Spectrophotometric Determination of Platinum with 3,4-Diaminobenzoic Acid LARRY D. JOHNSON' and GILBERT H. AYRES The University o f Texas, Austin, lex.
b Platinum in solution reacts with 3,4diaminobenzoic acid when the mixture, at pH 9.8 to 12.2, is heated; the blue to blue-green solution has maximum absorption at 715 mg. The absorbance is not highly sensitive to reagent concentration, pH, or normal room temperature fluctuations. Developed samples are stable for at least 4 hours. The system conforms to Beer's law over the optimum concentration range of 0.35 to 1.20 p.p.m. of platinum for measurement at 1 .OO-cm. optical path. The only ions which interfere enough to require separation are palladium, osmium, and ruthenium. Separation procedures are easily applied. Spectrophotometric solution studies show that 3,4-diaminobenzoic acid reacts with platinum to form a 2 to 1 complex. Ion exchange studies show that the complex is anionic in alkaline solution and cationic in acid solution. Products thought to b e potassium salts of the anion and chloride salts of the cation were synthesized, but could not b e obtained with consistent composition.
T
excellent review articles (3, 4) have summarized the state of platinum metals spectrophotometry up to early 1964. At that time, the tin(I1) chloride ( 2 ) and the p-nitrosodimethylaniline ( I S ) methods were preferred. Since February 1964, three new spectrophotometric methods for platinum have appeared (6, 11, 16), but have not yet been subjected to the test of time and critical evaluation. Because relatively few spectrophotometric procedures for platinum have been published, more work in this field was desirable. Thirty-five reagents were investigated briefly for ability to form colors with platinum. During this study additional facts not mentioned in the original paper (15) were learned about o-phenylenediamine; by briefly heating an acidic sample, cooling, and then making it strongly alkaline, an intensely colored purple precipitate was formed. Very similar behavior was shown by solutions of platinum and 3,4-diaminotoluene. Several reagents containing two amino groups adjacent t o one another on an HREE
1 Present address, Chemstrand Research Center, Durham, N. C.
121 8
ANALYTICAL CHEMISTRY
aromatic ring, and also containing some feature to increase water solubility, were tested. Platinum formed watersoluble colored complexes with 3,4diamino-5-hydroxypyrazole sulfate, 2,3 - diaminopyridine, 4,5 - diaminopyrimidine, and 3,4-diaminobenzoic acid, A colored complex formed with oaminobenzenethiol was insoluble in aqueous solutions, but soluble in many common organic solvents and thus was extractable. From the preceding group of compounds 3,4-diaminobenzoic acid (DBA) was chosen for further study as a spectrophotometric reagent for platinum, EXPERIMENTAL
Apparatus. Absorbance measurements a t varying wavelengths for the temperature effect studies were recorded with a Beckman Model DK-1 spectrophotometer equipped with a cell thermostat. All other absorbance measurements at varying wavelengths were recorded with a Cary Model 14 spectrophotometer. Absorbance measurements a t a fixed wavelength were made with a Beckman Model DU quartz spectrophotometer set a t high and constant sensitivity. Matched silica cells of 1.00-cm. optical path were used. A Beckman Zeromatic pH meter equipped with a saturated calomel-glass electrode system was used for all pH measurements. All analytical weighings were made with a Mettler Type H5 analytical balance or a Mettler Type 115 microbalance. Reagents. 3,4-Diaminobenxoic acid was obtained from the Aldrich Chemical Co. arid used as received. Solutions were prepared by dissolving the desired amount (usually 0.3 gram) in 10 ml. of dimethylformaniide and diluting to .50 ml. with distilled water. N,N-Dimethylformamide (DMF), Eastman Yo. 5870, was used as received. tetrachloroplatinate(I1) Potassium was obtained from the Fisher Scientific Co. It had been checked for platinum content (1) and found to contain the theoretical amount. Standard platinum(1V) solution was prepared by dissolving 1.OOOO gram of grade 1 platinum thermocouple wire, 99.99% pure, in hot aqua regia. The resultant solution was taken almost to dryness, a small amount of hydrochloric acid was added, and the solution was again taken to dryness. This treatment was repeated three times in order to
