co., 1939. (2) Fletcher, M.H., ANAL.CHEM.,21, 173 (1949). (3) Fletcher, 11. H., White, C. E., and Sheftel, M. S.,IND. ENG. CHEM.. ANAL. ED.. 18. 1’79 11946). (4) Zbid., p . 204. ( 5 ) Kallmann. S., Ibid., 16, 712 (1944). (6) Merritt, L. L., Ibid., 16, 758 (1944)
(‘7) Scott, W. W., “Standard Methods of Chemical .%nalysia.” ed. by N. H. Furman, Vol. 1, 5th ed.. p. 890, New Yoik, D. (8)
Van Nostrand Co., 1939. TT’hite, C. E., and Lowe, C. S..IND.ENG.CHEX.,ANAL.ED., 13, 809 (1941).
Zinc. Merritt ( 6 ) has shown that oxine is a very sensitive quantitative reagent for zinc. It is necessary, therefore, that the zinc be renioved as far as possible. Experiments showed that on starting with 0.046 gram of zinc, about 400 miwograms remained in solution after a hydrogen sulfide precipitation; after precipitation of 0.046 gram of zinc with oxine only 7 3 micrograms remained in solution. Tests also showed that if residues from these separations were made up to 100 ml. with alcohol, aliquots of the solution from the hydrogen sulfide separation greater than 0.2 ml. would disturb the lithium determination; more than 1 ml. of the solution from the oxine separation could be tolerated. I n an actual determination not over 0.3 ml. of the sample solution would be used; therefore, oxine provides a safe separation. The amount of zinc left in solution by the hydrogen sulfide separation is much greater than the solubility product would indicate. This may be d u to ~ colloidal zinc sulfide passing through the filter; however, usual care was used in this procedure. SEPARATION OF LITHlUM
Several so1vent.s were investigated for the separation of lithium chloride from the chlorides of sodium and potassium. After extraction with the solvent, it was necessary to heat the mixture
RECRIVLD
AIas 18. 19.50.
Determination of Nitrate in Plant Material-Correction In the paper “Determination of Nitrate in Plant Material” by C. 31. Johnson and Albert Ulrich [ A N ~ LCHEM., . 22, 1526-9 (1950)], the folloiving errors in statistical terminology appear: In TaLle I and also in Tahle 11, footnote should read, u = ____ iristcad of standard yi.cc’y3, 1;. The remainder of the footnote is correct.
st,andard deviation = I ---= deviation = __
v‘;
7 1 - 1
u =
In Table 111,footnote d l a portion should read, C.L. = niean * t
z,
rather than 4%
C.L.
=
mean *t%.
The remainder of the
footnote is correct. All calculations were hased on thc correct formulas and are correct. C. M. J O H N ~ O N
