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Extraction spectrophotometric determination of niobium in rocks with sulfochlorophenol S
Analytica
Chimica
0 Elsevier
Scientific
Short
Acta.
116 (19SO)
Publishing
185-190 Company, Amsterdam
-Printed
in The Netherlnnds
Communication
EXTRACTION SPECTROPHOTOMETRIC DETERMI~_;:XTJO~ SIOBIUM IX ROCKS WITH SULFOCHLOROPHESOL S
X. E. CHILDIIESS*
and L. P. GREESLAND
C-.5’. Gcologicul
Surccy.
Reston,
(Received
October
1979)
96th
OF
17a. 32092
fC..S._A.)
Sunmary. After acid decomposition and potassium p?.-rosulfote fusion, niobium (IF.26 ppm) is separated from interfering elements by estraction into methvl isobutyl ketone from 6 \I H,SO,-_” 11 HF and back-extracted into xvatcr. The niobium-slll(.ochl~,,-ophenol S complcts is estmcted into amyl alcohol.
An cstraction spectrophotometric method utilizing -I-( 2-pyriclylazo Iresorcinol (PAR) has been in routine use by the US. Geological Survq- for the past several years [l] , but has nut proven sufficiently sensitive to allow precise measurement of niobium in the l-5 ppm range. This limitation and a recently noted systematic bias between spectrophotomctric and neutron activation values [2, 31 led to investigation of alternative m&hods of :1iobium determination. The sulfochlorophenol S method described in t.his paper provides sensitivity down to 1 ppm, but fails to resolve thct discrepancy betxeen previous spect.rophoton1etric and neutron activation values.
A*iobiunz standard solution. \Veigh a I,ol-tion (\dd 1 ml of concentrated sulfuric acid plus 0.2 ml of 1% potassium chloride solution and
186
evaporate to dryness_ _4dd 5 ml of 6 M i-ICI to the residue, cover and heat on a hot plate at 175°C for 30 min. Add 1.6 ml of 5% tartaric acid in 4 M HCl, 15 ml of water and O-1 ml of thioglycollic acid. Prepare a blank by making these same additions to a beaker containing no niobium. Heat each solution for 15 min at 60°C. _Add 0.3 ml of 0.3 M EDTA and 0.2 ml of an aqueous 0.1%
sulfochlorophenol S solution, and heat for another 15 min. Transfer solution to a separatory funnel, add 0.5 ml of 10% diphenylguanidine
the in
concentrated HCI (freshly prepared), and extract with 6 ml of amyl alcohol. Discard the aqueous layer and add 1 ml of ethanol to the organic phase. >Ieasure the absorbance of the organic phase in l- or 5-cm cells at 656 nm against a blank taken through the whole procedure. Determine the concentration of niobium by comparison with the standards carried through the entire procedure_
ResrcIts and discussion This method combines two estensively studied techniques, the separation of niobium from interfering elements by estraction from H,SOq-HF solutions with %IIBK [4-Y] and the extraction spectrophotometric determination of niobium with sulfochlorophenol S [S-13]. The MIBK estraction separates niobium from most elements found in rocks. Only elemental halogens, selenium and tellurium are coextracted to any appreciable extent [6] _ After an evaporation step to remove hydrogen fluoride, the sulfochlorophenol S color is developed in an acidic medium, avoiding t!le heavy reliance on masking necessary with the less selective P-AR. Although the selectivity of PAR increases dramatically in dilute mineral acid solutions, the corresponding loss of sensitivity makes this reaction of little practical value in rock analysis_ Thus the current method provides higher sensitivity because of the 150% higher molar absorptivity than PAR and better selectivity because of the MIBK extraction and the ability of sulfochlorophenol S to form stabIe complexes with niobium in acidic media. To provide an estimate of the precision of the method, three bottles each of nine U.S. Geological Survey standard rocks were randomized and sample portions analyzed over several weeks. The analyses, reported in Table 1, indicate that the standard deviation of a determination is generally better than 5% for concentrations of more than 9 ppm of niobium and 219% for 1.6 ppm. These data also permit a one-way analysis of variance to test for homogeneity among bottles of the standard rocks. As shown by the F-ratios, the bottles of standards were found to be homogeneous at the 95% confidence level. The results of this work are compared in Table 2 with those obtained in previous analyses_ There is a systematic bias both between the two spectrophotometric techniques and between these and the neutron activation values; this is illustrated in Fig. 1. Several possible sources of bias were investigated in some detail. The single niobium pentoxide sample used in previous work at the U.S. Geological
187 T-ABLE Niobium
1 determined
Standard rock
in USGS
Standard
Bottle (Split/position)