destroy any nitroso complexes. After the final evaporation, 10 ml. of hydrochloric acid was added and the solution was made up to 1 liter with distilled water. More dilute platinum solutions were prepared as needed by volumetric dilution of this stock. This procedure has been shown by Ayres and McCrory (1) and Janota (9), as well as earlier workers in this laboratory, to yield solutions of the concentration calculated from the weight of platinum taken. To check against mechanical loss or some other error in the dissolution step, two standard platinum solutions were prepared by exactly the same procedure and found to give exactly the same spectrophotometer readings when carried through the standard analysis scheme. All other reagents were ACS reagent grade. Recommended Procedure. A sample containing 35 to 120 jtg. of platinum in a volume of 25 ml. or less was placed in a 100-ml. borosilicateglass volumetric flask; 2 ml. of the DBA reagent solution was added and the pH of the solution was adjusted below 3 in order t o avoid hydrolysis effects during the heating step. The flask containing the solution was floated for 15 minutes in a large beaker of boiling water, then removed and cooled to room temperature. At this point the sample was light amber in color and sometimes contained a precipitate. The sample was made alkaline by adding an excess of either 0.1M or l d l sodium hydroxide, depending upon the amount of acid prcsent. The sodium hydroxide was added from a buret until a green color appeared in the solution. An amount of sodium hydroxide equal to one third of the amount already added was then run in. At this point the sample was blue-green and the precipitate was completely dissolved. The sample was made up to 100 ml. with distilled water, and its absorbance was measured within 1 hour a t 715 mjt. The reagent blank was negligible a t 715 mjt, but was employed as an added precaution. RESULTS A N D DISCUSSION
Calibration, Range, and Sensitivity. Data for calibration and a check for conformity to Beer's law were obtained by measuring the absorbance at different concentrations of platinum after treatment by the recommended procedure (Table I). Each absorbance
value given is the average of three closely agreeing replicates. The system follows Beer's law over the calibrated range, The optimum concentration range for measurement at 1.00-cm. optical path is 0.35 to 1.20 p.p.m. of platinum. The mean specific absorptivity for the calibrated range is 0.585 p.p.m.-l cm.-l, corresponding to a molar absorptivity of 1.14 X lo5 liter mole-' cm.-l Reproducibility. Twenty-three identical samples, each with a final platinum concentration of 0.80 p.p.m., were treated according to the recommended procedure, and their absorbances were measured. The mean absorbance was 0.468, with a standard deviation of 0.002 absorbance unit. This study was an attempt to measure the inherent precision of the method and was performed under ideal and constant conditions; the precision obtained in actual analyses of samples will probably be somewhat lower. Effect of pH. When the p H was varied by the use of appropriate buffers, no appreciable absorbance (at 715 mp) was produced a t p H below 4. Between pH 6 and 8 the absorbance increased rapidly, and leveled off at a constant high value between pH 9.8 and 12.2. The recommended procedure produced samples of p H about 11.5. Development Time. The time required for full color development in the hot water bath varied with sample volume, sample concentration, DBA concentration, and pH. For the conditions used in the recommended procedure, full color development took about 10 minutes. Continued heating up to 30 minutes produced no further change in absorbance. Effect of Temperature. After color development b y the standard procedure, absorbance was measured a t three temperatures (25O, 42O, and 75') in a thermostatically controlled