Determination of lithium in Spodumene by Flame Photometry ROBERT J. BRUMBAUGH and WILLIAM E. FANUS Research and Development Laboratories, Foote M i n e r a l Co., Berwyn, f a .
The slow, costly classical gravimetric methods for lithium assays in minerals initiated the search for a more rapid method. A accurate method has been developed for determining lithium in spodumene using the Beckman nlodel DU spectrophotometer with flame attachment of the perkin-~lmerflame photometer* amounts Of iron, calciurn, and magnesium . do not interfere. As much as 6% sodium oxide and 12% potassium oxide in the ore sample can be tolerated. Interferences caused by and solution acidity are compensated by appropriate addition to standard solutions. A method for burning the sample solutions which eliminates errors caused by unexpected variations in instrument response is presented. The Same technique n ~ a yalso be applied to lithium assays in lepidolite, petalite, and other lithium aluminum cates.
dryness on a sand bath. After cooling, about 10 ml. of distilled water are added and heated to dissolve salts and the solution is filtered into the original volumetric flask. The crucible and residue are washed several times with distilled water, the washings being added to the flask. The solution is diluted to the mark with distilled water, mixed, and poured into beakers for burning. The appropriate standards are also poured into the small beakers and the solutions are allowed to stand for 20 minutes to e ualiae their temperatures before burning. The burning procejure is described later. Gas pressures of 4 pounds per square inch of hydrogen and 17 pounds per square inch of oxygen are used. The slit opening is varied from 0.15 to 0.25 mm. for high to low lithium concentrations. EXPERIMEYTA L
The interfering effects of the various constituent elements on the lithium flame intensity Jvere studied by using synthetic SOlutions prepared from Baker and Adamson reagent grade sulfates of the elements concerned. A Beckman Model DU spectrophotometer with flame attachment, containing a hydrogen-oxygen burner, was used in the N THE search for a method to supplant the time-consuniing original investigation. Later, analyses n-ere performed by the classical methods of Smith (11) and ~~~~h ( 8 ) for lithium outlined method using a Perkin-Elmer flame photometer with assays in silicate minerals, a revie\v of the literature shelled exresults indicating that either instrument would be satisfactory. tensive use of the flame photometer for rapid determinations of The emission values given were read from the transmission dial alkalies and alkaline earths (1-7, 9, 10, 12-16), strange (13) of the instrument and are only indicative of relative intensities reported a flame photometric procedure for lithium in ternary of the lithium spectrum a t 671 mp. SO background or blank magnesium-lithium-aluminum alloys ahereby the effects of corrections were made for the data presented since the burning magnesium, acidity, and aluminum by approProcedure outlined does not require such corrections. priate addition to standard solutions. Kilberg (16)discussed the Effect of Potassium and Sodium. Because of the general presfeasibility of determining lithium in the presence of sodium, and sodium in lithium minerals, their possible calcium, and potassium by flame photometry. ~ ~ ~ dand~ ~ ence i ~ of ,potassium k interfering effects were initially considered. In solutions conan analysis of variance technique to determine the Zack ( 4 ) taining 10 and 50 P.p.m. of lithium, the potassium content was effects of lithium, sodium, and potassium on each other and provaried from 0 to 50 P.P.m. and 0 t o 600 p.p.m., respectively. pose a flame photometric method for their determination in The intensity of the lithium flame \vas constant in each series of glasses. S o n e of the glasses analyzed contained alumina. solutions, indicating that potassium does not interfere mith the of the previous flame photometry Tvorkwas specifilithium spectrum a t 671 mp. This is not in agreement with cally concerned with lithium assays in spodumene or other Ivilberg ( I 5 ) and Broderick and Zack (4). lithium aluminum silicates, the interfering effects of constituent h similar study shoivecl sodium to depress the lithium flame inelements and methods for burning the samples were investitensity when present in excess of 100 p.p.m. in solutions containing gated prior to establishing a procedure. 