ANALYTICAL CHEMISTRY
1174 The completeness of combustion and the over-all behavior of the apparatus were tested and proved satisfactory by burning succinic acid as a standard. There remained, however, the question of whether a relatively large quantity of ether, as was liberated from such materials as beryllium hydride etherate, would be burned completely or whether only partial combustion B-ould occur upon its passage through the combustion furnace. This was tested by burning a sample of purified ether in the apparatus. The ether was introduced from a sealed tube equipped with a breakoff seal, which was broken after the tube had been sealed into the system. The ether \\*asthen vaporized and carried into the combustion region by means of helium passing over the opening of the tube. By this method, a sample of ether weighing 63.3 mg. gave 99.17~of the expected amount of carbon dioxide and 99.3% of the expected amount of mater. Also, completeness of combustion was indicated when the amount of active hydrogen determined by hydrolysis checked with that found by combustion on compounds where this comparison was possible. Some workers ( 4 ) have found that successive cycles of heating a quartz tube above l l O O o C. with subsequent cooling cause porosity in the quartz, which might eventually develop into leaks in the tube. Also, the use of copper oxide in quartz a t high temperatures tends to weaken the quartz eventually to the point where a slight strain will cause it to crack.
tions were negligible, they were not made on the runs; however, they were included in calculating the per cent error. The weighings were carried out on an Ainsworth Chainomatic balance (Wm. Ainsworth and Sons, Denver, Colo.). An attempt v a s made to estimate the weights to a few hundredths of a milligram so that their reproducibility was within the limits of 5 0 . 1 mg. Hence, for each weight recorded a possible error of f 0 . 2 mg. mas allowed (because weighings were made by difference). I n addition to the weighing uncertainty, an allowance was made for the carbon dioxide and water blanks, 0.05 and 0.02 mg. per hour, respectively, including the uncertainty in the blanks. The per cent errors in the carbon dioxide and water yields were figured separately. The total uncertainty (in per cent) for a given determination was taken as the square root of the sum of the squares of the uncertainties in the weight of the product-Le., carbon dioxide or water-and in the original sample weight. From these considerations it is possible to predict the expected precision for a combustion of any sample of given size. Because different substances yield varying amounts of carbon dioxide and water, a specific sample size n-hich would ensure a certain per cent accuracy cannot be given. The sample size should be so chosen that the per cent uncertainty for the product of lowest yield is within the desired limits. ACKNOWLEDGMENT
PRECISION AND ERROR
The absorbents used were Ascarite and Anhydrone. It has been reported ( 1 ) that Ascarite will absorb carbon dioxide completely a t a flow rate as high as 0.5 liter per minute until its weight has increased about 20%. Anhydrone has been reported ( 2 ) to be as good a desiccant as phosphorus pentoxide a t a maximum flow rate of about 5 liters of gas per hour and absorbs up to 60% of its weight of water. Even though an initial copper oxide furnace and an absorption tube containing Anhydrone and Ascarite were used, a small blank wa9 found, as has been shown. Because the blank correc-
Acknowledgment is made to T. W. Newton and J. F. Lemons for their helpful suggestions concerning this work. LITERATURE CITED
Altieri, V. J., “Gas Analysis and Testing of Gaseous Rfaterials,” p. 98,American Gas Association, New York, 1945. (2) Hillebrand, W. F., Lundell, G. E. F., “Applied Inorganic dnalyses,” pp. 44-5,Wiley, New York, 1944. (1)
(3) Ibid., pp. 627, 630. (4) Lundell, G. E. F., Hoffman, J. I., Bright, H. A., “Chemical Analysis of Iron and Steel,” p. 161,Wiley, New York, 1931. RECEIVED for review January 27, 1956. Accepted April 4,1956. Work done under t h e auspices of the U. S. Atomic Energy Commission.
Assay for Platinum Metals in Ores and Concentrates 1. HOFFMAN, A. D. WESTLAND, C. L. LEWIS, and F.
