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The determination of molybdenum by atomic absorption spectroscopy
Analytica Chimica Acta
437
ElacvicrPublhhingCompany,Ametardnm Printedin The Nethcrl.wds
SHORT The
COMMUNICATIONS
determination
of molybdenum
by atomic
absorption
spectroscopy
KIRKBRIGHT, SMITH AND WEST have recently reported the determination of molybdenum in alloy steels’ and of niobium and tantalum” with a nitrous oxideacetylene flame. These authors did not find interference from foreign ions, either bccause of the presence of large concentrations of the matrix element iron which causes severe depression of molybdenum absorptionl, or because of a rather selective extraction of molybdenum(V1) as its oxinate from a medium containing both EDTA and fluoride. Earlier work on the determination of aluminum3 and vanadium” in nitrous oxide-acetylkne flames indicated that the presence of certain ions caused considerable enhancement in the absorption signal of these metals. This observation prompted us to ascertain the effect of various ions on the determination of molybdenum in aqueous samples. The results indicated that numer,ous ions interfere. This communication describes the details of this investigation and a simple and rapid procedure is proposed to overcome the interferences.
Apparatacs A Perkin-Elmer (Model 303) atomic absorption spectrophotometer was used for absorption measurements. The standard burner head on the Perkin-Elmer ‘burner mount was replaced by a nitrous oxide-acetylene burner head with a slot length of 3 in and a width of 0.015 in. In addition to the two-stage regulators on the gas tanks, pressure gauges were used in the nitrous oxide lines and a flow meter was used in the acetylene line. A Perkin-Elmer hollow-cathode lamp was used as radiation source.
Reagents A stock solution containing I mg/ml of molybdenum(V1) was prepared by dissolving 1.840 g of ammonium molybdate tetrahydrate in I 1 of distilled water. Standard working solutions were prepared by appropriate dilution of the stock solution. Solutions for interference studies were prepared from reagent-grade chemicals. TABLE OPERATING
I CONDITIONS
Wavelength: 3232.6 A Slit: 0.3 mm (2 A) Lamp, current: 35 mA
Operating
Nitrous oxide Atomizer: 35 p.s.i. Auxiliary: 15 p.9.i. Acctylcnc: 5 l/min.
conditiom
The optimum operating conditions listed in Table I were established with a zo-p.p.m. solution of molybdenum. Absorption measurements were made at the rose-red inner cone of the flame about I cm above the burner head. Water WV first sprayed to set the zero of the instrument and, while the standard molybdenum Anal. Chim. Acta,
44 (1969)
437-439
SHORT
438
COMMUNICATIONS
solution was aspirated, the burner position was adjusted to give maximum absorption. Under these conditions the sample uptake was about 4 ml per min and the sensitivity was I p.p.m. of molybdenum for I% absorption. Interferewe
stwi?ies has reported the suppressing effect of certain ions in the molybdenum absorption in an air-acetylene flame which were completely eliminated by the addition of aluminum chloride. MOSTYN AND CUNNINGHAM~ later recommended the use of ammonium chloride as an interference-suppressing agent for the analysis of molybdenum in ferrous alloys. Our examination of the effect of various foreign ions in a nitrous oxide-acetylene flame showed that most ions affect molybdenum absorption. The ions examined and their effect on molybdenum absorption are shown in Table II. The cations were added as chlorides or nitrates, or, in certain instances, as sulfates, and the anions as sodium or potassium salts. In these investigations the concentration of molybdenum was 20 p,pm. and that of interferent 200 p.p.m. DAVIDI
TABLE
II
INTERFERENCES
(Molybclcnum:
FROM 20
VARIOUS
p.p.m.)
SUBSTANCES
ON MOLYBDENUM
-
----
Iatevfercd (a00 p.p.m.)
Absorption C%)
Interfevent (a00 p.p.m.)
A bsorptio9b (% )
None BrIF-
19.8 22.6 22.4 16.7
HCOaHAsO.~=Li+ Ns+
23.5 24.5 19.8
zwSO&
23.8
NOaHPOJaseonnTeOa+ VV04=IhO,+ vonSiOP coca-
19.6
ABSORPTION
20.0
Inter/event
A hovption
(200
(%)
Ccl 2+
p.p.m.)
