Flame Spectrophotometric Determination of Lithium in Lithium Minerals JAMES L. KASSNER Deparfmenf of Chemistry, University o f Alabama, University, Ala. VIRGIL M. BENSON’ and ELLIS E. CREITZ Bureau of Mines, U. S. Deparfmenf o f the Inferior, Universify, Ala.
p Increased production of lithium has required developing a relatively rapid and reliable method to determine lithium in lithium-bearing minerals ranging from 0.10 to over 8.OY0 lithia. A procedure is proposed in which the sample is decomposed with a mixture of nitric, hydrofluoric, and perchloric acids. Removal of interfering ions (principally ferric, chromic, and aluminum) before analysis is not necessary. The effects of these ions on the intensity of the lithium spectrum are largely eliminated by buffering the solution within the p H range of 1 to 4 with a citric acid-ammonium citrate buffer. Under these conditions, beryllium is the only element commonly found in pegmatitic materials that produces an error. One analyst can start 12 samples and complete the analysis of another set of 12 samples each 8hour working day. Precision and accuracy are very good.
T
HE FLAME SPECTROPHOTOMETER has been used for determining lithium in silicate rocks ( 5 ) ,glass (a),magnesium alloys (9), and spodumene (3). These procedures involved either removal of interfering elements before analysis (2, 5 ) or addition of these elements to the reference standards in amounts similar to those present in the test samples (1, 3, 8-10). This investigation was prompted by the need for a method that would be relatively rapid, simple, and easily applicable to the wide range of lithium concentrations encountered in lithium minerals and other materials. APPARATUS AND REAGENTS
Flame Photometer. Beckman spectrophotometer, Alodel DU, with 9200 flame attachment and hydrogenoxygen burner. Buffer Solution. A buffer (about p H 3.68) containing 1 gram mole of citric acid and 1 gram mole of ammonium citrate per liter of solution v-as prepared. The gron-th of mold, Present address, University of Mis-
kipsippi, Oxford, Miss.
which clogged the atomizer of the burner, was retarded for about 3 months by boiling the solution for 30 minutes and storing it in a refrigerator a t about 40” F. Stock Lithium Solution. Recrystallized spectrographically pure lithium fluoride (0.4672 gram) was transferred to a platinum dish, 10 ml. of 62% perchloric acid was added, and the mixture was evaporated t o moist dryness on a low-temperature hot plate. Lithium perchlorate was dissolved in water, transferred to a volumetric flask, and diluted to make 500 ml. of solution containing 0.250 mg. of lithium per milliliter. Standard Working Solutions. Standard solutions ranging from 0.5 to 50.0 p.p.m. of lithium were prepared by adding appropriate aliquots of the stock solution to 250-ml. volumetric flasks containing 25 ml. of the citric acid-ammonium citrate buffer. The p H of these solutions was between 1 and 4. PROCEDURE
Preparation of Sample. A sample containing approximately 10 to 20 mg. of lithium oxide was decomposed in a platinum dish by treating i t with 10 ml. of concentrated nitric acid (specific gravity 1.42), 10 ml. of 48y0 hydrofluoric acid, and 5 ml. of 62% perchloric acid. Cold treatment with hydrofluoric acid for 30 minutes or longer before evaporation gave more complete decomposition (6). The sample then was evaporated on a lowtemperature hot plate to fumes of perchloric acid. I t was removed from the hot plate and allowed t o cool. Five milliliters of 48y0 hydrofluoric acid was added and was allowed to react 30 minutes or longer before again evaporating to fumes. The addition of hydrofluoric acid and subsequent evaporation were repeated twice more, or until most of the silica ameared to be vola.. tilized. After the fourth evaDoration. the sample was allowed to h m e to‘near dryness, but was not baked. Enough perchloric acid was left in the residue to permit solution of the salts on adding water. After cooling, 30 to 40 ml. of water was added and the mixture was warmed on the hot plate until the
