W A V E L E N G T H (Mp)
Figure 5. -Recorder tracings of excitation and fluorescence spectra of two urine fractions: Left, patient on thioridazine; right, patient on chlorprothiexene. - - -- Fluorescence of drugs in 0.1 N sulfuric acid after potassium permanganate treatmenr.
on chromatograms for subsequent identification of their fluorescence characteristics. Drugs with the same substitution on position 2 of the phenothiazine nucleus could not be distinguished from each other by their fluorescence characteristics. Their different radicals on position 10 facilitated further identification by paper electrophoresis and especially, by thin layer chromatography with silica gel. These procedures
LITERATURE CITED
are described in detail in another paper (1). ACKNOWLEDGMENT
The fluorescence spectra of chlorpromazine-sulfoxide and trifluoperazine-sulfoxide were performed with substances kindly supplied as thorazinesulfoxide and stehzine-sulfoxide, respectively, by R. s. Johnson of Smith Kline & French Laboratories, Philadelphia.
(1) Mellinger, T. J., Keeler, C. E., J . Pharm. Sci. 51, 1169 (1962). (2) Udenfriend, S., “Fluorescence Assay
in Biology and hledicine.” Academic Press. New York. 1962. (3) Udenfriend, S:, Duggan, D. E., Vasta, B. M., Brodie, B. R., J . Pharrnacol. Exptl. Therap. 120, 26 (195i).
RECEIVEDfor review August 6, 1962. Accepted January 25, 1963.
Amperometric Determination of Vanadium in Vanadium Metal ROBERT A. LANNOYE Technology Analytical laboratory, Technology Deparfment, Union Carbide Metals Co., Division o f Union Carbide Corp., Niagara Falls, N. Y.
b A rapid and precise method for the determination of vanadium in essentially pure vanadium metal i s presented. Since the end point of the titration i s determined amperometrically, the usual difficulties of a visual end point are overcome. After dissolution of the sample and oxidation to vanadium(V), a weighed small excess of ferrous ammonium sulfate hexahydrate i s added. This small excess i s then titrated amperometrically with standard 0.02N potassium dichromate solution. No interference i s found from dissolving acids or other reactants used in the method. In a precision study on a specially prepared standard sample, the average of 12 determinations by amperometry
558
ANALYTICAL CHEMISTRY
was 99.727% with a range of 0.04 1 % and a standard deviation of 0.0136. These data indicate a higher level of precision than by the visual method. Time for analysis i s one half of that for other known procedures.
A
and rapid method has been needed for a direct assay of vanadium in essentially pure vanadium metal. The practice of determining all the possible trace impurities and calculating vanadium by difference is both laborious and unacceptable for material when a contained-metal content must meet rigid specifications. Dietrich ( I ) devised a method for the direct assay of vanadium, using a small excess of PRECISE
ferrous ammonium sulfate salt as reductant after dissolution of the metal in concentrated perchloric and sulfuric acids. The end point of the reaction between Fe(I1) and V(V) was determined indirectly by titrating the excess Fe(I1) with 0.05N potassium dichromate solution and using diphenylaminesulfonate as the indicator. Visual titration methods required more time than ivas desired and high precision was difficult to achieve, since they lack the advantage of instrumentation. The indicator color change is somewhat obscure because of the intensely colored vanadium solution. This paper points out the effectiveness of the amperometric method in obtaining the required precision with a
reduction in time. Eight determinations can be completed in one working day. Many investigators have shown that very high sensitivity is possible with an amperometric titration. For example, Parks and Lykken ( 5 ) used 0.001N potassium dichromate solution to titrate samples containing 0.005 mg. of Fe(I1). Keily, Eldridge, and Hibbits (3) used amperometry to obtain a high degree of precision for the determination of chromium in chromium metal as well as for the determination of purity of ferrous ammonium sulfate. Vanadium a t about the 2y0 level in titanium-base alloys and alloy steels was titrated amperometrically by Rulfs, Lagowski, and Bahor (6) using standard ferrous sulfate solution. -4preliminary valence-state adjustment of vanadium was made by sequential addition of potassium permanganate, potassium nitrite, and urea according to the technique of Walden, Hammett, and Edmonds (8). Studenskaya and Songina (7) used twin microelectrodes to determine vanadium in steel without removal of excess permanganate. S o reference was found for the amperometric determination of a large amount of vanadium. Experiments were made to design a method lvhich would solve this problem. EXPERIMENTAL
