Determination of Gold in Rocks by Neutron Activation Analysis Using Fire-Assay Preconcentr ation F. 0.Simon and H . T . Millard, Jr. U.S . Geological Surcey, Washington, D . C. 20242 TWO RELIABLE TECHNlQUES for the determination Of gold in geologic materials are neutron activation analysis and classical fire assay. The high sensitivity of activation analysis has allowed the measurement of gold at the parts-per-billion concentration level in rocks (1-5). In conventional activation analysis, 0.1- to 1-gram samples and standards are irradiated with neutrons dissolved in the presence of gold carrier, and the carrier plus activated gold are separated from other radioactivity by radiochemical procedures. Counting of the lg8Au and measurement of the chemical yield of the carrier allows calculation of the amount of gold in the samples. Several factors contribute to the uncertainty in results obtained by this technique. The relatively small samples used may not be representative for gold (6, 7). In addition, the gold standards may not be exposed to exactly the same neutron flux as the samples, and the chemical exchange between lg8Au and the carrier may be incomplete. On the other hand, contamination of the carrier gold by gold from the reagents can be ignored, thus eliminating the necessity for a reagent gold blank. Bugbee (8) and Beamish (9) have discussed classical fire assay procedures. Variations of the classical technique have involved the estimation of the gold content of the silver bead by spectrography (IO) and atomic absorption (11), but the sensitivities are not sufficient to measure nanogram quantities of gold and therefore, gold cannot be determined in rocks a t the parts-per-billion concentration level. Although larger, more representative samples are used in fire assay, the possibility exists for incomplete recovery of gold in the fusion and cupellation steps. With low levels of gold, contamination by gold from the reagents necessitates carrying a blank through the procedure. It is a logical step to combine fire-assay preconcentration of gold with the measurement of gold by rapid instrumental neutron activation analysis. Washington and Holman (12) developed such a procedure in which they irradiated the final silver bead and determined lg8Auby gamma counting. Their detection limit, with a precision of 50%, was only 10 ppb gold
(1) E. A. Vincent and A. A, Smales, Geochim. Cosmochim. Acta, 9,
154 (1956). (2) E. A. Vincent and J. H. Crockett, ibid., 18, 130 (1960). (3) G. A. Perezhogin and I. P. Alirnarin, J. Anal. Chem., U.S.S.R., 20,870 (1965). (4) A. R. DeGrazia and L. Haskin, Grochim. Cosmochim. Ac/a, 28, 559 (1964). ( 5 ) P. A. Baedecker and W. D. Ehmann, ibid., 29, 329 (1965). (6) A. Chow and F. E. Beamish, Talatita, 14,219 (1967). ( 7 ) A. D. Wilson, Aualysr, 89, 18 (1964). (8) E. E. Bugbee, "A Textbook of Fire Assaying," Wiley, New York, 1922. (9) F. E. Bearnish, "The Analytical Chemistry of the Noble Metals," Pergamon Press, New York, 1966, pp 162-76. (IO) Ibid., pp 490-3. (11) C. Huffman, Jr., J. D. Mensik, and L. B. Riley, U. S. Geol. Survey, Circ. 544, 1967. (12) R. A. Washington and R. H. C. Holman, Geol. Survey of
Canada, Puper 657,1966. 1 150
ANALYTICAL CHEMISTRY
because of the large amount of 253-day llomAg,generated during the neutron irradiation, and a sizable reagent gold blank. The technique we have developed involves irradiation of the lead button obtained by the fire-assay procedure prior t o the addition of silver. This approach provides a uniform, dilute, and reproducible matrix for the neutron irradiation of gold, reduces the counting background due to llornAg,and avoids the possibility of introducing gold contamination with the silver. After irradiation, silver is added, the lead button cupelled, and the 1g8Aucounted instrumentally without further separations. The sources of error in this technique differ from those in conventional neutron activation analysis; therefore, if both methods give the same result for gold, confidence in the reliability of each is increased. EXPERIMENTAL
