V O L U M E 2 4 , NO. 11, NOVEMBER 1 9 5 2 Table VII.
a b
Sensitivity of Determinations
Element Weight, Gram Arsenic 1 x 10-9 Phosphorus 3 x 10-0 Copper 3 x 10-6 Iron“ 5 x 10-8 Potassiumb 1 x 10-7 Sodiumb 1 x 10-7 Strontium 2 x 10-6 Calcium 2 x 10-6 Chromium 5 x 10-6 Sulfur 1 X 10-6 Irradiated in Canadian pile a t Chalk River, Ontario. Calculated for normal transit time from Oak Ridge.
1815 Boyd, G. E., ANAL.CHEM.,21,335-47 (1949). Brown, H., and Goldberg, E., Science, 109, 347-53 (1949). Calvin, M., Heidelberger, C., Reid, J. C., Tolbert, B. M., and Yankwich, P. F., “Isotopic Carbon,” pp. 283-8, New Tork, John Wiley & Sons, 1949. Dauben, W-.G., Reid, J. C., and Yankwich, P. E , ANAL.CHEM., 19, 828-32 (1947).
Goldberg, E. D., and Brown, H.. Ibid., 22, 308-11 (1950). Hahn, P. F., IND.ENG.CHEM.,AKAL.ED.,17, 45-6 (1945). Hillebrand, W. F., and .Lundell, G. E. F., “Applied Inorganic Analysis,” Kew York, John Wiley & Sons, 1929. Hudgens, J. E., Jr., and Cali, P. J., ANAL.CHEM.,24, 171-4 (1952).
Hudgens, J. E., Jr., and Dabagian, H. J., .Vucleonics, 10, Y o . 5, 25-7 (1952).
Kolthoff, I. M., Stenger, V. A., and Moskovite, B., J. .4m. were investigated by irradiating samples of pure sodium chloride and lithium sulfate monohydrate. The chemical separations were carried out as previously described. For the conditions under which the magnesium samples were irradiated it was found that 1 microgram of chlorine would increase the apparent sulfur content of a sample by 42.08 micrograms and the apparent phosphorus content by 0.005 microgram. One microgram of sulfur would increase the apparent phosphorus content by 0.015 microgram. The first reaction may be utilized to set an upper limit for the amount of chlorine present in a sample of pure metal if the sulfur content is very low and if the assumption is made that all S36 in the irradiated sample was produced from Cl36. Artivation analysis cannot be used to determine small amounts of sodium in magnesium metal, because of the formation of radioactive sodium from the magnesium sample according to the reaction Mg*4(n,p) S a z 4(1, 16). A magnesium metal sample with a sodium content of 10 p.p.m. according to spectrographic analysis gave results of 123 and 116 p.p.m. by activation analysis. LITERATURE CITED
(1) Amaldi, E., D’Agostino, O., Fermi, E., Pontecorvo, B., Rasetti, F., and SergB, E., Proc. Roy. SOC.,A149, 522 (1935).
Chem. Soc., 56, 812-15 (1934).
Leddicotte, G. W., and Reynolds, S. A,, Nucleonics, 8 , Xo. 3, 62 (1951).
Libby, F. W., J . Am. Chem. SOC.,62, 1930-43 (1940). RlacKenzie, A. J., and Borland, J. W., ANAL.CHEM.,24, 176-9 (1952).
hfeinke, W.W., “Chemical Procedures Vsed in Bombardment Work a t Berkeley,” U. S. Atomic Energy Commission, UCRL-432, AECD-2738 (Oct. 15, 1949). Riesler, W., Phvstk, Z., 45, 191-2 (1944). Scott, W.IT-., “Standard Methods of Chemical Analysis,” 1’01. I, 287, 5th ed., New York, D. Van Xostrand Co., 1939. Seaborg, G. T., English, S. G., Wilson, V. C., and Coryell, C. D , U. S. Atomic Energy Commission, MDDC-763, 15 (1946). Seren, L., Friedlander, H. N., and Turkel, S. H.. Phys. Rea., 72, 888-901 (1947).
