Flame Photometric Determination of lithium, Rubidium, and Cesium in Silicate Rocks E. L. HORSTMANl Rock Analysis Laboratory, University o f Minnesota, Minneapolis, M i n n .
For the flame photometric determination of the trace alkali elements, lithium, rubidium, and cesium, in silicate rocks, the alkali metals and magnesium are separated from the other rock constituents by sulfuric acid and hydrofluoric acid decomposition followed by precipitation of the ammonia group with solid calcium carbonate. Calcium is removed from the filtrate by precipitation of the sulfate in 507, ethyl alcohol. The alcohol is removed from the final filtrate by evaporation and the salts are dissolved in water. Flame interference f r o m sodium and potassium is compensated by use of appropriate standard solutions. The method shows a precision and accuracy within 10% in the range from 10 to 200 p.p.ni. of the alkali metal and is sensitive to 5 p.p.m. of lithium, 10 p.p.m. of rubidium, and 5 p.p.1". of cesium.
T.
HE determination of the trace alkalies in silicate rocks has
been restricted to optical spectrographic techniques until the recent application of radioact,ivation ( 6 , 6 ) ,mass spectrometer (J), and flame photometric methods. Flame spectrophotometric techniques for the determination of lithium have been discussed by Ellestad and Horstman ( 2 ) . Williams and Adams ( 7 ) determined rubidium and cesium in glasses in amounts and associations quite different from natural silicate materials. The purpose of the present investigation was to develop a rapid Hame photometric procedure for the determination of lithium, ruliidium, and cesium on a single sample with accuracy suitable for geochemical interpretation. The method described here has been used for such deterniinations in a large number of igneous and sedimentary rocks. Results of this study nil1 be published elsen here. An acid decomposition method is preferred because of rapidity. The removal of the combined oxide group and calcium from the solution obtained from the decomposition is essential when determining traces of alkiili metals. Difficulty in removing amnionium sulfate from the final solution eliminates any method using an ammonium hydroxide precipitation. The ammonium ion has a serious quenching effect on alkali emission. The method of Ellestad and Horstman ( d ) , although satisfactory for lithium, slion-s serious losses of sodium, potassium, and ruhitlium caused by occlusion and adsorption of t,he alkalies by the lead sulfate precipitate. To prevent these losses calcium carlionate is substituted for the lead carbonate, the more soluble precipitate preventing much of the contamination. Aging the precipitate furt'her reduces loss of the alkali metals. L s r of calcium carbonate to precipitate the ammonia group raises the calcium content of the filtrate t o the point where it interferes spectrally with the determination of lit,hium. Calcium is removed from the solution by precipitation as the sulfate in 50% et'hyl alcohol. The small amount of alkali sulfates lost through coprecipitation is reduced by aging overnight. This precipitation must be carried out a t a pH of 7.6 to 7.8 to minimize coprecipit,ation of the less soluble alkali sulfates. hlagnesium sulfate accompanies the alkali sulfates in the final solution, but !