Reagents. STANDARDGOLDSOLUTIONS. Dissolve 1 .OOO gram of reagent grade gold metal in aqua regia and dilute to 1.000 liter with 6M HCl. A stock solution, containing 10 ppm of gold, is prepared by diluting 5.00 ml of this solution to 500 ml with 6M HCl. This solution is stable for several months. A working solution, containing 0.100 ppm of gold is prepared fresh daily by diluting 2.00 ml of the stock solution to 200 ml with 6 M HCl. RADIOGOLDTRACERSOLUTION. Prepare lQ8Au tracer by irradiating 10 Mg of gold leaf until a specific activity of approximately 10,000 cpm/pg gold is obtained. Dissolve the gold leaf in aqua regia and dilute to 20 ml. FIRE-ASSAY FLUX.Purify the fire-assay flux as follows: Thoroughly mix 2000 grams of PbO, 500 grams of Na2C03, 100 grams of Na2B40i,and 60 grams of flour (reducing power equivalent to 10 grams of lead per gram of flour). Fill several 250-ml alumina crucibles (Coors Porcelain Co., ceramic AD-99) with flux and heat at 900" to 1000" C for 1 hr. Pour the molten flux into an iron mold, allow the melt to cool, and discard the lead buttons containing the noble metal impurities. Grind the slag (purified flux) to -60 mesh in a disk grinder, homogenize in a porcelain ball mill, and store in a desiccator or air-tight container. Procedure. PREPARATION OF STANDARDS, SAMPLES,AND BLANK. Weigh five 90-gram portions of purified flux in 250-1111 alumina crucibles. To two of these, after making a deep depression in the flux, add 1.00 ml of the working gold solution, and dry overnight at 110" C. Any lumps formed by the addition of the gold solution should be broken up. To the flux in two other crucibles, add 10 t o 25 grams of sample. If larger samples are to be analyzed, a proportionately larger amount of flux should be used in all five crucibles. The fifth crucible serves a s the blank. Add 3.0 grams of flour to each of the five crucibles, mix thoroughly, and cover each with 3.0 grams of Na2B40i. FIRE-ASSAY FUSION.Fill standard 30-gram fire-assay clay crucibles to within 1 inch of the top with bone ash or silica powder, place the alumina crucibles on the bone ash, and add additional bone ash or silica for support. This arrangement protects the alumina crucibles from thermal shock. Place the crucibles in a furnace preheated to 900" to 1000" C and
heat for 1 hr. Pour the melts into an iron mold and allow to cool. Remove the slag from the metallic lead with a hammer and stiff brush. This operation should be done carefully as the slag contains large amounts of sodium which, when irradiated, will raise the level of radioactivity. The lead buttons should weigh from 20 to 36 grams. If any fall outside this range, repeat the fusion, altering the amount of flour accordingly. IRRADIATION OF LEADBUTTONS. Hammer the lead buttons into a cylindrical shape, 1.5 cm in diameter and approximately 1 cm high, and drill a 1/i6-inch diameter hole along the axis of the cylinder. String the five lead buttons, separated by aluminum washers, o n a n aluminum wire in the order: standard, sample, blank, sample, standard. We irradiated the lead buttons for 1 hour in a water-filled glory tube in the nuclear reactor at the Naval Research Laboratory, Washington, D. C., where the neutron flux is 5 X 1 0 I 2 neutrons/cm2/ second. The water-filled tube is required t o dissipate heat produced by absorption of gamma rays by the lead. The buttons are stored overnight under water to allow the decay of 3.3-hour *O9Pb. After this period of decay, the 24Na activity of the five buttons is less than 1 mCi. CUPELLATION. Insert one inquart (100 mg of lead wire containing 2.1 silver) into the hole in each lead button and crimp the edges of the hole to prevent the inquart from falling out. Cupel the lead buttons using the classical fire-assay technique (13). GAMMA-RAY SPECTROSCOPY.Place the silver beads in plastic vials and count