Precision. On the same day, a single penicillin V fermentation sample of a series was analyzed 71 times. The following results were obtained: Average value of 8764 units per milliliter with a relative standard deviation of 0.9%. Dayto-day ( N = 50) analyses of penicillin V fermentation samples gave an average value of 7710 units per milliliter with a relative standard deviation of 2.4%. The AutoAnalyzer Method us. the Manual Method. The two methods agreed with a correlation coefficient of 0.97 ( N = 50). Interference. Only a few substances are able to reduce a molybdoarsenic acid-mercuric chloride reagent at room temperature. Among these are strongly reducing salts such as ferro and stanno salts, as well as ascorbic acid and hydroquinone. These substances were not found in any measurable concentrations in the fermentation substrates which were used. However, degradation products of penicillin, such as penicilloic acid, penilloic acid, penillic acid, and penicillamine may be found. Penicilloic acid, the active agent in the method, reacts with molybdoarsenic acid-
mercuric chloride t o the greatest extent. Penilloic acid interfered slightly (less than 10 %), whereas penillic acid and penicillamine exerted strong interference, e.g., 10 mg of penicillamine per milliliter gave a reading corresponding t o 10000 units of penicillin G per milliliter. However, this interference did not affect the final calculated penicillin concentration, since the blank corrected for interference of this nature. Conclusion. The AutoAnalyzer modification of Pan’s procedure results in a method with a high order of specificity, accuracy, precision, and speed of analysis, as well as a high sensitivity. In addition, only very small amounts of sample are required for each analysis. ACKNOWLEDGMENT
The author is very grateful to P. W. Hansen for his valuable help during the preparation of this manuscript. RECEIVED for review September 12, 1971. Accepted December 7,1971.
Indirect Spectrophotometric Determination of Oxalate Using Uranium and 4-(2-Pyridylazo) resorcinol Robert E. Neasl and John C. Guyon2 Department of Chemistry, University of Missouri, Columbia, Mo. 65201 A method that exhibits operational simplicity is presented for the indirect spectrophotometric determination of the oxalate ion. The diminishment in absorbance effected by oxalate on the red uranium(1V)-4(2-pyridylazo)resorcinol (PAR) complex allows oxalate determination at the ppm level in the presence of many foreign ions. The diminishment in absorbance at 515 nm is linear with oxalate in the range 0 to 3 ppm in the final solution.
VERYFEW PROCEDURES are available for the determination of the oxalate ion in parts-per-million concentrations. The methods that do currently exist are essentially limited to spectrophotometric measurements. The major existing systems and their undesirable aspects may be summarized as follows. The direct iron(II1)-oxalate method ( I ) is insensitive and subject to a variety of interferences. An indirect iron(IIIF3hydroxy-1-p-sulfanatophenyl-3-phenyltriazene system (2) cannot tolerate fluoride, phosphate, citrate, and tartrate. The indirect copper(I1)-benzidine approach (3) is very insensitive, includes tartrate, formate, and citrate as serious interferences and furthermore, benzidine is carcinogenic (4). An indirect Present address, Department of Chemistry, Western Illinois University, Macomb, Ill. 61455 Present address, Department of Chemistry, Memphis State University, Memphis, Tenn. ( 1 ) T. Nozaki, F. Hori, and H. Kurhiara, Nippon Kugaku Zusshi,
82,713 (1961). (2) G. Mehra and N. C. Sogani, J. Indian Chem. SOC.,39, 145 (1967). (3) Z . D. Draganic, Anal. Chim. Acta, 28, 394 (1963). (4) T. G. Whiston and G. W.Cherry, Anulysr, 87,819(1962).
chloranilic acid scheme ( 5 ) presents temperature, time, and pH as critical variables and suffers from serious interferences by citrate and tartrate. The systems utilizing scandium(II1)monochromium Bordeaux C and zirconium(IV)-2-carboxybenzene-3,4-dihydroxybenzene(6) are subject to numerous interferences. The use of 4-(2-pyridylazo)resorcinol as an analytical reagent for uranium was first suggested by Pollard, Hanson, and Geary in 1958 (7). In 1963, Florence and Farrar published a method for the spectrophotometric determination of uranium with PAR at pH 8.0 using triethanolamine as buffer (8). Sommer, Ivanov, and Novotna published both a modification of this method using triethanolamine and a method using formate buffer or 20-30% vjv DMF at pH 3.6 (9). At the lower pH, the interference of rare earths and a number of anions is decreased but the interference due to oxalate isgreatly enhanced. This was the basis for preliminary laboratory studies to determine whether this interference could be utilized in a method for oxalate. No previous use of the uranium(1V) interaction with 4-(2pyridy1azo)resorcinol (also referred to as PAR) to determine oxalate was found. This paper reports the experimental results of work performed in the development of a new simple, ( 5 ) J. de Oliveira Meditsch, Eng. Quim., 15(8), 9 (1963). (6) N. V. Zaglyadimova and Z. M. Gur’eva, Tr. Khim. Khim. Teknol., 1,119 (1967). (7) F. H. Pollard, P. Hanson, and W. J . Geary, Anal. Chim. Acta.
