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Substoichiometric separation in activation analysis
1967.Vol.
T&ma.
14. pp. 109 to 119.
Pergamon Press Ltd.
Printedin NorthernIr&md
SUBSTOICHIOMETRIC SEPARATION ACTIVATION ANALYSIS EXTRACTION
IN
OF CO-ORDINATIVELY-UNSOLVATED
SALTS
I. P. ALIMARINand G. A. PEREZHOGIN V. I. Vemadsky Institute of Geochemistry
and Analytical Chemistry, Academy of Sciences, Moscow, U.S.S.R.
(Received 19 January 1966. Accepted 2 August 1966)
Summary--The necessary conditions have been examined for the use of extraction of coordinatively-unsolvated salts for substoichiometric separations in activation analysis, especially the influence of interfering ions and of the concentration of the carrier. A study has been made of the substoichiometric extraction of halide ions, oxyanions and anionic metal complexes by means of the tetraphenylarsonium ion into chloroform and dichloroethane, and of the extraction of alkali metal ions into nitrobenzene with tetraphenylborate. Substoichiometric extraction can be combined with neutron-activation analysis for the determination of traces of iodine, caesium, rubidium, manganese, rhenium, chromium, thallium, gold, gallium and tantalum.
suggested a new method of separation in NOT long ago, RSZiSiEkaand Sta$ activation analysis, called substoichiometric separation. For this purpose2s3 the extraction of chelate compounds is very satisfactory, but it is limited to those metals which form stable chelates. The extraction of co-ordinatively unsolvated salts (i.e., those containing large ions4) extends the range of the method, because the element to be determined can be present in either the cationic or the anionic part of the salt extracted.5*s The necessary conditions for satisfactory substoichiometric separation by extraction of unsolvated salts can be derived in two ways if the ions in the salt have the same degree of charge. We can use the extraction constant K,, for the system KAB = Wl,/[Al[Bl (1) where [AB], is the equilibrium concentration of the ion-pair (AB), extracted into:the organic phase, and [A] and [B] are the equilibrium concentrations of the two ions in the aqueous phase, and use it in conjunction with the initial concentrations of the reagent and the carrier of the element to be determined.6 Alternatively, we can use the equations for the distribution ratio for the reagent, EA,5 which relate it to the original concentration of reagent C, and carrier C,, A+B+(AB),;
&dG
EA = hJBl=
- CAI
(2)
and satisfy the requirement that at least 99 % of the reagent added should be extracted, EB. > 99 v/v,,
(3)
where v and v,, are the volumes of the aqueous and organic phases respectively. Solution of equations (2) and (3) gives the value of the extraction constant:
log K&B > 2 + log(v/vlJ - log(C, - C,). 8
109
(4)
I. P. ALMARINand G. A. Pmzmcm
110
If the ions constituting the salt do not bear the same degree of charge, the conditions for substoichiome~ic separation to be satisfactory are more complicated. It is understood that the roles of A and B may be reversed. The significance of the . extractron constant K,, depends on the size, charge and structure of the ions, the nature of the solvent, and on other factors. 4,7 The solvent must be little soluble in water, acid, or alkali, and must itself not be able to extract the element to be determined, unless the reagent is present. TAJSLE
I.-SoI_.uBlJ.‘lTYOF TETRAPHENYLA
RSONlUM
SALTS
IN
CHLOROFORM
AND
I,%DiCHLORO-
ETIUNE
Solubility, M 1,2-dichloroethane
Chloroform
Salt
44 1.1 0.7 0% o-2
R*AsAuCIa RhAsReOl R,AsMnO, R,,AsClOd R&As1 (RdAsMJr107
x x x x
10-s 10-a IO-’ 10-S
>2*8 x 6.1 x 3-1 x l-1 x >3*.5 x
-
10-s 10-s 10-S 10-Z 10-a
Concentration of the carrier In activation analysis, the concentration of carrier to be used is chosen arbitrarily. The upper limit is set by the solubility of the extracted species, and hence if Cn = 2CA the concentration of B is determined by the condition
where S,, is the solubility of AB in the organic phase. In Table I the solubi~ties of some tetraphenylarsonium salts in chloroform and 1,Zdichloroethane are quoted. The lower limit depends on K,n and U/V,; if C, = ZC,, then log c, 2, 2.3 + log(+J
- log &n.
(6)
Thus rhenium~1) for example, can be extracted substoichiometrica~y as its tetraphenylarsonium salt into chloroform over the concentration range 4 x 104M to In principle, a specific amount of carrier can 2 x 10_9Mifv=v~andCn=2C~ be isolated even when the rhenium concentration is less than 4 x IWM, but the concentration of carrier must be controlled precisely, whereas at concentrations of rhenium carrier greater than 2 x lO?V a change in the volume of the aqueous phase and loss of part of the carrier does not substantially influence the chemical yield. A wider range of carrier con~n~ations can be extracted with 1,2-dichlor~thane (a’b’ in Fig. l), and so this solvent is preferred. The effect of interfering ions Significant interference can be expected primarily from ions with the same charge as the carrier. The concentrations of the interfering ions (including any complexing ions) must not exceed a predetermined fraction of the concentrations of the reagent and of the carrier of the element to be determined, and can be derived by substituting
Substoichiometricseparation in activation analysis
111
,j; 05i _$ H 03B 2 z 2, C 2 t a 0.1
I 10-1
I
10-z
I
I
I
I
16'
10-4
10-5
10-s
cfieo;.M FIG. I.-The intluence of rhenium concentration of perrhenate
on the substoichiometric extraction with tetraphenylarsonium chloride (50 y0 stoichiometry). 0 with chloroform 0 with 1,2dichloroethane
the initial concentrations of the carrier and of the interfering ion (B’) into the ratio of the extraction constants for the two species: K AB W%[B’l 4B’ = [B][AB’], K
To maintain the condition of substoichiometric
*
(7)
separation it is necessary that
[AB],z+,> 0.99 CA0 [B]v < C,U - 0.99 C,v [AB’&, < 0.01 CAD [B’]v > C,*v - 0.01 C,v.
