justed to the conditions described for aluminum (Table I). Curve A was run in this manner a t a slit width of 0.015 mm. Curve B was run in the same manner as A , but with the light pipe removed. This illustrates the improvement that can be obtained by the use of a light pipe. Curve C was run in the same manner as A , but with the flame emission chopped mechanically by a chopper between the flame and light pipe. This illustrates the flame background which would be observed in making emission measurements under these flame conditions when a light pipe is used. Because the copper used blocked out effectively half of the flame emission, curve C was run a t a slit width of 0.021 m1.-Le., 0.015 X .\/2. Curve D was run in the same manner as C, but with the light pipe removed. This illustrates the flame background which would normally be observed in emission measurements under these
flame conditions. Comparison of curves C and D also illustrates the improvement attainable by the use of a light pipe. Comparison of curves B and D illustrates the improvement attainable by the use of an a x . amplifier in atomic absorption measurements in these flames. In curve A , one notes the absence of prominent OH band spectra. It is also interesting to note that the aluminum line used (309.3 mp) occurs on the peak of one of the most intense OH bands. The method and apparatus described should lend themselves well to the determination of many elements having a strong tendency to form compounds in the flame. One disadvantage of these fuel-rich flames aspirating organic solvents is the high acoustical noise and intense white radiation. For prolonged operation it is recommended that the burner be encased in some type of housing.
LITERATURE CITED
(1) Box, G. F., Walsh, A., Spectrochim. Acta 16,255 (1960). 12) Carnes. W. J.. Dean. J. A.. Anulvst 87, 748 (1962). ’ (3) Corliss, C. H., Bosman, W. R., Natl. Bur. Std. (U. S.),Monograph 53, July 20, 1962. (4) Eshelman, H. C., Dean, J. A., Menis, 0..Rains. T. C.. ANAL.CHEM.31. 183 (1959). ’ (5) Fassel, V. A., Mossotti, V. G., Ibid., 35,252 (1963). (6) Freiser, H., Chemist-Analyst 51, 62 (1962). (7) Slavin, W., Manning, D. C., ANAL. CHEM. 35,253 (1963).
.,
J. D. WINEFORDNER CLAUDE VEILLON
Department of Chemistry University of Florida Gainesville, Fla. TAKEN in part from the M.S. thesis of Claude Veillon, submitted t o the Graduate School of the University of Florida in November 1963.
Spectrophotometric Determination of Vanadium in EthylenePropylene Rubber Solutions with 3,3’-Diaminobenzidine SIR:Our interest in the determination of traces of vanadium in solutions of experimental ethylene-propylene rubbers (EPR) in organic solvents prompted us to search for a rapid and simple analytical procedure. Until recently, two methods were in general use in our laboratory. A colorimetric method dependent on formation of the phosphotungstate complex required the elimination of interfering ions through the use of a mercury cathode cell (4). This method while precise and accurate is time-consuming. Because of the nature of the samples, a rapid emission spectrographic method (9) did not always yield results of suitable precision and accuracy. Analysis by neutron activation while fast and accurate requires expensive equipment that is not always available. The present method is based on a procedure proposed by Cheng (1) for the determination of traces of vanadium through the use of diaminobenzidine (DAB). The reaction is specific for vanadium, and interfering substances such as strong oxidizing agents, highly colored substances, and certain anions (1) can easily be avoided. For readily ashed material, the rapid finkh enables the analysis of about ten samples a day. Properties of the reagent and conditions of the reaction have been thoroughly explored by Cheng. Since this method is relatively new, a comparison of results obtained by this method with 944
ANALYTICAL CHEMISTRY
those obtained by neutron activation analysis should be of interest. Our first attempt to analyze vanadium consisted of wet ashing the elastomer solution with nitric and perchloric acids followed by reacting the resulting solution with DAB. Further investigation indicated that the hydrochloric acid and hydrogen peroxide that are formed during the wet oxidation step reduce vanadiumw) as suggested by McCoy (2). Dry ashing the sample in a platinum crucible, however, proved quite satisfactory. To increase sensitivity, cells of a 5-cm. path length were used for some analyses. The use of these cells permitted the preparation of a calibration curve ranging from 5 to 50 pg. vanadium per 25 ml. The calibration curve similar to that described by Cheng was used for samples of higher vanadium content. EXPERIMENTAL