Apparatus. The titrations were made in a 600-nil. Griffin beaker using a Fisher Elecdropode in conjunction with a rotating platinurn microelectrode. The coinbination calomel reference electrode-salt bridge was a special design. Construction details are given in Figure 1. Sample solutions were stirred magnetically with 4-cm. Teflon-coated stirring bars. Purging of the system with nitrogen was unnecessary. A 10-ml. buret, with a long offset tip, was used to deliver the titrant. The stirring motors and reference electrode n-ere mounted on a suitable ring stand for convenience in adjusting the immersion of the electrodes. Reagents. PRIRIARY STANDARD DICHRORIATE SOLUTION (0.0200hr). Transfer evactly 0.98074 gram (for 100.OO~coxidant) of NBS 50.136 or equivalent primary standard K ~ C 9 0 7 to a 1-liter volumetric flask. Adjust the weight of potassium dichromate according t o the reported oxidation value. Dissolve in n ater, dilute to the mark, and mix. FERROUS A M M O X I ~SULFATE M HEXAHYDRATE (FAS). Spread the contents of a 5-pound bottle of reagent grade F d S on a large sheet of glazed paper and break up any lumps. Remove any frosty white particles, 15 hich are crystals that hare been partially dehydrated. Return to the bottle, mix by rolling, and store in a cool, dark place. AIallinckrodt reagent 5068 was found to be very stable over several months. Standardize the F.\S as follows:
Trznsfcr about 1 gram of primary standard K2Cr207,weighed to the nearest 0.05 mg , to a dry 600-ml. beaker. Unless otherm-ise known, assume 100% purity for the F.IS and calculate the amount required by multiplying the corrected weight of KzCr20i by 7.9973. Increase the calculated weight of PAS by 0.020 gram to have an excesq for titration. Transfer to the beaker containing the KzCr207and add a stirring bar and 350 ml of H2S04 ( 5 t o 95). Stir magnetically until the solids are dissolved, then add 10 ml. of H3P04 and 5 ml. of "03 (1 to 4). Insert the tips of the salt bridge and platinum microelectrode about 1 inch below the surface of the stirred solution and as far away as possible from the ~ o r t e x . To the indicator electrode apply a positive potential of 1.0 volt us. S.C.E Select a sensitivity shunt that nil1 give a full-scale galvanometer reading When the current is constant, indicating that equilibrium has been attained, add 1.00 ml. of 0.0200h7 dichromate solution. After stirring 15 to 20 seconds, record the galvanometer reading. Continue the incremental additioii of titrant until the current is nearly zero and constant, when excess dichromate has been added. On coordinate paper plot galvanometer readings us milliliters of titrant. Extrapolate to a point where the two straight lines of the graph intersect. Record the volume of titrant a t this point. Calculate the purity factor, K ,for the FXS as follows:
stopcocks. Ten washings were made nith ethyl ether by shaking and rvithdrawing the dissolved oil. Drying was accomplished by heating the flask a t 120' C. under a flow of argon for 1 hour. A protective working area tent was constructed from a 1-cubic yard polyethylene bag which mas continuously purged with argon. The millings mere screened under this tent on a 35mesh sieve and carefully treated with a niagnet to remove any pieces of metallic iron introduced by the milling operation. The '/,-inch by 35-mesh material nas stored under an argon atmosphere a t all times to prevent oxidation. Recommended Procedure. Obtain a representative sample of the metal t o be analyzed, by any suitable means. If particle sizes vary between 4 and 40 mesh, it may be necessary to riffle as much as 10 grams. I n this case divide the 10-gram sample approximately into four weighed portions, analyze each separately, and average the results. Transfer each portion to a GOO-ml. beaker; add 30 ml. of HzS04 (1 to l), 15 nil. of HN03, and 5 drops of HF. Cover and let stand in a warm place until the vigorous action ceases. Digest 011 a hot plate a t 100' to 150' C. until bran-n fumes are no longer evolved (10 to 15 minutes). Add more HK03 and digest, if necessary. Dilute to about 150 ml. with mater and add 1 gram of ammonium persulfate. -4dd a boiling rod and boil gently for 10 minutes to decompose excess persulfate. Cool to room temperature. A m Bn K = ___--Carefully rinse the cover and beaker Fx sides with water, add a magnetic stirring bar, and dilute to 350 ml. Khile = grams of KzCr20i stirring, add KMn04 qolution (10 = oxidizing power of K2CrZO7 grams per liter) clropnise until an expressed as a decimal orange-tinted pink color lierqists for 2 ( l O O . 