Smales, A. h., and Pate, B. D., AXAL.CHEM.,24, 717-21 (1952). Taylor, T. I., and Havens, W.W., Jr., Nucleonics, 6, No. 4 , 5466 (1950).
Tobias, C. A , , and Dunn, R. W.,Science, 109, 109-13 (1949). Vay, K., Fano, L., Sott, M. R., and Them, K., Natl. Bur. Standards, Circ. 499 (1950). Willard, H. H., and Diehl, H., “Advanced Quantitative Analysis,” Kew York, D. Van Nostrand Co., 1943. Willard, H. H., and Goodspeed, E. W., IND.ENG. CHI:M., ANAL.ED., 8, 414-18 (1936). RECEIVED for review June 30, 1852. Accepted August 27, 1952
Microgravimet ric Methods for Platinum A Comparative Study A . P. BLACKMORE, M . A. RIARKS, R. R. BAREFOOT, AND F. E. BEAMISH University of Toronto, Toronto, Ontario, Canada
M
A S Y methods for the gravimetric determination of platinum have been recorded in textbooks of quantitative analysis ( 5 , 8-10). When the methods are used for the determination of small quantities of platinum, however, there is a lack of accuracy and precision. No detailed examinations of this problem have been reported until recently. Jackson and Beamish (6) studied the hydrogen sulfide precipitation, which was found to produce positive errors. This is also true of most of the organic precipitants which have been studied, with the exception of thiophenol (1). In this paper, an investigation of some other familiar and widely used gravimetric methods for platinum is described. REAGENTS
Standard platinum solutions were prepared by dissolving weighed quantities of reagent grade platinum sponge, which had been dried a t 110” C., in a minimum of aqua regia. The solutions were evaporated t o dryness on a steam bath and the resulting residues were treated three times with concentrated hydrochloric acid and evaporated each time t o dryness. The final residue mas taken up with a few milliliters of concentrated hydrochloric acid, diluted to 50 ml. with water, filtered, and diluted further t o a known volume. The solution contained 0.05 ml. of hydrochloric acid for each 10 mg. of platinum; the pH was 1.3. Spectrographic examinations of both the starting material and
some evaporated portions of the stock solutions showed the presence of negligible traces of copper and zinc. All other chemicals were of reagent grade. Reagent grade ammonium chloride was recrystallized from boiling water. The zinc used was in the form of zinc dust which was fine enough to pass a 200-mesh sieve. An analysis of the material showed that it contained 0.1% lead and 0.006% iron. In the experiments in which zinc was used, care was taken to include the same amount of zinc in a blank. A Coleman Electrometer was used for all pH measurements. DETERMIKATION OF PLATINUM BY REDUCTION WITH FORMIC ACID
Although formic acid has been used successfully for the gravimetric determination of platinum in the range 0.1 to 0.3 gram (2-4), it has been the experience of one of the authors that the results obtained m-ith small samples of platinum were not sufficiently precise. Experiments were performed in order to determine the most suitable conditions under which results of acceptable precision and accuracy may be obtained. The method outlined by Hillebrand and Lundell ( 5 )was followed. As the instructions being followed recommended “several hours’ digestion a t 90’ to 95’ C.,” experiments were carried out to determine the optimum period of digestion. Samples which contained 9.95 mg. of platinum yielded 9.35 mg. after a digestion
1816
ANALYTICAL CHEMISTRY A comparative study was made of a number of important procedures for the microgravimetricdetermination of platinum; in these, platinum is precipitated as ammonium hexachloroplatinate and reduced to the metal by means of zinc or formic acid. The work was undertaken to discover the cause of serious errors which occurred frequently in determinations of platinum. A method for the accurate determination of platinum over the range of 1 to 10 mg. by reduction with zinc is outlined. The ammonium chloride and formic acid methods for the determination of platinum in this range are shown to be unreliable and subject to large errors. This paper is the first which sets forth the results of a critical examination of these methods. The accuracy, the sources of error, and the ranges over which they can be applied successfully are stated.
