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Matrix problems in the determination of lithium by flameless (HGA) atomic-absorption spectrometry, and their solution
132
SHORT
COMMUNICATIONS
Raman des solutions niobiques’ et les conclusions de Miiller,5 la protonation s’effectue sur un oxygene termmal pour le niobate Na7HNbs0i9.15H20 et en position pontte pour les niobates Na6H2Nb6019. 14H20 et Na,H,NbeO,s. 13HsO (fig. 4). L’addition dune nouvelle entite Nb6019 proton&e s’effectue par la mise en commun dune arete (et elimination de deux molecules d’eau) avec l’entitb Nb6019 proton&e de base. Cette reaction d’addition est lente, ce qui explique l’evolution des solutions pendant plusieurs mois. La formation de polymeres est plus importante dans les solutions du se1 double K15H(NbS019)2 .24H10; en effet, sa structure implique l’existence dune liaison hydrogene entre deux entitts NbsOI a :6 d’autres entites semblables peuvent s’adjoindre plus facilement que dans le cas precedent. Ces observations corroborent celles que faisaient Guerchais’ ou l’un d’entre nous:’ la mise en solution de ce se1 provoque initialement la formation d’un louche soyeux qui disparait rapldement si le se1 est fraichement prepare, plus ddlicilement (un chauffage est ntcessaire) si le se1 a vieilli ou si la solution nest pas agitee immediatement. Conclusion
La proportion
de ces especes hautement
polymerisees
est faible dam les solutions de niobates alcalins. D’apres des premieres observations I1 semble qu’elle soit bien plus Importante dans les solutions de niobates de tttralkylammonium rtcemment isoles:s,9 la determination precise de la masse molaire par ultracentrifugation, methode Schlieren sera effectuee sur ces solutions. L’existence de ces esptces permet enfin de comprendre pourquoi les recherches de la masse molaire par ultracentrifugation et par spectroscopic UV des especes hexaniobiques conduisaient a des rtsultats disperses. BIBLIOGRAPHIE 1. K. H. Tytko et B. Schdnfeld, Z. Naturforsch., 1975, 30 b, 471. 2. B. Spinner, Rev. Chim. Miner., 1968, 5, 819. 3. A. Goiffon et B. Spinner, ibid., 1973, 10, 487. 4. Idem, ibid., 1974, 11, 262. 5. M. Miiller, ibid., 1970, 7, 359. 6. A. Goiffon et B. Spinner, Bull. Sot. Chim. France, 1975, 2435. 7. J. E. Guerchais, ibid., 1962, 103. 8. S. S. Larbi et B. Spinn 9. A. Marty, K. Abdm
Summary-The authors attribute to the existence of high polymers the *decrease in reactivity and the appearance of cloudiness in aged alkaline niobate solutions. Gel permeation and ultracentrifugation can be used to determine the order of magnitude (lo4 and 6 x 104) of the molecular weight of these particles and their proportion (5%). The structure of these ions is used to explain this exceptional behaviour of the solutions.
rtifonro.Vol 24.pp 132-134Pergamon
MATRIX
Press. 1977 Pnnted
m Great Bream
PROBLEMS IN THE DETERMINATION OF LITHIUM BY FLAMELESS (HGA) ATOMIC-ABSORPTION SPECTROMETRY, AND THEIR SOLUTION AMITAL KATZ and NURIT TAITEL Department of Geology, The Hebrew University, Jerusalem, Israel (Received 25 May 1976. Accepted 27 September 1976)
Summary-Flameless atomic-absorption analyses for Li in geological materials (limestones, carbonatebearing cherts, calcium containing subsurface brines and sweet waters) result in low values, which may amount to less than 10% of the real lithium concentration in the sample. The observed signal decrease is caused by the reaction between gaseous Li and Cl and the consequent formation of LiCl (b.p. = 132551360”).The Cl vapour is produced by the dissociation of CXl, molecules in the graphitetube cavity. The interfering capacity of CaCl, is marked because of its survival in the graphite tube at temperatures above the boiling points of both LiCl and Li metal (= 1317”). Hydrochloric acid represses the LI signal In a similar manner but to a lesser extent because most of it is removed during the pre-atomizing drying and ashing stages. The addition of Ca2+ to the analytical solution lowers the absorption signal only if chloride is present. This interference of Cl has been completely overcome by addition of sulphuric acid in moderate excess relative to the quantity of chloride present. The release from interference is in strict accord with the stoichiometry of the reaction: HISO + CaCl, -+ 2HC1 + CaSO,. The same remedy overcomes Cl interference introduced by the presence of hydrochloric acid in the analytical solution. The addition of sulphuric acid itself has no effect on the intensity of the atomioabsorption signal of lithium. An analogous reaction with phosphoric acid takes place, but the Li signal is weaker and less reproducible. Solutions in which the Na/Li ratio exceeds 3 x lo4 cannot be analysed for Li by the method described since a molecular sodium peak is superimposed on the major Li peak.
SHORT
COMMUNICATIONS
The importance of lithium has steadily increased since its first discovery in 1817. Its classical and modern applications, as well as its geological occurrence, geochemical behaviour, prospecting and source materials, have been described and discussed by several workerse6 Although Li can easily be determined by other analytical methods, especially by flame emission photometry, its determination by flameless atomic-absorption spectrophotometry is at least as sensitive and accurate. The concentration levels of Li in inorganic natural materials (rocks and waters) vary greatly. Flame atomic-absorption spectrophotometry is normally sufficient for analysis of silicate rocks and brines for Li, but provides insuficlent sensitivity for analysis of materials lower in Li, such as most sweet waters and certain sedimentary rock types (carbonates, cherts), for which the higher sensitivity of flameless atomicabsorption spectrometry is needed. Our preliminary attempts to determine Li in such materials by flameless atomic-absorption spectrometry revealed a very substantial signal depression when both calcium and chloride ions were present m the analytical solution. The purpose of this study is to explain the nature of this interference and to delineate a way to remove it. The physical constants of the chemical compounds used in the following discussion were taken from Weast.’
EXPERIMENTAL
Apparatus The equipment used consists of a Perkin-Elmer model 403 atomic-absorption spectrophotometer. connected to a heated graphite atomizer, model HGA-72. The signal was fed into a model 56 potentiometrlc recorder. The orlginal optics of the spectrophotometer were mochfied after Kerber et al.,’ resulting in a significant improvement in the signalto-background ratio. The spectrophotometer settings were as follows. Wavelength: 670.8 nm. Slit setting: 3 (= 0.4 nm spectral band-width). Light source: Perkin-Elmer “Intensltron” Li hollowcathode lamp, current 20 mA. Scale expansion: 1A. Recorder response-time setting: 1. Signal output mode : absorbance. The following programme for the HGA-72 was found by experiment to provide the optimal analytical conditions. First drying stage: 1 min at 82” (digital setting = 030). Second drying stage: 5 set at 140” (digital setting = 045). Heat-up stage: “Rate” was set to 7, so as to increase the temperature from 140” to 800” (digital setting = 169) within approximately 30 sec. Ashing stage: 15 set at 800”. Atomizing stage: 12 set at approximately 2660” (digital setting = 999). Purge gas: argon (left to flow continuously through the graphite furnace). Procedure Stock solutions (1N) were prepared from the following analytical-grade reagents: H,S04, HCI, CaCO, (dissolved in a minimal volume of hydrochloric acid before dilution to volume with water), H,PO& and Ca(N03)2.4Hz0. A solution contaning 2.50 x lo4 ppm sodium was prepared by dissolving sodium chloride in water. A stock solution containing 100 ppm lithium was prepared by dissolving *It IS advisable to add the sulphuric acid only after most of the water has been added to the volumetric flask, to minimize CaSO, preapitation. Also, analysis should be carried out as soon as possible after dilution. However. shght crystallization of CaSO, does not affect the Li results. because co-precipitation is negligible.
133
Li,CO, in a mmimal amount of hydrochloric acid and diluting to volume. All solutions were prepared in 50-ml Pyrex volumetric flasks and transferred, Immediately after preparation, into 50-ml double-seal plastic bottles. Seven sets of solutions were made up. (1) Aqueous solutions D.lN in CaC& and containing 60ppM (parts per milliard) Li. Appropriate amounts of sulphuric acid were added to yield final concentrations in the range 0-0.26N*. (2) Same as set (1) but with half the CaCI, concentration. This set was used to check the constancy of the H2S04/ CaCI, ratio at which the interference of CaCl, with the Li signal is removed by sulphuric acid. (3) Same as set (1) but chloride-free, calcium nitrate being added. These solutions served to show that the interference IS caused by CaCI, rather than by Ca. (4) Aqueous 0 1N HCI solutions, containing 60 ppM Li but no calcium. Sulphuric acid was added similarlv as m
(1). (5) Aqueous solutions contaming 60 ppM of LI plus various amounts of sulphuric acid similar to those applied in set (1). (6) Sulphurlc acid solution (0.2N) containing 60 ppM of Li. with Na+ concentrations varymg between 0 and 4000 ppm. (7) A set of solutions similar to set (1). but containing, instead of sulphuric acid, phosphoric acid in the concentration range 0-0.2N. A 50-ml portion of each solution was transferred into the graphite tube by means of an “Oxford” constantvolume micropipette. Calibration curves were prepared from solutions containing 20, 40, 60 and 80 ppM Li. This practice IS superior to that of analysing different sizes of aliquot from the same standard, because equal-volume aliquots ensure a constant geometry of the graphite surface from which atomization takes place. RESULTS
AND
DISCUSSION
The analytical results are presented m Table 1. It is obvious that the mere addition of sulphuric acid does not repress the atomic-absorption signal of Li. In fact, the signal yielded by a pure IJ solution is increased by some 15% by addition of a very small amount of sulphuric acid. Introduction of CaCI,, however. causes a marked decrease in the LI absorbance. On addition of sulphuric (or phosphoric) acid as well, the I,1 signal at first decreases even more, and stays low until the acid added is exactly equivalent to the CaCl, present in the solution, the Li signal then being restored to its full value and remaining steady regardless of further Increase m acidity. It should be noted that the sIgnal is steady with sulphuric acid but fluctuates considerably with H,PO,. This does not affect the uroposed mechanism. b;t is. of analytical sigmficance. ‘The exactly stoichiometric relationship between CaCl, and H,SO, (or H,PO,) suggests that the CaCl, matrix mterference is removed according to the two reactions H2S04 2H3P0,
+ CaCl,
+ CaSO,
+ 2HCI
+ 3CaC1, + 6HCl + Ca,(PO,),.
(1) (2)
Reaction (2) will be disregarded in the following discussion, because of its poor analytical reproducibility. Reaction (1) must take place anyway, because saturation with CaSO, is achieved during the drying stage of the analysis. The hydrochloric acid produced is removed on further heating at about 110”. long before the boiling ooint of metallic Li (1317”) or of LiCl-(132s1360”) is reached. The resultant CaSOa undergoes transformation into monoclinic anhydrite at around 119(r1200”, but does not melt below 1450”. i.e., some 100” above the temperature at which Li boils off. There IS a drop in the instrumental signal obtained in the initial steps of ad&ion of acid but the signal is
134
SHORT
Table
1. The atomic-absorption
LI60 ppM H2SO.,. N
Absorbance
0000 0010 0 020 0 060 0.100 0 140 0180 0 220 0 240
0 380 0 395 0 423 0 422 0 422 0 426 0 427 0423 0 422
signal
of Li as a function
LI 60 ppM 1” O.lN CaCl, Absorbance HISO,. N O.ooO 0.020 0040 0060 0 080 0.084 0.088 0092 0.096 0100 0104 0 108 0 120 0 140 0 180 0 220 0 260
0 120 0086 0047 0015 0017 0018 0015 0.017 0013 0 018 0 094 0.239 0.38 I 0.407 0.407 0.420 0.418
COMMUNICATIONS
LI60 ppM HSOa.
,n 0.05N C&l, Absorbance N
OOOO 0.010 0020 0 030
0 025 0016 0015 0015 0015 0 025 0314 0 349 0410 0 429 0 422 0 425 0 426 0 425
0.040
0048 0 052 0 056 0.060 0 080 0 120 0 160 0.200 0 240
stronger when a higher CaCl, concentration is used. It is suggested that this signal is produced by the residual unreacted CaCl, evaporating during the atomization stage, and causing light-scatter and/or molecular absorption. That chloride is the interferent can be demonstrated directly. The replacement of chloride by nitrate (Table 1) removes the interference completely, although the Ca level is the same. When chloride is introduced into the system as HCl rather than as CaCl, (Table 1) a very substantial decrease in absorbance again occurs, but the effect is removed by addition of a very small amount of sulphuric acid. The nature of the interference by chloride is easily explamed. Li and LiCl have similar boiling points (1317” and 13251360”, respectively). The dissociation of LiCl. which is a prerequisite for atomic absorption is thus very sensitive to the vapour pressure of chlorine in the graphite tube. Any chloride capable of vaporization and dissociation at a temperature below 1320” will drive the reaction LtCl * Li + Cl
of sulphuric
(3)
towards the undissociated, non-absorbing, molecular LiCl, which in the gaseous state is rapidly swept out from the graphite tube by the purge gas. CaCl, is an ideal agent for causing the observed interference. It melts at around 772” and boils at above 1600”. Thus, at the critical temperature range near 1320” it probably has sufficiently high vapour pressure to sustain a Cl-rich atmosphere in the graphite tube, but its high boiling point prevents its early selective volatilization. The conclusions arrived at in this study have found successful application in the determination of Li m geological materials such as chert, silicates, limestones and sweet ground-waters. However, sodium at relatively high concentrations was also found to interfere with the Li signal. A
acid concentration
LI 60 ppM I” 0 1N CalNO,), Absorbance H,SO+. N OOIN
0010 0 020 0 040 0 060 0.080 0.096 0100 0104 0 108 0 120 0.160 0.200 0 240
0390 0415 0 420 0.423 0 422 0.426 0419 0 425 0 426 0 428 0421 0 422 0 426 0 425
in various
solutions
LI60 ppM I” 0 1N HCI Absorbance HISO,. N OOCQ
0001 0002 OCQ6 0010 0 020 0.040 0 080 0 120 0160 0200 0 240
0 070 0 367 0 379 0 387 0400 0418 0.420 0 423 0.412 0.423 0 425 0 422
secondary peak of a molecular sodium species is superimposed on the Li peak and cannot be entirely resolved from it (by changing the heating procedure) without a probable loss of some of the Li. Unless this Interference can be removed, analysis of materials with Na/Li ratios exceedmg approximately 3 x lo4 should not be attempted. Acknowledgements-Thanks are due to Mr. Ehud Elron who took part in the early stages of this study and to Dr. A. Starinsky for his critical reading of the manuscript. This investigation was supported by a grant from the Israel Ministry of Industry and Commerce.
REFERENCES
1 A. M. Cummings, U.S. Bur. Mines Bull. 1970. No. 650, 1073. 2. K. S. Heier and G. K. Bilhngs. in Hundbook of Geochemistry, Chap. 3. Springer Verlag, New York, 1969. Am. Inst. Mining, Metall.. Petroleum 3. I. A. Kunasz, Engineers, 1975, 79 1. 4. J. J. Norton, U.S. Geol. Surrey Prof. Paper 820, 1973, 365. 5. A. B. Ronov, A. A. Migdisov, N. T. Voskresenskaya. and G. A. Korzina, Geochemistry Intematl., 1970, 7. 75. 6. J. D. Vine, .I. Res. U.S. Geol. Survey, 1975, 3. 479. 7 R. C. Weast, S. M. Selby, and C. D. Hodgman, Eds.. Handbook of Chemistry and Physics, 46th Ed. The Chemical Rubber Co., Cleveland, 1965. 8. J. D. Kerber, A. J. Russo, G. E. Peterson, and R. D. Ediger, At. Absorp. Newsletter, 1973, 12, 106.
SHORT
COMMUNICATIONS
Raman des solutions niobiques’ et les conclusions de Miiller,5 la protonation s’effectue sur un oxygene termmal pour le niobate Na7HNbs0i9.15H20 et en position pontte pour les niobates Na6H2Nb6019. 14H20 et Na,H,NbeO,s. 13HsO (fig. 4). L’addition dune nouvelle entite Nb6019 proton&e s’effectue par la mise en commun dune arete (et elimination de deux molecules d’eau) avec l’entitb Nb6019 proton&e de base. Cette reaction d’addition est lente, ce qui explique l’evolution des solutions pendant plusieurs mois. La formation de polymeres est plus importante dans les solutions du se1 double K15H(NbS019)2 .24H10; en effet, sa structure implique l’existence dune liaison hydrogene entre deux entitts NbsOI a :6 d’autres entites semblables peuvent s’adjoindre plus facilement que dans le cas precedent. Ces observations corroborent celles que faisaient Guerchais’ ou l’un d’entre nous:’ la mise en solution de ce se1 provoque initialement la formation d’un louche soyeux qui disparait rapldement si le se1 est fraichement prepare, plus ddlicilement (un chauffage est ntcessaire) si le se1 a vieilli ou si la solution nest pas agitee immediatement. Conclusion
La proportion
de ces especes hautement
polymerisees
est faible dam les solutions de niobates alcalins. D’apres des premieres observations I1 semble qu’elle soit bien plus Importante dans les solutions de niobates de tttralkylammonium rtcemment isoles:s,9 la determination precise de la masse molaire par ultracentrifugation, methode Schlieren sera effectuee sur ces solutions. L’existence de ces esptces permet enfin de comprendre pourquoi les recherches de la masse molaire par ultracentrifugation et par spectroscopic UV des especes hexaniobiques conduisaient a des rtsultats disperses. BIBLIOGRAPHIE 1. K. H. Tytko et B. Schdnfeld, Z. Naturforsch., 1975, 30 b, 471. 2. B. Spinner, Rev. Chim. Miner., 1968, 5, 819. 3. A. Goiffon et B. Spinner, ibid., 1973, 10, 487. 4. Idem, ibid., 1974, 11, 262. 5. M. Miiller, ibid., 1970, 7, 359. 6. A. Goiffon et B. Spinner, Bull. Sot. Chim. France, 1975, 2435. 7. J. E. Guerchais, ibid., 1962, 103. 8. S. S. Larbi et B. Spinn 9. A. Marty, K. Abdm
Summary-The authors attribute to the existence of high polymers the *decrease in reactivity and the appearance of cloudiness in aged alkaline niobate solutions. Gel permeation and ultracentrifugation can be used to determine the order of magnitude (lo4 and 6 x 104) of the molecular weight of these particles and their proportion (5%). The structure of these ions is used to explain this exceptional behaviour of the solutions.
rtifonro.Vol 24.pp 132-134Pergamon
MATRIX
Press. 1977 Pnnted
m Great Bream
PROBLEMS IN THE DETERMINATION OF LITHIUM BY FLAMELESS (HGA) ATOMIC-ABSORPTION SPECTROMETRY, AND THEIR SOLUTION AMITAL KATZ and NURIT TAITEL Department of Geology, The Hebrew University, Jerusalem, Israel (Received 25 May 1976. Accepted 27 September 1976)
Summary-Flameless atomic-absorption analyses for Li in geological materials (limestones, carbonatebearing cherts, calcium containing subsurface brines and sweet waters) result in low values, which may amount to less than 10% of the real lithium concentration in the sample. The observed signal decrease is caused by the reaction between gaseous Li and Cl and the consequent formation of LiCl (b.p. = 132551360”).The Cl vapour is produced by the dissociation of CXl, molecules in the graphitetube cavity. The interfering capacity of CaCl, is marked because of its survival in the graphite tube at temperatures above the boiling points of both LiCl and Li metal (= 1317”). Hydrochloric acid represses the LI signal In a similar manner but to a lesser extent because most of it is removed during the pre-atomizing drying and ashing stages. The addition of Ca2+ to the analytical solution lowers the absorption signal only if chloride is present. This interference of Cl has been completely overcome by addition of sulphuric acid in moderate excess relative to the quantity of chloride present. The release from interference is in strict accord with the stoichiometry of the reaction: HISO + CaCl, -+ 2HC1 + CaSO,. The same remedy overcomes Cl interference introduced by the presence of hydrochloric acid in the analytical solution. The addition of sulphuric acid itself has no effect on the intensity of the atomioabsorption signal of lithium. An analogous reaction with phosphoric acid takes place, but the Li signal is weaker and less reproducible. Solutions in which the Na/Li ratio exceeds 3 x lo4 cannot be analysed for Li by the method described since a molecular sodium peak is superimposed on the major Li peak.
SHORT
COMMUNICATIONS
The importance of lithium has steadily increased since its first discovery in 1817. Its classical and modern applications, as well as its geological occurrence, geochemical behaviour, prospecting and source materials, have been described and discussed by several workerse6 Although Li can easily be determined by other analytical methods, especially by flame emission photometry, its determination by flameless atomic-absorption spectrophotometry is at least as sensitive and accurate. The concentration levels of Li in inorganic natural materials (rocks and waters) vary greatly. Flame atomic-absorption spectrophotometry is normally sufficient for analysis of silicate rocks and brines for Li, but provides insuficlent sensitivity for analysis of materials lower in Li, such as most sweet waters and certain sedimentary rock types (carbonates, cherts), for which the higher sensitivity of flameless atomicabsorption spectrometry is needed. Our preliminary attempts to determine Li in such materials by flameless atomic-absorption spectrometry revealed a very substantial signal depression when both calcium and chloride ions were present m the analytical solution. The purpose of this study is to explain the nature of this interference and to delineate a way to remove it. The physical constants of the chemical compounds used in the following discussion were taken from Weast.’
EXPERIMENTAL
Apparatus The equipment used consists of a Perkin-Elmer model 403 atomic-absorption spectrophotometer. connected to a heated graphite atomizer, model HGA-72. The signal was fed into a model 56 potentiometrlc recorder. The orlginal optics of the spectrophotometer were mochfied after Kerber et al.,’ resulting in a significant improvement in the signalto-background ratio. The spectrophotometer settings were as follows. Wavelength: 670.8 nm. Slit setting: 3 (= 0.4 nm spectral band-width). Light source: Perkin-Elmer “Intensltron” Li hollowcathode lamp, current 20 mA. Scale expansion: 1A. Recorder response-time setting: 1. Signal output mode : absorbance. The following programme for the HGA-72 was found by experiment to provide the optimal analytical conditions. First drying stage: 1 min at 82” (digital setting = 030). Second drying stage: 5 set at 140” (digital setting = 045). Heat-up stage: “Rate” was set to 7, so as to increase the temperature from 140” to 800” (digital setting = 169) within approximately 30 sec. Ashing stage: 15 set at 800”. Atomizing stage: 12 set at approximately 2660” (digital setting = 999). Purge gas: argon (left to flow continuously through the graphite furnace). Procedure Stock solutions (1N) were prepared from the following analytical-grade reagents: H,S04, HCI, CaCO, (dissolved in a minimal volume of hydrochloric acid before dilution to volume with water), H,PO& and Ca(N03)2.4Hz0. A solution contaning 2.50 x lo4 ppm sodium was prepared by dissolving sodium chloride in water. A stock solution containing 100 ppm lithium was prepared by dissolving *It IS advisable to add the sulphuric acid only after most of the water has been added to the volumetric flask, to minimize CaSO, preapitation. Also, analysis should be carried out as soon as possible after dilution. However. shght crystallization of CaSO, does not affect the Li results. because co-precipitation is negligible.
133
Li,CO, in a mmimal amount of hydrochloric acid and diluting to volume. All solutions were prepared in 50-ml Pyrex volumetric flasks and transferred, Immediately after preparation, into 50-ml double-seal plastic bottles. Seven sets of solutions were made up. (1) Aqueous solutions D.lN in CaC& and containing 60ppM (parts per milliard) Li. Appropriate amounts of sulphuric acid were added to yield final concentrations in the range 0-0.26N*. (2) Same as set (1) but with half the CaCI, concentration. This set was used to check the constancy of the H2S04/ CaCI, ratio at which the interference of CaCl, with the Li signal is removed by sulphuric acid. (3) Same as set (1) but chloride-free, calcium nitrate being added. These solutions served to show that the interference IS caused by CaCI, rather than by Ca. (4) Aqueous 0 1N HCI solutions, containing 60 ppM Li but no calcium. Sulphuric acid was added similarlv as m
(1). (5) Aqueous solutions contaming 60 ppM of LI plus various amounts of sulphuric acid similar to those applied in set (1). (6) Sulphurlc acid solution (0.2N) containing 60 ppM of Li. with Na+ concentrations varymg between 0 and 4000 ppm. (7) A set of solutions similar to set (1). but containing, instead of sulphuric acid, phosphoric acid in the concentration range 0-0.2N. A 50-ml portion of each solution was transferred into the graphite tube by means of an “Oxford” constantvolume micropipette. Calibration curves were prepared from solutions containing 20, 40, 60 and 80 ppM Li. This practice IS superior to that of analysing different sizes of aliquot from the same standard, because equal-volume aliquots ensure a constant geometry of the graphite surface from which atomization takes place. RESULTS
AND
DISCUSSION
The analytical results are presented m Table 1. It is obvious that the mere addition of sulphuric acid does not repress the atomic-absorption signal of Li. In fact, the signal yielded by a pure IJ solution is increased by some 15% by addition of a very small amount of sulphuric acid. Introduction of CaCI,, however. causes a marked decrease in the LI absorbance. On addition of sulphuric (or phosphoric) acid as well, the I,1 signal at first decreases even more, and stays low until the acid added is exactly equivalent to the CaCl, present in the solution, the Li signal then being restored to its full value and remaining steady regardless of further Increase m acidity. It should be noted that the sIgnal is steady with sulphuric acid but fluctuates considerably with H,PO,. This does not affect the uroposed mechanism. b;t is. of analytical sigmficance. ‘The exactly stoichiometric relationship between CaCl, and H,SO, (or H,PO,) suggests that the CaCl, matrix mterference is removed according to the two reactions H2S04 2H3P0,
+ CaCl,
+ CaSO,
+ 2HCI
+ 3CaC1, + 6HCl + Ca,(PO,),.
(1) (2)
Reaction (2) will be disregarded in the following discussion, because of its poor analytical reproducibility. Reaction (1) must take place anyway, because saturation with CaSO, is achieved during the drying stage of the analysis. The hydrochloric acid produced is removed on further heating at about 110”. long before the boiling ooint of metallic Li (1317”) or of LiCl-(132s1360”) is reached. The resultant CaSOa undergoes transformation into monoclinic anhydrite at around 119(r1200”, but does not melt below 1450”. i.e., some 100” above the temperature at which Li boils off. There IS a drop in the instrumental signal obtained in the initial steps of ad&ion of acid but the signal is
134
SHORT
Table
1. The atomic-absorption
LI60 ppM H2SO.,. N
Absorbance
0000 0010 0 020 0 060 0.100 0 140 0180 0 220 0 240
0 380 0 395 0 423 0 422 0 422 0 426 0 427 0423 0 422
signal
of Li as a function
LI 60 ppM 1” O.lN CaCl, Absorbance HISO,. N O.ooO 0.020 0040 0060 0 080 0.084 0.088 0092 0.096 0100 0104 0 108 0 120 0 140 0 180 0 220 0 260
0 120 0086 0047 0015 0017 0018 0015 0.017 0013 0 018 0 094 0.239 0.38 I 0.407 0.407 0.420 0.418
COMMUNICATIONS
LI60 ppM HSOa.
,n 0.05N C&l, Absorbance N
OOOO 0.010 0020 0 030
0 025 0016 0015 0015 0015 0 025 0314 0 349 0410 0 429 0 422 0 425 0 426 0 425
0.040
0048 0 052 0 056 0.060 0 080 0 120 0 160 0.200 0 240
stronger when a higher CaCl, concentration is used. It is suggested that this signal is produced by the residual unreacted CaCl, evaporating during the atomization stage, and causing light-scatter and/or molecular absorption. That chloride is the interferent can be demonstrated directly. The replacement of chloride by nitrate (Table 1) removes the interference completely, although the Ca level is the same. When chloride is introduced into the system as HCl rather than as CaCl, (Table 1) a very substantial decrease in absorbance again occurs, but the effect is removed by addition of a very small amount of sulphuric acid. The nature of the interference by chloride is easily explamed. Li and LiCl have similar boiling points (1317” and 13251360”, respectively). The dissociation of LiCl. which is a prerequisite for atomic absorption is thus very sensitive to the vapour pressure of chlorine in the graphite tube. Any chloride capable of vaporization and dissociation at a temperature below 1320” will drive the reaction LtCl * Li + Cl
of sulphuric
(3)
towards the undissociated, non-absorbing, molecular LiCl, which in the gaseous state is rapidly swept out from the graphite tube by the purge gas. CaCl, is an ideal agent for causing the observed interference. It melts at around 772” and boils at above 1600”. Thus, at the critical temperature range near 1320” it probably has sufficiently high vapour pressure to sustain a Cl-rich atmosphere in the graphite tube, but its high boiling point prevents its early selective volatilization. The conclusions arrived at in this study have found successful application in the determination of Li m geological materials such as chert, silicates, limestones and sweet ground-waters. However, sodium at relatively high concentrations was also found to interfere with the Li signal. A
acid concentration
LI 60 ppM I” 0 1N CalNO,), Absorbance H,SO+. N OOIN
0010 0 020 0 040 0 060 0.080 0.096 0100 0104 0 108 0 120 0.160 0.200 0 240
0390 0415 0 420 0.423 0 422 0.426 0419 0 425 0 426 0 428 0421 0 422 0 426 0 425
in various
solutions
LI60 ppM I” 0 1N HCI Absorbance HISO,. N OOCQ
0001 0002 OCQ6 0010 0 020 0.040 0 080 0 120 0160 0200 0 240
0 070 0 367 0 379 0 387 0400 0418 0.420 0 423 0.412 0.423 0 425 0 422
secondary peak of a molecular sodium species is superimposed on the Li peak and cannot be entirely resolved from it (by changing the heating procedure) without a probable loss of some of the Li. Unless this Interference can be removed, analysis of materials with Na/Li ratios exceedmg approximately 3 x lo4 should not be attempted. Acknowledgements-Thanks are due to Mr. Ehud Elron who took part in the early stages of this study and to Dr. A. Starinsky for his critical reading of the manuscript. This investigation was supported by a grant from the Israel Ministry of Industry and Commerce.
REFERENCES
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