Spectrophotometric Determination of Pa I lad ium with Azide RAY G. CLEM and E. H. HUFFMAN Lawrence Radiation Laboratory, University of California, Berkeley, Calif.
b A spectrophotometric method for the determination of palladium utilizes the absorptivity of the tetraazidopalladium(ll) complex at 315 mk after extraction into n-butyl alcohol and back-extraction into water. The main factors which control the color development and extraction are discussed and the effects of some common foreign ions are shown. The method offers advantages in sensitivity, color stability, speed, and accuracy. The standard deviation over the optimum range from 1.3 to 4.6 p.p.m. is 0.03 p.p.m.
A
of colorimetric methods are known for the determination of small amounts of palladium (1-4, 9, I d ) , and three recent sensitive methods allow the simultaneous determination of palladium and platinum (8, 10, I S ) . In general, these methods have one or more of the following drawbacks: slow development of color, fading of the color within a short time so that standards must be made frequently, low sensitivity, oxidation of the colorforming reagent by air, narrow pH range for extraction and color development, and low tolerance for other cations. This paper presents a simple, rapid, and accurate extraction-stripping method for the spectrophotometric determination of palladium by azide ion, with an optimum range of 1.3 to 4.6 p.p.m. for a 1.00-cm. cell. The color is formed immediately and the calibration standards are stable for months. I n contrast to commonly accepted methods of comparable sensitivity, this method can tolerate the presence of centigram quantities of several of the more common base metals and milligram to decimilligram amounts of gold and the platinum metals, except platinum itself. Without extraction] nearly all of the cations listed in Table I1 interfere in milligram amounts or less, Silver, lead, and platinum interfere and must be removed prior to analysis. Spectrophotometric studies by Sherif and MichaiI (15) of the azide-palladium system in the visible spectral region revealed that the diazido, tetraazido, and possibly the inonoazido palladium (11) species exist in water or in water86
NUMBER
ANALYTICAL CHEMISTRY
acetone mixtures. The molar absorptivity reported does not appear to merit development of an analytical method using the visible spectrum. Investigations in this laboratory in the near-ultraviolet spectral region have been reported to show the probable existence of the penta- and hexaazido palladium(I1) species in ethanol ( 5 ) . The tetraazido species, used as the basis for the analytical method presented here, exhibited an absorbance peak a t 315 mp with a high molar absorptivity in aqueous solutions. I t can be extracted into n-butyl alcohol from acidic azide solutions and back-extracted into neutral azide solutions to aeparate palladium from many common interfering cations. Other methods of high sensitivity require more timeconsuming separations of trace palladium by ion exchange, carrier precipitation] or extraction when large amounts of interfering cations are present ( 2 4 ) . EXPERIMENTAL
Apparatus. All absorbance measurements and spectrum scans were made with a Beckman DU spectrophotometer equipped with a multiplier phototube, using matched 1.00cm. silica cells. T h e pH measurements were made with a Beckman Model 76 expanded scale pH meter. Separatory funnels with Teflon stopcocks were used to avoid contamination of the organic solvents with grease. Reagents. Reagent grade chemicals were used unless otherwise specified. .4 0.1000-gram quantity of shiny, spectroscopically pure palladium metal sheet was dissolved in hot aqua regia and boiled repeatedly with concentrated hydrochloric acid until all nitrate was expelled, as shown by the brucine test (6). After evaporation to near dryness, the residue was taken up in a minimum amount of hydrochloric acid and made to 100 ml. .A 'working solution containing 25 pg. of palladium per ml. was obtained by dilution of the stock solution. Practical grade sodium azide, with a 98.5% purity as determined by the Volhard method, was recrystallized by the Pepkoivitz procedure ( I d ) and a 0.5031 solution was prepared. h buffer of pH 6.5 was prepared from disodium phosphate heptahydrate and monosodium phosphate monohydrate, 0.551 in each. The pH may vary from
6.3 to 6.6, depending upon the exact wat,er content of the salts used, hut this variation is of no consequence. The common cations used in the interference studies were in the form of either chloride or sulfate Ralts. The anions were in the form of either sodium or potassium salts. Platinum metals. with a purity of 99.8% or higher, were made soluble by a chlorination procedure (7'). Preliminary Tests. At the outset it was observed that the yellow palladium azide complex, which forms immediately, could be extracted into n-butyl alcohol and isoamyl alcohol from acidic azide solutions, b u t only in the case of n-butyl alcohol could t h e color be back-extracted into neutral aqueous azide solutions. T h e isoamyl alcohol resisted stripping of palladium even a t pH 11 to 12. Among other solvents tried as extractants without success were ethyl and diisoyropyl ethers and diisobutyl and methyl ethyl ketones. The solubility of n-butyl alcohol in water is 10% and it forms an emulsion, when shaken with water alone, which