Spectrographic Examination of Assay Beads for Platinum, Palladium, and Gold J. SEATH
.AND
F. E. BEARZISH
t-niversity of Toronto, Toronto, Ontario, Canada
T
for 2 minutes after "the blick" to remove most (of the lead. The weights of silver and precious metals reported are, in all cases, the weights added to the lead boxes. .4 standard gold chloride solution, containing 0.25 mg. of gold per cc., was prepared from spectrographically pure metal and analyzed by the method of Beamish, Russell, and Seath ( 2 ) . Palladium and platinum chloride solutions, each containing 0.25 mg. of the metal per cc., were prepared from the spectroscopically pure metals. Standard iridium, rhodium, and ruthenium solutions, also containing 0.26 m&.of the metal per cc., TTere made by fusion of the metal sponges with sodium peroxide in a silver crucible as suggested by Beamish and Russell ( 1 ) . Silver was removed as the chloride. Further dilution of these solutions made possible the use of microburets for adding traces of the precious metals to the lead boxes. In both the arc and the spark treatments the beads were part of the lower electrode. Spectrographically pure gold, palladium, platinum, and silver wires of 3-mm. diameter were used in the production of the comparison spectra, arid the other electrodes employed were purchased from Adam Hilger, Ltd., London, England.
HIS research was undertaken to develop a qualitative
spectrographic procedure which n-ould reduce the time necessary for determining platinum, palladium, and gold in ores. De Laszlo ( 3 ) has described a method for estimating platinum in silver assay beads Iiy counting the number of platinum spark lines. The authors have had little success with this method for the determination of platinum and palladium in assay beads. Hon-ever, detection of the precious metals by their spark lines has the advantage of leaving the bead intact for subsequent \vet' analysis. T h e second spectrographic procedure developed consists in qualitatively detecting minute traces of the nietals by examination of their arc lines. Particular attention has been paid to the possible interference of the lines of platinum. palladium, gold, rhodium, iridium, and ruthenium; these. with silver and lead, are the only metals likely t o lie present in a silver assay bead. Gold beads containing platinum and palladium. comparable to the annealed residues from sulfuric acid partings of silver assay heads, were also treated by the arc method.
Spark Analysis In tlie production of bead spark spectra the secondary of tlie transformer delivered 10,000 volts, 60 cycles per second. The spark gap was kept constant at 3 mm. and the light from the spark n.as focused on the slit of a Hilger medium quartz spectrograph, ESIby means of a spherical icondensing leiis with a focal length of 17 em. Supersensitive H. and D. S o . 650 Zenith plates were used with the developer and fixer prescribed by the manufacturer. The time of derelopment was 2 minutes; fixation and washing reqiiiieil 30 niiiiutes each. The dried plates m r e studied over
Preparation of Electrodes In the preparation of silver beads containing the precioub metals, lead containers n-ere made from spectrographically pure lead foil. To each box, solutions of the precious metals were added from microburets. The solutions were carefully evaporated on hot plates, then the boxes xere rolled up with a weighed amount of silver foil, wrapped in more lead foil, and compressed under a pressure of about 100 kg. per sq. mi. The resulting 30-gram cylinders were cupeled at 900" C. and the bends left in the nlufflr X