100 p.p.m. of lithium; the depressing effect of sodium in solutions containing 20 p.p.m. of lithium was noticed only nith sodium PROCEDURE contents greater than 200 p.p.ni. Potassium and sodium in small A representative portion of ore is ground through 200 mesh, amounts do not exhibit a combination effect in the presence of and 0.5 gram of the ground sample is transferred to a platinum dish. The dish and contents are heated to 1900" F. for 0.5 hour sulfuric acid and aluminum. and cooled and 1 to 2 ml. of concentrated sulfuric acid are added. In order to extend the study t o include constituent elements Concentrated hydrofluoric acid (48%) is added drop%-ise,allowfound in lepidolite (8.0% potassium oxide), the effect of potasing sufficient time between drops for the vigorous reaction to sium alone on the lithium flame intensity in the presence of a h subside. J\rhen further additions of hydrofluoric acid produce no vigorous evolution of silicon tetrafluoride, about 5 ml. more minum and sulfuric acid was considered (Table I). of 48% hydrofluoric acid are added and the contents of the dish The constancy of the emission readings indicates that as much evaporated just to dryness. Care should be taken to avoid spatas 12% potassium oxide in the sample can be tolerated, The tering. Five milliliters of concentrated sulfuric acid are pipetted onto the residue, and the contents of the dish are heated just to fumes of sulfur trioxide on a hot plate. After cooling, acid and residue are rinsed into a 250-ml. beaker with distilled water. The dish Effect of potassium on Intensity of Lithium Table 1. is scrubbed with a rubber policeman and rinsed several times, Spectrum" adding all the rinses to the bulk of solution. Distilled water is Emission Potassium, added to bring the final volume to about 100 ml., and the resultReading P.P.M. ing solution is heated to boiling on a hot plate until the solution 32.6 0 is clear. The solution is filtered into a 250-ml. volumetric flask 32 5 100 and the residue is washed several times with water. 32 6 125 32.7 150 Residue and paper are charred, burned, and finally ignited to 175 32.6 red heat in a platinum crucible over a Meker burner. After 32 6 200 cooling, the residue is repeatedly treated with 4 to 5 dro s of 48% All solutions contained 40 p.P.m. of lithium, 300 P.P.m. of aluminum, hydrofluoric acid, evaporating to dryness after each a c i j addition and were 0.7N in sulfuric acid. until all silica is expelled. Two to three drops of concentrated sulfuric acid are then added and the contents evaporated to
I
463
ANALYTICAL CHEMISTRY
464
procedure should, therefore, be applicable for lithium determinations in lepidolite as well as spodumene or other lithium aluminum silicates as long as the necessary compensations are made in the standard solutions. Effect of Sulfuric Acid and Aluminum. It was established that an acidic solution was necessary to ensure complete dissolution of the sulfate residues obtained upon decomposing the sample with a mixture of sulfuric and hydrofluoric acids. Since initial work showed that the presence of sulfuric acid or aluminum independently depressed the intensity of the lithium spectrum, the combination interference effect of the two depressants was studied.
dissolution of the sample, the maximum permissible acid concentration ( 0 . 7 N ) was chosen for analytical work. A series of experiments was designed to study the over-all combination interference effect that might be caused by the principal consitituents of the ore (Table IV). The solutions were prepared to contain the final concentrations designated and ere burned as a set. The presence of sodium and potassium in concentrations normally expected in spodumene concentrates do not contribute to the combination interference effect caused by the presence of aluminum and sulfuric acid. It is interesting that the combined interference of aluminum (130 p.p.m.) and acid ( 0 . 7 5 ) is the same as that of the aluminum alone.
Table 11. Effect of .iluniinunl o n Intensity of Lithium Spectruni” I l u r n l n l l n l , I’.P 11. ~ _ _ _ ._ ~ _ _
20
0
Kortnality 0.25 0. I 1.4
a
It10
.iO
200
300
400
4.5.0 44 2 42.2
43.0 44.2
Eniiiiion Reading 42.2 41.5 37.6
42 .i 43 9
41 9 42 .5 37.7
39.3
43 8 44 3 40 3
45.0 44.2 41 3
43.0
.ill solutions contained 50 p.p.nl. of 1ithiii:ii
Table 111. Effect of Sulfuric l c i d on Intensity of Lithium Spectruni” Emission Reading
Sormality 0 1 0.:
40 8 41.1 40 9 39.6 38.7
0 ,
1. o I .5
a
All solutions contained 130 p
I5
40
50
60
70
LITHIUM CONCENTRATION 1P.P.M.)
oi aluniiniitii and 30 p.p.in. of lithium.
I).II~.
Figure 1.
