2253
Anal. Chem. 1983, 55, 2253-2255
Kinetic-Potentiometric Determination of Vanadium(V) and Iron(II) S. B. Jonnalangadda
Department of Chemistry, University of Nairobi, Nairobi, Kenya
A double-check klnetlc method of followlng the emf change (with piatlnum and calomel electrodes) Is described for the determlnatlon of vanadlum(V) and lron(I1) In the 5 X lo4 to 10 X lo4 M concentratlon range based on thelr influence on the Induction period and on the klnetlcs of reaction between hydroxylamlne sulfate and acldlc bromate. Vanadlum(V) and Iron( I I ) concentratlons In the mlxtures contalnlng Co( II), Mn(II), Ni(II), AI(III), Ce(IV), Mo(VI), and Os(VII1) Ions were determined accurately (+4 % ) by uslng triethanolamine and EDTA as masklng agents. I n mlxtures contalnlng Iron, vanadium(V) is determined after seiectlve extraction of iron with Isobutyl methyl ketone and 1-hexanol.
The concept of kinetic and catalytic methods of determination has gained importance in contemporary analytical chemistry, as is evident from the number of review articles (1) and inclusion of the topic in the textbooks (2). A recent literature survey shows certain kinetic methods which bromate as one of the reactants (1, 3-5). This paper describes a double-check kinetiepotentiometric method for determination of vanadium(V) and iron(I1) in the 5 x lo4 to 10 X M concentration ranges with no titrations involved, using the specific influences of these cations on the induction period and kinetics of the reaction between hydroxylamine sulfate and bromate ion in sulfuric acid medium.
EXPERIMENTAL SECTION A Pye Unicam pH meter with expanded scale and chart recorder attachment was used for the potential measurements with a platinum electrode immersed in the reaction solution and a calomel electrode in the saturated KCl solution connected by an agar gel-KNO, salt bridge. The reaction solution was magnetically stirred and maintained at constant temperature (25 "C) by circulating water from a thermostat through a water jacket around the vessels. AU the reagents were of AnalaR grade except the solvents, which were distilled before use, and the solutions were prepared in iron-free deionized distilled water. Standard solutions (0.1 M) were prepared in 0.1 M sulfuric acid dissolving requisite amounts of ammonium metavanadate for V(V), ammonium molybdate for Mo(VI), ceric ammonium sulfate for Ce(IV),and osmium oxide for Os(VII1) and diluted further to requisite concentrations. Other solutions were prepared following the accepted standard procedures. In all the experiments, stock solution of hydroxylamine sulfate, 0.1 M, potassium bromate, 0.01 M, and sulfuric acid, 2 M, were used. Reactants were taken in the following order: the requisite volume of water to keep the total volume of the reaction mixture 20 mL, sulfuric acid (8 mL), hydroxylamine sulfate (1 mL) followed by 0-9 mL of cation or synthetic mixture. The reaction was then started by the addition of 2 mL of bromate solution. RESULTS AND DISCUSSION Preliminary experiments were carried out by varying the concentrations of hydroxylamine, bromate, and sulfuric acid to obtain suitable control conditions where both the induction period (It)(time required for emf to rise to maximum) and the reaction rate (k,) (fall in emf value which is expressed as change in millivolt per second) are sensitive to vanadium(V)
and iron(I1) concentrations and the region where ko value is essentially a straight line. Only under certain concentrations of reactants is the potential vs. time plot linear. Figure 1shows typical potential vs. time curves; the peak potential attainment time can be marked precisely by use of a chart recorder. The detailed kinetics and mechanism of the reaction are discussed in a separate paper (7). Initially HOBr is produced by reaction of bromate with bromide in the presence of acid
--
(8).
+ Br- + 2H+ HOBr + HBrOz HBr02 + Br- + Hf 2HOBr
Br0,-
(1)
(2)
HOBr oxidizes hydroxylamine to nitrite and then to nitrate in a consecutive reaction, with an overall stoichiometric ratio of 1:l for B r 0 3 to hydroxylamine.