at the bottom of a 1-inch diamter well in a 3- X 3-inch sodium iodide detector. Three gamma spectra, taken over a period of 1 week, are accumulated on a multichannel analyzer. These spectra should be free of photopeaks other than those due to IQ8Auand IlornAg. CALCULATIONS. Compute the areas above the base lines for the 0.41-MeV 1g8Auphotopeaks for the standards, samples, and blank; reduce to counts per minute; and correct to zero decay time using the 2.7-day lg8A~ihalf-life. These corrected values should agree to within the limits of the counting statistics. Subtract the value of the blank from the standards and samples and calculate the weight of gold in the samples using the equation: Wsample
=
Aeampls
[Watandard]
Astandard
where W is the weight of gold and A is the net activity. A s h n d a r d is the average value for the two standards. RESULTS AND DISCUSSION
Recovery of Gold. Losses of consequence in the fire-assay process are due t o retention of gold by the crucible and slag, absorption by the cupel, and volatilization. Volatilization losses are of the order of 0.1 or less (14). Fulton (15) found the averages of the slag and cupel losses of gold, in the range of 1.17 to 493.8 mg of gold, to be 0.54% (0.1 to 2.173 and 0.44% (0.1 to 1.6%), respectively. Coxon, Vervey, and Lock (16) found a OSZ loss to the crucible and slag and a 0.7% loss t o the cupel for 1.2 to 7.2 mg of gold. Radiotracer experiments using Ig8Auwere performed to extend these data t o 100 ng of gold. I t was assumed that the behavior of the radiotracer lg8Auadded to the fire-assay flux as a solution would be (13) E. E. Bugbee, “A Textbook of Fire Assaying,” Wiley, New York, 1922, pp 93-7. (14) W. F. Hillebrand and E. T. Allen, U. S. Geol. Survey, Bull. 253,1905, p 21. (15) C. H. Fulton, “A Manual of Fire Assaying,” Hill Publishing, New York, 1907, pp 1268.
Table I. Recovery of Gold by the Fire-Assay Procedure Weight of Weight of silver used Per cent of radiogold found gold for Fusion Silver taken, cupellation, flux Cupel bead Pg mg 0.1 2 1.5 1.5 97.0 6 0.5 1.3 98.2 10 1.2 0.5 98.3 20 0.4 0.6 99.0 0.5 2 0.9 1.5 97.6 6 0.9 0.8 98.3 20 0.3 0.7 59.0 Table 11. Gold Blanks from Fire Assay for Unpurified Flux and Two Separately Prepared Portions of Purified Flux
Unpurified
Crucible Clay
Wt. of
Wt of Au, ng Average, runs grams Range f sigma 19 93 23-72 38 10 No. of
flux
flux
Purified flux AlzO3 Portion A Portion B AI2O3 Portions A AI2O3 and B mixed together and homogenized in a ball mill
6 6 4
93 5.7-9.2 93 10.0-28.0 93 8.9-10.0
6 . 9 i 1.2 13.9 i 2.2 9.3 f 0.5
Table 111. Results of Analyses for Gold in W-1 by the FireAssay-Neutron Activation Analysis Technique and by Conventional Neutron Activation Analysis Sample weight, Average, Method g Au, ppb f sigma Reference Fire-assay25 4 . 0 , 3 . 2 , 3 . 4 , 3 . 6 f 0 . 4 Thiswork neutron acti3.9 vation analysis
Conventional neutron activation analysis
0.5
0.5-1
2.9, 3.2, 4.2, 3 . 4 i 0 . 6 3.8, 3.8, 2.8
(17)
4.8, 4.9
(5)
4.8
identical to gold contained in samples. This assumption is also implicit in the procedure for preparing the standards for irradiation. It should be noted that Coxon et al. (16) found that after adding radiotracer lg8Auto ores containing gold in the parts-per-million concentration range and carrying these through a fire-assay procedure, the specific activity of lg8Auin the gold beads was not constant. They concluded that this discrepancy was due to a difference in behavior between the radioactive and inert gold. The results of our experiments, presented in Table I , are normalized t o the sums of the amounts of radiogold found in (16) C. H. Coxon, C. J. Vervey, and D. N. Lock, J. S. African h s t . Milling Mer., 62,546 (1962). (17) H. T. Millard, Jr., J. J. Rowe, and F. W. Brown, unpublished data, 1967. VOL 40, NO. 7, JUNE 1968
1151
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). 1 152
ANALYTICAL CHEMISTRY
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).