separates very slowly. The addition of a high concentration of an indifferent salt suppresses the water solubility and effects a rapid pha>e separation. Recommended Procedure. Evaporate samples containing 40 t o 115 pg. of palladium, from which excessive amounts of interfering elements have been removed, to near dryness twice under a heat lamp wit'h 2 to 3 drops of concentrated sulfuric acid or 6 to 8 drops of 85% phosphoric acid, and cool. The selection of acid depends upon the cation solubilities involved. Transfer the solution t'o a 125-nil. separatory funnel and add 2 nil. of 0.50M sodium azide and 5 ml. of either 1. O M sodium bisulfate or 1. O N monosodium phosphate. .Idjust the volume to 10 to 15 nil. and test the pH, which should be below 2.7, with pHydrion paper. -1dd 10 ml. of n-butyl alcohol and extract the palladium by shaking vigorously for 1 minute. Discard the aqueous layer. Rinse the ertract with 10 to 15 nil. of an aqueous solution containing azide, salt, and acid as above. Add 1 ml. of 0.50.1f sodium azide to the organic phase, swirl briefly, and then add 10 ml. of pH 6.5 buffer. Back-extract the palladium by shaking vigorously for 1 minute and drain the aqueous phase into a 25-ml. volumetric
0.400
1
sponding to a 1.3% relative error. study of deviation on 17 samples varying from 2 to 4 p.p.m. gave the results shown in Table I.
IO0 +
5
0.300
COLOR DEVELOPMENT
0)
V
E
0 L1
$ 0.200 v1
n
a 0.100
300 320 340 Wavelength - m p Figure 1 . Absorption spectra of analytical species and reagents 0
0 A
0
5 p . p m Pd in n-BuOiH; pH 2.25, aq. phase 5 p.p.m. Pd in n-BuOH; pH 1.81, aq. phase 2 p.p.m. Pd, back extracted, p H 6.5. 1.OM N O N S 1.OM phosphate
flask. Rinse the funinel once by shaking for 5 seconds with 3 to 5 ml. of water and once with 1 nil. of 0.50M sodium azide plus 5 ml. of t'he buffer solution. Add these rinsings to t.he flask and make to volume. T o each of five 125-ml. separatory funnels add 1 to 5 ml. of the solution contlaining 25 pg. of palladium per ml., then continue as described above for the extraction and stripping. Against a blank containing all components but palladium (the blank need not be extracted), obtain and plot the absorbances of the five solutions us. the concentrations. The same calibration curve is obtained regardless of whether sulfuric or phosphoric acid is used in the preparation of the solution for extraction. If it is known that no interfering ions are present, the color may be developed without extraction of either the sample or the standards for calibration; the absorbance values will be about 3% higher than for the extracted-stripped solutions. RESlllTS
Spectrum scans are shown in Figure 1 for the palladium azide complex in water and in n-butyl alcohol extracts, as well a5 for the reagents used in the color development. The concentrations of azide and of phosphate scanned are 25 and 1.7 times, respectively, the concentrations present after color development. 13eer'i law is obeyed by the aqueous solution ovier the concentration range from 1 to 5 p.p.m. The optimum range for 1.00-mi. cells i q from 1.3 to 4.6 p g . of palladium per ml. SeiTeral solutions prepared over the range of 1 to 5 p.p.m. gave an absor1)tivity of 0.155 1) 1 ) m - l cm.-l with a standard deviation of 0.002, corre-
The following tests were made with 100 pg. (4 p.11.m.) of palladium, except in the last series, as noted, and the effects were obtained by developing the color without extraction. Effect of Azide Concentration. When t h e azide concentration was varied from 0 0 0 1 X to 0 , l X in small increments, the color intensity increased rapidly u p to 0.008M azide, after which it became constant. Three solutions, with azide concentrations of O.OOlM, O.OlM, and O.lJI, were allowed to stand for 2 months. The absorbance values of the last two solutions did not change during this time but the O.OOl,\I azide solution contained many golden flaky crystals, and the absorbance of the supernatant solution did not differ from that of the blank. Half-molar solutions of sodium azide were effective for color development for a t least 2 months after preparation. Effect of pH. Figure 2 shows t h e effect on the color development, using 0 01.1.1 azide, of varying the p H from 3 3 to 10 0 Hydrazoic acid is weak (pK = 4 . 7 2 ) and a t low pH values the azide ion concentration is suppressed, giving low percentages of maximum color development. At high pH values the palladium complex probably hydrolyzes, thus accounting for the suppression of color development. The phosphate buffer system was chosen because it has a high buffer capacity in the optimum pH region from 4.6 to 7.3. Effects of Chloride and Phosphate. T h e tetrachloropalladium(11) complex, which has a dissociation constant of 6 x lo-" ( I ? ) , is well known, and both higher and lower chloro complexes have been reported (16). Phosphate apparently forms a highly dissociated complex with palladium, as shown by the following experiments. Palladium ion (as perchlorate) exhibits an absorbance peak a t 380 mp with a molar absorptivity of 86. The addition of phosphoric acid to 4 X 10-3M palladium perchlorate in 0.5M perchloric acid, holding the ionic strength a t 1.0 with sodium perchlorate, causes shifts
Table
Pd added, p.p.m. Pd found, p.p.m., av. Range No. of samples Standard deviation, p.p.m. Relative std. dev. Std. dev. pooled, p.p.m.