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does not interfere with the determination of the alkalies in the low-temperature flame. APPARATUS AND REAGENTS
-4Beckman Model DU spectrophotometer was used with an air-natural gas flame attachment described by Ellestad and Horstman ( 2 ) . Standard Solutions. LITHIEMSULFATE. This was prepared and standardized according t o Ellestad and Horstman ( 2 ) , except that the quantities were halved. T h e solution prepared in this manner contains approximately TOO p.p.m. of lithium. REBIDIEMSULFATE. Weigh out 1.4425 grams of rubidium sulfate, dissolve in m-ater, and dilute to 500 ml. Determine the potassium content of the solution by adding known amounts of standard potassium sulfate solution to the rubidium solution. Determine an emission value for each potassium concentration and ext,rapolate the resultant emission-pot,assium concentration curve back to zero. Then determine the rubidium content of the solution by subtracting the weight of potassium found from the original weight of the salt. This solution contains approximately 1800 p.p.m. of rubidium. CESIXMSULFATE.Weigh out approximately 1.2 grams of cesium nitrate in a 50-ml. platinum dish, dissolve in distilled water, and add a slight excess of sulfuric acid. Evaporate to dryness and bring t,he sulfate t o constant weight by repeated heating to dull redness over a low flame. Dissolve the salt in distilled water and dilute to 500 ml. Determine the weight of the sulfate by drying and weighing the empty dish. It is extremely difficult to remove the excess sulfuric acid from the large amount of salt during the conversion from nitrate to sulfate, and some losses by decrepitation are likely t o occur. For this reason it is necessary to know the final weight of the converted salt. The solution prepared as described contains approximately 1900 o.u.m. of cesium. Calcium Carbonate. Analytical reagent, low in alkalies. Ethyl Alcohol, 95%. I
I
PROCEDURE
JTeigh out a 0.5-gram sample and transfer t o a 50-ml. platinum dish. Moisten the sample with xater and add 0.8 ml. of 18N sulfuric acid. Add 2 drops of 16Y nitric arid and 10 to 15 ml. of 48% hydrofluoric acid. Evaporate, with occasional stirring, t o fumes of sulfuric acid. Cool the residue and take up in a few milliliters of water. -4dd 0.1 ml. of 36h' sulfuric acid, and evaporate the solution to fumes of sulfuric acid. Add 5 ml. of water and carry out a third evaporation. Multiple evaporations are required to remove the last traces of fluoride. Cool the residue and dissolve in 25 ml. of water. Transfer t o a 150-ml. beaker and dilute to TO to 80 ml. Add several drops of bromothymol blue indicator and neutralize the solution by the addition of solid calcium carbonate. Allow the resulting precipitate to stand overnight to minimize contamination and consequent loss of the alkalies. Heat the solution with the precipitate to boiling and filter through an 11-em. medium-grade filter paper, washing the precipitate with hot water until the volume of the filtrate and washings is 150 ml. Evaporate the combined filtrate and vashings to 50 ml. Cool and add 50 ml. of 95% ethyl alcohol to precipitate calcium sulfate. Allow the precipitate t o stand overnight and filter on an 11-em. medium-grade filter paper. Wash the precipitate with a 1 to 1 solution of ethyl alcohol and water until the volume of the filtrate and vashings is about 150 ml. Evaporate the filtrate and washings to drynees on a steam bath. Bring the residue into solution with a few milliliters of water, transfer to a 50-ml. volumetric flask, and dilute to volume. INTERFERENCES
I n the determination of the trace alkalies three important types of interference are encountered. Continuum or background interferences are positive and arise from continuous spec-
1417
ANALYTICAL CHEMISTRY
1418
Present
Lithium, P.P.M. .4dded Total
17
7 14 71
Rubidium, P.P..\I. Present Added Total Found
Found
24 31 88
22 30 87
205a
18 37
55 4 4 18
1:4
;; 73 110 146 256 3BG
223 242 260
218 257 267
158 158 172 191 209 227 264 300 410 520
158 151 174 188 220 204 263 302 424 513
Granitr. G-1
11
Table 11. Comparison of Kesulls by Different Decomposition and Separation Procedures Lithium, P . P . l I . .4cid decomposition Present (8) method 23 29 28 143 59 212
Rubidium, P.P.hl. J. Lawrence Smith Acid decomposition method
24 19 31 147, I 5 9 59 216
119 137 119 150 204a
18b
146 165 110 147 2050 18
Arerage of six determinations. of five determinations.
b Average ~~
~
~~~
~
lithium compare favorably, but serious differences were Ceqiuin, P.P.M. found for rubidium. The Present Added Total Found flame determinations of ru0 10 10 10 bidium compare favorably with 21 21 21 31 31 25 the results obtained by the iso42 42 43 tope dilution and neutron ac52 52 56 08 b2 62 tivation methods. The precision of the flame spectrophotometric determinations with different methods of scparation (Table 11) and in recovery of known amounts of rubidium added (Table I ) indicates a serious error in the determination of rubidium with the optical spectrograph. X o figures are available for comparison of cesium determinations.
IZecorery of Trace Alkali Metals
Table I.