(8) (9) (10) (11)
By combining equations (7) to (11) we obtain the value for the threshold concentration of the interfering ion:
c,. <
0.01
(~-+,I.
(12)
If CB = 2CA, this simplifies to (13) It is obvious from equation (12) that if the carrier ion and the interfering ion have the same charge, then to give an adequate separation their exchange constant, i.e.,
I. P. ALIMARINand G. A. PEREZHOGIN
112
must be greater than 100. The exchange constant can be found experiKABIKAB? mentally from the equation z8 KAB E,=-..---;-
W’lo
KAB'
(14)
PI
where EB is the distribution ratio of B when Cn < CA and Cu. > C,. In Fig. 2 the influence of chloride, nitrate, and perchlorate on the extraction of perrhenate as its tetraphenylarsonium salt into 1,Zdichloroethane is shown.
0
7-
5-
3-
I
I
10-5
10-4
I
10-j
lo-2
In terferinq
FIG. 2.-The rhenium(VI1)
mm,
I
I
10-I
IO0
M
influence of interfering anions on the substoichiometric extraction of with tetraphenylarsonium chloride into 1,2-dichloroethane (1O-8M perrhenate, 50% stoichiometry). 0 perchlorate 0 nitrate D chloride (chloroform extraction)
Selectivity When co-ordinatively-unsolvated salts are extracted with an excess of the reagent, the distribution ratio is proportional to the extraction constant and the equilibrium concentration of the reagent in the aqueous phase? EB
=
KAB[AI
EB, = KAu*[A].
and
(15)
If KAB > KAB, > [Al-l then both ions are being extracted quantitatively, and the degree of separation, /?, is unity (B = [AB],C,,/[AB’],C,). If, however, B is being extracted substoichiometrically, the degree of separation b8 will be much greater than unity; it can be shown that when CB = 2CA and the conditions of equation (12) are fulfilled, then 0, =
(KAB
It is seen that /?a changes from KAB/KAB’$1.
+
KAB,)/~KAB,.
(16)
l/2 when KAB/KA,, < 1, to KA&!KAB# when
113
Substoichiometric separation in activation analysis SUBSTOICHIOMETRIC SEPARATIONS TETRAPHENYLARSONIUM SALTS
OF
The extraction of large anions with tetraphenylarsonium cations (R,As+) as the reagent has been well studied. It is known9 that ions such as I-, NO,-, CN-, SCN-, MnO,, TeO,-, ReO,-, ClOs-, ClO,, and CrsO,% are easily extracted into chloroform with R,As+, whereas Br, Cl-, BrOs-, IO,-, and IO,- are less well extracted, and the majority of the multi-charged oxyanions such as SO,*, CrOda-, MoOd2-, WOas-, and PO,% are very badly extracted. Examples are also known of the extraction of anionic
30
-
20
/JY.
.s
IO 7/
e :, 2 .L ‘I 0 .;
3-
5
\
/
t
/ 07 ‘; 0.5
r-
I
10.:
I
10.’
100 Chloride,
I
IO
M
FIG. 3.-The influence of the concentration of potassium chloride and hydrochloric acid on the extraction of tetraphenylarsonium chloride with chloroform. 0 potassium chloride 0 hydrochloric acid
complexes (halides, cyanides, thiocyanates, etc.) with &As+ and other large cations. For example, Ueno lo studied the extraction of 53 elements with tetraphenylarsonium chloride into chloroform from various concentrations of hydrochloric acid.
metal
Halides
Tribalat says the extraction constant of tetraphenylarsonium chloride into chloroform is 200. This value was obtained from the solubility of the reagent in water and chloroform. We have investigated the dependence of the distribution ratio of this reagent on the chloride concentration, the initial concentration of the reagent being 5 x 104iV in the aqueous phase (Fig. 3). Over the range 004549M potassium chloride there is a linear relationship with a slope of 30. In hydrochloric acid medium the distribution ratio passes through a maximum when the hydrochloric acid is about 3*5M. The distribution ratio for the reagent in the water-chloroform system is about 0.2 and is independent of pH over the range O*1-12.s In Table II results are given for the extraction of tetraphenylarsonium iodide from chloride or bromide solutions. The iodide distribution was determined radiometrically by means of lslI. The exchange constants were calculated from equation
114
I. P. ALIMARIN and G. A.
PERR~I-IOOIN
It is clear that Kn4_44sI is much higher than KR,AscI and KrtAsnr and that the difference is accentuated when 1,2-dichloroethane is used as solvent. The value of K FL&II is about 105 for the chloroform system, and the extraction of iodide with a deficit of the tetraphenylarsonium reagent satisfies the conditions of equation (1) for substoichiometric separation if v = v, and Cr- > 2 x 10-3M. The high solubility of tetraphenylarsonium iodide in chloroform (Table I) allows an iodide concentration higher than 10-aM to be used. In this case the concentrations of bromide and chloride must not exceed O.OlM and O-15M respectively [Table II and equation (13)]. (14).
TABLE II.-EXTRACTION OF la11 FROM SOLUTIONSCONTAINING CHU)RIDE OR BROMIDEAND SUBSTOICHIOMETRIC AMOUNTS OF TETRAPHENYLARSONIUM IONS
Solvent Chloroform
1 ,Zdichloroethane
Eqnilibrium concentration of B’in the aqueous phase,
Equilibrium concentration of R,AsB’ in the organic
M
M
Distribution ratio of iodide
ClBr-
1.0 0.2
1.8 x 1O-a 1.5 x 10-a
5.4 1.5
3 x l@ 2 x 108
Cl-
1.0
1.8 x 1O-s
11
6 x lo*
Anion B’
phase,
Exchange constant
In an acidic medium there will be interference from metals such as bismuth, gold, mercury, etc., which form anionic iodide complexes. The total concentration of such metals must not exceed 1CHM. Apparently the extraction constants of the &As+ salts of these complexes (especially if they are singly-charged) are higher than the extraction constant for the iodide itself, and consequently trace quantities of these metals will be extracted quantitatively and may interfere radiometrically. The interference can be eliminated either by a preliminary conversion of the iodide into iodine which is then extracted with chloroform, or by a preliminary preferential extraction of the halo-complexes with an amount of &As+ that is not more than a tenth of the amount to be used for extracting the iodide. Oxyanions
In Table III are given results for the extraction of tetraphenylarsonium perrhenate into chloroform and 1,2-dichloroethane from solutions containing chloride, nitrate, thiocyanate, perchlorate, permanganate and dichromate. The distribution ratio of the rhenium was determined by means of IsaRe. Similar experiments were carried out with QQmT~ from perrhenate solution. The exchange constants KX,ABTC,,/KR,AsBeO, were 1.7 and 2-5 for chloroform and 1,Zdichloroethane respectively. The order of extractability into chloroform as tetraphenylarsonium salts is MnO,
> TcO,- > ReO,- > ClO,- > (I-, SCN-, Cr,OVZ-)> NO,- > Br > Cl-.