Reagents and Apparatus. Beckman Spectrophotometer DU with 1- and 5-cm. cells. Standard vanadium solution. Prepare 5 pg. per ml. of 50 pg. per ml. from ammonium metavanadate. 3,3’-Diaminobenzidine tetrachloride (DAB) solution. J. T. Baker reagent grade. Dissolve 0.1 gram of reagent in 20 ml. of water and store in refrigerator under an inert atmosphere. Procedure. Ash a 10-gram sample of elastomer solution in a platinum
crucible by charring on a hot plate followed by heating over a Meker burner. Add 10 ml. of water and 2 ml. of nitric acid to the crucible, then wash the contents into a 100-ml. beaker. To ensure complete removal of vanadium, melt a gram of potassium pyrosulfate in the crucible, cool, then wash the salt into the beaker with hot water. Evaporate the contents of the beaker to about 10 ml. and transfer the contents to a 25-ml. volumetric flask. Before diluting to the mark, add 1 ml. of 85% Dhosphoric acid and 1 ml. of DAB solution. Measure the absorbance a t 470 nib in a suitable cell, using a reagent blank, after standing 15 minutes in the dark. PreDare a calibration curve by adding suitable aliquots of standard vanadium solution to 100-ml. beakers that contain about 50 ml. of water, 2 ml. of nitric acid, and 1 gram of potassium pyrosulfate. Reduce the volume to about 10 ml. by boiling and transfer the contents to a 25-ml. volumetric flask. React with phosphoric acid and DAB solution as described for sample analysis. For vanadium contents from 5 to 50 pg.. measure the absorbance in a 5-cm. cell using a reagent blank. Cse a 1-cm. cell for 50 to 250 pg. of vanadium. DISCUSSION OF RESULTS
Table I presents data showing adequate recovery of vanadium that had been added to vanadium-free E P R solutions as ammonium vanadate.
Table 1. Colorimetric Analysis of Vanadium in EPR Solutions
v
Added, fig.”
126 126 63 63 25 25
v
found,
ug.
V recovered, %
125 126 60 60 26 28
99 100 95 95 104 112
a I’anadium added 2 ~ ammonium 8 metavanadate t o 10 grams of a 37, elastomer solution in toluene.
There was, however, some uncertainty as to whether an aralytical procedure for vanadium preqenf in this form could apply to samples whose vanadium content originated from catalytic quantities of trialkll vanadates. To verify our results, a number of samples were submitted for neutron activation analysis. Data in Table I1 showing agreement between the two methods give a good indication that this colorimetric procedure c etects the total vanadium content. For the colorimetric analyses in Table 11, 10 grams of sample were analyzed. Two-gram portions of samples that contained over 10 p.p.m. vanadium yielded results of almost equal accuracy. Results of 10
replicate analyses of sample A gave a standard deviation of 0.91. The occasional substantial variance between the two procedures is most likely the result of mechanical loss during ashing. The neutron activation data wherein this error is obviated are probably the more accurate. No attempt was made to attain agreeable results for samples that contained less than 1 pap.m. vanadium. Since no difficulty was encountered in analyzing amounts of vanadium ranging from 10 to 20 pg. in standard solutions of ammonium vanadate, high results of samples D and F are attributable to substances that do not interfere with vanadium a t higher levels. Cheng’s suggestion for the removal of some interfering ions might well increase the accuracy of analyses of vanadium below 1 p.p.m. For the analyses in Table 11, no attempt was made to remove any ions that might interfere. Aluminum, iron, and chloride were present in varying amounts ranging from 20 to 100 p.p.m. Acetone used for rinsing glassware interferes because it reacts with DAB in the presence of vanadium to form a blue colored solution. The use of dichromate cleaning solution should be avoided since Cheng points out that chromium(V1) oxidizes DAB. Glassware rinsed well with distilled water is all that is needed.