cell compartment. With increasing temperature only a slight decrease in peak absorbance was produced. Normal room temperature fluctuations were without measurable effect. Effect of Reagent Concentration. Samples containing 100 pg, of platinum (IV) were developed by the recommended procedure, except that the concentration of DBA was varied. Maximum color development required about a 50-fold molar excess of reagent. Better precision on multiplicate samples could be attained by the use of an even larger excess of reagent. The 2 ml. of DBA solution used in the recommended procedure supplies a ratio of 128 to 1 if the sample contains 120 pg. of platinum. Effect of D M F Concentration. When the recommended 2 ml. of DBA solution was used, up t o 10 ml. of D M F had little or no effect on the absorbance. With smaller amounts of
DBA, low results were sometimes produced when several milliliters of D M F were present. It was therefore thought best to keep the concentration of this solvent reasonably low, and t o exclude it completely during the continuous variations studies. Effect of Foreign Ions. The effect of foreign ions was studied by adding a solution of the ion t o a sample containing 80 fig. of platinum, and developing by the standard procedure. The tolerance to agiven ion was definedas the maximum concentration which could be present without causing an error of 0.010 in the absorbance reading of the sample. All tolerance values given are averages of very closely agreeing triplicate determinations. Anions were added as solutions of their sodium, potassium, or ammonium salts. Cations were added as solutions of their chlorides, nitrates, or sulfates. The platinum metals came from solutions left over from previous investigations, and were usually chloride complexes. Except for substances used as masking agents, the highest concentration levels tested were 100 p.p.m. for anions and 50 pap.m. for cations. Anions which did not interfere a t the 100-p.p.m. level were bromide, iodide, nitrate, perchlorate, sulfate, phosphate, molybdate, tungstate, tartrate, and formate. Cations which were tolerated at the 50-p.p.m. level without masking were zinc, lead, and ammonium. Nitrite, dichromate, and iron(II1) oxidized the reagent. Tolerances for other ions tested are listed in Table 11. When the tolerance for a foreign ion was below 0.8 p.p.m., a search was made for a suitable masking agent. Several of the heavy metals required masking to prevent precipitate formation, even though no reaction with DBA was evident. Cobalt slowly formed an interfering color even when masked; if the absorbance was measured within 10 minutes after sample development, relatively large amounts of cobalt(I1) could be tolerated. Removal of Interfering Ions. Interference from palladium(II), ruthenium(III), and osmium(II1) was sufficient to warrant their removal, which was accomplished a t the 10-p.p.m. level. Palladium was easily removed by two extractions of its complex with 1-(2-pyridylazo)-2-naphthol(PAN) into chloroform, and the chloroform layer is suitable for spectrophotometric determination of palladium (5,14). Osmium and ruthenium were volatilized from a dilute hydrochloric acid solution to which ammonium peroxydisulfate was added. This process produced a solution with fewer extraneous ions than other methods tried. The method was not tested for larger amounts of osmium and ruthenium, so its general applicability is unknown.
Table 1.
I
Platinum , p.p.m. 0.400 0.500 0.600 0.700 0.800
0.900 1.00
1.10 1.200
Calibration Data Specific Absorb- absorptivity, ance p.p.m.-' cm.-' 0.235 0.588 0.291 0.582 0.352 0.587 0.412 0.589 0.468 0,585 0.526 0.583 0.581 0.581 0.639 0.589 0.694 0.587 Av. 0.585
Table 11. Tolerance for Foreign Ions (Platinum concentration, 0.80 p.p.m.) Tolerance, p.p.m. Without With Ion masking masking Gold(111) 0.3 2a Vanadium(1V) 2 Precip. 106 Precip. 5OC Precip.
Precip. Precip. Precip. Precip. Precip.
1o c 20c
5OC 5OC 50d 504
4
6.2 0.8 0.1
0.5 Acetate 10 EDTA 10 a Masked with iodide, 300 p.p.m. * Masked wjth EDTA, 10 p . p m c Masked with tartrate, 3 g. per liter. d Masked with formate, 1 g. per liter.