XGV- 1
0.139
10019 68/-l ? 16/30
12.0,11.9, 11.8 11.5, 11.8, 13.2 12.2. 12.2, ll.s
11.9 5 0.3
0.696
216 41:30 47il
11.6, 12.3, 18.2 12.1, 11.1, 11.4 10.9. 12.1, 11.5
11.-i = 0.5
1.05
S1!23 -19;“s
19.4, 18.4, lS.9 18.3. 17.9, 1S.l lS.1, lS.9, 19.0
18.6 : 0.5
2.56
RG.\I- 1
s.-:. 9.6. 9.3,
BHVO-1
17.6 1s.1: lS.3,
1
X3! 16 5 1,: -1916
UXC-
1
OiS8 146-1 1304
____-_-.-_.
-
1.11
26.0 = 0.7
103;1-1
QLO-
13.8 5 0.6
26.1 26.6 25.4
43/23
SLIC-1
F-ratio
X.5, 26.3, 21.5, 26.2, 26.3, 26.3.
-l/23
8Oi2
BCR-1
Mean 2 s.d.
(ppm) 14.8 13.6 14.0
-1-l/l-i
G-2
Nb cont.
13.2. 13.1, 13.1, 13.6, 14.0, 14.7,
-57l2.l Ill-1
GSP- 1
Rocks
..-_ - . - --.
9.6, 9.5, 9 _2 ,
15.3. IS.3 17.3, 15.2 15.2. 17.9
10.1. 10.5. 11.9. 10.6, 9.-I. 10.0. 1.7, 1.6. 1.0, l.s. 1.“. 1.6.
9.7 9.4 s .5
1.3, 1.4. 1.4, 2.3, 1.4, 2.0,
--. - -- -- - - --.-.
9.t; 9.9 10.5 2.0 1.s 1.” 1..5 I.3 1.5
9.2 I 0.5
lS.O
: 6.1;
10.3 : 0.7
1.6
- 0.3
0.77s
0
;j 9 3
1:zo
0.339
-
Survey was compared with standards prepared from spectrographically pure niobium pentoside and from niobium metal. The results of this comparison were in excellent agreement. In addition, the niobium metal standard was checked by gravimetric analysis and found to be within 37 of t.he expected value. Because the neutron activation procedure relied on acid decomposition alone, whereas both spcctrophotometric techniques used a fusion as well as an acid decomposition, the lower activation values could possibly have reflected incomplete dissolution of the sample_ The spectrophotometric
18s TABLE
2
Comparison of standard rocks Standard
niobium
(ppm)
GSP-1
G-2
_4GV-1
rock
Spectrophotometxy. this work Spectrophotometry. PXR Ill Spectrophotometry, isotope dilution-PAR Spectrophotometry. thiocyanate [ 111 Neutron activation [ZI X.r.f. [ 161 X.r.f_ [ 161 X.r.f.”
“Ref
concentrations
[ ‘21, Table
obtained
by
BCR-1
various
techniques
for
SDC-1
RGM-1
BHVO-1
QLO
13.8
26.0
11.9
11.7
18.6
9.2
18.0
10.3
15.7
28.7
13.1
X5.6
21.1
9.1
21.0
11.7
15.0
29.4
13.3
13.5
16.0
27.5
12.6
23.6
12.3
20.1
9.8
10.6
16.0
8.1
16.3
9.1
I-r]
I3
2-1 25.-x 23.1 (19-28.3
15.0 (IO- -21-i)
12 10.6 (S-16.1)
1-l
11.3 19.6 (g---to)
1; mean and range are given.
I’
.
’
*’
,A. f
:’ .;,. y ,.’ _ : ’ ,,’
p
1 ,p
y’ L__
_ _.._.._
... ._.
___...
._.. __..,
Fig.
I_ Pi&
negative method.
_.