E. BEAMISH
University o f Toronto, Toronto, Ontario, Canada
T h e r e are n o data recorded t o indicate the precision achieved by t h e various methods used for the fire assay of p l a t i n u m ores. In the following report platinum m e t a l s ores and concentrates have been examined by direct assaying and by m e t h o d s involving leaching prior to fire assay. The silver-platinum metals beads were examined spectrographically. The data obtained suggest that for ores and concentrates no advantage is to b e gained b y t h e elaborate, time-consuming leaching methods.
I
still used by some analysts as a preliminary treatment before fire assay. This process converts the bulk of the minerals to simple dissolved constituents and with subsequent treatment there is some isolation of base metals. There is no published evidence that better results are obtained by these time-consuming procedures. A unique opportunity was presented to evaluate the efficiencies of fire assay and leaching practices by an invitation to take part in a reconnaissance survey. The fire assay for platinum ( 2 ) was being investigated in the authors’ laboratory and it was considered desirable to obtain some information regarding losses with assays of ores.
T IS generally recognized that there is an appreciable lack of
precision in platinum metal values obtained from various laboratories which use different methods of fire assay. Experience has shown that the numbers vary by as much as a factor of 10. These discrepancies may be due in part to variations in procedure and technique. Leaching processes were developed many years ago and are 1
Falconbridge Metallurgical Laboratories, Richvale, Ontario, Canada.
APPARATUS, REAGENTS, AND ORES
A pyrometrically controlled Williams and Wilson 15-kva. Globar-type assay furnace was used for all the fire treatments. Spectrographic examinations of the silver beads were made on an Applied Research Laboratory 2-meter grating spectrograph (36,600 lines per inch). Zinc metal dust, Purple Seal grade, obtained from City Chemical Co., New York, N. Y., was used. Litharge, soda ash, borax glass, and calcium oxide, used in the
V O L U M E 2 8 , NO. 7, J U L Y 1 9 5 6 Table I. Sample NO.
Litharge, Grams
Assay Charges for Direct Fire Assay Soda Ash, Grams
Borax Glass, Grams
60 60 60
100
Silica Sand, Grams
100 65
GO
40 40 40 40
0-5s 0-6s 0-7N 0-7Sa
150 150
80 80 80
16 16 16
P-1 P-2 P-3 P-3a P-4 P-40
130 135 136 30 135 GO
50 50 50
40 40 40
62 62 62
50
40
62
P-5N P-5Na P-6N P-6Na P-7N P-8S P-8Xa
150 50 175 30 225 225 30
50
10
30
40
10
60
40 40
10 10
60 60
H-1 H-2 H-20 H-3 H-30 H-4 H-4a
316 316 30 340 30 340 60
30 30
13.2 13.2
30
60
30
60
H-5N H-6N H-6Na
250 250 30
35 35
15 15
0-1 0-2
0-3 0-4 0-46
a
1175
100 100
150
65
22 22 22 22
Flour, Grams
KNOi, Grams
Button si20
GraAs 29 28 28 08 51
3.8 3.8 3.8 8.0 8 0
8.0
57 37 58 67
4 4 7.2 5.6 5.6 12.0 8.4
17 36 25 I4 80 67
5.4 12 0 5.0
26 6.0 3.0 4.2
50 50 54
3.3 3.3 3 3 3.3 4 2 9.0 6.5 3 .3
101 51 30 19 38 34 31
42 39
20 80 31
preparation of fluxes, were either technical or commercial grades. All other chemicals used were of reagent grade. The following three platinum-bearing ore samples were used: plant feed composite, designated 0 : pyrrhotite concentrate, P; and high grade conrcntrate, H. The percentage composition of the samples was determined, with the following results: 0
P H
Fe 23.10 55.50 36.30
cu 0.90 0.08 5.18
Ni 1.53 1.02 4.66
S 12.63 46.00 27.75
DIRECT FIRE ASSAY METHOD
Preparation of Assay Charge. For each determination one assay ton (29.166 grams) of sample was placed in a shallow silica dish. All samples, except those fluxed with potassium nitrate (niter assay), were placed in an electric muffle and the temperature was raised to 675" C. The door of the oven was left slightly open to allow easy oxidation and roasting was continued overnight. The contents of the silica dish were transferred to a large cellophane sheet by being passed through a Standard KO.45 sieve, and silver powder (10 mg.) was added. Various amounts of fluxing substances as shown in Table I were sieved before being added and the whole charge was intimately mixed by rolling. I n Table I the letter K following a sample type designates a niter assay. Assay Procedure. The charges were transferred to assay crucibles and fused in the furnace between the temperatures given below: 975-1150 975-1100 975-1150 975-1100 1100-1150 975-1150
27 29 23 22 22 79 55
Reassay of slag.