Standard Curve
0.15-mm. slit opening
The greatest tolerance for the presence of aluminum was obtained in solutions which were 0 . 7 5 in sulfuric acid (Table 11). After the aluminum concentration reached 100 p.p.m., no further interference was produced upon increasing the aluminum concentration up to 400 p.p.m. By preparing standard lithium solutions containing aluminum in concentrations ranging from 100 to 400 p.p.m., the alumina content of the sample t o be analyzed could vary from 9 to 36% without producing any error in the analysis. A similar noninterfering range exists for sulfuric acid in the presence of 130 p.p.m. of aluminum for acid concentrations from 0.1 to 0.7N (Table 111). For normalities beyond 0.7, the intensity of the lithium spectrum decreases rapidly. Also, a t the acid concentration of 1.4N (Table 11),increasing quantities of aluminum continually increase the lithium flame intensity. Escessive acid is, therefore, to be avoided. I n order to aid in the
Effect of Other Minor Constituents. Iron from 0 to 30 p.p.m. in 60-p.p.m.lithium solutions which were also 0.7N in sulfuric acid showed no interferences. Calcium eshibited an enhancing effect on the lithium solutions for calcium concentrations exceeding 70 p.p.m. Magnesium ranging from 50 to 250 p.p.m. in IO-, 20-, 30-, and 50-p.p.m. lithium solutions showed no interference. Standard Solutions. A series of standards was prepared to contain 2, 10, 20, 30, 40, 50, 55, 60, and 70 p.p.m. of lithium t)y weighing appropriate quantities of lithium sulfate monohydrate. All standards v-ere made to contnin 270 p.p.m. of aluminum (arbitrary choice), using aluminum sulfate octadecahydrate, and to be 0 . 7 5 in sulfuric acid. Figures 1 and 2 show standard
Table IV. Combined Effect of Principal Constituents of Sample on Intensity of Lithium Spectrum Solution CompoFition AI, Sa. p.p.m. Soriiialiry pp.111.
L1, p.p,m. -”
An
n
30 30 30 30
136 130 130 130 ~~
n
n
n
0 7 0 7 0 7
Emission,
n
0 0 20 20
0 0 0 30
7% 39 R
38 38 38 38
3 1 2
0
~
Table V.
Burning Procedure Illustration
Sample KO.
K, p.p.m.
Sample
Set 1 40.6 33.7 39.i 30.1 41.5 50.6
Emission Reading Set 2 Set 3 44.9 36.2 40.2 31.3 40.5 51.2
45.0
55.4 40.3 31.4 41.3 50.9
Set 4 45.0 35.7 40.2 31.1 41.5 51.0
I
IO
I I I e0 30 40 LITHIUM CONCENTRATION 1P.P.M.)
Figure 2.
Standard Curve
0.25-mm. slit opening
I
50
465
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4 curves prepared, using these solutions, a t slit openings of 0.15 and 0.25 mm., respectively. The very small change in slope indicates that straight-line interpolation between standards differing by 10 p.p.m. of lithium which bracket the unknown could be used for the final calculation without introducing any appreciable error in the determination. The maximum error encountered would be about +0.02y0 lithium oxide. For spodumene concentrates (6y0lithium oside), the error would be about 0.3y0,, which is within theprecisionof the method. BURXYIYG PROCEDURE
h burning procedure was devised to eliminate the necessity of continually reproducing a standard curve, the necessity of correcting for blanks or flame background, and errors caused by unexpected changes in meter response during sample aspiration. -4 series of solutions of unknowns and the standards necessary for bracketing are burned in consecutive order. The emission readings recorded a t 671 mp constitute the first set (Table V). For the second set the solutions are again burned in the same order. This process is repeated until adjacent sets check within about 0.4%. In this instance, the average of the emission values for the third and fourth sets is used for the final calculation. This technique assures that all samples are burned under identical conditions. The calculation of percentage lithium oxide is performed by straight line interpolation between the standards that bracket the unknown. PRECISION AND ACCUR4CY
B Foote Mineral Co. standard spodumene sample having a value of 5.69y0 lithium oxide (established by repcntrtl gravimetric
analyses)y a s analyzed 10 times in replicate by the flame photometer method described yielding 5.694 f 0.042% lithium oxide. The precision of the method is *o.i49;b; the accuracy is of the same order of magnitude. ACKNOU LEDG\IE\T
The authors wish to acknoi\leclge the able assistance of L. D. Zwone in obtaining many of the t ~ \ p c ~imrntxl i data. LITER .tTL'RE CITED