From Table III it is clear that manganese(VII), rhenium(W), and chromium(W) can be substoichiometricahy separated by extraction of their tetraphenylarsonium salts into chloroform or 1,Zdichloroethane. Manganese has been substoichiometrically separated after oxidation of manganese(I1) to manganese(VI1) by sodium bismuthate in sulphuric acid medium,
Substoichiometric
115
separation in activation analysis
Table III.-EXTRACTION OF rerRe FROMSOLUTION~~NTAIN~N~OTHERANIONSAND SUL3STOICHIOMJiTRIC
Solvent
Anion B’
Equilibrium concentration of B’ in the aqueous phase, M x 10s
CINO,SCNChloroform
ClO,MnO,CrgO,=ClNO,SCN-
1,Zdichloroethane
AhtOUNTS
ClOpMnOlCra0,2-
OF TETRAPHENYLARWNIUM
Equilibrium concentration of RIAs salt in the organic
ph=, M x lOa
Rhenium distribution ratio
IONS
Exchange constant
400* 800 250 100 0.76 2.76 1.8 3.9 0.61 0.81 20
0.28 0.30 0.31 0.31 0.24 0.24 0.31 0.31 0.39 0.195 0.31
8.0 3.9 0.56 1.4 4.4 1.3 0.53 0.25 0.22 0.09 1’4f
1.1 1.0 4.5 4.5
x 10’ x 10’ x 10’ x 10’ 14 15 3.1 3.1 0.35 0.37 -
800 800 500 250 1.8 7.7 0.67 4.5 061 1.2 20
0.31 0.15 0.39 0.39 0.24 0.31 0.39 0.54 0.39 0.78 0.31
31 16 1.0 2.1 3.3 1.0 0.58 0.13 0.1 0.11 0.95t
8.0 8.5 1.3 1.3
x loo x lo4 x 10’ x 1oL 25 25 1.0 1.1 0.16 0.17 -
l That is, 0.4M. t Determined in presence of 0.4.5N sulphuric acid (the distribution in pH).
ratio increases with increase
following a preliminary separation of the manganese from interfering elements and reducing agents by precipitation as the hydroxide or manganese(N) oxide.ll Similar conditions are used for the separation of chromium(VI). Technetium, formed by the reaction sequence 9sMo(n,y)9sMo % QQmTc,does not interfere with the substoichiometric separation of rhenium, because equation (13) always holds if the amount of technetium present is too small to be weighed, but QQmTccoextracted with rhenium (according to equation (16) and Table III) does interfere with measurement of lsaRe on a gamma-spectrometer because the yradiation energies of the two isotopes are very close (about 0.14 MeV). The difference in redox potential of rhenium(VI1) and technetium(VI1) was made use of for their separation. In Fig. 4 is shown the effect of tin(II), added as sodium stannite, on the extraction of technetium and rhenium as their tetraphenylarsonium salts into chloroform or 1,2-dichloroethane from 0.3M sulphuric acid. A similar separation occurs from alkaline medium. Anionic complexes of metals
Anions such as AuCl,-, TlCI,-, FeCl,-, GaCI, and SbCI,, are easily extracted from hydrochloric acid solution as their R,As+ salts. Doubly-charged complex
116
I. P. ALIMARINand G. A. PEREZH~GIN
Tin
(II),
M
FIG. 4.-The influence of the tin(H) concentration on the substoichiometric extraction of tetraphenylarsonium perryhz;;;ito 1,2-dichloroethane (50 % stoichiometry). _.- coextraction of technetium
z. ‘_ .z z I
01. 0
I 2
I
Hydrochlonc
I
I 4 ocld,
I 6
I
M
Fro. S.-The influence of iron(III) and hydrochIoric acid on the substoichiometric extraction of tetraphenylarsonium ;uriiehl;~~ I”‘” chloroform (50 % stoichiometry). 0 FeiAu l&:1 A Fe:Au 15OO:l
117
Substoichiometric separation in activation analysis
anions of this kind are very poorly extracted. The extent of formation of these ions depends markedly on the hydrochloric acid concentration, which explains why iron does not hinder the substoichiometric extraction of gold when the concentration of hydrochloric acid is < lM, even when the ratio of iron to gold is 1000: 1 (Fig. 5). In Table IV results are given for the extraction of tetraphenylarsonium tetrachloroaurate(II1) into chloroform or 1,Zdichloroethane from solutions containing TABLE IV.-EXTRACTION OF losAu FROM SOLUTIONSCONTAININGTHALL~(III) OR IRON CHLORO-COMPLEXES
Solvent Chloroform
1,2-dichloroethane
Anion B’
AND
SIJBSTOICHIOhiETRICAMOUNTS
OF TETRAPHENYLARWNIUM
IONS
Equilibrium concentration of B’ in aqueous phase, M x 1w
Equilibrium concentration of R,AsB’ in organic phase, M x lo8
Distribution ratio of gold
Exchange constant
O-487 44 22 22
0.52 0.54 0.54 1.4
2.1 0.08 0.17 0.43
2.9 6.6 6.3 6.5
0.48
0.52
0.94
1.0
TICltFeCl*-*
TlCl,-
* Determined in presence of 8M hydrochloric acid. t That is 0.48 X 10-sM.