Sample
spectra. These elements probably were present in very low concentration, but were below the limits of detection of the method used.
Insol.
...
3.30
I6 80
SPECTROGRAPHIC METHOD
The method used for quantitative spectrographic analysis of fire assay beads is a modification of the procedure described by H a r l e y and Rimsaite (1). The standards, made with a lead base, contained 10% of silver and concentrations of platinum, palladium, and rhodium varying between 1.0 and 0.001%. Master standards were prepared by adding platinum, palladium, and rhodium (in high concentrations to minimize weighing errors) t o granular lead. Each master standard was melted under hydrogen in a graphite crucible. These lead beads were filed with fine-toothed files, a new iYe being assigned to each standard and used only for that standard. Weighed amounts of filings from each master standard were diluted with lead to provide a series of lead bead standards with various proportions of platinum, palladium, and rhodium. The constant 10% of silver and the "scrambling" of platinum metals concentrations is intended to minimize possible spectrographic effects of one element on another. The precision of determinations on standard samples containing 0.001% of platinum, palladium, and rhodium was, respectively, 4, 6, and 15%. A fire assay bead was weighed and dissolved in sufficient lead BO that the final lead bead contained 10% of silver. The lead bead was weighed, sampled, and analyzed spectrographically to find the percentages of platinum, palladium, and rhodium. These percentages were multiplied by the weight of the lead bead to find the number of milligrams of each platinum metal in the fire assay bead. I n Tables 111, IV, and V, blank spaces were left in those cases where no platinum, palladium, or rhodium lines appeared in the
C. for 0 samples C. for OK samples C. for P samples C. for P N samples C. for H samples C. for H S sample8
The fusion mixtures were poured into iron molds and the lead buttons when cool were broken away from the slags and weighed. In some instances reassays were made on the slag to test whether a better recovery of precious metals could be obtained from the sample. I n these cases the slag from an assay was ground to pass a S o . 45 standard sieve and mixed with extra litharge, flour, and a little silver powder. This mixture was fused in the original pot and a second lead button was obtained. Scorification Procedure. The influence of oversize buttons on the recovery of platinum metals was tested in some cases and this resulted in reights of lead beyond the absorptive capacity of the cupels. The combination of assay and reassay buttons for the formation of a single silver bead also resulted in too much lead for a cupel. Reduction in n eight of lead was made by heating at 900" C. on scorifying dishes (3 or 4 inches in diameter) for appropriate lengths of time. The fusion mixtures were poured into iron molds, and when cool the lead buttons were separated from the slag (chiefly fused litharge). For the cupellation of lead buttons, bone ash cupels preheated at 900" C. for a t least 10 minutes were used to form the silverplatinum metals bead.
Table 11.
Direct Fire Assay Spectrographic Analyses of Fire Assay Beads (Figures represent Troy ounces per ton)
Sample KO.
