the fusion flux, cupel, and silver bead. The sums of the measured activities ranged from 97 to 103% of the standards. They differed from 100% because of the standard deviation associated with the level of Ig8Auactivity in the silver bead. The crucibles were not examined for activity. Quartz was used to simulate the rock sample but this material contained negligible quantities of gold. The results indicate that a 97% or better recovery may be expected if 2 mg or more of silver are used during cupellation, a finding not significantly different from that reported by Fulton (15) and Coxon et al. (16) for larger amounts of gold. Since the recovery of gold is close to 100% and the standards, blanks, and samples are carried through the same procedure, no yield correction was made. Evaluation of the Gold Blank. Preliminary experiments indicated that the detection limit would be determined by the magnitude and reproducibility of the gold blank. The primary sources of gold contamination were the reagents used for the fire-assay flux and the clay crucibles used t o fuse the sample. The contribution from both sources was about equal but varied considerably. The reagent blank was lowered by performing a preliminary fire-assay fusion on a modified flux yielding a flux composition close to that desired for the fire assay of the samples. The composition of a single charge of flux, before purification, is 100 grams of PbO, 25 grams of Na2C03,5 grams of Na2B40,, and 3 grams of flour. After purification, the composition is equivalent to 60 grams of PbO, 25 grams of Na2COJ, 5 grams of Na2B40,, and 0 gram of flour. The flux described here was satisfactory for most silicate rocks. If a sample requires a different flux
composition, then adjustment should be made prior to purification. The clay crucibles, which are subjected to varying degrees of attack, were replaced by alumina crucibles. The attack on the alumina crucibles is considerably less and they can be re-used. The results of measurements on the gold blank from both the unpurified flux and two separately prepared portions of purified flux are shown in Table 11. Sigma is the standard deviation of the average for the series of runs on each sample. If the limit of detection is defined as three times the standard deviation of the blank, then this technique should be able to detect 1.5 ng of gold or 0.1 ppb of gold in a 15-gram sample. Measurement of Gold in W-1. The procedure was tested on the U. S. Geological Survey’s standard diabase, W-1, for which data have also been obtained by conventional neutron activation analysis (5,17). This standard rock is supplied in bottles containing approximately 50 grams. Since 25-gram portions were used, the samples were not mixed nor quartered. The results, shown in Table 111, indicate good agreement between these two quite different techniques. ACKNOWLEDGMENT
We thank L. A. Harris and other members of the nuclear reactor staff, Naval Research Laboratory, Washington, D. C., for help in irradiating the lead buttons. RECEIVED for review on January 19, 1968, and accepted on March 25, 1968. Publication authorized by the Director, U.S. Geological Survey.
Determination of Traces of Antimony by Single Sweep Polarography Paul E. Toren Central Research Laboratories, Minnesota Mining and Manufacturing Co., St. Paul, Minn. 55101 A METHOD was required for the determination of 5 ppb of antimony in water. Relatively large samples (150-200 ml) were available, and concentration by a factor of about 20 appeared feasible, so determination of 0.1 ppm of antimony (1 pg in 10 ml of final solution) was sufficiently sensitive. Chemical determinations of antimony at the microgram level are usually done spectrophotometrically. The antimony is complexed with an organic reagent such as Brilliant Green ( I ) , methyl violet ( 2 ) , phenylfluorone (3),or Rhodamine B (4, and the colored complex is extracted into an organic solvent where its absorbance can be measured. The use of Pyrocatechin Violet in a sensitive photometric determination has been reported (5), and a method based on the enhancement of the absorbance of a molybdenum blue has been published (6). Neither of these two methods requires an (1) R. E. Stanton and A. J. McDonald, Analyst, 87, 299 (1962). (2) E. V. Silaeva and V. r. Kurbatova, Zuaodsk. Lab., 28,280 (1962). (3) E. A. Biryuk, ibid., 30, 651 (1964). (4) B. J. MacNulty and L. D. Woollard, Anal. Chim. Acta, 13, 64 (1955). (.5,) T. T. Bykhovtseva and I. A. Tserkovnitskaya, Zaaodsk. Lab., 30,943 (i964). (6) R. M. Matulis and J. C. Guyon, ANAL.CHEM., 37,1391 (1965).