thallium(I) or iron(II1). The distribution coefficient was determined by means of ls*Au. From Table IV it can be seen that the difference in the extraction constants is negligible. For the extraction of gold, the ratio KIt,AsAucl,/KR,Aaclis about 1.5 x 108, i.e., it is possible to separate gold, thallium, iron and gallium from hydrochloric acid solution substoichiometrically by this method. TABLE V.-EXTRACTION OF lWs~o NITROBENZENE FROM SOLUTIONSCONTAINING ALKALI METALS AND SUBSTOICHIOMETRIC AMOUNTS OF TETRAPHENYLBORATB
Cation B’
Equilibrium concentration of B’in the aqueous phase, M x 10’
Equilibrium concentration of R,BB’ in the organic phase, M x 10’
Distribution ratio of caesium
750’ 1500 600 1000 19 200 19 39 1.15 0.65
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.35 0.35
13 6 3.1 2.2 1.5 0.16 0.32 0.17 0.023 0040
Li+ Na+ K+ Rb+ Tl+
OTHER
Exchange constant 1.1 1 2.1 2.4 ;;
x x x x
10’ 104 lo” 10’
6.8 I.4 0.076 0.074
* That is 0*75OM.
The extraction of the anionic fluoride, bromide, iodide, cyanide and thiocyanate complexes is apparently suitable for the substoichiometric separation of such metals as tantalum, bismuth, molybdenum(V) and tungsten(V), sometimes with ions smaller than the tetraphenylarsonium ion. The thiocyanate complex of gold, for
118
I. P. M
and G. A. PERE~OOIN
example, can be extracted into 1,Zdichloroethane with the triphenylguanidine ion. Increase in the anionic radius, however, decreases the effect of anionic charge on selectivity of separation, and the permissible concentrations of complexing anions which themselves are easily extractable (such as iodide and thiocyanate) are limited according to equation (4). SUBSTOICHIOMETRIC SEPARATIONS SIMPLE AND COMPLEX CATIONS
OF
It is known that the alkali metal cations can be extracted into nitrobenzene as compounds with tetraphenylborate or dipicrylamine anions, or other large anions. Some results are given in Table V. The distribution coefficient was determined by means of l=Cs. The extraction constant for caesium tetraphenylborate exceeds 10s. It seems possible to separate thallium(I), caesium, rubidium, and possibly potassium, substoichiometrically by extraction of the tetraphenylborates. Small or multiplycharged cations are not extracted. Such ions may be extracted, however, in the form of their cationic complexes with certain chelating agents such as 2,2’-dipyridyl or l,lO-phenanthroline which form complexes with metals such as copper(I) and iron(I1). In that case it is best to use as counter-ions large anions with the same degree of charge as the cation concerned.’ CONCLUSION
We have already used substoichiometric separation by extraction of coordinativelyunsolvated salts for the neutron activation determination of traces of gold6 and manganesell in high purity metals, and of goldI and rhenium13 in rocks and meteorites. This new method of separation of these elements in neutron activation analysis is much simpler and quicker than any used previously. The method seems extremely promising for the separation of iodine, caesium, rubidium, thallium, chromium, tantalum, and other metals, Zusammenfassuq-Die notwendigen Bedingungen zut Extraktion koordinativ nicht solvatisierter Salze zu unterstochiometrischen Trennungen in der Aktivierungsanalyse wurden untersucht, speziell der EinfluD stiirender Ionen und der Trlgerkonzentration. Die unterstochiometrische Extraktion von Halogenidionen, Oxyanionen und anionischen Metallkomplexen mittels Tetraphenylarsoniumionen in Chloroform und Dichlor%than wurde studier? sowie die Extraktion von Alkalimetallionen mit Tetraphenyloborat in Nitrobenzol. Die unterstochiometrische Extraktion kann mit der Neutronenaktivierungsanalyse bei der Bestimmung von Spurenmengen Jod, Caesium, Rubidium, Mangan, Rhenium, Chrom, Thallium, Gold, Gallium und Tantal kombiniert werden. R&am&-On a examine les conditions qui sont necessaires rl l’usage de l’extraction de sels non solvatb par coordination pour les separations substoechiometriques dans l’analyse par activation, essentiellement les influences des ions g%ants et de la concentration du porteur. On a effectue une etude sur l’extraction substoechiometrique des ions halogenures, des oxyanions et des complexes metalliques anioniques au moyen d’ion t&rapMnylarsonium en chloroforme et dichlorethane, et sur l’extraction des ions alcalins metaliques en nitrobenzene au moyen de tetraphenylborate. On peut combiner l’extraction substoechiometrique avec l’analyse par activation de neutrons pour le dosage de traces diode, caesium, rubidium, manganese, rhenium, chrome, thallium, or, gallium et tantale.
Substoichiometric
separation in activation analysis
119
REFERENCES 1. J. R&Wka and J. Starp, Tulanru, 1963,10,287. 2. J. Starf, Solvent Extraction of Metal Chelates, Pergamon, Oxford, 1964. 3. J. Star9 and J. Ri%icka, Talantu, 1964,11,697. 4. R. M. Diamond and D. G. Tuck, in F. A. Cotton, Progress in Inorganic Chemistry, Vol. II, pp. 139-150. Interscience, New York, 1960. 5. I. P. Alimarin and G. A. Perezhogin, Zh. Analit. Khim., 1965,20,48. 6. J. Rt%%a and A. Zeman, Talanta, 1965,12,997. 7. V. I. Kuznetsov and L. I. Moseyev, Radiokhimiyu, 1964,6,280. 8. S. Tribalat, Anal. Chim. Acta, 1949, 3, 113. 9. R. Bock and G. N. Beilstein, Z. Anal. Chem., 1962,192,44. 10. K. Ueno and C. Chang, J. Atomic Energy Sot. Japan., 1962,4,457. 11. G. A. Perezhogin, Zh. Analit. Khim., 1966,21, 879. 12. G. A. Perezhogin and I. P. Alimarin, ibid., 1965,20,793. 13. G. A. Perezhogin, Zavodskaya Lab., 1965,31,402.