~~
~
~~
~- . ___
Table 111. Determinations of Lithium and Rubidium in Granite (G-1) and Diabase (W-1) by Different Methods Analyst
Method
Present method Ellestad. Horstman ( 2 ) Mitchell ( I ) Gorfinkle, Alirens (1) Xockolds ( 1 )
Flame spectrophotometer Flame spectrophotometer Optical spectrograph Optical spectrograph Optical spectrograph
Present method
Flame spectrophotometer
Ahrens ( I ) Mitchell ( 1 ) Nockolds ( I ) Herzog. Pinson ( 4 )
Optical spectrograph Optical spectrograph Optical spectrograph Stable isotope dilution
Smales (6)
h-eutron actiration
G-1
m.1
Lithium, P.P.M. 22 24 19 23 25
14 14 9 9 20
Rubidium, P.P.hI.
tral emission by the major alkalies and the solvent. Radiation interference involves the depression or enhancement of the emission from the measured constituent by another constituent and results in either a positive or a negative error. Line spectral interference is caused by the proximity of the spectral lines of two elements, the radiation from each contributing to the other and giving a positive error. The interference of sodium and potassium in the lithium determination has been investigated previously ( 2 ) . Approximate determinations of the alkali elements are made by using calibration curves and correcting for continuum interference. Continuum and line spectral interferences and radiation effects for lithium, rubidium, and cesium are compensated by adding appropriate amounts of interfering elements to the standard solutions. The sample is compared directly to these standards. PRECISION A S D ACCURACY
The sensitivity of the instrument at the settings used (full sensitivity; 0.4-mm. slit, 671 mp for lithium; 0.15-mm. slit, 795 mp for rubidium; 0.2-mm. slit, 852 mp for cesium) is 0.02 p,p.m. of the metal in solution, corresponding to 0.0002% metal in the sample. Because of the magnitude of errors in the decomposition and separation, this sensitivity is of use only in the determination of lithium. No standard samples n-ere available for analysis, and the only check on the method was the recovery of known amounts of the metal added to a sample on nhich replicate determinations had been made. The average value obtained without addition of the metals sought F a s used as the true value. Rubidium values obtained by a J. Laarence Smith decomposition and a chloroplatinate separation n-ere also compared to those obtained by acid decomposition. Lithium values were compared with those obtained by a slightly different method ( 2 ) . These results are given in Tables I and 11. A granite (G-1) and diabase (IT-I) (3)mere run as an additional check. These samples have been analyzed by a number of investigators using different methods (1, 2, 4,5 ) . As shown in Table 111 the flame and optical spectrographic determinations of
a
6 C
d
2050 204b 550 590 250 215,218
19c 25d 64 15 20 27.9, 29.1 221, 25.1, 27, 29, 243 26
Average of six determinations, chloroplatinate separation. Average of six determinations, acid decomposition. Average of five determinations, acid decomposition. Single determination, chloroplatinate separation.
The maximum deviation of a result from the average of two
or more results is about 10% of the amount present in the range u p to 200 p.p.m. of the metal. Above this range the maximum deviation drops to less than 5 % of the amount present. The method is considered accurate to 5 p.p.m. of lithium and cesium and 10 p.p.m. of rubidium over the range from 10 to 200 p.p.m. of the metal. ACKNOWLEDGMENT
The research v a s suggested by S. S. Goldich of thc Rock Analysis Laboratory of the University of Minnesota and supported by a grant from the Penrose Fund of the Geological Society of America for research in silicate analysis. The advice of R. B. Ellestad of the Lithium Corp. of America and of members of the faculty of the University of Minnesota concerning development of the procedure is gratefully acknowledged. The work was carried out with the financial assistance of the Gulf Oil Fellowship Plan. LITERATURE CITED
Ahren., L. H., “Quantitative Spectrochemical Analysis of Silicates,” Pergainon Press, London, 1955. Ellestad, R. B., Horstman. E. L.. A N A L . CHEJI. 27, 1229 (1955). Fairbairn. H. W..others. U. S . Gcol. Survey, Bull. 980, 1951. (4) Herzog:.L. F., Pinson. W. €I.,Jr., Geochim. et Cosmochim. Acta 8 , 295 (1955).
(5) Smales, A. A., Ihid., 300 (1955). ( 6 ) Smales, A. A, Salmon, L.. A m l y s t 80, 37 (1956). ( 7 ) Williams, J. P., ildama, P. B., J . A m . Chern. SOC. 37, 306 (1954).
RECEIVED for review January
3, 1933.
.iccepted May 25, 1956