Platinum
Palladium
Rhodium
0-1 0-2 0-3 0-4
0.0048 0.0134 0.0040 0.0047
0.0031 0.0033 0.0030 0.0031
0.0012 0.001A 0.0015 0,0010
0-5s 0-6N 0-7N
0.0051 0.0037 0.0032
0.0065 0.0027 0.0031
0,0059 0.0013 0.0011
P-1 P-2 P-3 P-4
0.0028 0.0031 0.0033 0,0029
0.0023 0,0022 0.0028 0.0043
0.0022 0.0017 0.0037 0.0017
P-5N P-6N P-7N P-8N
0.0048 0.0033 0,0033 0.0033
0.0030 0.0040
0.0027 0,0032
0.0019 0.0023 0.0015 0.0021
H- 1 H-2 H-3 H-4
0.0231 0.0264 0.0207 0,0220
0.0146 0.0142 0.0130 0.0156
0.0045 0.0041 0,0036 0.0042
H-5N H-6N
0,0209 0,0205
0.0142 0.0144
0,0038 0.0034
ANALYTICAL CHEMISTRY
1176 The results obtained from the spectrographic analysis of fire assay beads obtained by the direct method are shown in Table 11. .4verages for these results are shown in Tables 111,IV, and V. LEACHING PRIOR TO FIRE ASSAY
Leaching Procedure. One assay ton of each sample was weighed into a shallow silica dish. The sample was roasted overnight a t 675' C. in an electric muffle with the door slightly open. The contents of the silica dish were transferred t o a 400-ml. beaker and 50 ml. of concentrated hydrochloric acid were added. The beaker and contents were placed on the steam bath and left overnight. Fifty milliliters of water were added and the sample was filtered using a 9.0-cm. Whatman No. 40 filter paper, The residue was washed well with water and the hydrochloric acid treatment was repeated. After a second filtration and washing, the residue and paper were placed in a Coors No. 3 porcelain crucible and ashed in the muffle. It was noticed that during leaching the P sample was greatly reduced in bulk, but its color was unchanged. The H sample was slightly reduced in bulk and appeared bleached. The 0 sample took on a sandy appearance. These leached residues were assayed and the results are included in Tables 111, IV, and V. Treatment of Filtrate. METHODA. The filtrates from the hydrochloric acid treatments were combined and evaporated t o
Table 111.
Expt. la lb 2a 2b 3a 3b 4a 4b
Method A Method A Method B Method B
Platinum 0,0058 0.0038 0.0039 0.0046 0.0031 Av. Av.0
Palladium
0.0058
0.0012
0.0038
0.0006 0.0006 0.0007 0.0015
0.0039
0.0038
0.0038
0.0077 0.0031 0.0053 0.0043 i 0,0007
Table IV.
Sum
0.0022
a. b.
Method A Method B
0.0023
Method B
0.0040 0.0014 0.0021 Av. .4v.
Direct assay Leached residue. Leach precipitate.
Table V.
Borax Glass,
Calcium Oxide,
Soda .ish,
%
%
%
%
%
15.4
7.7
6.4
44.9
25.6
7.5
77.6
14.9
0.0013 0.0014 0.0011
Sum 0.0013 0.0014 0.0011 0.0016 0.0013 0.0013 f 0.0002
Results of Leaching Experiments
Platinum 0.0011 0.0010
Method B
Rhodium
0.0010 0.0031 0.0006 0.0023 0.0030 f 0.0002
(Figures represent Troy ounces per ton. Expt. la lb 2a 2b 3a 3h 4a 4b
Silica Sand,
0 sample)
Sum
Direct assay a. Leached residue. b. Leach precipitate. 4 One analysis reported in Table I1 was deleted.
Treatment of Filtrate
Type of Flux Neutral flux Base metals flux
Results of Leaching Experiments
(Figures represent Troy ounces per ton. Treatment of Filtrate
a volume of about 50 ml. Two hundred millilit~ersof water were added and the solution was heated t o hoiling. Hydrogen sulfide gas was bubbled in while the sample was allowed to cool to near room temperature. The precipitate F-as recovered by filtration on a 9.0-cm. No. 40 filter aper and washed with 1% ammonium chloride solution. The figer paper and contents were placed in a Coors No. 3 porcelain crucible and heated overnight a t 675" C. Filtrates from the sulfide preci itates were assayed and revealed only insignificant amounts of pfatinuni metals. METHODB. The filtrate was evaporated to dryness and 150 ml. of water were added. The mixture was heated to boiling and enough hydrochloric acid was added to dissolve the residue. Zinc dust n-as added until all reaction had ceased and about 15 grams were added in excess. The mixture which contained base metal hydroxides and zinc was boiled for 0.5 hour, filtered through a 9.0-cm. No. 40 filter paper, and washed Kith water. Ashing was done in a large porcelain crucihle as before and it was noticed that some zinc was eliminated by volatilization. Composition of Fluxes. The compositions of the fluxes used , were as follows:
Sum
P sample)
0.0021
Palladium 0.0008 0.0015
0.0023
0.0023
0.0024
0.0024
0.0040
0.0024
0.0024
0.0035 0.0617 0,0030 0.0034 f 0.0006
Sum
Rhodium 0.0024 0 . 0014 0.0009 0.0014 0.0006 0.0018
..