NH2OH
2HOBr
NO2-
HOBr
NOB-
(3)
The maximum (peak) value in the emf time curve (Figure 1) corresponds to the maximum attained concentration of HOBr as a result of decomposition of bromate in acidic medium. The overall potential value decreased with the depletion of HOBr through eq 3. Both vanadium(V) and iron(I1) were found to appreciably decrease the induction period and increase the reaction rate. Calibration curves were plotted with different initial concentrations of vanadium(V) and Fe(I1) solutions in the 10 X lo-' to 12 X 10" M range (Table I). Induction periods were found reciprocally proportional to concentrations of vanaonly, dium(V) and Fe(I1) in the range 5 X lo4 to 10 X while at other concentrations its values deviated from linear relation (Figure 2). Linear plots were obtained, when the ko values vs. the concentrations of vanadium(V) or Fe(I1) were plotted in the concentration range of 0 to 10 X M. The intercept values agreed with the values corresponding to the uncatalyzed reaction. The effect of cations which normally catalyze bromate ion oxidized reactions like Co(II), Mn(II), Ni(II), Al(III),Ce(IV), Mo(VI), and Os(VII1) were studied. It was observed that Mn(I1) and Co(I1) in the concentration range 0 to 20 X lo4 M and Ni(II), Al(III),Ce(IV), and OS(VII1) in the range 0 to 50 X lo4 M have no appreciable effect on the hydroxylamine-acidic bromate reaction parameters. At high concentrations (10 X 10" M), Co(I1) increased the induction period and decreased the reaction rate, while Mn(I1) behaved in the opposite way. Mo(VI), even at low concentrations, showed interference (Table I). To minimize the analysis time, influence of various organic and inorganic ligands on the reaction parameters was studied, to select the ligands that mask the interference of Mo(V1) and Mn(I1) in vanadium and iron determination. In the absence of cations, the reagents EDTA, triethanolamine (TRA), sodium cyanide, 1,lO-phenanthroline, sodium tartrate, and potassium pyrophosphate do not interfere with the uncatalyzed reaction up to 2.5 X lo4 M concentration, In the study of their masking efficiency on separate cations and synthetic mixtures of Mo(VI), Mn(II), and V(V), only TRA and EDTA were found to be selective in masking the interference of
0003-2700/83/0355-2253$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 14,DECEMBER 1983
Table I. Effect of Cations on the Induction Period ( I t )and Reaction Rate ( k , ) d b
a
106[V(V)], M
It, s (il0)
1.0 2.5 5.0 10.0 20.0 40.0 60.0 80.0 100.0 120.0
640 640 610 54 5 525 510 480 415 375 340 310 295
k,, mV/s 106(iron(II)), (kO.001) M
0.106 0.106 0.106 0.107 0.108 0.112 0.115 0.124 0.133 0.143 0.151 0.157
1.0 2.5 5.0 10.0 20.0 40.0 60.0 80.0 100.0 120.0
c
I,, s (i 10)
k,, mV/s (iO.001)
64 0 64 0 6 20 615 560 510 455 370 315 280 250 23 5
0.106 0.106 0.106 0.106 0.107 0.108 0.111 0.117 0.122 0.128 0.133 0.137
106(cation), M Co(I1) 5.0 50.0 Mn(I1) 20.0 100.0 Ni(I1) 5.0 50.0 Al(II1) 5.0 50.0 Ce(1V) 5.0 50.0 WV) 40.0 100.0 Os(VII1) 10.0 50.0
an valu 5 from three experiments. Mean values from two experiments. sulfuric acid 0.8 M, and potassium bromate 0.001 M, temperature 25 "C.
I,, s 10)
k,, mV/s 0.001)
(k
(k
630 59 5
0.107 0.109
635 610
0.108 0.119
630 610
0.107 0.108
640 6 20
0.107 0.109
630 59 5
0.107 0.109
605 520
0.110 0.120
63 0 615
0.108 0.112
Hydroxylamine sulfate 0.005 M,
Table 11. Determination of Vanadium(V) and Iron(I1) Concentrations from Calibration Curves with Selective Masking and Extraction Methods a
synthetic mixture 1. Mn, Mo, V (1:l:l) 2. Mn, Mo, Fe ( 1 : l : l )
3. 4. 5. 6.
