Pulse Polarographic Determination of Nickel and Vanadium D. D. GILBERT’ California Research Corp., Richmond, Calif.
b The increased sensitivity for reducible species obtained by pulse polarography, in contrast to direct-current polarography, allows microgram amounts of both nickel and vanadium to be determined with a single polarogram. Considerable time i s saved over conventional polarographic or two different colorimetric procedures. The method has been successfully applied to petroleum stocks containing 0.5 to 50 p.p.m. of either metal; lower levels can be determined with larger sample sizes and higher available instrumental sensitivity.
P
polarography has an inherently higher sensitivity for both reversible and irreversible reductions and yields better resolution between two electroactive species than conventional direct-current polarography ( I , 2, 7 ) . These characteristics have been applied advantageously to the determination of nickel and vanadium in petroleum stocks. The two metals are well known catalyst poisons, and rapid analytical methods are continually sought for them. The high sensitivity of pulse polarography allows a relatively small sample to be taken for decomposition, considerably reducing the elapsed time over a conventional polarographic analysis. Since both metals can be determined from one pulse polarogram, the analyses can be completed in less than one half the time necessary for most colorimetric methods. ULSE
EXPERIMENTAL
Apparatus. The peak or “derivative” mode of the Southern-Harwell Mark I1 Pulse Polarotrace (Southern Analytical, Ltd., Camberley, Surrey, England) was used to obtain all pulse polarograms. I n this mode a single, 40-millisecondJ 35-mv. voltage pulse is superimposed on a n increasingly negative d.c. ramp once during the life of each mercury drop. At the conclusion of the pulse, the drop is automatically knocked from the capillary; and there is a 1-second delay with the new drop until the next pulse is applied. The cycle repeats itself Present address, Dept. of Chemistry, Arizona State College, Flagstaff, Ariz. 1
1’102
ANALYTICAL CHEMISTRY
throughout a 1-volt d.c. potential sweep. Current measurements are taken during the last 20 milliseconds of the applied 40-millisecond pulse. Background current from the d.c. ramp is filtered out, and only the change in current resulting from the pulse is measured. A11 data were obtained with a 7.5minute, 1-volt sweep. The signal-to-noise ratio was enhanced hy using a smoothing circuit with an output voltage that was a function of the last three drop lives. d sensitivity was used such that the recorder deflection was 11 x 1 0 - ~ ampere per inch. The over-all sensitivity of the instrument can be increased by 20-fold. Cells with a mercury pool anode were thermostated a t 25’ f 0.3” C. Reagents. Nitrogen for deaeration was passed through a two-stage purification train, the first containing lead wool and a n alkaline 1,2-naphthoquinone-4-sulfonic acid solution and the second a sulfuric acid solution of chromous sulfate over amalgamated zinc. T h e unit is designed to regenerate chromous ion continuously in the second scrubber. Standard solutions were prepared from Fisher Certified ammonium metavanadate and Matthey nickel sponge. Distilled water was used after passing through a mixed bed ion exchanger. Procedure. Petroleum stocks, containing about 5 to 10 pg. each of nickel and vanadium, were first evaporated on a hot plate and then decarbonized in a muffle furnace a t 600” C. There was no evidence of the volatilization of nickel or banadium when compared to previous analyses using a sulfuricnitric acid digestion. Volatile nickel or vanadium compounds may be present in samples from other sources, in which case a wet-ashing or acid-digestion decomposition of the hydrocarbon would be preferable. Residues were treated with aqua regia, taken to dryness, and
Table 1.
Sample 1
a
Metal Ni
2
V Ni V
3
Ni
redissolved in 10 ml. of 0.1N hydrochloric acid. After quantitative transfer to a 25-ml. glass-stoppered cylinder, water was added to about 20 ml. and ammonium hydroxide (28 to 30%) added until the solution was p H 8 to 9 (pH paper). The solution was diluted to a final volume of 25 ml. with water. After a Bminute deaeration, the peak or “derivative” mode pulse polarcgram was obtained. RESULTS A N D DISCUSSION
Figure 1 is a typical polarogram of a hydrocarbon sample taken through the procedure (upper curve). The current peaks a t about -0.78 and -1.20 volts (us. Hg pool) are reductions of nickel and vanadium, respectively. The lower curve is that obtained with the reagents used to dissolve and treat the decarbonized sample. Over a period of time, the base lines of the polarograms begin to slope, and small periodic oscillations appear in the curves. These two effects present no problem in determining nickel or vanadium a t 0.08 kg. per ml.; but a t lower concentrations, both effects must be minimized before higher instrumental sensitivities can be used, This is readily done by installing a new capillary. The magnitude of the current a t its peak value, above that of the reagentsupporting electrolyte background, is proportional to the concentration of the nickel or vanadium. Measurements are made from the tops of the two curves. The spike between current measurements is just a discharge of a portion of the current that was stored in the measuring circuit before the next measurement is recorded. Linear calibration curves are obtained for both
Typical Nickel and Vanadium Determinations
Colorimetrica
Emission spectroscopic
polarographic
0 65, 0 55 2 3,2 1 1 9 13 59 50
0 52,O 40,O 56 2 0,2 4 , l 9 2 6.3 0.2 7 13, 14, 13 58, 53 49, 45
0 53, 0 48 2 1,2 5 2 8,2 9 13, 14
V
Modified procedure of Forrester and Jones ( 4 ) .