T&ma.
14. pp. 109 to 119.
Pergamon Press Ltd.
Printedin NorthernIr&md
SUBSTOICHIOMETRIC SEPARATION ACTIVATION ANALYSIS EXTRACTION
IN
OF CO-ORDINATIVELY-UNSOLVATED
SALTS
I. P. ALIMARINand G. A. PEREZHOGIN V. I. Vemadsky Institute of Geochemistry
and Analytical Chemistry, Academy of Sciences, Moscow, U.S.S.R.
(Received 19 January 1966. Accepted 2 August 1966)
Summary--The necessary conditions have been examined for the use of extraction of coordinatively-unsolvated salts for substoichiometric separations in activation analysis, especially the influence of interfering ions and of the concentration of the carrier. A study has been made of the substoichiometric extraction of halide ions, oxyanions and anionic metal complexes by means of the tetraphenylarsonium ion into chloroform and dichloroethane, and of the extraction of alkali metal ions into nitrobenzene with tetraphenylborate. Substoichiometric extraction can be combined with neutron-activation analysis for the determination of traces of iodine, caesium, rubidium, manganese, rhenium, chromium, thallium, gold, gallium and tantalum.
suggested a new method of separation in NOT long ago, RSZiSiEkaand Sta$ activation analysis, called substoichiometric separation. For this purpose2s3 the extraction of chelate compounds is very satisfactory, but it is limited to those metals which form stable chelates. The extraction of co-ordinatively unsolvated salts (i.e., those containing large ions4) extends the range of the method, because the element to be determined can be present in either the cationic or the anionic part of the salt extracted.5*s The necessary conditions for satisfactory substoichiometric separation by extraction of unsolvated salts can be derived in two ways if the ions in the salt have the same degree of charge. We can use the extraction constant K,, for the system KAB = Wl,/[Al[Bl (1) where [AB], is the equilibrium concentration of the ion-pair (AB), extracted into:the organic phase, and [A] and [B] are the equilibrium concentrations of the two ions in the aqueous phase, and use it in conjunction with the initial concentrations of the reagent and the carrier of the element to be determined.6 Alternatively, we can use the equations for the distribution ratio for the reagent, EA,5 which relate it to the original concentration of reagent C, and carrier C,, A+B+(AB),;
&dG
EA = hJBl=
- CAI
(2)
and satisfy the requirement that at least 99 % of the reagent added should be extracted, EB. > 99 v/v,,
(3)
where v and v,, are the volumes of the aqueous and organic phases respectively. Solution of equations (2) and (3) gives the value of the extraction constant:
log K&B > 2 + log(v/vlJ - log(C, - C,). 8
109
(4)
I. P. ALMARINand G. A. Pmzmcm
110
If the ions constituting the salt do not bear the same degree of charge, the conditions for substoichiome~ic separation to be satisfactory are more complicated. It is understood that the roles of A and B may be reversed. The significance of the . extractron constant K,, depends on the size, charge and structure of the ions, the nature of the solvent, and on other factors. 4,7 The solvent must be little soluble in water, acid, or alkali, and must itself not be able to extract the element to be determined, unless the reagent is present. TAJSLE
I.-SoI_.uBlJ.‘lTYOF TETRAPHENYLA
RSONlUM
SALTS
IN
CHLOROFORM
AND
I,%DiCHLORO-
ETIUNE
Solubility, M 1,2-dichloroethane
Chloroform
Salt
44 1.1 0.7 0% o-2
R*AsAuCIa RhAsReOl R,AsMnO, R,,AsClOd R&As1 (RdAsMJr107
x x x x
10-s 10-a IO-’ 10-S
>2*8 x 6.1 x 3-1 x l-1 x >3*.5 x
-
10-s 10-s 10-S 10-Z 10-a
Concentration of the carrier In activation analysis, the concentration of carrier to be used is chosen arbitrarily. The upper limit is set by the solubility of the extracted species, and hence if Cn = 2CA the concentration of B is determined by the condition
where S,, is the solubility of AB in the organic phase. In Table I the solubi~ties of some tetraphenylarsonium salts in chloroform and 1,Zdichloroethane are quoted. The lower limit depends on K,n and U/V,; if C, = ZC,, then log c, 2, 2.3 + log(+J
- log &n.
(6)
Thus rhenium~1) for example, can be extracted substoichiometrica~y as its tetraphenylarsonium salt into chloroform over the concentration range 4 x 104M to In principle, a specific amount of carrier can 2 x 10_9Mifv=v~andCn=2C~ be isolated even when the rhenium concentration is less than 4 x IWM, but the concentration of carrier must be controlled precisely, whereas at concentrations of rhenium carrier greater than 2 x lO?V a change in the volume of the aqueous phase and loss of part of the carrier does not substantially influence the chemical yield. A wider range of carrier con~n~ations can be extracted with 1,2-dichlor~thane (a’b’ in Fig. l), and so this solvent is preferred. The effect of interfering ions Significant interference can be expected primarily from ions with the same charge as the carrier. The concentrations of the interfering ions (including any complexing ions) must not exceed a predetermined fraction of the concentrations of the reagent and of the carrier of the element to be determined, and can be derived by substituting
Substoichiometricseparation in activation analysis
111
,j; 05i _$ H 03B 2 z 2, C 2 t a 0.1
I 10-1
I
10-z
I
I
I
I
16'
10-4
10-5
10-s
cfieo;.M FIG. I.-The intluence of rhenium concentration of perrhenate
on the substoichiometric extraction with tetraphenylarsonium chloride (50 y0 stoichiometry). 0 with chloroform 0 with 1,2dichloroethane
the initial concentrations of the carrier and of the interfering ion (B’) into the ratio of the extraction constants for the two species: K AB W%[B’l 4B’ = [B][AB’], K
To maintain the condition of substoichiometric
*
(7)
separation it is necessary that
[AB],z+,> 0.99 CA0 [B]v < C,U - 0.99 C,v [AB’&, < 0.01 CAD [B’]v > C,*v - 0.01 C,v.