Mn, Mo, V ( 1 : l : l ) Mn, Mo, Fe Fe, V ( 1 : l ) Fe, V(1O:l)
It, s (%10)
k , , mV/s (iO.001)
390 350 400 345 4 55 350
0.129 0.119 0.128 0.119 0.117 0.141
vanadium(V) or iron(I1) concn obtained value, expected Mx (r4%) value, M x lo+ from It from k o 5.0 5.0 5.0 5.0 2.5 7.5
5.1 4.9 4.8 5.1 2.6 7.4
5.0 4.8 4.9 4.9 2.5 7.7
a Hydroxylamine sulfate 0.005 M, sulfuric acid 0.8 M, and potassium bromate 0.001 M, temperature 25 "C. Results are mean values of two experiments. Cation determined is italicized. Synthetic mixtures 1and 2 were masked with 25 X M EDTA. In mixtures 5 and 6 Fe(I1) is extracted with two 10-mL M triethanolamine and mixtures 3 and 4 with 25 X portions of IBMK and 5 mL of 1-hexanol.
j. 0
L
x
8.0
-
'..,
e 0
NIO
I
I
I
I
I
1
I
LO
% ',
',
.O
.e
[VANADIUY(VI]
.O
1 100
AND [IRON Illl] ~ I O ' Y
1
--
4 10
r.0
Figure 2. Standard calibration curve: reciprocal of induction period
vs. concentration of vanadium(\/) and iron(I1).
Mn(I1) and Mo(VI), while the other ligands either interfered in the determinations or were not selective (8). Employing
2255
Anal. Chem. 1983, 55,2255-2260
2.5 X M of TRA or EDTA, both Mo(V1) and Mn(I1) in the concentration range 0 to 5 X M were effectively masked while determining concentrations of vanadium(V) or Fe(I1) (Table XI). For the determination of the concentration of vanadium(V) in samples containing iron, the masking technique proved futile. Hence, in the selection of and extraction technique, vanadium(V) is accurately determined by selective solvent extraction of Fe(1I) from the sample with isobutyl methyl ketone (IBMK) (9) (a 10 mL sample was shaken twice with 10 mL of IBMK for 3 min each and then with 5 mL of 1hexanol for 2 min to remove traces of IBMK miscible in sample). A perusal of Table I1 shows the sensitivity of the method. The overall potential measured with reference to the calomel electrode varied slightly from experiment to experiment, but the values of the induction periods and reaction rate were highly reproducible. Since only arbitrary values of potential are considered, cation concentration determinations were not affected. In conclusion the kinetics of oxidation of hydroxylamine by acidic bromate provide a double-check method for the accurate determination (maximum 4% deviation) of vana-
dium(V) and iron(I1) in the concentration range 5 X lo4 to M, even in the presence of different cations or either 10 X organic or inorganic ligands.
ACKNOWLEDGMENT The author is thankful to S. 0. Wandiga, Department of Chemistry, for the research facilities. Registry No. V, 7440-62-2;Fe, 7439-89-6;",OH, 7803-49-8; potassium bromate, 7758-01-2;hydroxylamine sulfate, 10039-54-0. LITERATURE CITED Mottola, H. A.; Mark, H. B., Jr. Anal. Chem. 1982, 5 4 , 62R. Harris, W. E.; Kratochvil, B. "An Introduction to Chemical Analysis"; Saunders College Publlshing: Philadelphia, PA, 1981; p 347. Fukasawa, T.; Yamane, T. Sunsekl Kagaku 1977, 26, 300. Dickson, E. L.; Svehla, G. Microchem. J . 1979, 24,509. Pantel, S. Anal. Chlm. Acta 1979, 104, 205. Sullivan, J. C.; Thompson, R. C. 1979, 18, 2375 Jonnalagadda, S. B., unpublished work. Schwarzenbach, G.; Flaschka, H. "Complexometric Titrations"; translated by H. M. N. H. Irving; Methuen & Co. Ltd.: London, 1969; p 129. Akama, Y.; Nakai, T.; Kawamura, F. Analyst (London) 1981, 106, 250.