period of 1 hour, 9.99 mg. after 3 hours, and 10.02 mg. after 5 hours. Heating for 3 hours or less produced precipitates which did not coagulate; the materials appeared to be colloidal. For this reason it was decided to use a digestion period of 5 to 6 hours. When this was done, most of the precipitates coagulated easily, and the supernatant liquid was clear and colorless. However, occasional samples did not coagulate well. Procedure. Measured amounts of standard platinum solutions were evaporated to sirups in 125-mI. conical beakers and taken up with water. Three grams of sodium acetate and 12 drops of formic acid were added; the final volume vias 60 ml. hfter the required period of digestion on a steam bath, the mixture was cooled and filtered through a 5.5-cm. Whatman S o . 42 filter paper. A small sector of 5.5-cm. filter paper was used to remove the last traces of solids from the beaker walls. The precipitate was washed with hot water until a test of the washings showed the absence of chloride. The precipitate was ignited a t 800" C., cooled, and weighed under conditions of constant humidity. This was repeated until a constant weight was obtained, A blank was subtracted. The filtrates were evaporated to dryness and were tested for platinum with stannous chloride ( 7 ) . The results are recorded in Table I (each entry represents the average of four determinations). Table I. Expt. NO.
Formic Acid Precipitation of Platinum
Platinum Added
N o . of Expts.
Platinum Recovered
Mo.
%
4 4 4 4
10.00 9.95 9.95 9.96 9.96 5.10 5.02 5.01 5.02 5.01 5.02 6.01 4.96 4.97 5.15 5.21
0.42 0.58 0.20 0.18 0.35 1 .oo 0.60 1.35 0.60 0.30 0.22 0.70 0.30 0.20 2.90 0.45 1.40 0.85
Mo. 1 2 3 4 5 6
9.95
4v. 5.00
7 8 9 10 11 12 13 14 15 1fi
Av.
4 4 4 4 4 4 4 4 4 4 4 4
5.15
5.05
Bv. Deviation
P t Filtrate Test Y
... ...
... ...
...
... ... ... 30
cipitation of platinum sulfide by forming a complex compound with chloroplatinic acid (6). It was concluded that the precision obtained by formic acid precipitations is not sufficient to justify its use in gravimetric determinations of small amounts of platinum. DETERMINATION OF PLATINUM BY PRECIPITATION A S AMMORIUM HEXACHLOROPLATINATE
The persistent use of ammonium chloride as a precipitant for platinum is due mainly to its value in separating platinum (and also iridium) from solutions of base metals and the other platinum metals (8). I t is well known that the recovery of platinum is incomplete. However, the literature contains no data on microdeterminations to indicate the precision or accuracy of the method.
Procedure. Standard platinum solutions were heated to approximately 80" C. on the steam bath and solid ammonium chloride was added to form a saturated solution. The mixtures were set aside in a cool place overnight, filtered through a Whatman Xo. 42 filter paper, and washed with either 20% ammonium chloride or 95% ethyl alcohol. The filter paper was folded compactly about the precipitate, placed in a porcelain crucible so that the threefold section of the paper was uppermost, and dried a t 70" C. Over a period of 24 hours the temperature was increased slonly to 230' C., the decomposition temperature of ammonium chloride. After several hours a t this temperature the crucible was transferred to a muffle a t 200' C., where the temperature was increased slowly and held a t 800" C. for 1 hour. Table I1 records some of the data obtained. The results indicate that larger samples produce a higher percentage recovery. All filtrates and washings showed strong tests for platinum.
... ...
1.50 150 75 85 100
Table 11. Precipitation by Ammonium Chloride Expt. No.
Platinum Recovered
Recovery
Mo.
Mo.