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extraction. Several of these procedures are reportedly applicable t o determination at the microgram level but all require preliminary treatment of the sample (in addition to any concentration steps) to bring the antimony to the proper state for measurement. I n all of these methods, for example, the antimony must be entirely in a single oxidation state before the color-forming reaction can be carried out, and a preoxidation or reduction is therefore required. The electrochemical properties of antimony are well known (7), and a number of procedures for its polarographic determination have been published. Trivalent antimony gives polarographic reduction waves in dilute mineral acid and in sodium hydroxide solution (8). Pentavalent antimony is reducible in strongly acidic chloride solution ( 9 ) and gives a double wave corresponding to the reductions Sb(V) + Sb(II1) + Sb(0). Consequently polarographic measurements in strong hydrochloric acid can be used to determine both trivalent and pentavalent antimony, and d o not require that the (7) I. M. Kolthoff and J. J. Lingane, “Polarography,” 2nd ed., Interscience, New York, 1952, p 545. (8) J. J. Lingane, IND.ENG.CHEM., ANAL.ED., 15, 585 (1943). (9) J. J. Lingane and F. Nishida, J. Am. Chem. SOC.,69,530 ( 1 947).
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E
E Figure 2. Reverse sweep currentvoltage curve of 0.5 ppm antimony solution 0.5 ppm Sb(II1) in 6 M HCI. Single cell. “Reverse” voltage sweep. Instrument sensitivity Initial voltage -0.4 V cs. Hg pool
E Figure 1. Effect of 60-mV change in initial voltage on current voltage curve of 0.5 ppm of antimony 0.5 ppm Sb(II1) in 6MHCI.
Single cell. “Forward” voltage sweep. Instrument sensitivity l/loo. (a) Initial voltage -0.01 V os. Hg pool (6)Initial voltage -0.07 V us. Hg pool
antimony be present in a specific oxidation state. In general, the polarographic determination of antimony compares very favorably with spectrophotometric methods with respect to convenience and simplicity, but conventional polarography is not sensitive enough for determinations at the microgram level. Several years ago, Davis and Seaborn described a “differential cathode ray polarograph” (IO) which greatly increases the sensitivity of dropping mercury electrode voltammetry. The operation of this instrument has been discussed by Davis and Rooney ( I I ) , and its use for the determination of relatively high levels of antimony has been described (12). The present paper describes the development of a method for the determination of traces of antimony in water by cathode ray (or single sweep) polarography. EXPERIMENTAL Apparatus. A Southern Analytical Model 1660 CathodeRay Polarotrace was used for this work. This is a commercial instrument based on the design of Davis and Seaborn. Reagents. Antimony trioxide (Sb203) used was analytical reagent grade. Standard antimony solutions containing
(10) H. M. Davis and Joyce E. Seaborn, “Advances in Polarography,” Vol. 1, Pergamon Press, New York, 1960, p 239. ( 11) H. M. Davis and R. C . Rooney, J. Polurog. SOC.,8,25 (1962). (12) R. C. Rooney, ibid., p 20.
about 1 mg of Sb/ml were prepared by dissoiving weighed amounts of SblOa in small amounts of concentrated HC1 and diluting with water. Solutions containing about 10 pg of Sb/ml were prepared by dilution of the 1 mg/ml solutions with water. Establishment of Analytical Conditions. CONCENTRATION OF SAMPLE.This procedure was used for the determination of traces of antimony in relatively pure water. For the sample t o be concentrated without loss of antimony it was necessary to add a small amount of H2SOa to the water before the evaporation step. SELECTION OF ELECTROLYTE. Because we were interested in determining total antimony, an acidic halide electrolyte [in which both antimony(II1) and antimony(V) are reduced] was chosen for this work. Both Lingane and Nishida (9) and Rooney (12) had found that highly acid chloride solutions were required for the best antimony waves. In general agreement with their work, we found that the peak current obtained from 0.5 ppm of pentavalent antimony was maximum in 6 M HC1. Accordingly, we selected 5M HC1 (actually a 1 :1 concentrated HC1-water mixture) as our electrolyte. CURRENT-VOLTAGE CURVES. At higher concentrations of antimony (10 ppm) direct measbrement with a forward sweep gave the expected current-voltage curve as reported by Rooney (12). At lower concentrations, however, the interference of a prewave, apparently caused by the reduction of a mercury chloride surface film on the electrode, became relatively more serious. The chloride film forms on the growing mercury drop prior to the voltage sweep and cannot be prevented except by making the initial electrode potential more cathodic. Because the antimony wave appears at about -0.15 V cs. mercury pool, the starting point cannot be shifted very much without losing the antimony peak. Figures l u and 16 illustrate the effect of a 60-mV change in the initial voltage on the current-voltage curve of 0.5 ppm of antimony. Because of this problem, the use of a reverse voltage sweep was investigated. By applying the voltage sweep from -0.4 to +0.1 V, the reaction during the drop growth prior to the voltage sweep is the diffusion-controlled reduction of both antimony(II1) and antimony(V) to the metal. The reverse voltage sweep then gives an anodic current corresponding to the stripping of the deposited metal from the mercury drop. The reverse sweep current-voltage curve of a 0.5-ppm antimony solution is shown in Figure 2. In addition to avoiding the mercury chloride interference, the sensitivity of the deVOL 40, NO. 7, JUNE 1968