Hga3’ H@+ Sna+
13.3 19.7 19.0 18.4
23.6 20.8 22.6 22.8
I<+
19.8
22.9 21.5
La&+ ca=+ Srs+
23.3 8.7 2.7
Fe3+ Cr3+ Pb”’ Sbs+
2G.8
BZl.=+
13.2
cc4*
12.3
21.4 22.8
M@‘* Mna+
19.8 19.8
uo+ Al=+
23.6
x9.8
coa+
21.4
Ti4+
s’
27.3 27.2
19.6
Ni3+
23.5
Zr4+
24.8
21.G
ma+
23.2
BC3’
27.0
25.0 23.5
cu=+ %n3+
24.3 23.7
The effect of acids was examined by adding 5 ml of the appropriate concentrated acid to the solution followed by dilution to xoo ml with distilled water. Both perchloric and acetic acids enhanced the absorption by about 30%, whereas sulfuric acid interfered by reducing the sensitivity by about 25 o/O.Hydrochloric acid and nitric acid’showed little affect on molybdenum absorption. An examination of Table II indicates that most ions caused an enhancement in molybdenum absorption. The few ions that showed suppressing effect included Caa+, Baz+, Sre+, Cd2+ and Ce+. It is also evident from Table II that the presence of ions Anal.
Chicn.
,Actn, 44 (1969)
437-439
SHORT
COMMUNICATIONS
,
439
such as Al3+ and Ti4+ which readily form stable oxides in the flame, caused maximum enhancement of molybdenum absorption. Later experiments confirmed that the interferences from these ions were not additive, as no further enhancement was noticed when more than one refractory metal ion was present in the sample. Although, in these instances, the enhancement in sensitivity could be attributed to the competitive reaction to form stable oxides, as was noticed in the case of aluminums and vanadium”, the reason for an enhancement in the presence of most other ions was not clear. An attempt was next made to determine the interfering effect of various ions on molybdenum absorption in the presence of a refractory metal ion. Aluminum was chosen for examination as it is readily available and has already been shown to be effective in overcoming the interferences in air-acetylene flame+. Although it was confirmed experimentally that 50 p.p.m. of aluminum is sufficient to produce maximum enhancement of molybdenum absorption, a higher concentration of aluminum (IOO p.p.m.) was present when the effect of various ions was re-investigated. The concentration of molybdenum and interferents were, as before, 20 p.p.m. and 200 p.p.m. respectively. None of the ions listed in Table II showed any effect on molybdenum absorption when the absorption values were compared to solutions containing identical concentrations of molybdenum and aluminum in the absence of extraneous ions. The interfering effect of the few ions that were found to suppress molybdenum absorption also disappeared in the presence of aluminum.
The results of this investigation indicated that the determination of molybdenum in nitrous oxide-acetylene flame was subject to interference from several extraneous ions and compounds. Depending upon the ion present, the interference was either an enhancement or a depression in the molybdenum absorption. However, the method could be made more selective by the addition of aluminum to the samples. The sensitivities in the absence and presence of aluminum were respectively I p.p.m. and 0.8 p.p.m. of molybdenum for 1% absorption. This work was supported by the US. Public Health Service Grant AP 00128 from the National Center for Air Pollution Control, Bureau of State Services. T. V. RAMAKRISHNA
Coates Chemical Laboratory, Louidhna State Ukx&ty, Batoti Rouge, La. 70803 (U.S.A.)
PHILIP
w.
WEST
J. W. ROBINSON
I G. F. KIRKBRIGHT, A. M. SMITH AND T. S. WEST, Analyst, 91 2 G. F. KIRKBRIGHT, M. K. PETERE AND T. S. WEST, Analyst, gr 3 T. V. RAMAKRISHNA, P. W. WEST AND J. W. ROBINSON, Anal. 4 S. L. SACHDEV, J. W. ROBINSON AND P. W. WEST, Ana2. Chim. :D. J. DAVID, Atialyst, 86 (x96x) 730. 6 R. A. MOSTYN AND A. F. CUNNINGHAM, Anal. Clrem., 38 (x966)
(1966) (1966) Chim. Acta.
700. 705. Act% 39 (1967) 81. 37 (1967) 12.
121.
(Received August xzth, 1968) Anal.