%
Ammonium Chloride Added G. 8.5 8.5 8.5 8.5
Wash Solution
20% WHiCl 4.81 96.3 97.4 20% NHiCl 4.87 2 98.5 20% NH4Cl 4.92 3b 95% E t O H 4.90 98.0 4 Av 4.87 97.5 Av. dev. 0.7% 9.82 98.7 3.5 20% X H L l 5 9.95 6 9.84 98.9 3.8 20% "4Cl 7 9.87 99.2 4.5 20% NHiCl 9.57 96.2 3.8 95% EtOH 8C 9.75 98.0 2.9 95% E t O H 9d 457. 9.77 98.2 Av. dev. 0.9% a Sample evaporated on steam b a t h until ammonium chloride began t o precipitate. Just enough water added to dissolve ammonium chloride. b Sample cooled below room temperature until ammonium chloride precipitated, and the? allowed t o stand a t room temperature until i t dissolved. C Platinum preclpitated from 41% ethyl alcohol solution. A large quant i t y of ammonium chloride precipitated which did not dissolve in ethylalcohol washings, so t h a t considerable ammonium chloride was present during i nition d Blatinim precipitated from 50% ethyl alcohol solution. Theoretical amount of ammonlum chloride requlred to form a saturated solution with 50% ethyl alcohol solution was added. No ammonium chloride precipitated. 1"
The average recover? of 10-mg. samples was good, but the poor precision indicates a compensation of errors. Both the precision and the accuracy of the recovery of 5-mg. samples were poor. Furthermore, without any discernible cause, significant amounts of platinum sometimes appeared in the filtrates (Kos. 9, 12 to 15). Washing the precipitates with a dilute solution of an electrolyte ( 8 , 4 ) instead of water did not prevent the occurrence of this phenomenon. It is the authors' opinion that the lack of precision is due to the variable size of the initial precipitate. A direct examination of the precipitates under high magnification might reveal the cause of their unpredictable character. A general investigation of this nature on platinum precipitates is anticipated by the authors. The presence of sodium chloride did not interfere in the reduction of platinum chloride solutions x-ith formic acid. However, under certain conditions, sodium chloride does affect the pre-
Platinum ddded
5.00
.
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
181’2
DETERMINATION OF PLATINUM BY REDUCTION WITH ZINC
It has been noted by various authors (5, 8, 9) that the reduction of platinum by zinc resulted in a contaminated precipitate. Because of this fact, zinc precipitation is not generally used as a gravimetric method-e.g., Schoeller and Powell’s procedure (8) for separating platinum from base metals produced results in error by as much as +lo%. Consequently it seemed advisable to examine the various factors which might influence the puritL- of the platinum precipitated by zinc. These data might also find applications in the separation of the noble metals from base metals, for which zinc has been widely used. Procedure. Hexachloroplatinic acid solutions were prepared for precipitation by diluting a measured volume of standard platinum solution to 85 ml. a i t h water and adding 1 ml. of 1 to 1 hydrochloric acid. The pH was 1.3. Then 270 mg. of a zinc suspension, which was prepared by quickly pouring 15 ml. of water over zinc dust in a dry beaker, was added to the platinum solutions heated on the steam bath. The zinc was added either directly from the beaker or from a microburet over periods ranging from 13 to 120 minutes. Both suspension and reaction mixture were agitated during the addition. Some mixtures were boiled gently for 1 hour; others mere left on the steam bath for several hours. The digested mixtures were filtered hot through a tared 2-ml. A2 Berlin filtering crucible and washed with 25 to 30 ml. of hot 1% hydrochloric acid, then with hot water. A small sector of 7-cm. Whatman KO.42 filter paper was used to remove the traces of solids adhering to the beaker walls. The residues were ignited a t 800” C. for several hours, cooled, then leached and ignited to constant weights. Leaching was performed between each weighing by drauing ten 1-ml. portions of hot 1% hydrochloric acid through each crucible after 45 seconds’ contact with the residue, followed by a hot water “flush.” A blank was subtracted in each case. Filtrates, wash, and leach solution after evaporation were treated for platinum n-ith stannous chloride ( 7 ) . The data from these experiments are reported in Table 111. Steam bath digestion resulted in finely divided precipitates which were difficult to manage, whereas a 1-hour boiling period produced a better coagulation. Because losses were significant, irrespective of the conditions under which platinum was precipitated, it was decided to avoid the leaching process. Alternative procedures for the purification of the precipitates were examined. The advisability of this was augmented by the results of a great many zinc precipitations made in the presence of base metal impurities-e.g., copper. Here the resulting high losses may have been due to the formation of a base metal-platinum couple. Effect of Ignition in Hydrogen. Spectrographic examination of the platinum residues revealed that the principal contaminant was zinc. The low melting and boiling points of zinc suggested the possibility of distilling the impurity from the precipitated platinum. Ignition in hj-drogen should allow volatilization, and qualitative experiments on a few milligrams of zinc confirmed this. Platinum was precipitated from heuachloroplatinic acid solutions, by adding zinc as described above. The precipitate was filtered through a 7-cm. Yo. 42 Whatman filter paper. The residues were washed by decantation, once with 5 ml. of hot 1% hydrochloric acid and twice with hot water, then transferred and n-ashed with a total of 60 to 70 ml. of hot water. The residue and paper were ignited slo\ily at 800” C. in a porcelain crucible, roasted overnight at this temperature, cooled, weighed, ignited
-
Table 111. Effect of Leachine Period of Addition of Zn .$fin.