residue dissolved, additional water or acid being added when necessary. The sample was removed from the hot plate, cooled, then transferred to a 250-ml. volumetric flask containing 25 ml. of 1M citric acid-ammonium citrate buffer solution, diluted to the mark a t room temperature, and mixed. Standardization Curve. Calibration curves were prepared by running 11 or more of the standard n-orking solutions a t a r a v e length of 671 mp. The instrument was set a t 50.0% transmittance and was balanced with the sensitivity control for reference solutions containing 20 and 10 p.p.m. of lithium, respectively. The instrument readings in per cent transmittance were plotted against concentrations in parts per million on two-cycle by three-cycle log-log paper. Sample Solutions. The sample solutions were burned under the same conditions as those used in preparing the standardization curve. The operating conditions of the instrument were checked each time a sample was burned by running standard lithium solutions which bracketed the instrument reading of the sample. DEVELOPMENT
OF
METHOD
Parks, Johnson, and Lykken (7) showed that certain cations and anions had a marked effect in determining sodium and potassium by flame photometry. They stated “that indiscriminate use of acids in a solution would result in appreciable unknown errors in the flame photometric determination of the alkali metals.’’ Some investigators (3, 8, 10) have attempted t o circumvent this difficulty by adding a fixed amount of acid to both standard and unknown. However, none has presented adequate information on the magnitude of the errors produced by the hydronium ion over an extended pH range. Effect of pH. The p H of a series of unbuffered solutions containing 30 p.p.m. of lithium as lithium perchlorate was varied by adding hydrochloric acid or barium hydroxide, and the relative intensity of the 671-mfi VOL 32, NO. 9, AUGUST 1960
1151
Table I. Effect of Cations on Relative Intensity of Lithium Spectrum (30 p.p.m. of Li) Error, Concn. of
Cation Cation, Tested P.P.3t.a Na + 50 K+ Ba +z Ca +2 h h +z hIg +z
Al +a Fe + 3 Cr Be
+J
+*
500 1000 50 500 1000 50 500 1000 500 1000 100 500 800 500 800 100 500 800 100 500 1000 100 500 750 10 50 100
Kithout bufferb
With buffer
0.0 0.8 1.2 0.4 0.8 2.4 0.2 0.0 0.4 0 6 0.8 0.4 0.8 0.4 0.2 0.4 0.8 -2.4 0.0 3.6 5.6 8.0 2.2 2.4 2.8
0.0 0.4 1.6 0.0 0.4 1.0 0.0 0.4 0 8 0.0 0.8 0.2 1.0 1.2 0.0 0.4 -0.2 0.0 0.0 0.0 0.4 1.0 0.0 0.2 0.6 0.0 0.8 1.6 5.0 14.0 0.0 0.2 0.2 0.0
500 H+
1000 500 1000 500 1000
-0.6 -4.6 XH4 4.0 2.0 a In dilutions used, 1000 p.p.m. of cation is e uivalent to 50% of a 0.5000-
gram samde. * p H of these solutions was not controlled or measured.
Table II. Relative Intensity of Spectrum for Different lithium Compounds
(30.0 p.p.m. of Li)
Compounds Relative Intensity Used" No bufferb Buffer LiOH 58.0 62.8 LiCl 58.2 62.6 LilJO3 59.0 62.5 LiClOr 58.4 62.5 Li2COs 57.8 62.5 Li F 57.4 61 _-8 LiZSO4 56.5 63.0 LiC2H302 57.2 62.5 a Reagent grade chemicals. * pH of these solutions vas not controlled or measured. Table 111.