0 0 ~=~ 1) minutes (3 or 4 dropsj. .Zdd 9aN02 = ml. of 0.0200N titrant a t solution ( 3 . 5 grams per liter) dropwise equivalence point until the pink color is nearly discliarged. = grams of KzCr2O7contained Then add 1.0 ml. of nitrite solution, in 1 ml. of titrant followed immediately by 1 gram of = grams of FAS taken urea. iifter stirring 2 minutes, add the = molar equivalent ratio of FXS salt, calculated as follo K2Cr207to FAS ( = 0 125042) Grams of FAS requircd =
+
\%hereA m
B n
F 2
Triplicate determinations should agree within 1 part in 10,000. Standardize daily to maintain precision. Preparation of Standard Vanadium Sample. d bar of vanadium metal by 15 inches) which had been vacuum-melted and cold-wrought mas given a surface cleaning by removing about 0.001 inch in a lathe under a constant flow of cutting oil (Bacnul KO. 15--4-10). After careful wiping, the bar was completely immersed in a cutting oil bath and mounted on the bed of a milling machine. The cutter was adjusted to produce chips about inch in area and 0.010 inch by in thicknebs. Repeated passes n-ere macle along the axis of the bar until about 95y0 of it was reduced to chips. The bulk of the oil was removed from the chips by filtration on a Buchner funnel. The chipb were transferred to a 1-liter flask, which nas capped with a ground-glass fitting containing two
_-___ (grams of sample) i7 6969 I K
+ 0020
where K = previously deter~ni~ied purity factor for FAAS. Stir until the salt is di>sol\eci, tlicn add 10 ml. of H3P04. Insert the platinum electrode and salt bridge whicli hare been flushed with a few milliliters of KCl solution from the reseri oir. -idjust the potential to + O S Lolt 2s. S.C.E.Perform the amperonietric titration and graphic determination of the end point a'. described under standardization. Calculate the per cent vanadium as follow : vanadium
where
K
=
G
=
purity factor determined for FAS grams of FAS taken
VOL. 35, NO. 4, APRIL 1963
559
C
w fi
of 0.0200N titrant a t equivalence point y = grams of FAS equivalent to 1 ml. of titrant 2 = molar equivalent ratio of vanadium to FAS (= 0.129921) D = grams of sample = ml.
; 5
--A
DISCUSSION
Variables Affecting Amperometric Titration. The theoretical requirements for an amperometric titration of Fe(I1) with dichromate have been thoroughly covered by Lingane (4) and others. It was necessary merely t o substantiate these conditions in the presence of large amounts of V(IV) and Fe(II1). Although the titration proceeded quantitatively over a wide range of applied potential, +0.8 volt US. S.C.E. was chosen because of greatest sensitivity to small additions of titrant. It is not necessary to remove excess nitric and hydrofluoric acid after sample dissolution. Ten milliliters of HNOs and 1.5 ml. of H F did not cause a measurable interference. In an experiment using ammonium metavanadate of known purity, 100.0% recovery of vanadium was observed when a threefold excess of all reagents (persulfate, permanganate, nitrite, and urea) was added in the proper order. This conclusion is implied in the data of Rulfs, Lagowski, and Bahor ( 6 ) . Chromium is partially osidized by boiling with ammonium persulfate and will give a positive error. If more than 0.01% is known to be present, a correction must be applied to the vanadium determination after using a silver nitrate catalyst to oxidize the chromium completely. At least 5 grams of vanadium may be present, since only the excess Fe(I1) is titrated. h 2- to 3-gram sample is most convenient because of FAS requirements. Thus, samples may be
D
K H Figure 1 . Calomel electrodesalt bridge A. 60-1111.separatory funnel, 175mm. stem 6. Rubber tubing connectors C. 7-mm. 0.d. glass tee D. Rubber stopper E. 10-mm. 0.d. X 100-mm. tube with medium porosity glass frit ar porous Vycor bulb (Corning Glass No. 7930) F. 125-ml. flask
STATISTICAL STUDY
G. Platinum wire sealed in tip of 7-mm. 0.d. glass tube containing bare Cu wire and Hg contact H. Hg layer 1. HgzClz salt layer J. KCI salt layer K. Fill flask and tubing completely with saturated KCI solution (no air bubbles)
large enough to be truly representative without interfering with end point detection. When the material is nonhomogeneous, this fact gives an increase in precision for the method compared with other procedures.