_._
_’
illustrating
.
_ .._
_.:_
_.
__-
.;.
.
positive
systematic
bias
of
spectrophotometric
systematic bias of neutron activation results (0) in relation Perfect agreement between methods is represented by the dashed
results to the line_
(i- ) and current
results shown in Table 3 compare this acid decomposition-MIBK extraction procedure [ 2, 31 with the acid decomposition plus fusion and MIBK extraction used in this method, both followed by sulphochlorophenol S determination_ These data demonstrate the adequacy of acid decomposition for the standard rocks analyzed in this work. Because of the widespread occurrence of niobium in acid-resistant minerals, however, the procedure involving fusion must still be regarded as more generally satisfactory for samples of unknown composition_
189 TABLE
3
Comparison Standard
of spectrophotometric
rock
-_.--~-
l-1.4 26.1 11.7 11.6 8.S
(3) (3) (3) (-1) (3)
18.3 10.0
(6) (5)
of niobium” Present procedure
Acid decomposition
_______~_ AGV-1 GSP-1 G-2 BCR-1 RGhI-1 BHVO-1 QLO-1
determinations
[2, 3
j 13.6 26.0 11.9 11.7 9.2 1s.o 10.3
_--.-
aNumbers in parentheses indicate number valties given are the means of nine replicate
of replicates. determinations.
For the present
---
procedure,
the
Table 2 shows that the three previous spectrophotometric techniques used at the U.S. Geological Survey are in good agreement. with one another. They are also independent of one another in the sense that different separations and color reagents were used. Thus an elemental interference is unlikely to be common to all three methods and to be of similar magnitude in each case. Also, the fact that the bias between methods is systematic,
regardless of rock type, further decreases the likelihood that an elemental interference is the source of the discrepancy. Although a comparison of mean s-ray fluorescence values [2] (Table 2) demonstrates as good an as with the agreement with the spectrophotometric values, the range is such that no meaningful comparison
neutron activation can be made. Iiow-
ever, although the bias cannot be accounted for, it is small enough to assert that the absolute accuracy of the present method is rl3Z of the true value. \Ve are grateful to E. Y’. Campbell comparing the niobium standards_
for her many helpful comments
and for
REFEREXCES 1 2 3 1 5 6 7 8 9 10
L. R. R. L. F. P. H. S. T. S.
P. Greenland and E. Y. Campbell, J. Res. U.S. Geol. Surv., r? (19i-!:) 333. 0. Allen and E. Steinnes, Geostandards NewsI., 3 (19i9) 57. 0. Men and E. Steinnes, Anal. Chem., 50 (197s) 903. P. Greenland and E. Y. Campbell, Anal. Chim. Acta, -19 (1970) 109. Nelson, R. &I. Rush and K. A. Kraus, J. Am. Chem. Sot.: SL? (1960) 339. C. Stevenson and H. G. Hicks, Atxl. Chem., 25 (1953) 1517. G. Hicks and R. S. Gilbert, Anal. Chem., 26 (1954) 1205. V. Elinson, Russ. Chem. Rev., 4-1 (1975) 70’7. Sakaki, J. Jpn. Inst. Metals, 33 (1969) 1092. B.
Savin,
21 (1966) 11 E. 39
N.
I.
D.
Paschenko,
(1973)
Pisarenko,
E.
I. Ynrchenko
and
Yu.
51.
Declkov,
Zh.
and
N.
P. Volkovn,
ArmI.
Khim..
669. 129i.
L.
A.
Vasil’eva,
V.
F.
bIal’tsev
Zavod.
Lab..
190 12 18 1-I 15 16
S. B. Savin, P. N. Romanov and Yu. G. Eremin, Zh. Anal. Khim., 12 (1966) 1423. S_ B. Savin, V_ A. Bortsova and E_ N_ Malkina. Zh. Anal. Khim.. 20 (1965) 947. F. S. Grimaldi. Anal. Chem., 32 (1960) 119. E. Jagoutz and C. Palme, Anal. Chem., 50 (19T8) 1555. J. P. Willis, L. H. Ahrens, R. V. Danchin, A_ J. Erlank, J. J. Gurney, P. K. Hofmeyr. T. S. McCarthy and 31. J. Orren, Proc. 2nd Lunar Science Conf.. Geochim. Cosmochim. Acta, Suppl. 2. (1971) 1123.