20,26 (1959).
( 8 ) T. M. Florence and Y.Farrar, ANAL.CHEM., 35, 1613 (1963). (9) L. Sommer, V. M. Ivanov, and H. Novotna, Tulunta, 14, 329 ( 1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
799
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0.5 I
360
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400
440
I
480 Wavelength, nm
I 520
Figure 2. Spectral curve of uranium-PAR complex
0.4
Absorbance, 7.00 X 10-5Muranyl nitrate reagent and 6.60 X 10-5M PAR in 40 DMF, pH 4.8
z
0.2
I
I
340
I
I
380
I
I
I
420
1
1
460
500
Wavolongth, nm
Figure 1. Spectral curve of PAR Absorbance, 6.6 X 10-+MPAR in 40z DMF, pH 4.8
sensitive method for determining oxalate that circumvents many problems accompanying existing methods. EXPERIMENTAL Apparatus. Absorbance measurements were made with a Cary Model 12 recording spectrophotometer with matched 1-cm quartz cells using deionized water as the reference, Blanks were measured and subtracted for all data presented unless noted otherwise. A Beckman Zeromatic pH meter equipped with a Beckman No. 39183 long thin probe combination electrode was used for all pH measurements. Glass hypodermic syringes (without needles) of 1-, 2-, 5-, lo-, 20-, 30-, and 50-1111 capacities were used for the additions of the various reagents except for the standard oxalate solutions which were added with volumetric pipets and burets. Volumetric glassware was also used for uranyl nitrate and 4-(2-pyridylazo)resorcinol additions in the mole ratio and continuous variations studies and for the addition of foreign ions in that study. A water bath equipped with a Sargent Model 3554 thermoregulating unit was used to control reaction temperature at 25 f 1 OC during preliminary studies. No temperature control is required in the recommended procedure as long as room temperature does not deviate more than about ten degrees from 25 "C. Reagents, Aqueous uranyl nitrate reagent was prepared to be exactly 0.100M UO2(NO& using Fisher Certified Reagent. Exactly 50.21 grams of U02(N03)2.6Hz0 was combined with 100 ml of 1.00M HC104 and diluted to exactly 1 liter with deionized water. Uranyl nitrate reagents of lesser concentrations were made by volumetric dilution of the 0.100M reagent. 800
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, A P R I L 1972
Aqueous potassium oxalate stock solution was prepared using Fisher Certified Reagent by dissolving exactly 5.2328 grams of K2C204.HzO and diluting to volume in a 250-ml volumetric flask with deionized water. Solutions of lesser concentrations were made by volumetric dilutions. PAR reagent was prepared using 4-(2-pyridylazo)resorcinol obtained from K K Laboratories without further purification. The 1.00mM solution was prepared by dissolving exactly 0.2152 gram of CllH9N302and diluting to 1 liter with dimethylformamide (DMF). Formate buffer solution was prepared using Baker Analyzed Reagent assayed to be 90.1 formic acid. The buffer solution was prepared as 0.10Mformic acid and 0.10Mformate by adding 10.22 grams of 90.1 formic acid to approximately 900 ml of deionized water. Sufficient NaOH pellets and solution were then added to the solution with magnetic stirring to adjust the pH to about 3.68. The final adjustments of volume to 1 liter and pH to 3.68 were accomplished using deionized water and NaOH and/or HCIOa solutions. Master reagent was prepared by combining exactly 100 ml of 1.00mM uranyl nitrate reagent, 200 ml of 1.00mM PAR reagent, 500 ml of 0.10M formic acid-0.lOM formate pH 3.68 buffer, 1 liter of D M F and diluting to exactly 2 liters with deionized water. The master reagent was sufficiently stable for more than three weeks as long as standards were carried through the procedure along with samples. No special storage conditions were used. Deionized water was obtained by passing steam condensate through Barnstead No. 8902 mixed bed and No. 8904 organic removal ion-exchange columns. Mallinckrodt A.R. HC104,NaOH, and dimethylformamide (DMF) were used. Procedure. Accurately transfer the requisite amounts of oxalate standard solutions to 50-ml volumetric flasks to cover the concentration range 0 to 3 ppm Cz042-. Using hypodermic syringes (sans needles), to each flask add sufficient deionized water and exactly 35 ml of master reagent to yield a reaction volume of about 45 ml in each flask. After 10 minutes, dilute to volume with deionized water, allow to stand 5 minutes, and measure the absorbance at 515 nm us. deionized water, Subject a reagent blank to the same procedure and construct a calibration curve of diminishment in absorbance us. oxalate concentration. Samples to be analyzed should be subjected to the same procedure and the