Stability of Developed Samples. Absorbance measurements a t frequent time intervals showed the samples to be stable for a t least 4 hours. Stability of DBA Reagent Solutions. The 0.6% reagent in 20y0 DMF-water solution could be used for from 2 days to a week; in practice, fresh solution was prepared every other day. In O.lyoconcentration, the reagent was more stable, and solutions in pure DMF were even more stable; but this amount of D M F in the final solution was undesirable. Solutions of DBA in sodium hydroxide were even less stable than the reagent solution recommended. In any case, slight instability of the reagent solution is not a serious difficulty. STUDY OF REACTIONS AND PRODUCTS
Absorption Spectra. Figure 1 shows the absorption spectrum of a sample containing 80 pg. of platinum(1V) developed by the recommended procedure. Absorbances a t both peak wavelengths were proportional to the platinum concentration. Another sample containing 80 pg. of platinum(I1) VOL 38, NO. 9, AUGUST 1966
1219
I
0.5
0.6
0'4
0.5
0.3 w
w 0.4
z a m 0.2
a
u
Y
0
z
LT
0.3
0
m
0
m
m m
a 0,I
a 0.2 0 0. I 400 500 600 700 WAVELENGTH, MILLIMICRONS Figure 1. Absorption spectrum of platinum complex with 3,4-diaminobenzoic acid
was treated in the same manner; its spectrum was identical with that in Figure 1 except that there was no absorption in the 40040 500-mp region. The DBA blank absorbed very slightly in this region, but samples of DBA which had been oxidized by either air or ammonium peroxydisulfate absorbed strongly. The maximum of the blank and of the two oxidized samples of DBA coincided exactly in wavelength with the maximum in that region of Figure 1. It appears, therefore, that the absorption in the 40040 500-mp region is due primarily to oxidized DBA. Since this absorption increased with an increase in platinum(1V) in the starting material but remained a t blank level for all concentrations of platinum(I1) tested, it also appears that the DBA reduced the platinum(1V) to platinum (11) and was oxidized in the process. Ullmann and Mauthner (18) state that when DBA is oxidized with iron (111) the product is 7-amino-6-hydroxyphenazine - 2 - carboxylic acid. Their claim is neither supported nor disputed in the present work. If this compound is the correct product for the reaction of DBA with iron(III), it is likely that the same oxidation product would be produced by reaction with platinum (IV). Reactions and products of a similar nature have been reported (7, 8 , 17) for the oxidation of o-phenylenediamine with iron(II1). Method of Continuous Variations. The method of Job (10) as modified by Vosburgh and Cooper (20) and others ( l g , 19,21) was applied to a study of this system, first to a series of solutions of total concentration of platinum(I1) 1220
e
ANALYTICAL CHEMISTRY
0 0 Figure 2. complex
0.2 0.4 0.6 0.8 MOLE F R A C T I O N D B A
I .o
Continuous variations plot for platinum-DBA
The plus DBA = 2.05 X l O - 5 M . continuous variations plot (Figure 2) has a maximum a t 0.67 mole fraction of DBA, indicating a 2 to 1 DBAplatinum ratio. Absorbance measurements at three different wavelengths produced no shift of the position of maximum, and the spectra of all samples above mole fraction 0.5 were identical except for concentration differences. Existence of other complexes is therefore improbable. The curvature in the neighborhood of 0.20 mole fraction might be an argument for the existence of another complex in the system. This possibility cannot be completely ruled out, but it seems very doubtful. Even a mole fraction value of 0.25 would correspond to a complex with three platinum atoms and one ligand molecule, and smaller mole fraction values would correspond to even less probable arrangements. The curvature near mole fraction 0.20 is more probably due to a fairly high dissociation of the complex or to hydrolysis of the excess platinum in the alkaline solution. A series of solutions of total concentration of platinum(I1) plus DBA of 5.13 X 10-5iz1 produced a continuous variations plot identical with the first except for higher absorbance values. The method of continuous variations was applied to two series of platinum (IV) samples corresponding in total concentration to the values previously used with platinum(I1). The plots obtained had inflections between mole fraction 0.30 and 0.40, and maxima a t 0.73. By use of more concentrated solutions of platinum(1V) and DBA and