Table 11. Determination of Vanadium in Toluene Solutions of EPR
Vanadium. u.u.m. Neutron DABa activation
Sample A B C D E F G H
10.2 14.0 14.6 0.5 13 0 0 9 15 2 18 2
~
9.9 & 0.2 14.1 & 0 . 1 15.6 f 0 . 3 0.14 f 0 . 0 1 1 4 8 f O 2 0 27 f 0 01 188*03 179*03
Average of three determinations.
ACKNOWLEDGMENT
General Atomics Division of the General Dynamics Corp. furnished the neutron activation analyses. LITERATURE CITED
(1) Cheng, K. L., Talanta 8, 658 (1961). (2) McCoy, J. W., “The Inorganic Analvsis of Petroleum,” Chemical Publishing, 1962, p. 236. (3) Roza, J. T., Zeeb, L. E., Petrol. Process 8, 1708 (3953).
(4) Snell, C., Snell, F. D., “Colorimetric
Methods of Analysis,” Vol. 11.4, p. 347, Van Yostrand, S e w York (1959).
Sun Oil Co. Marcus Hook, Pa.
A . J. SMITH
The Dead-Stop Titration of Thallium SIR: The dead-stop titration method originated by Salomon (7) and developed by Foulk and Bawden (3) has been applied to the analysis of many metals including zinc by Swinehart (8), gallium by Fetter and Swinehart ( 2 ) , and indium by Martens and Frye (5). An apparatus similar to the simple, rugged devics used by these workers has been employed in the thallium determination. The galvanometer employed had a sensitivity of 1 X ampere pcsr millimeter. A number of presentations of the theory have been made, induding those of Reilley, Cooke, and Furman ( 6 ) ,Gaugin and Charlot (4),and 1)elahay ( I ) . EXPERIMENTAL
Standard solutions of thallium(II1) and hexacyanoferrate(I1) were prepared. The thallium(II1) solution was prepared by first removing the oxide from Fisher reagent grade thallium metal with nitric acid. then washing in distilled water and drying thoroughly under nitrogen, and h a l l y dissolving in aqua regia to ensure quantitative oxidation of the thalliuni(1) to thallium
(111). The excess oxidant was removed by evaporation of the solution to dryness. Baker and Adamson reagent grade potassium ferrocyanide was pulverized and dried to constant weight a t 100’ C. The anhydrous salt was then weighed and dissolved in doubly distilled water; 2 grams per liter of reagent grade sodium carbonate was added to stabilize the solution. Volumes of the standard thallium(II1) solution were dispensed from a calibrated buret into the titration vessel, the mechanical stirrer was started, and current readings were taken a t constant time intervals after each addition of the hexacyanoferrate( 11) solution; the latter solution was added by means of a precision calibrated buret. No supporting electrolyte, in the polarographic sense, was used. DISCUSSION
The reduction of thallium(II1) is irreversible a t the potentials used in these determinations, whereas that of hexacyanoferrate( 111) is reversible. The hexacyanoferrate(I1) solution doubtless contains a little hexacyanoferrate(”, and current will flow
when the hexacyanoferrate(I1) is in excess. The sharpness of the end point depends on the low solubility of the thallium( HI) - hexacyanoferrate(I1) and the consequent low concentration of hexacyanoferrate(I1). As soon as the concentration of thallium(II1) has been reduced sufficiently, current will begin to flow because hexacyanoferrate(I1) is
Table 1.
Moles x 111 106:I T1(
Results of Titrations“
&fOle5 Fe(x 105 --4
l f d e ratio Fe(Tl(II1) CN)s- 4 :
1.343 1,075 1:1.249 1.343 I . 083 1:I ,240 1.343 1.067 1:1.258 1.343 1.083 1:1.240 1.343 1.070 1:1.255 1.343 1.074 1:1.250 1 ,078 1:1.246 1.343 1.343 1.070 1 : l 255 1.343 1.071 1:1.254 Av. mole ratio: 1: 1,250. Std. dev.: 0.0066. 5 Data in Table I are all at a potential of 250 mv.
VOL. 36, NO. 4, APRIL 1964
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