Table I. Determination of Vanadium in Vanadium Metal Standard Per cent vanadium Amperometric method Visual method, Lab. A Lab. B Lab. B 99.736 99.707 99.737 99 726 99.735 99.702 99.735 99.743 99.732 99.732 99.713
99.724 99.740 99.741 99.713 99.664 99.749 99.724 99.745 99.720 99.723 99.786 99.725 ... 99.730 Mean 99.727 0.122 0.041 Range 0,02830 0.01355 Std. dev. O.OOO18369 0.00080089 Variance 99.730 f 0.018 0 . 9 5 confidence interval 99.727 & 0.009 99.730 + 0.075 99.727 f 0.037 0.95% tolerance interval Calculations based on 1961 atomic weight8 with 0 = 16.
560
ANALYTICAL CHEMISTRY
A probable trouble spot in the amperometric titration may be found in the calomel half-cell or salt bridge. During the course of experiments, anomalies were observed which could be traced to this source. After several unsuccessful attempts to convert gelatinfilled tubes and p H electrodes to the required conditions, a calomel cell-salt bridge combination was designed as in Figure 1. The resistance of the circuit remains constant (350 ohms), provided the system is free of air bubbles. Thus, the usual difficulties of handling gelatin bridges are eliminated. There is no depletion of KCl or Hg2C12 in the calomel half-cell and the system is easily freed of diffused contaminants by flushing with KC1 solution after each titration. When stored properly, FAS is a very stable salt. Keily, Eldridge, and Hibbits (3) found a variation of not, more than 5 parts in 10,000 in the purity factor after 6 months’ storage. This fact has been confirmed. A daily standardization will detect this minor variation.
99.665 99.734 99.640 99.626 99.659 99.750 99.671 99.799 99.796 99.708 99.802 99.802 99.721 0.176 0.06815 0.0046439 99.721 f 0.043 99.721 + 0.181
An interlaboratory statistical study of the amperometric method was made in cooperation with the Y-12 Plant operated by Union Carbide Nuclear Co., Oak Ridge, Tenn. Because a certified standard of vanadium metal was not available for this purpose, it was necessary to prepare one from production-grade metal as described in the experimental section. Twelve 1-gram samples of the vanadium metal standard ryere analyzed over a period of 4 days by each laboratory. The results are recorded in Table I. Agreement between the two laboratories with the amperometric method (first txvo columns of Table I) is very good. There is no significant difference a t the 0.05 probability level for either the mean or the variance. Khile there is no doubt whatever concerning the agreement for the mean values, the difference in the variance approaches significance a t the 0.01 level. This means that there is only about one chance in 10 that the results from Laboratory B deviate more than those from Laboratory A. All this is in sharp contrast with the comparison between the two methods run by Laboratory B (columns 2 and 3), where the difference in variance is strongly significant. The difference in the mean values for the two methods is 0.0097, vanadium, and, while this appears much larger than the 0.003% vanadium (difference between laboratories with the amperometric method), it cannot be tested statistically because of the significant difference in the variance.
The 0.95, 90% tolerance intervals (2) indicate the degree of reproducibility available by the amperometric method. From the data for Laboratory h we can predict with 0.95 confidence that at least 90% of all determinations run on a similar material would lie in the interval fif f 0.037y0vanadium, where M indicates the average of a large number of such determinations. In other words, a t least 90% of all determinations which may be run on a single sample would be within a range of 0.074y0 vanadium. A similar range calculated from column 2 is 0.150y0 vanadium, while for the visual method it would be 0.380% vanadium. It is apparent that precision can be maintained which is equal to classical weight-buret titrimetry, since a salt reductant, with a favorable molar equivalent ratio, is weighed. The backtitration using amperometry is a very exact means of end point detection. A summation of the complete analysis of the vanadium metal standard is included in Table 11. These data are reported for reference as to the probable content of minor impurities. Accountability of components appears to be very nearly complete (99.987%). ACKNOWLEDGMENT
The author thanks I. H. S. Fraser, A. R. Gahler, F. N. Hopper, C. XI.
Table
II.
Analysis of Vanadium Metal Standard
Found, % 99.727
Element Vanadium
0.065
Oxygen Hydrogen Nitrogen Carbon Iron Silicon Calcium Magnesium Chromium Aluminum Manganese Molybdenum Titanium Lead Cobalt Nickel
No. of detns. 12
0.004
2 2 2 3
0.036
Single
0,030 0.052 0.032 0,020 0.007 0.007 0.007