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termination is increased because the anodic peak corresponds to the oxidation of all the antimony deposited during the 5-sec growth of the drop before the start of the voltage sweep. Although the antimony peak is clearly visible at the 0.5-ppm level as shown in Figure 2, the background current is increasing rapidly in this region also, and measurement of the peak height is difficult. At 0.1 ppm, a quantitative measurement using this mode of operation is essentially impossible. This difficulty was resolved by using the subtractive mode of operation, with 6 M HC1 in the second cell. Figure 3 shows the current-voltage curve of 0.5 ppm of antimony measured in this way. (Note that a higher instrument sensitivity can be used when the background is subtracted.) Using this reverse sweep subtractive technique, antimony concentrations of 0.1 ppm were easily measured, and the required sensitivity of analysis was obtained. Analytical Procedure. Place the sample in a 250-ml beaker and add 0.5 ml concentrated H2SO4. Evaporate the solution to about 0.5 ml on a hot plate (surface temperature about 150 “C). The solution should not boil. (If the sample is larger than 200 ml, add a portion of the sample first, then add the rest as the evaporation proceeds.) Allow the beaker to cool. To the material in the beaker, add 10.0 ml of 6 M HCl and mix well. Transfer 3 t o 5 ml of the resulting solution t o a polarograph cell and place it in the “Cell I” (Sample) position in the polarograph. Place a cell containing the same volume of 6 M HC1 in the “Cell 11” (Reference) position in the polarograph. After deaerating both solutions for 10 min, adjust the “Potential Balance” control of the instrument t o give the best cancellation of the large mercury chloride anodic current. Use a reverse sweep starting at -0.4 V. Measure the height of the Sb peak at about -0.06 V. Determine the S b concentration of the solution by comparison of the peak height of the unknown with that of standards measured in parallel. Calculate the Sb concentration of the original sample by use of the appropriate volume ratio. DISCUSSION Range of Applicability. Linear calibration curves were obtained with both Sb(II1) and Sb(V) solutions containing 0.1 to 10 ppm of antimony. The calibrations passed through the origin, and both the Sb(II1) and Sb(V) calibrations had essentially the same slope (to within 1075). The peak was still detectable in solutions containing 0.01 ppm of antimony. Precision. Six 200-ml portions of water containing 5 ppb of added antimony were carried through the procedure as described above. The relative standard deviation of the results was 6%. Interferences. Because we were concerned with antimony in relatively pure water, interfering elements were not a problem in this determination. In general, no interference would be expected from materials more cathodic than -0.4 V (the start of the reverse voltage sweep), or more anodic
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E Figure 3. Current-voltage curve of 0.5 ppm of antimony measured by subtractive mode 0.5 ppm Sb(II1) in 6 M HCI. Sample cs. 6 M HCI. “Reverse” voltage sweep. Instrument sensitivity Initial voltage -0.4 V OS. Hg pool
than the antimony oxidation a t -0.06 V. On the basis of reported half-wave potentials in chloride solution, it appeared that bismuth, copper, and tin could interfere with this determination. In 6 M HCI, the peak potentials of Bi(II), Cu(II), and Sn(1V) were -0.08, -0.20, and -0.42 V (reverse voltage sweep, us. Hg pool). The tin peak is almost in the voltage sweep range, and if tin is present, the antimony current-voltage curve has a steeply sloping base line; copper gives a n anodic peak prior t o the antimony oxidation and will interfere if present in excess; and the bismuth peak interferes completely with that of antimony. Advantages and Disadvantages. This method is as sensitive as the best spectrophotometric methods and is considerably easier t o use, since no extractions or chemical pretreatments are needed. A determination can be made in 15 min. Because of the differential measurement, reagent impurities are not sources of trouble, and the method is reasonably selective and free from interferences. On the minus side are the requirements for specialized instrumentation, which is not as yet widely available; and the specific interference of certain metals. These interferences could be removed chemically, but some of the speed and convenience of the method would thereby be lost. ACKNOWLEDGMENT
The assistance of W. A . Durheim and W. F. Peterson in carrying out the experimental work is gratefully acknowledged.
RECEIVED for review November 13, 1967. Accepted March 6, 1968.