?? h
Y
Y
Y
Y YY
Y
h
h
Y
XI
Y
SPECTRA OF 5 0 - ~ I GRILVER-P.~LL.ADIU\I . BEADSK I T H GOLD.eTHE UPPERELECTRODE FIGURE 1. SPARK 1. 2-8,
Palladium spark s p e c t r u m Spectra of gold a n d bead containing: : 2 1 2.0 per ce:iG of palladium: ( 3 : 2 . 5 per cent of palladium: ( 4 13.0 per cent of pallad i u m ; f5) 3 . 5 per cent of palladiutri: ( 6 ' 4.0 per cent of palladium; ( 7 1 4 . 5 per cent of palladium; ( 8 15.0 p e r c e n t of palladium il. Gold s p a r k s p e c t r u m 10. Silver spark s p e c t r u m
53.5
ISDUSTRIAL A S D El-GIKEEHING CHEMISTRY
536
an aperture in a closed box with interior lighting, by iiieans of a magnifying glass. The wave lengths for the lines reported are giveii in International Angstrom p i t s ($, 6). The spectral region studied Tyas between 4000 A , and 2000 A. Fifty-milligram silver-platinum and silver-palladiuiii beads were investigated by the spark method. I n order to photograph a series of beads rapidly, a simple container Tvas designed, consisting of a sinal1 silver cup niade of silver foil) iiiserted in one end of a glass tube of 3-mm. diaiiieter and attached lieneath to a silwr wire threaded through the 6-cni. tube anti connected to the loll-er terminal of the spark stand. The heads were easily removed from this holder after sparking. As only a small part of tlie lseatl ivas \-aporizetl it was difficult to conclude what Tveight of the preciouq metal the spark would detect. Table I records the lines consistently olit,ained for the loll-est concentrations idien a silver xire v-as the upper electrode. Figure 1 represents spectra of silverpalladiuni heads in n-hich the upper electrode was a gold wire.
nick for the bead. Before putting the bead iii place the arc was struck for a few seconds to reinow surface contamination. I n every case the loxer electrode ivas iiiade the anode, since the beads decoriiposetl rapidly and the blackening of the carbon bands v a s reduced. .Ifter 2 iiiinutes' arcing 50- and 100ing. silver beads and 10-mg. gold heads n.ere only partly decomposed but 10-nig. silver beads were almost' coiiipletely decomposed. T-IBLE11. ARC SPECTRA 1Tai-e I.engt1i
A. 3064.7 3042 ti 2998.0 2830 3 3894 2 3i18 0
TABLE I. SPARKSPECTR-I W a v e 1;engtli '1
.
3204 3064 3042 2998 2893 2830 2i94 2771 2733 2719 2705
3894 3799 3718 3690 3634 3609 33i1 3553 3517 3489
Lowest Concent rat inn Observed
7 7 1
7 6 0 9 3 2 i 9 0 9 2 2 Y 4
7 j
2 1 0 8
\\-are Length
A.
Platiriuni l i n e s . Silver-Platinum Beads 0 08 2702 4 0 01 2659.4 0 04 2630, !i 0 02 264G 9 0 04 2572 7 0.04 231.5 i 0.02 2514 0 0 04 2487 4 0 40 2450.8 0 08 2442 7 0 04 Palladium Lines, Silver-Palladium Beads 0.08 3481 2 0.60 3460.8 0.20 3433.4 0.08 3421.2 0 02 3404. G 0 02 3373 0 0.20 3242.7 0 30 3114 B 0 08 3085. 3 0 80 , . . .
I.o\r-eat C'nncent I a t ion Obeerved
% 0 02
0 0' 0.08 0 02 0 80 0 80 0 80 (1. 20 n.40
36'30 4 3634 7 300'3, G 331.2 3553,l 3517 0 3489.8 3481 2
The s e n d i v i t y of this iiiethod is much greatei than that of the method of surface effects, the inadequacies of 11-hich have been pointed out by Forlie. and Reaiiii~h( 5 ) .