0.0017 0.0022 0 . 0 0 3 1 f 0.0008
Sum 0.0024 0.0023 0.0020 0.0018 0.0021 0.0021 f 0 . 0 0 0 7
Results of Leaching Experiments H sample)
(Figures represent Troy ounces per ton. Expt. la lh 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b
a. b.
Treatment of Filtrate Method A Method A Method A Method B Method B Method B
Direct assay Leached residue. Leach precipitate.
Platinum 0.0170 0.0010 0.0202
o.0isi 0.0i6i 0,0043 0.0221 0.0039 0.0195 0.0058 Av. Av.
Sum 0.0180 0.0202 0.0191 0.0204 0.0260
Palladium 0.0067 0.0114 0.0027 0.0076 0.0036 0.0107 0.0017 0.0100 0.0034 0.0133 0.0014 0.0153
0.0253 0.0215 0.0223 f 0.0023
Sum 0.0181 0.0103 0.0143 0.0117 0.0167
Rhodium 0.0031 0.6027 0.0029 0.0015 0.0031 0.0031 0.0010 0.0021 0.0015
0.0167 0.0146 0.0143 f 0.0009
Sum 0.0031 0.0027 0.0029 0.0046 0.0041 0.0036 0.0035 0.0039 f 0.0004
Litharge,
P r e p a r a t i o n of A s s a y Charge. The residues remaining in the porcelain crucibles after ignition were transferred t o a large cellophane sheet by being passed through a G. S. Standard S o . 45 sieve. The following additions were made to all samples except the filtrate residues o b t a i n e d b y Method B : Xeutral Bus, 80 grams Litharge, 28 grams Flour, 2.8 grams Silver powder, 10 mg.
After being intimately mixed 11) rolling on the cellophane sheet, the mixture was transferred to an assay crucible and covered with 20 grams of the neutral flux. Filtrate residues obtained by Method B were mixed with 320 grams of the base metals flux, extra litharge, f l o u r , a n d s i l v e r powder. Eighty grams of the flux were used as a cover. Because the amount of residue n-as small for 0 samples by Method B, half of the above weights of base metals flux were used. Fire Assay Procedure. The pots were placed in the furnace at 975" C. and the temperature was raised a t the maximum rate to 1200' C. The fusion mixtures were poured into conical iron molds and allowed to cool. The lead button was broken away and any slag adhering to the button was removed by gentle tapping with an iron rod. Where a reassay was made on the slag, it was ground t o pass a No. 45 standard sieve and fused in the original pot with flour and extra litharge to replace that lost to the first h u t t o n . The lead-platinum metals button was cupelled a8 described above for the direct fire assay method.