RECEIVED for review February 22, 1983. Accepted June 15, 1983.
Laser Microprobe Mass Analysis of Plastic-Contaminated Asbestos Fiber Surfaces Johan K. De Waele,* J. J. Gybels, E. F. Vansant, and F. C. Adams
Department of Chemistry, Universitaire Instelling Antwerpen (U.I.A.),Universiteitsplein 1, B-2610 Wilrijk, Belgium
Standard U.I.C.C. asbestos samples are often contamlnated wlth organic addltlves of polyethylene packing material. Laser microprobe mass analysis when used at low laser lrradlance eondltlons is able to detect specific organlc surface contaminants on indivldual fibers wlth detectlon llmlts of better than 500 pg g-' and wlth a shot-to-shot reproduclblllty of ca. 20%.
ena were investigated semiquantitatively with laser microprobe mass analysis (LAMMA) and quantitatively with gas-liquid chromatography; gas chromatography/mass spectrometry was necessary for the identification of the particular impurity. It proved to be phthalates originating from the polyethylene bags in which the material had been stored.
The hazards for human health associated with the extraction and handling of various members of the asbestos materials are now well established. There is strong evidence that associates occupational exposure to asbestos fibers to a high incidence of lung cancer, pleural and peritoneal mesothelioma, and an excess risk of gastrointestinal cancer (1-4). However, a new issue has recently come to the forefront of environmental toxicology concerning the possible health hazard from inhalation or ingestion of asbestos fibers. I t has been found that there exists a synergetic effect of a number of organic pollutants when they are associated with asbestos pollution (5,6).In this respect, compounds in tobacco smoke appear to be one of the more active (7). Because of the hypothesis that the toxicity of the asbestos fibers is correlated with the adsorption power of carcinogenic organic pollutants or precursors of carcinogenic compounds (8,9), the intention of our asbestos research work is to characterize the fiber surface (10-15). In this paper we report the results of an investigation on a particular case of organic surface contamination of standard asbestos fibers which was mentioned in a previous article in this journal (1I ) . The characteristic contamination phenom-
EXPERIMENTAL SECTION Apparatus. The commercially available laser microprobe mass analyzer (LAMMA) (LAMMA-500, Leybold-Heraeus, Cologne, Germany) is described in detail in the literature (16-19). The primary advantage claimed for the LAMMA instrument is its ability to analyze very rapidly and with high sensitivity both organic and inorganic species present in a microscopic region of a sample. A variable ionization source in either positive or negative ion detection modes provides sensitive elemental and molecular information (18,19). Asbestos fibers can be held onto a Formvar f i i fixed to a standard copper electron microscope grid. Organic impurities adsorbed onto the surface of asbestos fibers can be detected at low laser irradiance, in the laser desorption (LD) mode (10-12,20-22). For details on sample preparation for LAMMA analysis we refer to De Waele et al. (11). All gas-liquid chromatographic (GLC) analyses were performed with a Hewlett-Packard, Model 3380 A, gas chromatograph, equipped with a flame ionization detector (FID). The data were collected by a H-P 3380 A integrator. A glass column packed with 3% OV-225 on 80/100 mesh Chromosorb WHP was used for the quantitative analysis. The gas chromatographic conditions were as follows: injector temperature, 300 "C; FID detector temperature, 350 "C; carrier gas, argon at 30 mL min-l; isotherm temperature adjustment for DEP analysis, 160 OC, internal standard C&38, and for DOP analysis, 250 O C , internal standard C,,Hw The GC/MS analyses were performed with a Hewlett-Packard 5990-A GC/MS instrument using a glass column packed with
0003-2700/83/0355-2255$0 1.50/0
0 1983 American Chemical Society