Platinum Steam Bath Di- Boiling Filtrate Wash a n d gestion Digestion test leach test HOUTS Hours y Y
Platinuni Taken
.Wg.
Platinum Recovered .bfQ.
in an atmosphere of hydrogen for 1 hour a t Meker burner temperature, cooled in nitrogen and air, then weighed again. The process was repeated until constant a eight was achieved. The results, which are reported in Table IV, show that considerable zinc distilled from the platinum residues. Nos. 8 and 9 indicate that the volatilization was much less complete for relatively large amounts of zinc. It was concluded, however, that the process was a desirable substitute for leaching. Effect of Rate of Addition of Zinc. .4n attempt was made to produce a purer precipitate through a variation of the rate of addition of zinc. T o each platinum solution were added 220 mg. of zinc suspended in various volumes; 0.5-ml. portions of the suspension were added per minute over a 10- to 15-second period. The resulting mixtures were boiled for 1 hour and filtered, and the residue was washed, ignited, and neighed as described in th- previous section. Tests for platinum in filtrate and wash solutions indicated negligible losses in each case. The results are listed in Table V. The precision and accuracy of both experiments 3 and 4 were good; 2.8 mg. per minute was selected as the optimum rate on the basis of coagulation.
Table 1V. Ignition i n Hydrogen (Platinum taken = 9.97 mg.)
Expt.
Zinc Added
KO.
Final pH (Cold Filtrate)
MQ.
7. 6_
1.4 1.4 1.5 1.6 1.8 1.8 1.8 6.7 6.7
78 93 126 183 203 207 255 275
Platinum Filtrate Wash test test
Weight before Ignition
Weight after Ignition
I
Y
MQ.
MQ
20 30 10
10 10 10 5-10 5 5
10.05 10.08 10.06 10.32 10.59 10.75 10.79 12.66 12.27
9.87 9.87 9.83 9.91 10.04 10.10 10.15 11.34 11.05
‘8
10 5-10
.. ..
5
.. ..
~
~~
TabIe V.
Effect of Rate of -4ddition of Zinc
(Platinum taken = 9.97 mg.) Platinum No. of Rate of Addition Recovered Detns. .Mu./Man. MU. 8.4 10.17 2 4.2 10 06 4 2.8 10.00 10 00 3 2 . 11 9.98
Expt. SO.
1 2 3 4
4V. Dev. 70 1 1 . 110 0 0.50 0 . 2200 0 . 117 7
Separation of Zinc from Highly Contaminated Precipitates. Because excess zinc resulted in positive errors, and on the other hand excessive boiling of precipitated platinum in acid media resulted in dissolution of platinum, it seemed desirable to learn the relationships between the final pH of the precipitating medium, the amount of zinc added, and the degree of positive error. The procedure used was similar to that described above under leaching, except that the zinc was added over a period of 0.5 to 0.75 hour and the mixture was boiled gently for 1 hour. The p H of the cold filtrate was measured. The results are recorded in Figures 1 and 2. Solid zinc must have been present a t completion of reaction in all cases where more than 250 mg. of zinc was added. The term “excess zinc” refers to this condition. The addition of large excesses of zinc produced residues which could not be purified adequately by volatilization ( S o s . 8 and 9, Table IV). This Detns. No. of Av, suggested the possibility that during ignition zinc did not remain a mechanically R 1 .. mixed impurity. Experiments were con1 ducted to determine the influence of plati4 1 1 2 0.7 num on the volatilization of zinc. 1 1
.. ..