V O L U M E 2 8 , NO. 7, J U L Y 1 9 5 6
1177
The results obtained for the spectrographic analysis of fire assay beads in the leaching experiments are shown in Tables 111, IV, and V. DISCUSSION
The variables that xere altered in an attempt to find the conditions which would result in maximum recovery of platinum metals can be seen in Table I. Attempts were made to correlate these variables with the results of analysis shown in Table 11. S o definite advantage was gained for the 0 or P samples by the use of large buttons, by reassaying, or by niter assays. In addition to these variables, large alterations were made in the amount of litharge used in the niter assays without obtaining improved recovery. Slterations in the amount of silica present in the flux, as well as oversize buttons and reassaying, failed to produce any outstanding advantage in the case of H samples. Results obtained by niter assay for H samples were very similar to those obtained by assaying the roasted samples. All the evidence above can be interpreted to mean that a wide variety of conditions give equally good recoveries of platinum, palladium, and rhodium from the samples tested. One result for an 0 sample mas deleted from Table 111. It was considered to be due to the presence of a grain of platinum metals mineral. The presence of small, isolated grains of sperrylite could also account for the lack of precision with the ore samples. Scattered results are characteristic of analyses performed on samples in which the constituent sought is not intimately dispersed throughout the sample. Ideally, a fen- micrograms of platinum metals mineral must be distributed evenly in 30 grams of ore. This is not possible with any knoxn methods of sampling and mixing. I t is thus necessary to attach significance only to
very large assays or to the averages of several small assays. Leaching and normal assay results agree closely when the ore is concentrated. Averages of platinum, palladium, and rhodium recovered from H samples by leaching (Table V) agree well with the normal assaying results. In the case of the 0 and P samples the data in Tables I11 and IV may be interpreted only to indicate that leaching processes do not, in general, provide values higher than those obtained by normal fire assay. The results are not intended to define the precision which may be obtained by leaching processes. Undoubtedly more precise values could be obtained with 0 and P samples through the use of larger samples. However, the difficulties incident to the wet treatment of vary large amounts of ore encouraged the authors to limit their objective here to the question of the superiority of the leaching process. The peculiar variations between the proportions of leachable and unleachable platinum metals are difficult to explain satisfactorily. Undoubtedly difference in grain size is an important factor. ACKNOWLEDGMENT
Appreciation is also expressed to the Canadian Department of Agriculture, Science Service, for financial support and leave of absence given to I. Hoffman. LITERATURE CITED
(1) Hawley, J. E., Rimsaite, Y., Am. JIl.line?.aZogist38, 163 (1963). (2) Hoffman, I., Beamish, F. E., AXAL.CHEM.28, 1188 (1956).
RECEIVED for review December 9, 1955. Accepted March 5, 1956. Work supported by a grant from the National Research Council (Canada).
Determination of Polyphenol Oxidase Activity by Rotating Platinum Electrode LLOYD L. INGRAHAM W e s t e r n Utilization Research &a&
U. S. Department o f Agriculture, Albany 70, Calif.
Use of a polarized rotating platinum electrode enables polyphenol oxidase activity to be measured at various ascorbic acid concentrations, which is not possible with the commonly used chronometric method of Miller and Dawson. With this new method a continuous potentiometer record of oxygen consumption can be made.
T
HE catalytic activity of polyphenol oxidase, which is respon-
sible for enzymatic darkening of fruits, is commonly described by a chronometric method that measures the time required for oxidation of a given amount of ascorbic acid (3, 9) with catechol as substrate. During studies in this laboratory it became necessary to determine the activity of polyphenol oxidase a t various concentrations of ascorbic acid, which is impossible with the chronometric method. Because the rate of the reaction catalyzed by polyphenol oxidase is not constant, but falls off rapidly with time from reaction-inactivation (S), it is desirable to be able to measure the rate of reaction during the first few minutes. A polarized electrode for measuring the oxygen consumed in the reaction seemed to satisfy this requirement. The first attempt with a polarized electrode was made with an alternating polarizing and depolarizing potential (IO). Al-
though this method was stable and accurate, 5 or 6 minutes were required for the cell to reach equilibrium after addition of the enzyme or substrate to initiate the reaction. This method was developed for photosynthesis studies (b) and would probably serve well in any determination where the first 5 minutes of the reaction are not so critical as with polyphenol oxidase. However, a rotating polarized platinum electrode was found to reach equilibrium within 5 seconds. The use of a rotating platinum electrode to determine oxygen in solution is well known (6-7, 11); its use in determining polyphenol oxidase activity by measuring the oxygen consumed in the reaction is described here. EQUIPMENT
A schematic diagram of the equipment is shown in Figure 1. The reaction cell containing the rotating platinum electrode ia polarized from the potentiometer with 0.800 volt. The current is measured by measuring the iR drop across R with a recording potentiometer. The variable resistance, R, is a standard decade resistance box which may be varied from 0 to 2000 ohms. The recording potentiometer has a range from 0 to 10 mv. and chart speeds of 11/2 and 6 inches per minute. Switch S is added to prevent erratic motions of the recorder pen when the cell is filled or emptied. The cell, which contains 5 ml. of solution with the electrode inserted, is shown in Figure 2. The electrode is 2 mm. long and