Barnes, R. B., Berry, J. IT., and Hill, W. B., Eng. Mining J . , 149, No. 9, 92 (1948). Barnes, R. B., Richardson, David, Berry, J. W., and Hood, R. L., IND.ESG.CHEM.,-4w.4~.ED.,17, 605 (1945). Berry, J. W., Chappel, D. G., and Barnes, R. B., Ibid., 18, 10 (1946). Rroderick. E. J.. and Zack. P. G.. A N A L .CHEM..23. 1455 (19513. Diamond, J. J., and Bean, L., Sm. SOC.Testing Materials, Preprint 128 (1951). FQX,C . L., A N A L .CHEY.,23, 137 (1951). C;illiland, J. L., Am. SOC.Testzng Materials, Preprint 129 (1951). W c h , F. A., Am. Chenr. J . , 9, 33 (1887). Ilosher, R. E. Bird, E. J., and Boyle, -4. J., A N ~ LCHEM., . 22, 715 (1950). Myers, A. T., Dyal, H. S., and Barlnnd, J. W., Soil Sci. SOC. Anaer., Proc., 12, 27 (1947). Smith, J, L., Am. J . Sci., (2) 50, 269 (1871). Standford, G., and English, L., Argon J., 41, 446 (1949). Strange, E. E., ASAL. CHEM.,25, 650 (1953). West, P. W., Folse, P., and hlontgomery, Dean, Ibid., 22, 667 (1950). Wilberg, E., Z. anal. Cheni., 131, 405 (1950). RECEIVED for review .4upmt 28, 1953
Acrepted December 18, 1'253,
Effects of Anions on Calcium Flame Emission in Flame Photometry GRAEME L. BAKER and LEON H . JOHNSON Department o f Chemistry Research, Montana State Collage, Bozeman, M o n t .
This investigation was prompted by variations in calcium flame intensity as measured by the flame photometer. Perchlorate, phosphate, sulfate, and dichromate ions have been examined for the influence that they exert on the calcium flame. The results obtained indicate that pyro- ions of phosphorus or sulfur may be responsible for the flame anomalies resulting from calcium solutions containing phosphate or sulfate ions. Perchlorate ions intensify the flame emission from calcium chloride solutions, whereas mixtures containing sulfate or phosphate ions in addition to the perchlorate ions may show lower values than those containing sulfate or phosphate alone. These effects must be considered in the use of the flame photometer for calcium analyses. Methods of obviating the difficulties which accompany these effects are outlined.
V
ARIOUS ions, both cations and anions, will affect the inten-
sity of the characteristic calcium flame emission. The quantitative appects of cationic influence have been investigated by many workers, but consideration of the effect of anions (1-3, 5-8) has been limited. Some quantitative data for phosphate ( 3 , 5, 6 ) and sulfate effects (1,d, 6) have been presented, but the data provided by these have been insufficient to the fundamental study of anionic action. This paper presents an accumulation of data illustrative of the calcium flame variation to be found upon the addition of increments of the following acids: hydrochloric, acetic,
boric, nitric, phosphoric, arsenic, sulfuric, chromic, periodic, or perchloric. EXPERIMENTAL
A Beckman DU spectrophotometer equipped with Model 9200 flame photometry attachment and using acetylene as a source of fuel was used for the cursory examinations of most of the anions considered, but was supplemented with a photomultiplier attachment before completion of the work. This change did not invalidate any of the preliminary work and all strictly comparative data were taken with the photomultiplier attached. All readings were made with standard calcium chloride solutions containing increments of the various acids. iicids were selected as a means of introducing the appropriate anions only after the hydrogen ion influence had been shown t o be negligible. The hydrogen ion effect was established as insignificant on the evidence that hydrochloric acid had no significant effect when added to a calcium acetate solution; acetic acid produced no significant variation when added to a solution of calcium chloride, nor did increments of hydrochloric acid introduce significant deviations in the calcium flame intensities resulting from calcium chloride solutions. It was felt that if the hydrogen ion did exert any appreciable influence upon the calcium flame intensity, one of these systems should yield results showing the effect. Since no significant variations were to be noted in the calcium flame emissions from any of these systems, it seemed reasonable to assume that any variations that might be produced by added acids could be attributed solely to the added anion and not to the influence of hydrogen ion.