Pulse
T
0.012 p a
-1.4
I
-1.2
-1.3
I
-1.1
I
-1.0
I
-0.9
-0.7
-0.8
-0.6
-0.5
Vults V e r s u s Hg Pool Figure 1 . Upper. lower.
Typical pulse polarographic data
Sample taken through procedure, 5.7 X 10% Electrolyte-reagents blank
nickel and vanadium between 0 and 0.4 gg, of Ni per ml. and 0 and 0.8 gg. of V per ml. with maximum currents of 80 and 75 nanoamperes, respectively. The linear relationship holds for even higher concentrations of these metals, but they are of no interest in this work. Typical results for three different samples are shown in Table I. There is good agreement with colorimetric and emission spectroscopic analyses. Sample 3 was analyzed for both nickel and vanadium, pulse polarographically, in about 90 minutes using 0.1 gram of sample. No. 1 required 10 grams of sample and about 4 hours; solution volumes can be decreased and instrumental sensitivity increased so that less than 2 grams would be sufficient, reducing the elapsed time to less than 2 hours. No interference has been encountered from metals ordinarily present in petroleum stocks. The interpretation of many peak-type pulse polarograms is rather difficult because the shapes of the curves are a function of the electrode process kinetic parameters (2). The reduction of vanadium at the dropping mercury electrode in a n ammoniacal medium is particularly complex (3, 6 ) , and no attempt has
Ni+2, 4.3 X 1 O-%
been made in this work to determine the electrode process. Only one peak is observed for the vanadium reduction, in about 0.01M NH3-0.01M NH,CI, up to a concentration of 3 X 10-5.\f vanadium(V) with a linear relation between peak current and concentration. Higher concentrations were not studied. The vanadium pulse polarogram does not change after the solution is heated. Evidently, equilibrium between the various vanadium(V) species is established rapidly at these concentrations, whereas millimolar vanadium(V) solutions must be heated for a short time to hasten equilibrium before obtaining a polarogram (5). The use of a dilute ammonia-ammonium chloride supporting electrolyte is beneficial on two counts. First, there is a n increased separation between the nickel and vanadium peaks, in contrast to a solution of 1M ammonia and 1M ammonium chloride (470 mv. us. 110 mv.). This is particularly advantageous should there be a high ratio of nickel LO vanadium, in which case the vanadium could be easily determined. Very high ratios of one metal to the other were not encountered in this work. The second advantage of the more dilute
Vis
supporting electrolyte is the minimization of contamination from reagents. ACKNOWLEDGMENT
The experimental assistance of P. I). E. Nelson is gratefully acknowledged, and appreciation is expressed to J. 0. Larson for many helpful discusslons LITERATURE CITED
( 1 ) Barker, G. C., Anal. Chim. Acta 18, 118 (1958).
(2) Barker, G. C., Gardner, A. W., United Kinedom Atomic Enerev Authoritv. Repi. C/R 2297 (Augustq958). ( 3 ) FilipoviC, I., Hahl, Z., Gabparac, Z., KlemenEiC, V., J . A m . Chem. Soc. 76, 2076 (1954). (4) Forrester, J. S., Jones, J. L., ANAL. CHEM.32, 1443 (1960). (5) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., Vol. 2, p. 448, Interscience, New York-London, 1952. (6) Schmid, R. W.,Reilley, C. N., J . A m . Chem. Soc. 80, 2087 (1958). ( 7 ) Schmidt, H., von Stackelberg, M., “Modern Polarographic Methods,” Academic Press, New York-London, 1963. “
?
RECEIVEDfor review May 26, 1965. Accepted June 15, 1965. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1965. VOL. 37, NO. 9, AUGUST 1965
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