Chimica
0 Elsevier
Scientific
Short
Acta.
116 (19SO)
Publishing
185-190 Company, Amsterdam
-Printed
in The Netherlnnds
Communication
EXTRACTION SPECTROPHOTOMETRIC DETERMI~_;:XTJO~ SIOBIUM IX ROCKS WITH SULFOCHLOROPHESOL S
X. E. CHILDIIESS*
and L. P. GREESLAND
C-.5’. Gcologicul
Surccy.
Reston,
(Received
October
1979)
96th
OF
17a. 32092
fC..S._A.)
Sunmary. After acid decomposition and potassium p?.-rosulfote fusion, niobium (IF.26 ppm) is separated from interfering elements by estraction into methvl isobutyl ketone from 6 \I H,SO,-_” 11 HF and back-extracted into xvatcr. The niobium-slll(.ochl~,,-ophenol S complcts is estmcted into amyl alcohol.
An cstraction spectrophotometric method utilizing -I-( 2-pyriclylazo Iresorcinol (PAR) has been in routine use by the US. Geological Survq- for the past several years [l] , but has nut proven sufficiently sensitive to allow precise measurement of niobium in the l-5 ppm range. This limitation and a recently noted systematic bias between spectrophotomctric and neutron activation values [2, 31 led to investigation of alternative m&hods of :1iobium determination. The sulfochlorophenol S method described in t.his paper provides sensitivity down to 1 ppm, but fails to resolve thct discrepancy betxeen previous spect.rophoton1etric and neutron activation values.
A*iobiunz standard solution. \Veigh a I,ol-tion (
186
evaporate to dryness_ _4dd 5 ml of 6 M i-ICI to the residue, cover and heat on a hot plate at 175°C for 30 min. Add 1.6 ml of 5% tartaric acid in 4 M HCl, 15 ml of water and O-1 ml of thioglycollic acid. Prepare a blank by making these same additions to a beaker containing no niobium. Heat each solution for 15 min at 60°C. _Add 0.3 ml of 0.3 M EDTA and 0.2 ml of an aqueous 0.1%
sulfochlorophenol S solution, and heat for another 15 min. Transfer solution to a separatory funnel, add 0.5 ml of 10% diphenylguanidine
the in
concentrated HCI (freshly prepared), and extract with 6 ml of amyl alcohol. Discard the aqueous layer and add 1 ml of ethanol to the organic phase. >Ieasure the absorbance of the organic phase in l- or 5-cm cells at 656 nm against a blank taken through the whole procedure. Determine the concentration of niobium by comparison with the standards carried through the entire procedure_
ResrcIts and discussion This method combines two estensively studied techniques, the separation of niobium from interfering elements by estraction from H,SOq-HF solutions with %IIBK [4-Y] and the extraction spectrophotometric determination of niobium with sulfochlorophenol S [S-13]. The MIBK estraction separates niobium from most elements found in rocks. Only elemental halogens, selenium and tellurium are coextracted to any appreciable extent [6] _ After an evaporation step to remove hydrogen fluoride, the sulfochlorophenol S color is developed in an acidic medium, avoiding t!le heavy reliance on masking necessary with the less selective P-AR. Although the selectivity of PAR increases dramatically in dilute mineral acid solutions, the corresponding loss of sensitivity makes this reaction of little practical value in rock analysis_ Thus the current method provides higher sensitivity because of the 150% higher molar absorptivity than PAR and better selectivity because of the MIBK extraction and the ability of sulfochlorophenol S to form stabIe complexes with niobium in acidic media. To provide an estimate of the precision of the method, three bottles each of nine U.S. Geological Survey standard rocks were randomized and sample portions analyzed over several weeks. The analyses, reported in Table 1, indicate that the standard deviation of a determination is generally better than 5% for concentrations of more than 9 ppm of niobium and 219% for 1.6 ppm. These data also permit a one-way analysis of variance to test for homogeneity among bottles of the standard rocks. As shown by the F-ratios, the bottles of standards were found to be homogeneous at the 95% confidence level. The results of this work are compared in Table 2 with those obtained in previous analyses_ There is a systematic bias both between the two spectrophotometric techniques and between these and the neutron activation values; this is illustrated in Fig. 1. Several possible sources of bias were investigated in some detail. The single niobium pentoxide sample used in previous work at the U.S. Geological
187 T-ABLE Niobium
1 determined
Standard rock
in USGS
Standard
Bottle (Split/position)