absorbance measurements a t both 715 and 445 mp, it was found that a sharp change in slope occurred a t 0.38 mole fraction, which corresponds to oxidation of DBA by reduction of platinum(1V) to platinum(I1). When successive complexes are formed with a given metal and ligand, each peak will still occur at, or near, the mole fraction value corresponding to its particular stoichiometry (22, 19, 20). The present case is different, in that the ligand is destroyed (for our purposes) by the redox reaction. Thus, the major break in the curve must occur a t a mole fraction corresponding to a reaction ratio including the total amount of ligand in both the redox and the complexation steps. It therefore appears that 3 moles of platinum(1V) are reduced by 2 moles of DBA (mole fraction 0.4). The 3 moles of platinum(I1) formed require 6 additional moles of DBA for complexation, giving a total reaction ratio of DBA to platinum of 8 t o 3 (mole fraction 0.73). The redox behavior agrees with that observed for the previously cited reactions of iron(II1) with DBA and with o-phenylenediamine (7, 8, 17, 18). Mole Ratio Method. This method was inconclusive as to the reaction ratio, on account of the very great rounding of the curve of absorbance against mole ratio of DBA to platinum. Ion Exchange Studies. An alkaline solution of the colored complex was not retained by a cation resin, but was retained very strongly on an anion resin. An acidic solution was retained
quantitatively on a cation resin, and partially retained on an anion resin; the acidic form was easily removed from the cation exchanger by use of dilute sodium hydroxide; the material on the anion column was slowly eluted with 1 to 1 hydrochloric acid. Isolation of Product. By use of much more concentrated solutions of reactants, a t p H about 3, heating the mixture for 30 minutes just below the boiling point produced a yellow-brown precipitate. The mixture was made alkaline (pH 10.5) with potassium hydroxide; the precipitate dissolved and the solution turned an intense blue. Cooling in an ice bath and addition of isopropyl alcohol to decrease the solubility resulted in formation of a precipitate (probably the potassium salt of the anionic complex). The precipitate was filtered, washed, and dried. The product was usually very dark blue (almost black), although occasionally a bright purple solid was obtained. Elemental analysis for carbon, hydrogen, and nitrogen varied considerably from batch to batch. Portions of the salts were titrated with hydrochloric acid, with potentiometric and conductometric detection of the end point;
although sharp breaks were obtained in the titration curves, results were variable from batch to batch. The yellow-brown precipitate originally formed in acidic solution likewise gave variable analyses for carbon, hydrogen, and nitrogen for different preparations. LITERATURE CITED
(1) Ayres, G. H., McCrory, R. W., ANAL. CHEM.36, 133 (1964). (2) Ayres, G. H., Meyer, A. S., Jr., Ibid., 23, 299 (1951); J. Am. Chem. SOC.77, 2671 (1955). (3) Beamish, F. E., Talanta 12,743 (1965). (4) Beamish, F. E., McBryde, W. A. E., Anal. Chim. Acta 9,349 (1953); 18,551 (1958). (5) Busev, A. I., Kiselva, L. V., Vestnik Moskov. Univ.,Ser. Mat., Mekh., Astron., Fiz.. Khim. 13. No.4.179 (1958): Chem. Absir., 53, 11105f (1959). (6) Chechneva, A. N., Podchainova, V. N., Izv. Bysshikh Uchebn. Zavedenii, Khim. i Khim. Tekhnol. 7 (5), 731 (1964); Chem. Abstr. 62, 9760c (1965). (7) Fischer, O., Hepp, E., Ber. 22, 356 (1889). (8) Greiss, P., J. Prakt. Chem. 3, 143 (1871). (9) Janota, H. F., Ph.D. dissertation, University of Texas, 1963. (10) Job, P., Ann. Chim. 9 (lo), 113 (1928); 6 ( l l ) , 97 (1936). (11) Karpova, L. V., Alyanchikova, V. N., I
,
Simirnov, P. P., Gut’ho, A. D., Ryserva,