I.
90
-k 0
0,
5
80
'0 L
-0 E
7c
t
C
a
$
a
6C
50 I
I
I
I
I
I
4
5
6
7
0
9
PH Figure 2. Effect of p H on color development
in the absorbance. .In increase in absorptivity is observed in O.OL1.1 phosphoric acid solution but the absorbance peak remains a t 380 mp. A peak shift becomes perceptible when the concentration of phosphate is increased to O.lM, and the peak shifts to near 390 mp a t a phosphate concentration of 0.5M, with a corresponding molar absorptivity of 106. These changes are shown in Figure 3, except for the O.OL1.1 phosphoric acid solution. Although phosphate and chloride both form complexes with palladium(t1) phosphate does not interfere with the azide color development under the conditions for analysis and chloride interferes only slightly. With constant concentrations of 2 1i.p.m. of palladium, 0.04M azide, and 0.6M phosphate buffer, a t pH 6.5, the chloride concentration was varied from 0.016M to 0.64M. C p to a chloride concentration of 0.08M the absorbance was the same as in the absence of chloride, but a t , or above, 0.16W chloride there was a constant negative deviation of 3Yc. The same absorbances were obtained in the absence of phosphate, with the pH adjusted to the region of the plateau (Figure 2). Gross variations in ionic strength thus have no effect on the absorbance. ~
Reproducibility
2.00 2.03 2.01-2.04 5 0.046 2.3 0,032
3.00 3.00 2.95-3.07 5 0.044 1.5
4.00
3.94 3,77-4.04 7 0.029 0.7
VOL. 37, NO. I , JANUARY 1965
87
a 0.400 0 C
0
n L
0
2 0.350 a
0.300
380 390 Wavelength - m p
370
Figure 3. Effect of phosphate on absorbance-wave length curves of 4 X
10-3M Pdf2 0 0
A
0.5M H3PO4 0.1M H3P04 O.OM HaPo4
EXTRACTION
Palladium was held a t 100 p g . in all extraction studies except those for determination of the extraction range. Color intensity was obtained after extraction and back-extraction, as described under the recommended procedure, unless otherwise specified. Effect of pH. Figure 4 shows t h e effect of p H on extraction into n-butyl alcohol when the volumes of aqueous and organic phases were 10 ml. each, and the concentrations of azide and monosodium phosphate were 0.1 M and 0.5M, respectively. T h e p H was adjusted by dropwise addition of 85% phosphoric acid or 10M sodium hydroxide. Ai similar curve was obtained when sodium sulfate and sulfuric acid were used. The pH of the aqueous phase was obtained each time after extraction and the absorbance values were read either for the back-extracted color, if more than 50% of the color was
Table
II. Tolerance for Foreign Cations
Foreign cation Ala Fe
co
Ni
cu Zn
.4g b Cd AU
Pb Ru Rh OS
Ir Pt
Max. mg. giving less than 2.3Yc error 10 >50 10 40 20
>50 50
0.5 (10 0.5 6.0 0.8 1 Tichail, K. F., J . Inorg. Xucl. Chem. 25, 999 (1963). (16) Sundarum, A. K., Sandell, E . B., J . Am. Chem. SOC.77, 855 (1955). (17) Templeton, I). H., Watt, G. W.. Garner, C. S., Ibzd., 6 5 , 1608 (1943).