XGV- 1
0.139
10019 68/-l ? 16/30
12.0,11.9, 11.8 11.5, 11.8, 13.2 12.2. 12.2, ll.s
11.9 5 0.3
0.696
216 41:30 47il
11.6, 12.3, 18.2 12.1, 11.1, 11.4 10.9. 12.1, 11.5
11.-i = 0.5
1.05
S1!23 -19;“s
19.4, 18.4, lS.9 18.3. 17.9, 1S.l lS.1, lS.9, 19.0
18.6 : 0.5
2.56
RG.\I- 1
s.-:. 9.6. 9.3,
BHVO-1
17.6 1s.1: lS.3,
1
X3! 16 5 1,: -1916
UXC-
1
OiS8 146-1 1304
____-_-.-_.
-
1.11
26.0 = 0.7
103;1-1
QLO-
13.8 5 0.6
26.1 26.6 25.4
43/23
SLIC-1
F-ratio
X.5, 26.3, 21.5, 26.2, 26.3, 26.3.
-l/23
8Oi2
BCR-1
Mean 2 s.d.
(ppm) 14.8 13.6 14.0
-1-l/l-i
G-2
Nb cont.
13.2. 13.1, 13.1, 13.6, 14.0, 14.7,
-57l2.l Ill-1
GSP- 1
Rocks
..-_ - . - --.
9.6, 9.5, 9 _2 ,
15.3. IS.3 17.3, 15.2 15.2. 17.9
10.1. 10.5. 11.9. 10.6, 9.-I. 10.0. 1.7, 1.6. 1.0, l.s. 1.“. 1.6.
9.7 9.4 s .5
1.3, 1.4. 1.4, 2.3, 1.4, 2.0,
--. - -- -- - - --.-.
9.t; 9.9 10.5 2.0 1.s 1.” 1..5 I.3 1.5
9.2 I 0.5
lS.O
: 6.1;
10.3 : 0.7
1.6
- 0.3
0.77s
0
;j 9 3
1:zo
0.339
-
Survey was compared with standards prepared from spectrographically pure niobium pentoside and from niobium metal. The results of this comparison were in excellent agreement. In addition, the niobium metal standard was checked by gravimetric analysis and found to be within 37 of t.he expected value. Because the neutron activation procedure relied on acid decomposition alone, whereas both spcctrophotometric techniques used a fusion as well as an acid decomposition, the lower activation values could possibly have reflected incomplete dissolution of the sample_ The spectrophotometric
18s TABLE
2
Comparison of standard rocks Standard
niobium
(ppm)
GSP-1
G-2
_4GV-1
rock
Spectrophotometxy. this work Spectrophotometry. PXR Ill Spectrophotometry, isotope dilution-PAR Spectrophotometry. thiocyanate [ 111 Neutron activation [ZI X.r.f. [ 161 X.r.f_ [ 161 X.r.f.”
“Ref
concentrations
[ ‘21, Table
obtained
by
BCR-1
various
techniques
for
SDC-1
RGM-1
BHVO-1
QLO
13.8
26.0
11.9
11.7
18.6
9.2
18.0
10.3
15.7
28.7
13.1
X5.6
21.1
9.1
21.0
11.7
15.0
29.4
13.3
13.5
16.0
27.5
12.6
23.6
12.3
20.1
9.8
10.6
16.0
8.1
16.3
9.1
I-r]
I3
2-1 25.-x 23.1 (19-28.3
15.0 (IO- -21-i)
12 10.6 (S-16.1)
1-l
11.3 19.6 (g---to)
1; mean and range are given.
I’
.
’
*’
,A. f
:’ .;,. y ,.’ _ : ’ ,,’
p
1 ,p
y’ L__
_ _.._.._
... ._.
___...
._.. __..,
Fig.
I_ Pi&
negative method.
_.
_._
_’
illustrating
.
_ .._
_.:_
_.
__-
.;.