Arc Analysis
In the production of bead arc spectra the current was 2 amperes, the arc gap about 3 iiiiii., and the voltage approsiinately 40 volts. Since the method did not' require tlie exact reproduction of electrical conditions, slight variations m r e not det,riinental. I n all other respects the photography of the arc was similar to that of the spark, except that' arc exposures x e r e 2 minutes. Silyer, gold, bismuth, zinc, and tin were not satisfactory as electrodes because they melted a t tlie arc temperature. Copper, nickel, and molybdenum introduced too iiiaiiy interfering lines. Soft carbon electrodes burned too rapidly and made it impossible to keep the bead on the lower electrode for more t'han a few seconds. Graphit,e electrodes of 6.3-inm. diaiiieter were found satisfactory since they burned slowly, did not melt a t t,he arc temperature, ai?d produced very few interfering lines in the coiiiposite spectrum. The deficiency of silver arc lines also assisted in the detection of the precious metals as their lilies were not masked. The upper graphite electrode n-as sliaqiened after each exposure ani1 the lower one n-as rounded off, leaving a small
r.o\\-est Concellti a t i o n Ohserred
{Val-e Length 70 I: . P l a t i n u m Lines, S:lver-Platinum Beads 0 004 '733.9 0 004 2 i 0 , J .9 2702. 4 0 004 0 008 2C.59.4 Palladium l i n e s , Silver-Palladium Beads 0.006 3400.9 0.03 3433.4 3421.2 0 OOG 0.001 3404 6 0 02 3302.1 0.006 3242. i 3114 1 0.02 3027. '3 13 OOG 2i83.1 0 03 ..., 0 004
Louest L'oiirenti ation
Olwerved
% 0 008 0.04 0.04 0 04
0 004 0.004 0.001 0.01)l 0 . 006 0.001 0.004 0.008 0,004
...
The lines tabulated in Tables I1 and I11 were consistently found to he present in the spectra of beads niade up to contain the recorded percentages.
Binary Systems
0.80
0.20 0 30 0.30 0.20 0 0" Q.20 0.20 0 60 0.20
\ OL. 10, KO. 9
A series of 10-my. silvcr-platinum beads SILVEH-PLATISU~I. !vas arced aiid the platinum lines for the lowest concentrations are recorded in Table 11. Thus as little as 0.0004 my. of platinum can be detectrd in a 10-mg. silver bead and there are no interfersensitive than the stannous chloride spot ent lines for platinum were found to be 3064.7, 3042.6, 2998.0, 2830.3, and 2733,.9. SIL~ER-P.ILI..IDI~;.\~. The most persistent palladium lines observed in the spectra of a series of 10-mg. silver-palladium beads are also tabulated in Table 11. The arc will detect at least 0.0001 nig. of palladium in a 10-mg. silver bead, which is as sensitive as the dimethylaminobenzilidine rhodanine >pot tebt. Silver and graphite lines did not interfere. The most persistent lines were 3634.7, 3421.2, 3404.8, and 3242.7. SILVER-GOLD.Ten-milligram silver-gold beads Jvere arced and the gold lines for the lowest concentrations are recorded in Table 111. The arc nil1 detect at least 0.0001 mg. of gold in a 10mg. silver bead. Figure 2 represents a typical series of silvergold heads., the two most persistent, lines being 2428.0 aiid 2676.0.
TABLE111. ARC SPECTRA \\-areel,engrli A.
Lowest C oncentrarion Observed
70
Lowest
K a v e Length
.
.-I
Concentrax Ohserve'
%
Gold Lines, Silver-Gold . \ s a y Beads 3122 8 3020.2 2748.3 2700 9
0.04 0.40 0.02 0 08
2688. i Xiti. 0 2641. ,5 2428 0
30ti4. i 3042 6 2'398 0 2830 3
P l a t i n u m Lines, Gold-Platinum Beads 0 006 2733 .!1 0.006 2705, '3 0.006 2702.4 0 03 2659.4
38'34. 2 3718 '3 3ti9[1.4 3Ci34.i 3ii0'3.0 3371.2 3333. 1 3317.0 3488.8 3481.2
Palladiuiii Lines, Gold-Palladium Beads 3460.8 0.03 3433.4 0 04 3421 2 0.02 34u4 (i 0 OOG 3302.1 0 04 3242. i 0 0% 3114.1 0 04 302i .9 0 02 2i83 1 0 03 0 01
0.80 0.001 0.40 o 001 0 03 0 04 0.03 0.05
0.01 0.01
0 .0lKi 0 OOb 0 02 I 1 006
0.01 O,O2 0 03
...