(8) (9) (10) (11)
By combining equations (7) to (11) we obtain the value for the threshold concentration of the interfering ion:
c,. <
0.01
(~-+,I.
(12)
If CB = 2CA, this simplifies to (13) It is obvious from equation (12) that if the carrier ion and the interfering ion have the same charge, then to give an adequate separation their exchange constant, i.e.,
I. P. ALIMARINand G. A. PEREZHOGIN
112
must be greater than 100. The exchange constant can be found experiKABIKAB? mentally from the equation z8 KAB E,=-..---;-
W’lo
KAB'
(14)
PI
where EB is the distribution ratio of B when Cn < CA and Cu. > C,. In Fig. 2 the influence of chloride, nitrate, and perchlorate on the extraction of perrhenate as its tetraphenylarsonium salt into 1,Zdichloroethane is shown.
0
7-
5-
3-
I
I
10-5
10-4
I
10-j
lo-2
In terferinq
FIG. 2.-The rhenium(VI1)
mm,
I
I
10-I
IO0
M
influence of interfering anions on the substoichiometric extraction of with tetraphenylarsonium chloride into 1,2-dichloroethane (1O-8M perrhenate, 50% stoichiometry). 0 perchlorate 0 nitrate D chloride (chloroform extraction)
Selectivity When co-ordinatively-unsolvated salts are extracted with an excess of the reagent, the distribution ratio is proportional to the extraction constant and the equilibrium concentration of the reagent in the aqueous phase? EB
=
KAB[AI
EB, = KAu*[A].
and
(15)
If KAB > KAB, > [Al-l then both ions are being extracted quantitatively, and the degree of separation, /?, is unity (B = [AB],C,,/[AB’],C,). If, however, B is being extracted substoichiometrically, the degree of separation b8 will be much greater than unity; it can be shown that when CB = 2CA and the conditions of equation (12) are fulfilled, then 0, =
(KAB
It is seen that /?a changes from KAB/KAB’$1.
+
KAB,)/~KAB,.
(16)
l/2 when KAB/KA,, < 1, to KA&!KAB# when
113
Substoichiometric separation in activation analysis SUBSTOICHIOMETRIC SEPARATIONS TETRAPHENYLARSONIUM SALTS
OF
The extraction of large anions with tetraphenylarsonium cations (R,As+) as the reagent has been well studied. It is known9 that ions such as I-, NO,-, CN-, SCN-, MnO,, TeO,-, ReO,-, ClOs-, ClO,, and CrsO,% are easily extracted into chloroform with R,As+, whereas Br, Cl-, BrOs-, IO,-, and IO,- are less well extracted, and the majority of the multi-charged oxyanions such as SO,*, CrOda-, MoOd2-, WOas-, and PO,% are very badly extracted. Examples are also known of the extraction of anionic
30
-
20
/JY.
.s
IO 7/
e :, 2 .L ‘I 0 .;
3-
5
\
/
t
/ 07 ‘; 0.5
r-
I
10.:
I
10.’
100 Chloride,
I
IO
M
FIG. 3.-The influence of the concentration of potassium chloride and hydrochloric acid on the extraction of tetraphenylarsonium chloride with chloroform. 0 potassium chloride 0 hydrochloric acid
complexes (halides, cyanides, thiocyanates, etc.) with &As+ and other large cations. For example, Ueno lo studied the extraction of 53 elements with tetraphenylarsonium chloride into chloroform from various concentrations of hydrochloric acid.
metal
Halides
Tribalat says the extraction constant of tetraphenylarsonium chloride into chloroform is 200. This value was obtained from the solubility of the reagent in water and chloroform. We have investigated the dependence of the distribution ratio of this reagent on the chloride concentration, the initial concentration of the reagent being 5 x 104iV in the aqueous phase (Fig. 3). Over the range 004549M potassium chloride there is a linear relationship with a slope of 30. In hydrochloric acid medium the distribution ratio passes through a maximum when the hydrochloric acid is about 3*5M. The distribution ratio for the reagent in the water-chloroform system is about 0.2 and is independent of pH over the range O*1-12.s In Table II results are given for the extraction of tetraphenylarsonium iodide from chloride or bromide solutions. The iodide distribution was determined radiometrically by means of lslI. The exchange constants were calculated from equation
114
I. P. ALIMARIN and G. A.
PERR~I-IOOIN
It is clear that Kn4_44sI is much higher than KR,AscI and KrtAsnr and that the difference is accentuated when 1,2-dichloroethane is used as solvent. The value of K FL&II is about 105 for the chloroform system, and the extraction of iodide with a deficit of the tetraphenylarsonium reagent satisfies the conditions of equation (1) for substoichiometric separation if v = v, and Cr- > 2 x 10-3M. The high solubility of tetraphenylarsonium iodide in chloroform (Table I) allows an iodide concentration higher than 10-aM to be used. In this case the concentrations of bromide and chloride must not exceed O.OlM and O-15M respectively [Table II and equation (13)]. (14).
TABLE II.-EXTRACTION OF la11 FROM SOLUTIONSCONTAINING CHU)RIDE OR BROMIDEAND SUBSTOICHIOMETRIC AMOUNTS OF TETRAPHENYLARSONIUM IONS
Solvent Chloroform
1 ,Zdichloroethane
Eqnilibrium concentration of B’in the aqueous phase,
Equilibrium concentration of R,AsB’ in the organic
M
M
Distribution ratio of iodide
ClBr-
1.0 0.2
1.8 x 1O-a 1.5 x 10-a
5.4 1.5
3 x l@ 2 x 108
Cl-
1.0
1.8 x 1O-s
11
6 x lo*
Anion B’
phase,
Exchange constant
In an acidic medium there will be interference from metals such as bismuth, gold, mercury, etc., which form anionic iodide complexes. The total concentration of such metals must not exceed 1CHM. Apparently the extraction constants of the &As+ salts of these complexes (especially if they are singly-charged) are higher than the extraction constant for the iodide itself, and consequently trace quantities of these metals will be extracted quantitatively and may interfere radiometrically. The interference can be eliminated either by a preliminary conversion of the iodide into iodine which is then extracted with chloroform, or by a preliminary preferential extraction of the halo-complexes with an amount of &As+ that is not more than a tenth of the amount to be used for extracting the iodide. Oxyanions
In Table III are given results for the extraction of tetraphenylarsonium perrhenate into chloroform and 1,2-dichloroethane from solutions containing chloride, nitrate, thiocyanate, perchlorate, permanganate and dichromate. The distribution ratio of the rhenium was determined by means of IsaRe. Similar experiments were carried out with QQmT~ from perrhenate solution. The exchange constants KX,ABTC,,/KR,AsBeO, were 1.7 and 2-5 for chloroform and 1,Zdichloroethane respectively. The order of extractability into chloroform as tetraphenylarsonium salts is MnO,
> TcO,- > ReO,- > ClO,- > (I-, SCN-, Cr,OVZ-)> NO,- > Br > Cl-.