4
3.0
1
EXPERIMEXT 1. A mechanical mixture of platinum and zinc was prepared by igniting approximately 20 mg. of platinum
ANALYTICAL CHEMISTRY
1818 to constant weight in a crucible, and sprinkling over this about 2 mg. of zinc, tapping the crucible gently in order to mix the components. After ignition in the muffle overnight a t 800’ C., the mixture was reduced for 1 hour in hydrogen, cooled, and weighed. The process was repeated to constant weight. In other experiments quantities of zinc were treated in the same manner with no platinum present.
Table VI.
Separation of Zinc from Highly Contaminated Precipitates Length of hluffle Ignition Hours
Expt. No.
Weight of Platinum
Weight of Zinc
Zinc Volatilized
Mg.
Mg.
70
The results of these experiments given in Table VI, show that platinum does interfere with the volatilization of zinc. This could be the result of the formation of platinum-zinc “alloys.” 76 -
tion 5.5 nig. per ml. by pouring water quickly over zinc dust in a dry beaker and stirring immediately. To the platinum solution on the steam bath add 100 mg. of zinc, dropwise, in 0.5-ml. portions over a 10- to 15-second period each minute, stirring both suspension and reaction mixture in the interim. Add another 100 mg. of zinc at approximately twice this rate. Boil the resulting mixture gently for 1 hour, filter hot, and wash the residue once by decantation with 5 ml. of hot 1%hydrochloric acid, followed by a total of 60 to 70 ml. of hot water or preferably the same volume of hot 1% ammonium chloride solution. A small piece of filter paper moistened with 1% hydrochloric acid cleanses the beaker walls efficiently. Ignite the residue and paper slowly to 600’ C. in the muffle, and continue heatin a t this temperature for 1 hour. Cool and ignite in an atmosptere of hydrogen a t Meker burner temperature for 1 hour, cool in nitrogen and air, and Tveigh as metallic platinum. Subtract a blank which is determined by adding zinc to a hydrochloric acid solution of pH 1.3, heating until all visible reaction has ceased, then boiling, filtering, and igniting as described above. Samples which contain 1 t o 5 mg. of platinum are treated as in the procedure described above, except for the following changes. Add a suspension of 50 mg. of zinc in 20 ml. of water over the period of 1 hour. Boil the mixture for 15 minutes, add 0.5 ml. of 3 N hydrochloric acid, and continue to boil the mixture for 45 minutes. The rest of the procedure is the same.
5-
I, a4-
Q
z
= 3 9 -
1-
The results of a number of determinations are shown in Table VII. A spectrographic examination of the recovered platinum showed the presence of only insignificant amounts of impurities on comparison -with spectra of the starting material and the analysis blanks.
Table VII. Expt. so. I
I
1
2 3
2 4
4
b
5 6 7 8 9 10
Bp1
12 13 14 15
$ 3 OL
11
100
150
900
950
300
Precipitation of Platinum by Zinc Platinum Recovered
Pt. Filtrate and Wash Test
Me.
Me.
Y
1.00 1.00 1.99 1.99 3.97 3.97 3.97 3.97 5.96 5.96 9.97 9.97 9.97 9.97 9.97
1.00 0.99 1.99 1.99 3.96 4 00 3.97 3.96 6.03 5.98 9.98 9.92 9.92 9.92 9.97
5 5 5 5 15 15 10 20 10 10 10 10 10 10 10
Platinum Taken
SUMMARY
ZINC, MG.
Figure 2. Correlation of Per Cent Error of Recovery of Platinum with Quantity of Zinc Added
RECOMMENDED PROCEDURE
The following procedure gave the most accurate and precise recovery of platinum from samples containing 5 to 10 mg. of platinum. Evaporate the hexachloroplatinic acid solution containing approuimately 10 mg. of platinum to a sirup on the steam bath, add 0.5 ml. of concentrated hydrochloric acid, and dilute to 85 ml. with water. Prepare an aqueous suspension of zinc of concentra-
The precipitation of platinum from chloride solutions by reduction to the metal by means of zinc has been successfully applied in the accurate microgravimetric determination of platinum. Procedures which employ formic acid and ammonium chloride for the quantitative precipitation of platinum in the same range were not satisfactory. ACKNOWLEDGMENT
The authors are indebted to J. G. Fraser, who aided in the research. This work was supported in part by a grant from the Kational Research Council (Canada).