+
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1
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0.4
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i I 0.2
0.2
0.4
0.6
0.0
1.0
0.0
ml 10.0 m M UOz(NO,):, Reagent
9.0
0.0
7.0
6.0
P”
Figure 3. Effect of uranium:PAR ratio A. Absorbance at 510 nm,no oxalate E . Absorbance at 510 nm,2.00 mg C2042C. Absorbance difference, A minus B
oxalate concentration determined by comparing the diminishment in absorbance by samples with the calibration curve. RESULTS
Preliminary Studies. Examination of a variety of combinations of uranyl nitrate reagent, PAR, DMF, formate buffer, and oxalate at various pH levels confirmed that oxalate reliably interfered with the red uranium-PAR complex at about pH 4. Figure 1 shows the spectral curve of PAR at pH 4.8 in 40 DMF, with an absorbance maximum at about 390 nm. Figure 2 shows the spectral curve obtained from a combination of uranyl nitrate reagent and PAR in 40z DMF at pH 4.8. The absorbance maximum of the uranium-PAR complex occurs at about 515 nm and is reliably diminished in the presence of oxalate. The optimum conditions for utilizing this diminishment in absorbance to determine small amounts of oxalate were investigated. All concentrations indicated as ppm are based on the final 5 0 4 solution unless stated otherwise. Effect of Uranium:PAR Ratio. This study was made in buffered aqueous medium to determine the approximate ratio of uranyl nitrate reagent :PAR allowing the largest diminshment in absorbance by a fixed amount of oxalate. Figure 3 reveals that the maximum diminishment in absorbance occurred at about 1:1 mole ratio of uranium:PAR. Later studies revealed that a lower ratio was optimum for the lower total analytical concentrations of uranium plus PAR dictated by optimum absorbance considerations and incorporated in the recommended procedure. Effect of Dimethylformamide Concentration. As a result of insoluble red residue encountered in the previous study in aqueous medium, it was necessary to incorporate dimethylformamide in all subsequent work. At least 3 0 z vjv D M F
z
Figure 4. Effect of pH
5.0
ml 1.00 m M PAR Reagent
A. Absorbance at 515 nm,7.00 X 10-6M uranium, 6.60 X 10-6M PAR, no oxalate B. Absorbance at 515 nm, 0.250 mg Ce042C. Absorbance difference, A minus B
in the final solution was necessary to ensure that no red residue formed on standing. Unless stated otherwise, a final concentration of 35 vjv DMF was used for all further work to ensure solubility, yet accommodate an adequate sample volume. A small enhancement in the diminishment in absorbance was effected by the addition of DMF to the system. Effect of Total Analytical Concentration of Uranium plus PAR. A fixed quantity of 0.2500 mg of oxalate and a fixed 1.08 ratio of U:PAR was used to study this effect over the range 1.93 to 40.5 micromoles of total uranium plus PAR in the final 50-1111 solution. The maximum diminishment in absorbance occurred when about 5 micromoles of total uranium plus PAR was used. Effect of Formate Buffer Concentration. A study was performed to determine the optimum amount of formate buffer necessary to control pH effects from samples and DMF. At least 5.0 ml of pH 3.68 formate buffer was required to obtain a final solution of reproducible “apparent” pH in 35 vjv DMF and 10.0 ml was considered optimum. Larger amounts of formate buffer resulted in decreased sensitivity to oxalate. Effect of Time between Addition of Oxalate and Dimethylformamide. Very little effect on the diminishment in absorbance was found when the time between oxalate and D M F addition was varied from 0 to 70 minutes. Essentially no time lapse was necessary so long as all samples were treated similarly. Effect of pH. The effect of pH on color development of the uranium-PAR complex and on the diminishment of this color by oxalate was studied. The pH was maintained at the desired value by HC1O4 and/or NaOH additions. No D M F was used in this study. Reagent blanks were subjected to the same procedure except that no oxalate was added. Figure 4 presents the results of this study. The difference curve reveals a maximum diminishment in absorbance at about pH
z
z
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
801
1.2
I
1
I
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I
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
i.0
0.8
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Figure 5. Effect of uranyl nitrate concentration
0 P
A . Absorbance at 515 nm, 6.60 X 10-’M PAR, no oxalate B. