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The Alkali Metals (Lithium, Sodium, Potassium, Rubidium, Caesium)
CHAPTER 5 The Alkali Metals (Lithium, Sodium, Potassium, Rubidium, Caesium)
Sodium and potassium are always included in the list of elements determined in any 'complete' rock analysis.
The range of alkali metal contents varies from 100 ppm
or less in certain ultrabasic rocks, such as dunites and peridotites to as much as 10 per cent K 0 or 15 per cent Na 0 in rocks composed largely of felspar minerals. Gravimetric methods have been used for the determination of all five of the alkali metals.
These required a most careful separation of the total alkali metals from
silica, aluminium, calcium and other elements present.
The most frequently used (1) involved decomposing
procedure for this, described oaginally by Lawrence Smith,
the rock sample by ignition with ammonium chloride and calcium carbonate.
The alkali
metals were recovered by leaching with water, and were then separated from the small amount of calcium taken into solution.
Sulphates were converted into chlorides and (2) Some authors
the introduced ammonium salts were removed by volatilisation.
considered that special precautions were necessary to ensure a complete recovery of lithium with the remaining alkali metals.
The mixed chloride residue obtained after
the expulsion of ammonium salts was carefully ignited and weighed before the separation of the individual alkali metals. An alternative procedure for decomposing the silicate rock material and recovering the alkali metals as chlorides was based upon evaporation of the sample with hydro fluoric acid and precipitation of iron, aluminium, calcium and other elements with ammonia and ammonium carbonate, a method derived from work by Berzelius. The separation of lithium from the other alkali metal chlorides was based upon the solubility of lithium chloride in organic solvents such as isobutanol, pentanol, pyridine or ether-ethanol mixtures.
The determination was often completed by
converting the lithium chloride to sulphate prior to weighing.
This gravimetric
procedure for lithium is not sufficiently sensitive for application to the majority of silicate rocks, and lithium could only be reported in those samples rich in this element. Potassium was usually determined following the precipitation with chloroplatinic acid, perchloric acid or sodium cobaltinitrite.
The insoluble potassium salts were
collected and weighed directly, although a number of indirect procedures were also in common use.
Sodium was generally determined by difference, although some analysts
-59-
Chemical Methods of Rock Analysis
60
preferred to precipitate as a triple acetate of uranium and zinc or other divalent metal.
In all these precipitation procedures, corrections for the solubility of
the potassium or sodium salts were necessary, and none of the weighing forms could be considered ideal. Even when present in quantity, the separation of rubidium and caesium from potassium and from each other was seldom attempted , and on these few occasions the results obtained were not always very reliable. All these gravimetric procedures for the five alkali metals have now been rendered obsolete by the development of instrumental methods, such as flame photometry and atomic absorption spectroscopy.
The emission spectra of the alkali metals are simple,
consisting of a prominent line or doublet known as the resonance line(s), correspond ing to the transition between the lowest excited state and the ground state, together with weaker lines relating to other transitions.
In many flames, lithium and sodium
are only slightly ionised; this is in contrast to the remaining alkalis, where the degree of ionisation increases in the order K, Rb, Cs, until with caesium a large part of the metal in tie flame is in the ionised state.
This accounts for the poor
sensitivity to caesium, and the general decrease of sensitivity from sodium to caesium. Lithium is about as sensitive as rubidium. One way of increasing sensitivity to a particular element in a given solution is to introduce a second, more easily ionised element such as another alkali metal into the flame.
This serves to decrease the extent of ionisation of the required element and
thus increase the number of atoms available for the transition which gives rise to the resonance line(s).
At high alkali concentrations some loss of emission (and
consequent calibration curvature) occurs by self absorption.
This effect has been
noted particularly with lithium and sodium. Early designs of flame photometer used low temperature flames such as air-propane, or air-coal gas, together with a filter system for wavelength selection.
These
instruments could not be used for the determination of the rarer alkali metals (lithium, rubidium and caesium) as the characteristic emission could not be isolated by the filters available.
Considerable improvements were obtained with the introduction
of prism monochromators, enabling all five metals to be determined although, as described below, interferences between the metals gave rise to difficulties in determining lithium and rubidium.
These could be minimised by plotting emission
spectra thus enabling background corrections to be made.
These difficulties disappear
with the increased wavelength resolution that is obtainable using a grating monochromator.
The Alkali Metals
61
In general terms, the chemical interferences in the determination of the alkali metals are best minimised by using either a very cool (air-propane) or a very hot (acetylene-nitrous oxide) flame.
Much work remains to be done on the determination
of alkalis in silicate rocks, particularly following that of Hildon and Allen and Luecke.
A number of authors have described the determination of sodium and potassium in silicate rocks by atomic absorption spectroscopy, as well as the determination of (rp\
lithium
/ON
and rubidium.
As with flame photometric methods it is necessary to
add potassium to the solutions before determining rubidium.
Vosters and Deutsch
also recommended adding lanthanum as a flame buffer, a practice commonly used to limit inter-element effects. Rock solutions can be prepared in the same way as those for the flame photometric determination of the alkali metals. One of the most successful ways of separating mixtures of the alkali metals is by (9) (10) ion-exchange chromatography.
Resins such as Dowex 50
or Amberlite 120
can
be used for this separation, which requires the use of a cation-exchange resin as all five elements form well-characterised positive ions in solution. The separation of (11) sodium from potassium is well established, and the successful separation of (12) rubidium and caesium has also been reported. However, none of these procedures is ideal for the routine separation of all five alkali metals, and are now little used. Determination of Sodium and Potassium in Silicate Rocks by Flame Spectrophotometry The method described in detail below is based upon work by one of the authors (1*0 by Eardley and Reed.
(13)
and
The rock material is decomposed by evaporation with hydro
fluoric, nitric and sulphuric acids.
Sulphuric acid is added to assist in the removal
of fluoride ion which will otherwise depress the flame emission of potassium.
In most
cases the small amount of residue that remains after this treatment can be ignored, although for really accurate work this residue should be collected and analysed separately.
Caesium sulphate solution is added as an ionisation buffer. Method
Reagents:
Sodium sulphate, anhydrous. Potassium sulphate. Caesium sulphate solution, dissolve 0.205 g of pure caesium sulphate, Cs SO,, in 500 ml of water.
62
Chemical Methods of Rock Analysis
Procedure.
Accurately weigh approximately 0.1 g of the finely powdered silicate
rock material into a platinum dish (Note 1 ) , moisten with a little water and add 10 ml of concentrated nitric acid, 5 ml of concentrated sulphuric acid and 10 ml of hydrofluoric acid. sulphuric acid.
Place the dish on a cool hotplate and evaporate to fumes of
Cool, rinse down with a little water and add a further 5 nil of
hydrofluoric acid and evaporate until the contents of the dish are fuming well. Again allow to cool, rinse down with water, evaporate, this time until all fumes of sulphuric acid have ceased and a dry residue is obtained. Add 1 ml of concentrated nitric acid to the cool residue, followed by 25 to 30 ml of water. beaker.
Warm the dish to detach the residue and rinse the contents into a ^tOO-ml
Digest until all soluble material has passed into solution, and collect any
small residue on a small close-textured filter paper and wash with a little water. Discard the residue (Note 2) and dilute the filtrate and washings to volume with water in a 250-ml volumetric flask. Transfer 25 ml of this solution (Note 3) by pipette to a 50-ml volumetric flask, add 5 ml of the caesium sulphate solution, dilute to volume with water and mix well. Prepare also a reagent blank solution in the same way as this sample solution, using the same quantities of reagents but omitting the powdered rock material. Set the flame spectrophotmeter base line whilst spraying water, and the sensitivity at 10 ppm full-scale deflection spraying the 10 ppm standard solution, with the wavelength control set at 589 nm for sodium and 766.5 nm for potassium.
The
adjustment of sensitivity will affect the base line position and it may be necessary to reset the controls alternatively two or three times.
Once this adjustment is
complete, spray also the reagent blank solution and the sample solution, noting the flame emission recorded at both wavelengths. The flame emission of a series of standards should also be measured.
This should span the composition range 0 to 10 ppm.
The calibration is not a straight line. Where the flame spectrophotometer is fitted with a wavelength drive motor and a pen recorder, the flame response for either element can be obtained by scanning the spectrum.
The reagent blank solution and the standard solutions should similarly be
examined.
For sodium scan from about 56Ο nm to 620 nm and for potassium from 730 nm
to 820 nm, (Note *+). Calibration.
Dry small quantities of both sodium and potassium sulphates (Note 5)
in an electric oven at a temperature of 120
and allow to cool in a desiccator.
63
The Alkali Metals
Weigh 0.229 g of the anhydrous sodium sulphate and Ο.185 g of the potassium sulphate into the same beaker, dissolve in water and dilute to volume in a litre volumetric flask.
This solution contains 100 μg Na 0 and 100 μg K 0 per ml.
Prepare a 20 ppm
standard by diluting 50 ml of this stock solution, measured with a pipette, and 1 ml of concentrated nitric acid to 250 ml with water and mixing well.
Transfer 25 ml of
this solution by pipette to a 50-ml volumetric flask, add 5 ml of the aluminium sulphate solution and 5 ml of the caesium sulphate solution and dilute to volume with water.
This gives a 10 ppm standard for setting the sensitivity control of the
flame spectrophotometer.
Notes: 1.
The weight of sample material taken for the analysis and the volume of
the final solution may be varied in relation to the alkali content of the sample, but the final solution for measurement should be of the correct acid concentration and contain the recommended concentration of caesium sulphate. 2.
This small residue contains much of the zircon and some of the tourmaline
present in the rock, together with varying proportions of other minerals that are difficult to decompose with hydrofluoric acid.
These minerals seldom contain more
than trace amounts of alkali metals and the error involved in neglecting these small amounts is often within the limits of the experimental error of the method.
If the
amount of residue indicates that an appreciable part of the sample has not been attacked, an alternative procedure, such as fusion with lithium borate, should be used for the sample decomposition.
Obviously lithium cannot then be determined in this
sample portion. 3.
This volume of solution is suitable for determining up to 5 per cent of
sodium and potassium oxides in the rock sample. *t. There are a number of ultrabasic rocks that contain only small amounts of sodium and potassium.
The importance of these rocks is out of all proportion to the
extent to which they occur.
If reliable results are to be obtained for the alkali
metals at these levels, particular attention must be paid to the details of the (15) determination. It has been pointed out that at the high instrument sensitivities required for determining these small amounts of alkali metals, a large percentage of the total light emission can be due to background radiation.
The simplest way of
obtaining a background correction is to record the flame emission over the ranges involved using an instrument fitted with a motorised wavelength drive and a pen recorder. 5:
More dilute standards will also be required for these materials. Laboratory reagent grade sodium and potassium sulphates were found to be
sufficiently free from cross contamination to be used as standards in this way. more rigorous standards Specpure carbonates may be used.
For
64
Chemical Methods of Rock Analysis Determination of Potassium in Micas
The need for very accurate potassium determinations has been underlined by the simple way in which geological ages can be obtained from an accurate knowledge of the A
:K
ratios.
The small amounts of argon present can readily be recovered and
determined isotopically with a mass spectrometer.
Potassium can also readily be
determined by flame spectroscopy, but the accuracy and precision obtained by the usual procedures such as the one given in detail above, although adequate for most petrographic purposes, do not match that obtainable for argon. A flame photometric procedure for this accurate determination has been given by Abbey and Maxwell;
the sample material is decomposed with hydrofluoric acid and
sulphuric acid, and the excess hydrofluoric acid removed by fuming.
After the
addition of a known excess of magnesium sulphate, the excess sulphuric acid is in turn removed by evaporation to dryness.
The sulphates are ignited, the cooled
residue is leached with warm water, and the resulting suspension filtered, giving a neutral sulphate solution containing the alkali metals, magnesium and little else. The approximate relative proportions of the alkali metals are then determined and a neutral sulphate standard solution prepared approximating in composition to that of the sample solution.
The final potassium determination is made with a flame
photometer using this solution with added potassium as standard. The need to eliminate small systematic errors in the potassium determination was also (17) underlined by Rice , who compared dissolution methods using atomic absorption spectroscopy for the final determination.
A cation-exchange separation was used
based on the work of Strelow et al« Determination of Lithium The simple method described above for sodium and potassium can be extended to include also the lithium present in the rock material.
A somewhat greater instrument
sensitivity-setting is required, but this is well within the range of most modern instruments.
The solution prepared as described above for sodium and potassium, can
be used directly for the lithium measurement, but for rocks containing only a few ppm of lithium, a more concentrated solution is desirable. Of all the alkali metals, lithium has the least effect on the flame response of the (19) For most purposes this small effect upon
others, and is least affected by them.
the lithium response can be ignored, but for accurate determinations, the amounts of
65
The Alkali Metals
sodium and potassium present in the rock solution can be measured and similar amounts added to the standard solutions used for the lithium calibration. The most sensitive lithium line is the resonance line at 67Ο.8 nm.
Filter-type
instruments cannot be used for measurement at this wavelength as emission from potassium will also be recorded.
Similarly emission from sodium will provide a
sloping background against which the lithium emission can be recorded if a prism monochromator is used for wavelength selection.
These problems do not arise when
an instrument incorporating a grating monochromator is used.
A procedure for
determining lithium, together with rubidium and caesium, as described by Abbey is given below. The use of atomic absorption spectroscopy for the determination of lithium in silicate rocks has been described by a number of authors including 01Gorman and (21) (22) (23) (2*0 Suhr,
' Stone and Chesher,
' Zelyukova et_aLv
Ji
and Abbey.
'
The rock
solution for the determination can be prepared by either acid digestion (HF, HpSO, , etc) or by borate fusion. The procedure given below, adapted from that given by (21) O1Gorman and Suhr, uses a fusion with sodium borate. Procedure.
Accurately weigh approximately 0.1 g of the finely powdered silicate
rock material into a graphite fusion crucible, add 0.5 g of anhydrous sodium borate (Na B.0 ) , and fuse for 10 minutes in an electric muffle furnace at a temperature of 1000 . Pour the molten bead directly into a beaker containing exactly 50 ml of a 3 per cent nitric acid solution. dissolution is complete.
Add a magnetic stirring bar and stir until
Use this solution without further treatment for direct
nebulisation into the flame of the atomic absorption spectrophotometer in the usual way. To prepare standard solutions use portions of a silicate rock known to be essentially lithium-free (eg USGS Peridotite PCC-1) fused, as above, with sodium borate and add aliquots of a lithium standard solution prepared from pure dried lithium carbonate. Two ranges of standards are recommended equivalent to 0-200 ppm and 0-2000 ppm Li in the silicate rock. Matrix problems that arise with the use of a graphite furnace for the determination of lithium by atomic absorption spectroscopy have been recorded by Katz and Tartel. Determination of Rubidium and Caesium In theory there is no reason why the methods described above f o r the determination
(25)
Chemical Methods of Rock Analysis
66
of sodium and potassium should not be used for rubidium and caesium. However since these elements are seldom present in more than trace amounts, and since their flame response is very dependent on other elements present in the solution, a modified technique is preferred. ( 7>r \
Horstman
has described a method based upon the precipitation of oxide elements
(iron, aluminium etc) with calcium carbonate, and of calcium by precipitation as sulphate in aqueous alcoholic solution. The sodium and potassium contents of the rock solution (and lithium where this is present in quantity) are determined, and similar amounts added to the rubidium and caesium standards used to calibrate the flame spectrophotometer. This is given in detail below. Method Procedure.
Accurately weigh a sample of 0.5-2 g of the finely powdered silicate
rock into a large platinum dish, moisten with water and add sufficient sulphuric and hydrofluoric acids to give complete decomposition of the sample. Add 2-3 drops of concentrated nitric acid and evaporate on a hot plate to fumes of sulphuric acid. Allow to cool, add 5 ml of water and again evaporate, this time until all excess sulphuric acid is removed, leaving only a moist residue. Allow the dish to cool and dissolve this residue, by warming if necessary, in 25 ml of water. Transfer the
λ, nm
FIG. h
Flame emission of rubidium and caesium from pollucite.
The Alkali Metals
67
solution with any unattacked mineral grains (and sulphate precipitate if any) to a 150-ml beaker and dilute with water to a total volume of 70-80 ml.
Add a few drops
of bromothymol blue indicator solution and neutralise by adding solid calcium carbonate.
Allow the precipitate to stand overnight.
Heat the solution and precipitate to boiling and filter on to a medium-textured filter paper.
Wash the residue well with hot water until the volume of combined
filtrate and washings reaches about 150 ml.
Discard the residue and evaporate the
solution to a volume of 50 ml, cool and add 50 ml of ethanol to precipitate calcium sulphate.
Allow the precipitate to stand overnight and then collect on a close-
textured filter paper.
Wash the precipitate with a mixture of equal volumes of water
and ethanol until the volume of the combined filtrate and washings again reaches 150 ml. Discard the sulphate residue and evaporate the solution to dryness on a steam bath. Dissolve the dry residue in a little water and dilute to volume in a 50-ml volumetric flask. Using a flame spectrophotometer set according to the manufacturer's instructions, determine the sodium and potassium content of the solution and also the approximate values of lithium, rubidium and caesium.
For rocks containing only traces of the
rarer alkali metals prepare a series of calibration solutions for each of these metals spanning the approximate values and containing the same amounts of sodium and potassium as the rock solution.
For those rocks and minerals containing appreciable
amounts of rubidium or caesium add each of these elements to the standard solutions used to calibrate for the other. The flame photometer trace shown in Fig. k
was obtained with an instrument fitted
with a prism monochromator and a solution prepared from a sample of pollucite, decomposed as described above.
It illustrates the difficulty of determining small
amounts of rubidium in the presence of potassium with this type of instrument. Clearly the improved wavelength resolution obtainable with a grating instrument can be used with advantage for this determination. The determination of alkali metals by direct injection of the powdered silicate rock (27) More recently this has been (?8^ used with atomic absorption spectrophotometry by Govindaraju et al. for rubidium (29) and by Langmyhr and Thomassen y for rubidium and caesium. This is a method that material into a flame has been described by Lebedev.
should be capable of further development but its application to a wider range of rocks and minerals needs further study.
68
Chemical Methods of Rock Analysis
The method given in detail below is that due to Abbey.
The rock sample is
decomposed by a conventional acid digestion procedure and there are no chemical separations.
Following the addition of potassium to suppress ionisation, lithium,
rubidium and caesium are determined by direct emission or absorption (lithium and rubidium only) in an air-acetylene flame. can be used to overcome matrix effects.
When required a standard addition technique
Aliquots of the same sample solution can
be used for the determination of strontium and barium by atomic absorption spectroscopy with a nitrous oxide-acetylene flame. Procedure.
Accurately weigh approximately 0.5 g of the finely powdered rock
material in duplicate into 100-ml platinum dishes and add to each 5 ml of concentrated nitric acid, 2 ml of concentrated perchloric acid and 5 ml of hydrofluoric acid. Transfer to a hot plate and evaporate to copious fumes of perchloric acid.
Allow to
cool, rinse down the walls with a little water and evaporate on the hot plate once more, this time to complete dryness.
Allow to cool, add 2 ml of concentrated
hydrochloric acid, again rinse down the walls with a little water and evaporate to complete dryness (Note 1 ) . Using a safety pipette, add 2 ml of concentrated hydrochloric acid, rinse down with a little water, warm to dissolve the chlorides and transfer with water to a 50-ml volumetric flask.
To one of the duplicates add alkali solutions containing 25 μg
lithium, 250 μg rubidium and 2.^> μg caesium (Note 2 ) .
Add 1 ml of potassium buffer
(76.3 g potassium chloride per litre) to both spiked and unspiked samples and dilute each to volume with water (Note 3)· Determine the flame emission at 67Ο.8 nm for lithium, 78Ο.Ο nm for rubidium and 852.1 nm for caesium, preferably by recording over a wavelength range containing the complete emission peak.
An air-acetylene flame should be used.
Lithium and rubidium
can be determined in the same solution by atomic absorption spectroscopy, also using an air-acetylene flame. Notes: 1.
If barium is to be determined in the same sample solution - as described
by Abbey - the residue should be collected, fused with a little anhydrous sodium carbonate and extracted with water.
Discard the aqueous extract and acidify the
residue with a very little diluted hydrochloric acid.
This can then be added to the
main rock solution before its final evaporation to dryness. 2.
In the final 50-ml volume, these quantities correspond to an additional
50 ppm lithium, 500 ppm rubidium and 5·0 ppm caesium. 3·
If barium and strontium are to be determined in this solution, pipette
The Alkali Metals
69
10 ml into a clean 50-ml volumetric flask, add 2 ml of potassium buffer solution, 10 ml of a lanthanum buffer solution (50 g La 0
per litre), dilute to volume with
water, mix well and determine the barium and strontium by atomic absorption using a nitrous oxide-acetylene flame. References 1.
SMITH J L, Amer. J. Sei. 2nd Ser. O871) 50, 269
2.
HERING H, Anal. Chim. Acta (1952) J5, 3*+0
3.
BERZELIUS J J, Lehrbuch der Chemie. 3rd ed. (18VQ JO, k6
k.
DEAN J A, Flame Photometry, McGraw-Hill, New York, i960
5.
HILDON M A and ALLEN W J F, Analyst (1971) 96, ^δθ
6.
LUECKE C, Chem. Erde (1979) ^ 8 , 1
7.
OHRDORF R, Geochim. Cosmochim. Acta (1968) ^ 2 , 191
8.
VOSTERS M and DEUTSCH S, Earth Planet Sei. Lett. (1967) £, ^ 9
9.
BEUKENKAMP J and RIEMAN W, Analyt. Chem. (1950) 22, 582
10.
RIECHEN L E, Analyt. Chem. (1958) 30, 19^8
11.
ELLINGTON F and STANLEY L, Analyst (1955) 80, 313
12.
COHN W E and KOHN H W, J. Amer. Chem. Soc.
13.
JEFFERY P G, unpublished work
( 1 9 W 70, 1986
1*f.
EARDLEY R P and REED R A, Analyst (197D 96, 699
15.
EASTON A J and LOVERING J F, Anal. Chim. Acta (196*0 30, 5^3
16.
ABBEY S and MAXWELL J A, Chem. in Canada (i960) J2, 37
17.
RICE T D, Anal. Chim. Acta (1977) 21» 221
18.
STRELOW F W E, TOERIEN Von S and WEINERT C H S W, Anal. Chim. Acta (1970)
19. 20.
50, 399 GROVE E L, SCOTT C W and JONES F, Talanta (1965) J2, 327 ABBEY S, Geol. Surv. Canada Paper No 71-50 (1972)
21.
O'GORMAN J V and SUHR N H, Analyst (1971) 96, 335
22.
STONE M and CHESHER S E, Analyst (1969) 2it» 1063
23.
ZELYUKOVA Yu V, NIKONOVA M P and POLUEKTOV N S, Zhur. Anal. Khim. (1966)
2k. 25. 26. 27. 28. 29.
ABBEY S, Geol. Surv. Canada Paper 67-37 (1967) KATZ A and TARTEL N, Talanta (1977) 2jf, 132 HORSTMAN E L, Analyt. Chem. (1956) 28, 1^17 LEBEDEV V I, Zhur. Anal. Khim. (1969) 21., 337 GOVINDARAJU K, MEVELLE G and CHOUARD C, Chem. Geol. (1971) £, 131 LANGMYHR F J and THOMASSEN Y, Z. Anal. Chem. (1973) 26*1, 122
21_, 1*K>7
Sodium and potassium are always included in the list of elements determined in any 'complete' rock analysis.
The range of alkali metal contents varies from 100 ppm
or less in certain ultrabasic rocks, such as dunites and peridotites to as much as 10 per cent K 0 or 15 per cent Na 0 in rocks composed largely of felspar minerals. Gravimetric methods have been used for the determination of all five of the alkali metals.
These required a most careful separation of the total alkali metals from
silica, aluminium, calcium and other elements present.
The most frequently used (1) involved decomposing
procedure for this, described oaginally by Lawrence Smith,
the rock sample by ignition with ammonium chloride and calcium carbonate.
The alkali
metals were recovered by leaching with water, and were then separated from the small amount of calcium taken into solution.
Sulphates were converted into chlorides and (2) Some authors
the introduced ammonium salts were removed by volatilisation.
considered that special precautions were necessary to ensure a complete recovery of lithium with the remaining alkali metals.
The mixed chloride residue obtained after
the expulsion of ammonium salts was carefully ignited and weighed before the separation of the individual alkali metals. An alternative procedure for decomposing the silicate rock material and recovering the alkali metals as chlorides was based upon evaporation of the sample with hydro fluoric acid and precipitation of iron, aluminium, calcium and other elements with ammonia and ammonium carbonate, a method derived from work by Berzelius. The separation of lithium from the other alkali metal chlorides was based upon the solubility of lithium chloride in organic solvents such as isobutanol, pentanol, pyridine or ether-ethanol mixtures.
The determination was often completed by
converting the lithium chloride to sulphate prior to weighing.
This gravimetric
procedure for lithium is not sufficiently sensitive for application to the majority of silicate rocks, and lithium could only be reported in those samples rich in this element. Potassium was usually determined following the precipitation with chloroplatinic acid, perchloric acid or sodium cobaltinitrite.
The insoluble potassium salts were
collected and weighed directly, although a number of indirect procedures were also in common use.
Sodium was generally determined by difference, although some analysts
-59-
Chemical Methods of Rock Analysis
60
preferred to precipitate as a triple acetate of uranium and zinc or other divalent metal.
In all these precipitation procedures, corrections for the solubility of
the potassium or sodium salts were necessary, and none of the weighing forms could be considered ideal. Even when present in quantity, the separation of rubidium and caesium from potassium and from each other was seldom attempted , and on these few occasions the results obtained were not always very reliable. All these gravimetric procedures for the five alkali metals have now been rendered obsolete by the development of instrumental methods, such as flame photometry and atomic absorption spectroscopy.
The emission spectra of the alkali metals are simple,
consisting of a prominent line or doublet known as the resonance line(s), correspond ing to the transition between the lowest excited state and the ground state, together with weaker lines relating to other transitions.
In many flames, lithium and sodium
are only slightly ionised; this is in contrast to the remaining alkalis, where the degree of ionisation increases in the order K, Rb, Cs, until with caesium a large part of the metal in tie flame is in the ionised state.
This accounts for the poor
sensitivity to caesium, and the general decrease of sensitivity from sodium to caesium. Lithium is about as sensitive as rubidium. One way of increasing sensitivity to a particular element in a given solution is to introduce a second, more easily ionised element such as another alkali metal into the flame.
This serves to decrease the extent of ionisation of the required element and
thus increase the number of atoms available for the transition which gives rise to the resonance line(s).
At high alkali concentrations some loss of emission (and
consequent calibration curvature) occurs by self absorption.
This effect has been
noted particularly with lithium and sodium. Early designs of flame photometer used low temperature flames such as air-propane, or air-coal gas, together with a filter system for wavelength selection.
These
instruments could not be used for the determination of the rarer alkali metals (lithium, rubidium and caesium) as the characteristic emission could not be isolated by the filters available.
Considerable improvements were obtained with the introduction
of prism monochromators, enabling all five metals to be determined although, as described below, interferences between the metals gave rise to difficulties in determining lithium and rubidium.
These could be minimised by plotting emission
spectra thus enabling background corrections to be made.
These difficulties disappear
with the increased wavelength resolution that is obtainable using a grating monochromator.
The Alkali Metals
61
In general terms, the chemical interferences in the determination of the alkali metals are best minimised by using either a very cool (air-propane) or a very hot (acetylene-nitrous oxide) flame.
Much work remains to be done on the determination
of alkalis in silicate rocks, particularly following that of Hildon and Allen and Luecke.
A number of authors have described the determination of sodium and potassium in silicate rocks by atomic absorption spectroscopy, as well as the determination of (rp\
lithium
/ON
and rubidium.
As with flame photometric methods it is necessary to
add potassium to the solutions before determining rubidium.
Vosters and Deutsch
also recommended adding lanthanum as a flame buffer, a practice commonly used to limit inter-element effects. Rock solutions can be prepared in the same way as those for the flame photometric determination of the alkali metals. One of the most successful ways of separating mixtures of the alkali metals is by (9) (10) ion-exchange chromatography.
Resins such as Dowex 50
or Amberlite 120
can
be used for this separation, which requires the use of a cation-exchange resin as all five elements form well-characterised positive ions in solution. The separation of (11) sodium from potassium is well established, and the successful separation of (12) rubidium and caesium has also been reported. However, none of these procedures is ideal for the routine separation of all five alkali metals, and are now little used. Determination of Sodium and Potassium in Silicate Rocks by Flame Spectrophotometry The method described in detail below is based upon work by one of the authors (1*0 by Eardley and Reed.
(13)
and
The rock material is decomposed by evaporation with hydro
fluoric, nitric and sulphuric acids.
Sulphuric acid is added to assist in the removal
of fluoride ion which will otherwise depress the flame emission of potassium.
In most
cases the small amount of residue that remains after this treatment can be ignored, although for really accurate work this residue should be collected and analysed separately.
Caesium sulphate solution is added as an ionisation buffer. Method
Reagents:
Sodium sulphate, anhydrous. Potassium sulphate. Caesium sulphate solution, dissolve 0.205 g of pure caesium sulphate, Cs SO,, in 500 ml of water.
62
Chemical Methods of Rock Analysis
Procedure.
Accurately weigh approximately 0.1 g of the finely powdered silicate
rock material into a platinum dish (Note 1 ) , moisten with a little water and add 10 ml of concentrated nitric acid, 5 ml of concentrated sulphuric acid and 10 ml of hydrofluoric acid. sulphuric acid.
Place the dish on a cool hotplate and evaporate to fumes of
Cool, rinse down with a little water and add a further 5 nil of
hydrofluoric acid and evaporate until the contents of the dish are fuming well. Again allow to cool, rinse down with water, evaporate, this time until all fumes of sulphuric acid have ceased and a dry residue is obtained. Add 1 ml of concentrated nitric acid to the cool residue, followed by 25 to 30 ml of water. beaker.
Warm the dish to detach the residue and rinse the contents into a ^tOO-ml
Digest until all soluble material has passed into solution, and collect any
small residue on a small close-textured filter paper and wash with a little water. Discard the residue (Note 2) and dilute the filtrate and washings to volume with water in a 250-ml volumetric flask. Transfer 25 ml of this solution (Note 3) by pipette to a 50-ml volumetric flask, add 5 ml of the caesium sulphate solution, dilute to volume with water and mix well. Prepare also a reagent blank solution in the same way as this sample solution, using the same quantities of reagents but omitting the powdered rock material. Set the flame spectrophotmeter base line whilst spraying water, and the sensitivity at 10 ppm full-scale deflection spraying the 10 ppm standard solution, with the wavelength control set at 589 nm for sodium and 766.5 nm for potassium.
The
adjustment of sensitivity will affect the base line position and it may be necessary to reset the controls alternatively two or three times.
Once this adjustment is
complete, spray also the reagent blank solution and the sample solution, noting the flame emission recorded at both wavelengths. The flame emission of a series of standards should also be measured.
This should span the composition range 0 to 10 ppm.
The calibration is not a straight line. Where the flame spectrophotometer is fitted with a wavelength drive motor and a pen recorder, the flame response for either element can be obtained by scanning the spectrum.
The reagent blank solution and the standard solutions should similarly be
examined.
For sodium scan from about 56Ο nm to 620 nm and for potassium from 730 nm
to 820 nm, (Note *+). Calibration.
Dry small quantities of both sodium and potassium sulphates (Note 5)
in an electric oven at a temperature of 120
and allow to cool in a desiccator.
63
The Alkali Metals
Weigh 0.229 g of the anhydrous sodium sulphate and Ο.185 g of the potassium sulphate into the same beaker, dissolve in water and dilute to volume in a litre volumetric flask.
This solution contains 100 μg Na 0 and 100 μg K 0 per ml.
Prepare a 20 ppm
standard by diluting 50 ml of this stock solution, measured with a pipette, and 1 ml of concentrated nitric acid to 250 ml with water and mixing well.
Transfer 25 ml of
this solution by pipette to a 50-ml volumetric flask, add 5 ml of the aluminium sulphate solution and 5 ml of the caesium sulphate solution and dilute to volume with water.
This gives a 10 ppm standard for setting the sensitivity control of the
flame spectrophotometer.
Notes: 1.
The weight of sample material taken for the analysis and the volume of
the final solution may be varied in relation to the alkali content of the sample, but the final solution for measurement should be of the correct acid concentration and contain the recommended concentration of caesium sulphate. 2.
This small residue contains much of the zircon and some of the tourmaline
present in the rock, together with varying proportions of other minerals that are difficult to decompose with hydrofluoric acid.
These minerals seldom contain more
than trace amounts of alkali metals and the error involved in neglecting these small amounts is often within the limits of the experimental error of the method.
If the
amount of residue indicates that an appreciable part of the sample has not been attacked, an alternative procedure, such as fusion with lithium borate, should be used for the sample decomposition.
Obviously lithium cannot then be determined in this
sample portion. 3.
This volume of solution is suitable for determining up to 5 per cent of
sodium and potassium oxides in the rock sample. *t. There are a number of ultrabasic rocks that contain only small amounts of sodium and potassium.
The importance of these rocks is out of all proportion to the
extent to which they occur.
If reliable results are to be obtained for the alkali
metals at these levels, particular attention must be paid to the details of the (15) determination. It has been pointed out that at the high instrument sensitivities required for determining these small amounts of alkali metals, a large percentage of the total light emission can be due to background radiation.
The simplest way of
obtaining a background correction is to record the flame emission over the ranges involved using an instrument fitted with a motorised wavelength drive and a pen recorder. 5:
More dilute standards will also be required for these materials. Laboratory reagent grade sodium and potassium sulphates were found to be
sufficiently free from cross contamination to be used as standards in this way. more rigorous standards Specpure carbonates may be used.
For
64
Chemical Methods of Rock Analysis Determination of Potassium in Micas
The need for very accurate potassium determinations has been underlined by the simple way in which geological ages can be obtained from an accurate knowledge of the A
:K
ratios.
The small amounts of argon present can readily be recovered and
determined isotopically with a mass spectrometer.
Potassium can also readily be
determined by flame spectroscopy, but the accuracy and precision obtained by the usual procedures such as the one given in detail above, although adequate for most petrographic purposes, do not match that obtainable for argon. A flame photometric procedure for this accurate determination has been given by Abbey and Maxwell;
the sample material is decomposed with hydrofluoric acid and
sulphuric acid, and the excess hydrofluoric acid removed by fuming.
After the
addition of a known excess of magnesium sulphate, the excess sulphuric acid is in turn removed by evaporation to dryness.
The sulphates are ignited, the cooled
residue is leached with warm water, and the resulting suspension filtered, giving a neutral sulphate solution containing the alkali metals, magnesium and little else. The approximate relative proportions of the alkali metals are then determined and a neutral sulphate standard solution prepared approximating in composition to that of the sample solution.
The final potassium determination is made with a flame
photometer using this solution with added potassium as standard. The need to eliminate small systematic errors in the potassium determination was also (17) underlined by Rice , who compared dissolution methods using atomic absorption spectroscopy for the final determination.
A cation-exchange separation was used
based on the work of Strelow et al« Determination of Lithium The simple method described above for sodium and potassium can be extended to include also the lithium present in the rock material.
A somewhat greater instrument
sensitivity-setting is required, but this is well within the range of most modern instruments.
The solution prepared as described above for sodium and potassium, can
be used directly for the lithium measurement, but for rocks containing only a few ppm of lithium, a more concentrated solution is desirable. Of all the alkali metals, lithium has the least effect on the flame response of the (19) For most purposes this small effect upon
others, and is least affected by them.
the lithium response can be ignored, but for accurate determinations, the amounts of
65
The Alkali Metals
sodium and potassium present in the rock solution can be measured and similar amounts added to the standard solutions used for the lithium calibration. The most sensitive lithium line is the resonance line at 67Ο.8 nm.
Filter-type
instruments cannot be used for measurement at this wavelength as emission from potassium will also be recorded.
Similarly emission from sodium will provide a
sloping background against which the lithium emission can be recorded if a prism monochromator is used for wavelength selection.
These problems do not arise when
an instrument incorporating a grating monochromator is used.
A procedure for
determining lithium, together with rubidium and caesium, as described by Abbey is given below. The use of atomic absorption spectroscopy for the determination of lithium in silicate rocks has been described by a number of authors including 01Gorman and (21) (22) (23) (2*0 Suhr,
' Stone and Chesher,
' Zelyukova et_aLv
Ji
and Abbey.
'
The rock
solution for the determination can be prepared by either acid digestion (HF, HpSO, , etc) or by borate fusion. The procedure given below, adapted from that given by (21) O1Gorman and Suhr, uses a fusion with sodium borate. Procedure.
Accurately weigh approximately 0.1 g of the finely powdered silicate
rock material into a graphite fusion crucible, add 0.5 g of anhydrous sodium borate (Na B.0 ) , and fuse for 10 minutes in an electric muffle furnace at a temperature of 1000 . Pour the molten bead directly into a beaker containing exactly 50 ml of a 3 per cent nitric acid solution. dissolution is complete.
Add a magnetic stirring bar and stir until
Use this solution without further treatment for direct
nebulisation into the flame of the atomic absorption spectrophotometer in the usual way. To prepare standard solutions use portions of a silicate rock known to be essentially lithium-free (eg USGS Peridotite PCC-1) fused, as above, with sodium borate and add aliquots of a lithium standard solution prepared from pure dried lithium carbonate. Two ranges of standards are recommended equivalent to 0-200 ppm and 0-2000 ppm Li in the silicate rock. Matrix problems that arise with the use of a graphite furnace for the determination of lithium by atomic absorption spectroscopy have been recorded by Katz and Tartel. Determination of Rubidium and Caesium In theory there is no reason why the methods described above f o r the determination
(25)
Chemical Methods of Rock Analysis
66
of sodium and potassium should not be used for rubidium and caesium. However since these elements are seldom present in more than trace amounts, and since their flame response is very dependent on other elements present in the solution, a modified technique is preferred. ( 7>r \
Horstman
has described a method based upon the precipitation of oxide elements
(iron, aluminium etc) with calcium carbonate, and of calcium by precipitation as sulphate in aqueous alcoholic solution. The sodium and potassium contents of the rock solution (and lithium where this is present in quantity) are determined, and similar amounts added to the rubidium and caesium standards used to calibrate the flame spectrophotometer. This is given in detail below. Method Procedure.
Accurately weigh a sample of 0.5-2 g of the finely powdered silicate
rock into a large platinum dish, moisten with water and add sufficient sulphuric and hydrofluoric acids to give complete decomposition of the sample. Add 2-3 drops of concentrated nitric acid and evaporate on a hot plate to fumes of sulphuric acid. Allow to cool, add 5 ml of water and again evaporate, this time until all excess sulphuric acid is removed, leaving only a moist residue. Allow the dish to cool and dissolve this residue, by warming if necessary, in 25 ml of water. Transfer the
λ, nm
FIG. h
Flame emission of rubidium and caesium from pollucite.
The Alkali Metals
67
solution with any unattacked mineral grains (and sulphate precipitate if any) to a 150-ml beaker and dilute with water to a total volume of 70-80 ml.
Add a few drops
of bromothymol blue indicator solution and neutralise by adding solid calcium carbonate.
Allow the precipitate to stand overnight.
Heat the solution and precipitate to boiling and filter on to a medium-textured filter paper.
Wash the residue well with hot water until the volume of combined
filtrate and washings reaches about 150 ml.
Discard the residue and evaporate the
solution to a volume of 50 ml, cool and add 50 ml of ethanol to precipitate calcium sulphate.
Allow the precipitate to stand overnight and then collect on a close-
textured filter paper.
Wash the precipitate with a mixture of equal volumes of water
and ethanol until the volume of the combined filtrate and washings again reaches 150 ml. Discard the sulphate residue and evaporate the solution to dryness on a steam bath. Dissolve the dry residue in a little water and dilute to volume in a 50-ml volumetric flask. Using a flame spectrophotometer set according to the manufacturer's instructions, determine the sodium and potassium content of the solution and also the approximate values of lithium, rubidium and caesium.
For rocks containing only traces of the
rarer alkali metals prepare a series of calibration solutions for each of these metals spanning the approximate values and containing the same amounts of sodium and potassium as the rock solution.
For those rocks and minerals containing appreciable
amounts of rubidium or caesium add each of these elements to the standard solutions used to calibrate for the other. The flame photometer trace shown in Fig. k
was obtained with an instrument fitted
with a prism monochromator and a solution prepared from a sample of pollucite, decomposed as described above.
It illustrates the difficulty of determining small
amounts of rubidium in the presence of potassium with this type of instrument. Clearly the improved wavelength resolution obtainable with a grating instrument can be used with advantage for this determination. The determination of alkali metals by direct injection of the powdered silicate rock (27) More recently this has been (?8^ used with atomic absorption spectrophotometry by Govindaraju et al. for rubidium (29) and by Langmyhr and Thomassen y for rubidium and caesium. This is a method that material into a flame has been described by Lebedev.
should be capable of further development but its application to a wider range of rocks and minerals needs further study.
68
Chemical Methods of Rock Analysis
The method given in detail below is that due to Abbey.
The rock sample is
decomposed by a conventional acid digestion procedure and there are no chemical separations.
Following the addition of potassium to suppress ionisation, lithium,
rubidium and caesium are determined by direct emission or absorption (lithium and rubidium only) in an air-acetylene flame. can be used to overcome matrix effects.
When required a standard addition technique
Aliquots of the same sample solution can
be used for the determination of strontium and barium by atomic absorption spectroscopy with a nitrous oxide-acetylene flame. Procedure.
Accurately weigh approximately 0.5 g of the finely powdered rock
material in duplicate into 100-ml platinum dishes and add to each 5 ml of concentrated nitric acid, 2 ml of concentrated perchloric acid and 5 ml of hydrofluoric acid. Transfer to a hot plate and evaporate to copious fumes of perchloric acid.
Allow to
cool, rinse down the walls with a little water and evaporate on the hot plate once more, this time to complete dryness.
Allow to cool, add 2 ml of concentrated
hydrochloric acid, again rinse down the walls with a little water and evaporate to complete dryness (Note 1 ) . Using a safety pipette, add 2 ml of concentrated hydrochloric acid, rinse down with a little water, warm to dissolve the chlorides and transfer with water to a 50-ml volumetric flask.
To one of the duplicates add alkali solutions containing 25 μg
lithium, 250 μg rubidium and 2.^> μg caesium (Note 2 ) .
Add 1 ml of potassium buffer
(76.3 g potassium chloride per litre) to both spiked and unspiked samples and dilute each to volume with water (Note 3)· Determine the flame emission at 67Ο.8 nm for lithium, 78Ο.Ο nm for rubidium and 852.1 nm for caesium, preferably by recording over a wavelength range containing the complete emission peak.
An air-acetylene flame should be used.
Lithium and rubidium
can be determined in the same solution by atomic absorption spectroscopy, also using an air-acetylene flame. Notes: 1.
If barium is to be determined in the same sample solution - as described
by Abbey - the residue should be collected, fused with a little anhydrous sodium carbonate and extracted with water.
Discard the aqueous extract and acidify the
residue with a very little diluted hydrochloric acid.
This can then be added to the
main rock solution before its final evaporation to dryness. 2.
In the final 50-ml volume, these quantities correspond to an additional
50 ppm lithium, 500 ppm rubidium and 5·0 ppm caesium. 3·
If barium and strontium are to be determined in this solution, pipette
The Alkali Metals
69
10 ml into a clean 50-ml volumetric flask, add 2 ml of potassium buffer solution, 10 ml of a lanthanum buffer solution (50 g La 0
per litre), dilute to volume with
water, mix well and determine the barium and strontium by atomic absorption using a nitrous oxide-acetylene flame. References 1.
SMITH J L, Amer. J. Sei. 2nd Ser. O871) 50, 269
2.
HERING H, Anal. Chim. Acta (1952) J5, 3*+0
3.
BERZELIUS J J, Lehrbuch der Chemie. 3rd ed. (18VQ JO, k6
k.
DEAN J A, Flame Photometry, McGraw-Hill, New York, i960
5.
HILDON M A and ALLEN W J F, Analyst (1971) 96, ^δθ
6.
LUECKE C, Chem. Erde (1979) ^ 8 , 1
7.
OHRDORF R, Geochim. Cosmochim. Acta (1968) ^ 2 , 191
8.
VOSTERS M and DEUTSCH S, Earth Planet Sei. Lett. (1967) £, ^ 9
9.
BEUKENKAMP J and RIEMAN W, Analyt. Chem. (1950) 22, 582
10.
RIECHEN L E, Analyt. Chem. (1958) 30, 19^8
11.
ELLINGTON F and STANLEY L, Analyst (1955) 80, 313
12.
COHN W E and KOHN H W, J. Amer. Chem. Soc.
13.
JEFFERY P G, unpublished work
( 1 9 W 70, 1986
1*f.
EARDLEY R P and REED R A, Analyst (197D 96, 699
15.
EASTON A J and LOVERING J F, Anal. Chim. Acta (196*0 30, 5^3
16.
ABBEY S and MAXWELL J A, Chem. in Canada (i960) J2, 37
17.
RICE T D, Anal. Chim. Acta (1977) 21» 221
18.
STRELOW F W E, TOERIEN Von S and WEINERT C H S W, Anal. Chim. Acta (1970)
19. 20.
50, 399 GROVE E L, SCOTT C W and JONES F, Talanta (1965) J2, 327 ABBEY S, Geol. Surv. Canada Paper No 71-50 (1972)
21.
O'GORMAN J V and SUHR N H, Analyst (1971) 96, 335
22.
STONE M and CHESHER S E, Analyst (1969) 2it» 1063
23.
ZELYUKOVA Yu V, NIKONOVA M P and POLUEKTOV N S, Zhur. Anal. Khim. (1966)
2k. 25. 26. 27. 28. 29.
ABBEY S, Geol. Surv. Canada Paper 67-37 (1967) KATZ A and TARTEL N, Talanta (1977) 2jf, 132 HORSTMAN E L, Analyt. Chem. (1956) 28, 1^17 LEBEDEV V I, Zhur. Anal. Khim. (1969) 21., 337 GOVINDARAJU K, MEVELLE G and CHOUARD C, Chem. Geol. (1971) £, 131 LANGMYHR F J and THOMASSEN Y, Z. Anal. Chem. (1973) 26*1, 122
21_, 1*K>7