From Table III it is clear that manganese(VII), rhenium(W), and chromium(W) can be substoichiometricahy separated by extraction of their tetraphenylarsonium salts into chloroform or 1,Zdichloroethane. Manganese has been substoichiometrically separated after oxidation of manganese(I1) to manganese(VI1) by sodium bismuthate in sulphuric acid medium,
Substoichiometric
115
separation in activation analysis
Table III.-EXTRACTION OF rerRe FROMSOLUTION~~NTAIN~N~OTHERANIONSAND SUL3STOICHIOMJiTRIC
Solvent
Anion B’
Equilibrium concentration of B’ in the aqueous phase, M x 10s
CINO,SCNChloroform
ClO,MnO,CrgO,=ClNO,SCN-
1,Zdichloroethane
AhtOUNTS
ClOpMnOlCra0,2-
OF TETRAPHENYLARWNIUM
Equilibrium concentration of RIAs salt in the organic
ph=, M x lOa
Rhenium distribution ratio
IONS
Exchange constant
400* 800 250 100 0.76 2.76 1.8 3.9 0.61 0.81 20
0.28 0.30 0.31 0.31 0.24 0.24 0.31 0.31 0.39 0.195 0.31
8.0 3.9 0.56 1.4 4.4 1.3 0.53 0.25 0.22 0.09 1’4f
1.1 1.0 4.5 4.5
x 10’ x 10’ x 10’ x 10’ 14 15 3.1 3.1 0.35 0.37 -
800 800 500 250 1.8 7.7 0.67 4.5 061 1.2 20
0.31 0.15 0.39 0.39 0.24 0.31 0.39 0.54 0.39 0.78 0.31
31 16 1.0 2.1 3.3 1.0 0.58 0.13 0.1 0.11 0.95t
8.0 8.5 1.3 1.3
x loo x lo4 x 10’ x 1oL 25 25 1.0 1.1 0.16 0.17 -
l That is, 0.4M. t Determined in presence of 0.4.5N sulphuric acid (the distribution in pH).
ratio increases with increase
following a preliminary separation of the manganese from interfering elements and reducing agents by precipitation as the hydroxide or manganese(N) oxide.ll Similar conditions are used for the separation of chromium(VI). Technetium, formed by the reaction sequence 9sMo(n,y)9sMo % QQmTc,does not interfere with the substoichiometric separation of rhenium, because equation (13) always holds if the amount of technetium present is too small to be weighed, but QQmTccoextracted with rhenium (according to equation (16) and Table III) does interfere with measurement of lsaRe on a gamma-spectrometer because the yradiation energies of the two isotopes are very close (about 0.14 MeV). The difference in redox potential of rhenium(VI1) and technetium(VI1) was made use of for their separation. In Fig. 4 is shown the effect of tin(II), added as sodium stannite, on the extraction of technetium and rhenium as their tetraphenylarsonium salts into chloroform or 1,2-dichloroethane from 0.3M sulphuric acid. A similar separation occurs from alkaline medium. Anionic complexes of metals
Anions such as AuCl,-, TlCI,-, FeCl,-, GaCI, and SbCI,, are easily extracted from hydrochloric acid solution as their R,As+ salts. Doubly-charged complex
116
I. P. ALIMARINand G. A. PEREZH~GIN
Tin
(II),
M
FIG. 4.-The influence of the tin(H) concentration on the substoichiometric extraction of tetraphenylarsonium perryhz;;;ito 1,2-dichloroethane (50 % stoichiometry). _.- coextraction of technetium
z. ‘_ .z z I
01. 0
I 2
I
Hydrochlonc
I
I 4 ocld,
I 6
I
M
Fro. S.-The influence of iron(III) and hydrochIoric acid on the substoichiometric extraction of tetraphenylarsonium ;uriiehl;~~ I”‘” chloroform (50 % stoichiometry). 0 FeiAu l&:1 A Fe:Au 15OO:l
117
Substoichiometric separation in activation analysis
anions of this kind are very poorly extracted. The extent of formation of these ions depends markedly on the hydrochloric acid concentration, which explains why iron does not hinder the substoichiometric extraction of gold when the concentration of hydrochloric acid is < lM, even when the ratio of iron to gold is 1000: 1 (Fig. 5). In Table IV results are given for the extraction of tetraphenylarsonium tetrachloroaurate(II1) into chloroform or 1,Zdichloroethane from solutions containing TABLE IV.-EXTRACTION OF losAu FROM SOLUTIONSCONTAININGTHALL~(III) OR IRON CHLORO-COMPLEXES
Solvent Chloroform
1,2-dichloroethane
Anion B’
AND
SIJBSTOICHIOhiETRICAMOUNTS
OF TETRAPHENYLARWNIUM
IONS
Equilibrium concentration of B’ in aqueous phase, M x 1w
Equilibrium concentration of R,AsB’ in organic phase, M x lo8
Distribution ratio of gold
Exchange constant
O-487 44 22 22
0.52 0.54 0.54 1.4
2.1 0.08 0.17 0.43
2.9 6.6 6.3 6.5
0.48
0.52
0.94
1.0
TICltFeCl*-*
TlCl,-
* Determined in presence of 8M hydrochloric acid. t That is 0.48 X 10-sM.