G. V., Morozova, N. F., Labkovskaya, D. B., Vopr. Analiza Blogorodn. Metal., Akad. Nauk SSSR.,Sibirsh. Otd., Tr.6go (Pyatago) Vses. Soveshch. 1963, 30; Chem. Abstr. 61, 10038b (1964). (12) Katzin, L. I., Gebert, E., J . Am. Chem. SOC.72, 5455 (1950). (13) Kirkland, J. J., Yoe, J. H., ANAL. CHEM.26, 1340 (1954). (14) Sawada, T., Kato, S., Bunseki Kagaku 11, 544 (1962); Chem. Abstr. 57, 5297h (1962). (15) Sen‘Gupia, J. G., Anal. Chim. Acla 23, 462 (19tiO). (16) Senise, P., Pitombo, L. R. M., Talanta 11, 11885 (1964). (,17) Ullmann, F., Mauthner, F., Ber. 35, 4302 (1902). (18) Ibid., 36, 4032 (1903). (19) Underwood, A. L., Toribara, T. Y., Neuman, W. F., J. Am. Chem. SOC.72, 5597 (1950). (20) Vosburgh, W. C., Cooper, G. R., Ibid., 63, 437 (1941). (21) Woldbve. F.. Acta Chem. Scand. 9, ‘ 299 (1955”). ’ ‘ RECEIVED for review May 16, 1966. Accepted June 3, 1966. Condensed from a dissertation submitted by Larry D. Johnson t o the graduate school, The University of Texas, in partial fulfillment of the reguirements for the doctor of philosophy egree, June 1966. The authors gratefully acknowledge the financial support of National Science Foundation grant G14479, and fellowships provided by Procter and Gamble, Monsrtnto Co., and the. Dow Chemical Co. \ - - - - I -
Analysis of a High-Molecular-Weight Phenolic Inhibitor H.
S. KNIGHT and HERBERT SIEGEL
Shell Development Co., Emeryville, Calif. Gas chromatography and phase solubility analysis were combined to achieve a more definitive analysis of a high-molecular-weight phenolic inhibitor than would have been possible by either technique alone. Phase solubility analysis showed that the material was about 99% pure, and about 1% of a related phenol wos found by gas chromatography. The agreement of the two techniques showed that this was the only significant impurity present.
T
HE OXIDATIOK-IKHIBITIKG PROPERTIES of certain hindered phenols
have given these materials widespread commercial interest. In the manufacture of films and fibers which may be subjected to high temperature during processing, it is desirable that the inhibitor be of high molecular weight and correspondingly low vapor pressure to avoid evaporation losses. High molecular weight also contributes to low extractability of the inhibitor from the polymer on contact, for example, by foods packaged in films.
A phenolic inhibitor having a molecular weight of 775 and a vapor pressure of 0.014 mm. at 180’ C., suitable for use in food packaging materials, is 1,3,5trimethyl - 2,4,6 - tri(3,5 - di - tertbutyl-4-hydroxybenzyl)benzene, Shell trade-named Ionox 330. The application of this inhibitor in the food and drug industry leads to the need for detailed and precise knowledge concerning its purity, which was believed to be 98% or more. Two complementary techniques were employed to obtain a more definitive analysis than would have been possible by either technique alone or by other available procedures. Phase solubility analysis, a known but little used technique, was applied to determine nonvolatile impurities but was not suitable for light materials. Gas chromatography was employed for the volatile components but could not be used to determine polymers or other nonvolatile impurities. Final agreement of the two techniques showed that only one impurity was present in significant concentration. This was identified by gas chromatography on the basis of its re-
tention volume compared with that of a known compound. I n the course of the gas chromatographic study, it was found possible to determine the Ionox 330 itself. This is believed to be the highest-molecularweight phenol to be successfully eluted from a gas chromatographic column. Analytical techniques that have been successfully used for lower-molecularweight phenols include various forms of spectrometry, determination of weak acidity, gas chromatography, and determination of physical properties such as melting point. Spectrometry and titration were not specific for Ionox 330 which might be expected to contain phenolic impurities, although nuclear magnetic resonance could detect these impurities if they were present a t a sufficiently high level. KO gas chromatographic method was available in the 775 molecular weight range. The melting point of high-molecular-weight materials is strongly influenced by traces of low-molecular-weight impurities and therefore does not provide a practical measure of purity. Phase solubility analysis consists of VOL. 38, NO. 9, AUGUST 1966
0
1221