Claim. Acta. 44 (1969) 437-439
437
ElacvicrPublhhingCompany,Ametardnm Printedin The Nethcrl.wds
SHORT The
COMMUNICATIONS
determination
of molybdenum
by atomic
absorption
spectroscopy
KIRKBRIGHT, SMITH AND WEST have recently reported the determination of molybdenum in alloy steels’ and of niobium and tantalum” with a nitrous oxideacetylene flame. These authors did not find interference from foreign ions, either bccause of the presence of large concentrations of the matrix element iron which causes severe depression of molybdenum absorptionl, or because of a rather selective extraction of molybdenum(V1) as its oxinate from a medium containing both EDTA and fluoride. Earlier work on the determination of aluminum3 and vanadium” in nitrous oxide-acetylkne flames indicated that the presence of certain ions caused considerable enhancement in the absorption signal of these metals. This observation prompted us to ascertain the effect of various ions on the determination of molybdenum in aqueous samples. The results indicated that numer,ous ions interfere. This communication describes the details of this investigation and a simple and rapid procedure is proposed to overcome the interferences.
Apparatacs A Perkin-Elmer (Model 303) atomic absorption spectrophotometer was used for absorption measurements. The standard burner head on the Perkin-Elmer ‘burner mount was replaced by a nitrous oxide-acetylene burner head with a slot length of 3 in and a width of 0.015 in. In addition to the two-stage regulators on the gas tanks, pressure gauges were used in the nitrous oxide lines and a flow meter was used in the acetylene line. A Perkin-Elmer hollow-cathode lamp was used as radiation source.
Reagents A stock solution containing I mg/ml of molybdenum(V1) was prepared by dissolving 1.840 g of ammonium molybdate tetrahydrate in I 1 of distilled water. Standard working solutions were prepared by appropriate dilution of the stock solution. Solutions for interference studies were prepared from reagent-grade chemicals. TABLE OPERATING
I CONDITIONS
Wavelength: 3232.6 A Slit: 0.3 mm (2 A) Lamp, current: 35 mA
Operating
Nitrous oxide Atomizer: 35 p.s.i. Auxiliary: 15 p.9.i. Acctylcnc: 5 l/min.
conditiom
The optimum operating conditions listed in Table I were established with a zo-p.p.m. solution of molybdenum. Absorption measurements were made at the rose-red inner cone of the flame about I cm above the burner head. Water WV first sprayed to set the zero of the instrument and, while the standard molybdenum Anal. Chim. Acta,
44 (1969)
437-439
SHORT
438
COMMUNICATIONS
solution was aspirated, the burner position was adjusted to give maximum absorption. Under these conditions the sample uptake was about 4 ml per min and the sensitivity was I p.p.m. of molybdenum for I% absorption. Interferewe
stwi?ies has reported the suppressing effect of certain ions in the molybdenum absorption in an air-acetylene flame which were completely eliminated by the addition of aluminum chloride. MOSTYN AND CUNNINGHAM~ later recommended the use of ammonium chloride as an interference-suppressing agent for the analysis of molybdenum in ferrous alloys. Our examination of the effect of various foreign ions in a nitrous oxide-acetylene flame showed that most ions affect molybdenum absorption. The ions examined and their effect on molybdenum absorption are shown in Table II. The cations were added as chlorides or nitrates, or, in certain instances, as sulfates, and the anions as sodium or potassium salts. In these investigations the concentration of molybdenum was 20 p,pm. and that of interferent 200 p.p.m. DAVIDI
TABLE
II
INTERFERENCES
(Molybclcnum:
FROM 20
VARIOUS
p.p.m.)
SUBSTANCES
ON MOLYBDENUM
-
----
Iatevfercd (a00 p.p.m.)
Absorption C%)
Interfevent (a00 p.p.m.)
A bsorptio9b (% )
None BrIF-
19.8 22.6 22.4 16.7
HCOaHAsO.~=Li+ Ns+
23.5 24.5 19.8
zwSO&
23.8
NOaHPOJaseonnTeOa+ VV04=IhO,+ vonSiOP coca-
19.6
ABSORPTION
20.0
Inter/event
A hovption
(200
(%)
Ccl 2+
p.p.m.)