V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2 LITERATURE CITED (1) Currah, J. E., McBryde, u’.A. E., Cruikshank, A. J., and Beamish, F.E., IND.ENQ.CHEM.,ANAL.ED.,18, 120 (1946). (2) Gflchrist, R., Bur. Standards J . Research, 20, 745 (1938). (3) Gilchrist, R., Chem. Reas., 32, 322 (1943). sot., 57, 2565 (4) Gilchrist, R,,and \Tichers, E., J , A ~ them, , (1935). (5) Hiliebrand, R-. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” pp. 289, 290, New York, John Wiley & Sons, 1929. (6) Jackson, D. S., and Beamish, F. E., -$SAL. CHEM.,22, 813 (1950).
1819 (7) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 358, New York, Interscience Publishers, 1944.
(8) Schoeller, W.R., and Powell, A. R., “Analysis of Minerals and Ores of the Rarer Elements,” pp. 254, 280, London, C. Griffin & Co., 1940. (9) Scott, W.W.,“Standard Methods of Chemical Analysis,” 5th ed., Vol. I, New York, D. Van Nostrand Co., 1939. (10) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” Vol. 11,pp. 137, 145, New York, John Wiley & Sons, 1942. RECEIVED for review February 11, 19.52.
Accepted August 32, 1952.
Determination of Trace Oxygen in Gases D. D. WILLIAMS, C. H. BLACHLY, AND R. R. MILLER Naval Research Laboratory, Washington 25, D. C. .4rapid and accurate method for the determination of oxygen in inert cover gases was required for experiments involving oxygen-sensitive materials. The method developed is spectrophotometric, utilizing the oxygen-pyrogallol complex. The apparatus is constructed in a manner which allows for direct calibration. Blank, or reference solution, values are determined for each analysis. Equilibrium conditions are established with the aid of a 70” C. controlled temperature bath. Beer’s law is obeyed for a range of 0 to 800 micrograms of oxygen. The method has been applied to the determination of traces of oxygen in nitrogen, argon, helium, and hydrogen.
R
E C E N T projects at this laboratory involving high purity inert gas “blankets” necessitated analysis of samples for trace oxygen on a routine basis. Various difficulties were encountered in applying the methods proposed in the literature. A variety of colorimetric methods based on oxidation of ferrous iron and subsequent determination of ferric iron color complexes is reported by Mellan (4). Most of these complexes were found t o be somewhat unstable and nonreproducible. The method of Shaw ( 5 ) showed promise, but vas rather involved for routine application, as was that of Hand ( 3 ) . The latter was, however, the most sensitive and consistent of all methods tried. One
significant lack of all methods was a means of determining a reliable reagent and procedural blank. Blanks could be determined on separate samples, but the variance between blanks was greater than the per cent of oxygen in the sample in question. Most of the methods investigated involved complicated apparatus and subsequent difficulty in applying the method routinely. IOC
0 - ixio-2gms O X Y G E N o-ix 1 0 - 3 g m s O X Y G E N -3X ~ o - ~ g m OXY s GEN --CORNING F I L T E R #50
8C
10 A S P I R A T O R W 0
z a
E-
6C
z
v)
2
a 0:
t-
TO SAMPLE SOURCE
4c
8 2c
C
LJ Figure 1. Apparatus
50
450 550 650 WAVELENGTH ( M k ) Figure 2. Transmittancy Curves
75c
The method here described was devised with an eye to accurate routine determination of traces of oxygen in gas samples. “Blanks” are determined for each run and represent true reagent and operational values. The method is colorimetric and is based on the oxidation of an alkaline solution of pyrogallic mid. The pyrogallol-oxygen complex undergoes a rearrangement before a ronstant optical density is reached. Willstatter and Heiss ( 7 ) have postulated that the oxidation of pyrogallol progresses through several intermediates. The authors have found