Analysis of Synthetic Lithium Minerals
hIinerals Spodumene Amblygonite Lepidolite Petalite Cryolithionite
1152
Liz07 %
Absolute
Added Found
Error,
%
8 . 6 1 8.59 -0.02 9.69 9.69 0.00 4 . 3 1 4.28 -0.03 5 . 3 8 5 . 4 3 +0.05 18.84 18.73 -0.11
ANALYTICAL CHEMISTRY
Figure 1 . Effect of hydrogen ion concentration on intensity of lithium flame All solutionr contained 30.0 p.p.m. of ti Relative intensity dropped to 50.4 at pH 0.53 and to 44.5 o t negative pH 0.30 (2000 p.p.rn, of hydiogen as hydronium ion)
lithium line was measured. The instrument was set at 85.0 relative intensity with a solution having a p H of 4.02. The results (Figure 1) indicate that the relation between relative intensity of the lithium spectrum and p H is complex. Buffer Solution. Figure 1 indicates t h a t t h e determination should be carried out within the p H range of about 1 t o 4 for maximum sensitivity and precision of results. Although several buffers suitable for use within this p H range were available, the citric acid-ammonium citrate buffer was selected because the citrate ion could serve t h e further desirable purpose of complexing some of the interfering ions, thus helping to keep them in solution. To determine if this buffer had sufficient capacity, the pH of 206 routine lithium mineral samples was measured. The total range of pH values mas 2.9 to 3.6, and 91y0 of these values fell within the range of 3.2 to 3.5. Interference. The cations of substances t h a t might be encountered in lithium minerals were tested for interference by preparing 12 series of solutions, each containing several known concentrations of t h e suspected elements and 30 p.p.m. of lithium. Solutions of reagent grade chlorides of t h e various elements were prepared in duplicate. One solution was tested with the buffer, the other without (Table I). These data indicate that when no buffer is used, Fe+3, Cr+3, &of, and XH4+ions produce errors of detectable magnitudes. It may be significant that, except for hydrogen, within this series, these ions (and Al+3) are the only ones that hydrolyze. These data show that beryllium is the only cation commonly found in pegmatitic materials that produces an error in the determination of lithium by the proposed procedure. Fortunately, beryl, Be3Alz(SiOa)8, the most common mineral of beryllium, is not appreciably decomposed by preliminary treatment with nitric, hydrofluoric, and per-
chloric acids. Thus, interference by this element is minimized for many beryllium samples. I n weathered material, a fem secondary minerals such as bertrandite may be present. These minerals, if present, would go into solution and produce a positive error. Solutions containing 30 p.p.m. of lithium were prepared in duplicate, using eight compounds of lithium. One mas tested with the buffer, the other without. The instrument was set at 58.0 relative intensity, with a 30.0 p.p.m. unbuffered lithium hydroxide solution (Table 11). These data show that the intensity of the lithium flame is increased by the presence of the buffer. Except for three anions, hydroxyl, sulfate, and fluoride, the intensity is constant within experimental error. The cumulative effect of cations was determined by analyzing synthetic samples of lithium minerals by the proposed procedure (Table 111). PRECISION AND ACCURACY
The statistical precision of this method was found by making 10 replicate determinations on eight samples of spodumene. The coefficients of variation calculated from these data are shown in Table IV. The accuracy of the method cannot be determined precisely because of the shortage of certified standards; however. a close estimation was obtained by
Table IV.
Sample KO.
Reproducibility of Method
LizO Coefficient Exptl. of VariaYo. of Runs Mean," yo tion, "c 10 0.0633 2 23 10 10 10 10 10 10 10
a
0.759 1.11 1.72 3.49 4.42 7.37 7.52
1.43 1.48 1.18 1.82 0 840 1.14 0.545 I
X o sample or result, was discarded.
Table V.
Analyses of Lithium Ores
Li&, 70 Exptl. meana
NBS Samples No. of Type Detns. Present 97 Flint clay 10 0 . 23b 181 Spodumene 6 6 .3gC 182 Petalite 10 4.34c 183 Lepidolite 10 4.12c Foote RIineral Co. 2 Spodumene 10 5.6gd KO results discarded. * Certified value. e Provisional value, issued Feb. 24, 1953. d Established by repeated gravimetric analyses, No.
analyzing a standard spodumene sample ( i ) ,one certified, and three provisional Bureau of Standards minwals. Each sample was analyzed in replicate b y the proposed procedure (Table V). Xdditional evidence of the reliability of the method was obtained by adding knon-n amounts of lithia to a standard spodumene sample before decomposition, and the recovery was determined (Table VI). These data indicate t h a t the accuracy and precision of the proposed method are good. The federal Bureau of Mines is engaged in additional studies of the method and will publish a comprehensive report covering its findings.
VI.