RECEIVEDfor review August 28, 1964, Accepted October 22, 1964. Work done under the auspice of the U. S. Atomic Energy Commission, AEC Contract W7405-eng-48.
Correlations and Anomalies in Mass Spectra Lactones W. H. McFADDEN, E. A. DAY,’ and M. J. DIAMOND Western Regional Reisearch laboratory, Western Utilization Research and Development Division, Agricultural Research Service,
U . S. Department of Agriculture, Albany, Calif. The mass spectra of 17 lactones are presented. Laciones with an aliphatic chain Cz or greater a t the point of ring closure show significant ion current due to loss of the hydrocarbon chain. For 5- and (5-membered rings these ions will geneirally be the most abundant. For larger rings it defines the point of ring closure or the number of carbons in, or at, the ring. With a chain Ca or greater, lactones rearrange to give ions due to loss of H20; with four or more carbons in the chain, an ion current due to loss of a second water molecule is common. These rearrangements aid in identification, particularly with low intensity spectra that may not yield a significant amount of the parenf ion. Rearrangement ions isobaric with esters were not observed in any abundance except for the terminally-closed cyclopentadecanolide which gives several such ions. Knowledge 0.F these anomalous rearrangements is necessary to avoid confusion in analysis of unknowns.
L
AXALOGS of aliphatic hydroxy acids are amenable to identification by infrared spectrophotometry ( I S ) , gas chromatography ( l e ) , and chromatography of derivatives (3, 8). The utility of combinations of these methods for unequivocal identification decreases, however, as one approaches the concentration levels of flavor constituents in foods. A case in point is milk fat, in which the lactones ACTONE
’ O n leave from -the Department of Food Science and Technology, Oregon State University, Corvallis, Ore.
occur in the parts-per-million range (3, 8). To identify these compounds it is necessary to resort either to elaborate, large scale extraction and distillation or to micro-qualitative methods such as mass spectrometry. The latter approach is increasingly attractive, especially with recent developments in combination of capillary gas chromatography with rapid-scan mass spectrometry (4, 6, 10, 11, 1 5 ) . The mass spectra of lactones have been discussed only briefly in previous work ( 2 , 7 , ) , which did not describe effects encountered as the side chain increases. Because these effects have important analytical implications, the mass spectra of 17 available lactones are presented. EXPERIMENTAL
The y-valerolactone was supplied by the Quaker Oats Chemical Co. The other ylactones were obtained from K and K Laboratories except for y-palmitolactone and y-stearolactone which were synthesized by Dr. J. S. Showell of the Eastern Regional Research Laboratory, Cnited States Department of Agriculture. &Lactones were supplied by Unilever Company, England. The 12-stearolactone was prepared from 12-hydroxystearic acid according to the procedure of Stoll and Gardner (14). The t-caprolactone was obtained from K and K Laboratories and the w-lactone, cyclopentadecanolide, was obtained from Aldrich Chemical Co. Impurities in the ylactones (up to CIJ were less than 2% by gas chromatography. The cyclopentadecanolide was checked by gas chromatography, infrared (IR) and nuclear magnetic
resonance ( S M R ) to confirm the struc ture and no significant impurities were detected. The other lactones were used as obtained. The mass spectra were obtained with a Bendix Time-of-Flight mass spectrometer, Model 12, operating with continuous ionization. Samples were introduced from a stainless steel inlet system a t 150” C. RESULTS A N D DISCUSSION
The mass spectra of the lactones are presented in Figures 1 to 3. A tabulation of these data will be circulated in the ASTM E-I4 Committee file of uncertified mass spectra. Background corrections were made in the usual way except when the air background fluctuated significantly. I n such cases, the established Oz/S2 ratio was used to correct the mass 28 ionization. Determination of mass 28 was not possible for y-stearolactone. Parent Ion. Only a small traction of the total ionization is collected as the parent ion. Table I shows t h a t the proportion of ions remaining as t h e parent ion decreases t o a minimum for Cloor CI1lactones and increases beyond that. This small percentage is sufficient with normal sample amounts to obtain the parent mass, but with small samples such as may come from capillary chromatographic columns the parent ion would not always be detectable. Ions Due to Loss of the Side Chain. The most important feature of the mass spectra of the lactones i i the amount of ion current due to the lactone ring-i.e., loss of the side chain. VOL. 37, NO. 1 , JANUARY 1965
89