SEPTETIBER 13. 1938
537
1
, 3 4
3
G
7
a 9 10
OF 10-MG. SILTER-GOLD BEADS FIGURE 2. ARC SPECTR.~
1. 2-9. 10.
Gold a r r spectrum Spectra of silver bead containing: ( 2 ) 0.75 mg. of gold; (3) 0.26 mg. of gold; 14: 0.12 nig. of gold; i2 0.08 mg. of gold; of gold: (7) 0.008 mg. of gold; 18) 0.004 mg. of gold; ( S i 0.002 mg. of gold I r e spectiurn of d r e r a n d graphite
GOLD-PLATISUX. The most persistent platinum lines observed in spectra of 10-mg. gold-platinum beads are also recorded in Table 111. After 2 minutes' arcing, at least half of the bead was intact; so the method will detect at least 0.0006 nig. of platinum in a 10-mg. gold bead. GOI.D-PALI..IDIUIf. Ten-milligram gold-palladium heads I r e r e arced and the palladium lines are recorded in Table 111. The arc method detects at least 0.0006 my. of palladium in a 10-mg. gold bead.
Silver and gold heads containing tn-o or iiiore of the precious metals in small quantities n-ere arced to determine tlie semitivity of the method for platinum, palladium, and gold in polycomponent' beads and to investigate interference of the precious-metal lines. Ten-milligram silver beads containing equal amounts of platinum and palladium v-ere arced and only platinum line 3064.7 and palladium line 3065.3 produced interference. -kt least 0.0004 mg. of both platinum and palladium can be detected in a 10-mg. silver bead. Similarly, it as found that platinum and palladium did not interfere with gold lines in spectra of 10-nip. silver heads, containing equal weights of the precious metals. At least 0.0001 mg. of gold could be detected in the 10-mg:. composite bead. Ten-milligram silver beads containing equal proportions of platinum, palladium, gold, and iridium n-ere arced. Iridium interfered n.ith certain platinum lines, especially Ivith line 3064.7, when more than 0.05 mg. of iridium !vas present in a 10-mg. composite bead. Otherwise there \vas 110 interference. Ten-milligram silver beads containing equal amounts of platinum? palladium, gold, and rhodium were similarly arced arid no rhodium lines were found to interfere. Ten-milligram silver beads containing equal \wights of platinum, palladium, gold, and ruthenium n-ere arced in the same way. Ruthenium lines did not interfere n-hen less than 0.05 mg. of the metal !vas present in a 10-mg. silver bead. Ten-milligram gold beads containing equal proportions of platinum and palladium were arced and a t least 0.0006 mg. of each metal could be detected with no line interference.
As the proportion of platinum, palladium, and gold usually greatly exceeds t h a t of the other associated platinum metals
0.04 m p .
in the coiiiiiioii sources: the interference caused by iridium, rhodium, and rutheniimi can generally lie igiioretl.
Summary
~~
Polycomponent Systems
(1;)
The autliors mggest the preparation of duplicate assay beatis for preliminary spectrographic examination wliich may modify the subsequent wet analysis for the precious metals. If line.. of pallarliuni, platinum, and gold a1.e not present in the Iiead qiectrmn, tlie t'etlious time-consiriiiiiig separations of these metals may lie avoided. Silver-platinum and silrer-palladim~ilieatlz- can be sparked for the detection of traces of platinuni antl palladium. Minute traces of platinum. pallatliuni, ant1 gold are more accurately detected by arcing silver assav l m t l s than hy spot tests. Traces of platiiiuni antl pallatliuni can be detected in gold beads coi1iparal)le to tlie annealetl residue3 fi,oiii sulfuric acid partings of .silver a+ay beads. W i e n the proportions of iridiuin rhodium, antl rutheniuin are low in tlie 1)ead. there are very fell- interfering arc line.