.
positive
systematic
bias
of
spectrophotometric
systematic bias of neutron activation results (0) in relation Perfect agreement between methods is represented by the dashed
results to the line_
(i- ) and current
results shown in Table 3 compare this acid decomposition-MIBK extraction procedure [ 2, 31 with the acid decomposition plus fusion and MIBK extraction used in this method, both followed by sulphochlorophenol S determination_ These data demonstrate the adequacy of acid decomposition for the standard rocks analyzed in this work. Because of the widespread occurrence of niobium in acid-resistant minerals, however, the procedure involving fusion must still be regarded as more generally satisfactory for samples of unknown composition_
189 TABLE
3
Comparison Standard
of spectrophotometric
rock
-_.--~-
l-1.4 26.1 11.7 11.6 8.S
(3) (3) (3) (-1) (3)
18.3 10.0
(6) (5)
of niobium” Present procedure
Acid decomposition
_______~_ AGV-1 GSP-1 G-2 BCR-1 RGhI-1 BHVO-1 QLO-1
determinations
[2, 3
j 13.6 26.0 11.9 11.7 9.2 1s.o 10.3
_--.-
aNumbers in parentheses indicate number valties given are the means of nine replicate
of replicates. determinations.
For the present
---
procedure,
the
Table 2 shows that the three previous spectrophotometric techniques used at the U.S. Geological Survey are in good agreement. with one another. They are also independent of one another in the sense that different separations and color reagents were used. Thus an elemental interference is unlikely to be common to all three methods and to be of similar magnitude in each case. Also, the fact that the bias between methods is systematic,
regardless of rock type, further decreases the likelihood that an elemental interference is the source of the discrepancy. Although a comparison of mean s-ray fluorescence values [2] (Table 2) demonstrates as good an as with the agreement with the spectrophotometric values, the range is such that no meaningful comparison
neutron activation can be made. Iiow-
ever, although the bias cannot be accounted for, it is small enough to assert that the absolute accuracy of the present method is rl3Z of the true value. \Ve are grateful to E. Y’. Campbell comparing the niobium standards_
for her many helpful comments
and for
REFEREXCES 1 2 3 1 5 6 7 8 9 10
L. R. R. L. F. P. H. S. T. S.
P. Greenland and E. Y. Campbell, J. Res. U.S. Geol. Surv., r? (19i-!:) 333. 0. Allen and E. Steinnes, Geostandards NewsI., 3 (19i9) 57. 0. Men and E. Steinnes, Anal. Chem., 50 (197s) 903. P. Greenland and E. Y. Campbell, Anal. Chim. Acta, -19 (1970) 109. Nelson, R. &I. Rush and K. A. Kraus, J. Am. Chem. Sot.: SL? (1960) 339. C. Stevenson and H. G. Hicks, Atxl. Chem., 25 (1953) 1517. G. Hicks and R. S. Gilbert, Anal. Chem., 26 (1954) 1205. V. Elinson, Russ. Chem. Rev., 4-1 (1975) 70’7. Sakaki, J. Jpn. Inst. Metals, 33 (1969) 1092. B.
Savin,
21 (1966) 11 E. 39
N.
I.
D.
Paschenko,
(1973)
Pisarenko,
E.
I. Ynrchenko
and
Yu.
51.
Declkov,
Zh.
and
N.
P. Volkovn,
ArmI.
Khim..
669. 129i.
L.
A.
Vasil’eva,
V.
F.
bIal’tsev
Zavod.
Lab..
190 12 18 1-I 15 16
S. B. Savin, P. N. Romanov and Yu. G. Eremin, Zh. Anal. Khim., 12 (1966) 1423. S_ B. Savin, V_ A. Bortsova and E_ N_ Malkina. Zh. Anal. Khim.. 20 (1965) 947. F. S. Grimaldi. Anal. Chem., 32 (1960) 119. E. Jagoutz and C. Palme, Anal. Chem., 50 (19T8) 1555. J. P. Willis, L. H. Ahrens, R. V. Danchin, A_ J. Erlank, J. J. Gurney, P. K. Hofmeyr. T. S. McCarthy and 31. J. Orren, Proc. 2nd Lunar Science Conf.. Geochim. Cosmochim. Acta, Suppl. 2. (1971) 1123.