thallium(I) or iron(II1). The distribution coefficient was determined by means of ls*Au. From Table IV it can be seen that the difference in the extraction constants is negligible. For the extraction of gold, the ratio KIt,AsAucl,/KR,Aaclis about 1.5 x 108, i.e., it is possible to separate gold, thallium, iron and gallium from hydrochloric acid solution substoichiometrically by this method. TABLE V.-EXTRACTION OF lWs~o NITROBENZENE FROM SOLUTIONSCONTAINING ALKALI METALS AND SUBSTOICHIOMETRIC AMOUNTS OF TETRAPHENYLBORATB
Cation B’
Equilibrium concentration of B’in the aqueous phase, M x 10’
Equilibrium concentration of R,BB’ in the organic phase, M x 10’
Distribution ratio of caesium
750’ 1500 600 1000 19 200 19 39 1.15 0.65
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.35 0.35
13 6 3.1 2.2 1.5 0.16 0.32 0.17 0.023 0040
Li+ Na+ K+ Rb+ Tl+
OTHER
Exchange constant 1.1 1 2.1 2.4 ;;
x x x x
10’ 104 lo” 10’
6.8 I.4 0.076 0.074
* That is 0*75OM.
The extraction of the anionic fluoride, bromide, iodide, cyanide and thiocyanate complexes is apparently suitable for the substoichiometric separation of such metals as tantalum, bismuth, molybdenum(V) and tungsten(V), sometimes with ions smaller than the tetraphenylarsonium ion. The thiocyanate complex of gold, for
118
I. P. M
and G. A. PERE~OOIN
example, can be extracted into 1,Zdichloroethane with the triphenylguanidine ion. Increase in the anionic radius, however, decreases the effect of anionic charge on selectivity of separation, and the permissible concentrations of complexing anions which themselves are easily extractable (such as iodide and thiocyanate) are limited according to equation (4). SUBSTOICHIOMETRIC SEPARATIONS SIMPLE AND COMPLEX CATIONS
OF
It is known that the alkali metal cations can be extracted into nitrobenzene as compounds with tetraphenylborate or dipicrylamine anions, or other large anions. Some results are given in Table V. The distribution coefficient was determined by means of l=Cs. The extraction constant for caesium tetraphenylborate exceeds 10s. It seems possible to separate thallium(I), caesium, rubidium, and possibly potassium, substoichiometrically by extraction of the tetraphenylborates. Small or multiplycharged cations are not extracted. Such ions may be extracted, however, in the form of their cationic complexes with certain chelating agents such as 2,2’-dipyridyl or l,lO-phenanthroline which form complexes with metals such as copper(I) and iron(I1). In that case it is best to use as counter-ions large anions with the same degree of charge as the cation concerned.’ CONCLUSION
We have already used substoichiometric separation by extraction of coordinativelyunsolvated salts for the neutron activation determination of traces of gold6 and manganesell in high purity metals, and of goldI and rhenium13 in rocks and meteorites. This new method of separation of these elements in neutron activation analysis is much simpler and quicker than any used previously. The method seems extremely promising for the separation of iodine, caesium, rubidium, thallium, chromium, tantalum, and other metals, Zusammenfassuq-Die notwendigen Bedingungen zut Extraktion koordinativ nicht solvatisierter Salze zu unterstochiometrischen Trennungen in der Aktivierungsanalyse wurden untersucht, speziell der EinfluD stiirender Ionen und der Trlgerkonzentration. Die unterstochiometrische Extraktion von Halogenidionen, Oxyanionen und anionischen Metallkomplexen mittels Tetraphenylarsoniumionen in Chloroform und Dichlor%than wurde studier? sowie die Extraktion von Alkalimetallionen mit Tetraphenyloborat in Nitrobenzol. Die unterstochiometrische Extraktion kann mit der Neutronenaktivierungsanalyse bei der Bestimmung von Spurenmengen Jod, Caesium, Rubidium, Mangan, Rhenium, Chrom, Thallium, Gold, Gallium und Tantal kombiniert werden. R&am&-On a examine les conditions qui sont necessaires rl l’usage de l’extraction de sels non solvatb par coordination pour les separations substoechiometriques dans l’analyse par activation, essentiellement les influences des ions g%ants et de la concentration du porteur. On a effectue une etude sur l’extraction substoechiometrique des ions halogenures, des oxyanions et des complexes metalliques anioniques au moyen d’ion t&rapMnylarsonium en chloroforme et dichlorethane, et sur l’extraction des ions alcalins metaliques en nitrobenzene au moyen de tetraphenylborate. On peut combiner l’extraction substoechiometrique avec l’analyse par activation de neutrons pour le dosage de traces diode, caesium, rubidium, manganese, rhenium, chrome, thallium, or, gallium et tantale.
Substoichiometric
separation in activation analysis
119
REFERENCES 1. J. R&Wka and J. Starp, Tulanru, 1963,10,287. 2. J. Starf, Solvent Extraction of Metal Chelates, Pergamon, Oxford, 1964. 3. J. Star9 and J. Ri%icka, Talantu, 1964,11,697. 4. R. M. Diamond and D. G. Tuck, in F. A. Cotton, Progress in Inorganic Chemistry, Vol. II, pp. 139-150. Interscience, New York, 1960. 5. I. P. Alimarin and G. A. Perezhogin, Zh. Analit. Khim., 1965,20,48. 6. J. Rt%%a and A. Zeman, Talanta, 1965,12,997. 7. V. I. Kuznetsov and L. I. Moseyev, Radiokhimiyu, 1964,6,280. 8. S. Tribalat, Anal. Chim. Acta, 1949, 3, 113. 9. R. Bock and G. N. Beilstein, Z. Anal. Chem., 1962,192,44. 10. K. Ueno and C. Chang, J. Atomic Energy Sot. Japan., 1962,4,457. 11. G. A. Perezhogin, Zh. Analit. Khim., 1966,21, 879. 12. G. A. Perezhogin and I. P. Alimarin, ibid., 1965,20,793. 13. G. A. Perezhogin, Zavodskaya Lab., 1965,31,402.