Hga3’ H@+ Sna+
13.3 19.7 19.0 18.4
23.6 20.8 22.6 22.8
I<+
19.8
22.9 21.5
La&+ ca=+ Srs+
23.3 8.7 2.7
Fe3+ Cr3+ Pb”’ Sbs+
2G.8
BZl.=+
13.2
cc4*
12.3
21.4 22.8
M@‘* Mna+
19.8 19.8
uo+ Al=+
23.6
x9.8
coa+
21.4
Ti4+
s’
27.3 27.2
19.6
Ni3+
23.5
Zr4+
24.8
21.G
ma+
23.2
BC3’
27.0
25.0 23.5
cu=+ %n3+
24.3 23.7
The effect of acids was examined by adding 5 ml of the appropriate concentrated acid to the solution followed by dilution to xoo ml with distilled water. Both perchloric and acetic acids enhanced the absorption by about 30%, whereas sulfuric acid interfered by reducing the sensitivity by about 25 o/O.Hydrochloric acid and nitric acid’showed little affect on molybdenum absorption. An examination of Table II indicates that most ions caused an enhancement in molybdenum absorption. The few ions that showed suppressing effect included Caa+, Baz+, Sre+, Cd2+ and Ce+. It is also evident from Table II that the presence of ions Anal.
Chicn.
,Actn, 44 (1969)
437-439
SHORT
COMMUNICATIONS
,
439
such as Al3+ and Ti4+ which readily form stable oxides in the flame, caused maximum enhancement of molybdenum absorption. Later experiments confirmed that the interferences from these ions were not additive, as no further enhancement was noticed when more than one refractory metal ion was present in the sample. Although, in these instances, the enhancement in sensitivity could be attributed to the competitive reaction to form stable oxides, as was noticed in the case of aluminums and vanadium”, the reason for an enhancement in the presence of most other ions was not clear. An attempt was next made to determine the interfering effect of various ions on molybdenum absorption in the presence of a refractory metal ion. Aluminum was chosen for examination as it is readily available and has already been shown to be effective in overcoming the interferences in air-acetylene flame+. Although it was confirmed experimentally that 50 p.p.m. of aluminum is sufficient to produce maximum enhancement of molybdenum absorption, a higher concentration of aluminum (IOO p.p.m.) was present when the effect of various ions was re-investigated. The concentration of molybdenum and interferents were, as before, 20 p.p.m. and 200 p.p.m. respectively. None of the ions listed in Table II showed any effect on molybdenum absorption when the absorption values were compared to solutions containing identical concentrations of molybdenum and aluminum in the absence of extraneous ions. The interfering effect of the few ions that were found to suppress molybdenum absorption also disappeared in the presence of aluminum.
The results of this investigation indicated that the determination of molybdenum in nitrous oxide-acetylene flame was subject to interference from several extraneous ions and compounds. Depending upon the ion present, the interference was either an enhancement or a depression in the molybdenum absorption. However, the method could be made more selective by the addition of aluminum to the samples. The sensitivities in the absence and presence of aluminum were respectively I p.p.m. and 0.8 p.p.m. of molybdenum for 1% absorption. This work was supported by the US. Public Health Service Grant AP 00128 from the National Center for Air Pollution Control, Bureau of State Services. T. V. RAMAKRISHNA
Coates Chemical Laboratory, Louidhna State Ukx&ty, Batoti Rouge, La. 70803 (U.S.A.)
PHILIP
w.
WEST
J. W. ROBINSON
I G. F. KIRKBRIGHT, A. M. SMITH AND T. S. WEST, Analyst, 91 2 G. F. KIRKBRIGHT, M. K. PETERE AND T. S. WEST, Analyst, gr 3 T. V. RAMAKRISHNA, P. W. WEST AND J. W. ROBINSON, Anal. 4 S. L. SACHDEV, J. W. ROBINSON AND P. W. WEST, Ana2. Chim. :D. J. DAVID, Atialyst, 86 (x96x) 730. 6 R. A. MOSTYN AND A. F. CUNNINGHAM, Anal. Clrem., 38 (x966)
(1966) (1966) Chim. Acta.
700. 705. Act% 39 (1967) 81. 37 (1967) 12.
121.
(Received August xzth, 1968) Anal.
Claim. Acta. 44 (1969) 437-439