Table
(yoLi20 in sample Lid% %
Probable error
0,225 6.40 4.33 4.12
&0,002 f O , 02 f 0 . 03 *0. 00
5.73
10.03
LITERATURE CITED
( 1 ) Berry, J. K., Chappell, D. G., Barnes,
R. B., ISD. ENG.CHEM., ANAL. ED. 18, 19-24 (1946). (2) Broderick, E. J., Znck, P. G., AXAL. CHEM.23,1455 (1951). (3) Brumbaugh, R. J., E’oote Mineral
Co., Berwyn, Pa., private communication. (4) Brumbaugh, R. J., E’anus, IT. E., AXAL.CHEM.26,463 (1954). ( 5 ) Ellestad, R. B., Horstman, E. L.,
Zbid., 27, 1229 (1955). (6) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic A4nalvsis.” 2nd ed.. Part 111. p. 8%, Wley, SeT’York, 1953. ( 7 ) Parks, T. D., Johnson, H. O., Lvkken. I,., AN.~L.CHEM.20, 822 (19-18). ”
Addition of Spodumene
Added None None 1 10 2 15 3 23 4 30 6 46 8.61
Total
Found
1.08 1.08 2 18 3 23 4 31 5 38 7 54 9.69
1.08 1.08 2 20 3 23 4 28 5 40 7 49 9.64
=
Lithium
to
1.08)
ilbsolute
Error, % 0.00 0.00 +o 02 0 00 -0 03 $0 02 -0 05 -0.05
(8) Redding, D. B., Phillips, R. C., Hiester, N. S., Anal. Chzm. .4cta 1 1 , 538 (1954) (9) Strange, E. E., . ~ N A L . CHEX 25, 650 (1953). (10) Killiams, J. P., Adams, P. B., J . A m Ceram. Soc. 37, 306 (1954).
RECEIVEDfor review July 13, 1959. Accepted June 2, 19GO. Presented in part Annual Southeastern Regional Meeting, ACS, Birmingham, Ala., October 1954. From the dissertation of Virgil I f . Benson to the Graduate School of the University of Alabama in partial fulfillment of the requirements for the degree of doctor of philosophy, May 1954. Work carried out under a cooperative agreement between the University of Alabama and the U. S. Bureau of Mines supported in part by- a grant from the University of Alabama research committee.
Least-Squares Treatment of Spectrometric Data H. A. BARNETT and A. BARTOLI U. S. Steel Applied Research laboratory, Monroeville, Pa.
,A least-squares treatment of spectrometric d a t a yields equations for calculating concentrations of each of n components in a mixture from absorbances measured a t n wave lengths. Absorption coefficients can b e derived by this treatment, if desired, and b e used in the conventional manner to derive the final equations. However, when numerical values for the coefficients are unnecessary, the final equations can b e derived directly. Correction terms for deviations from linearity in the absorbance-concentration relationships can b e derived directly. This treatment of data has been applied in the development of methods for the analysis of several coal-chemical systems. Corrections for deviations from linearity in the absorbance-concentration relationships were derived for certain compounds.
I
QUANTITATIVE spectrophotometry, the intensities of characteristic absorbances of the compounds in a given system a t selected w a e lengths are related to the concentrations of the compounds. These relationships are usually deduced through the use of working curves, successive approximations, or the Bouguer-Beer law of light absorption. Each of these techniques provides satisfactory solutions for simple systems in which the concentration-absorbance relationship is linear for each compound. However, serious difficulties are encountered in achieving optimum results for more complex systems; for systems in which the concentration-absorbance relationship is nonlinear for one or more compounds, or in which there is mutually interfering absorption among the components; and particularly for sys-
x
tems in which all these conditions occur simultaneously. Most of these difficulties can be minimized by the application of the least-squares procedure to develop equations which best represent the experimental data. The techniques ( 1 , 4 , 6), applications, and limitations (3, 7) of the leastsquares procedure and the simultaneous solution of equations (9,6) for the determination of coefficients have been presented by other authors. Sternberg, Stillo, and Schm-endeman (8) have recently applied a least-squares matrix method in the analysis of a fivecomponent system. I n their method, a least-squares matrix was used to determine concentrations of the five components from absorbances measured a t a large number of wave lengths. This paper shows the application of the least-squares procedures to spectroVOL. 32,
NO. 9,
AUGUST 1960
1153
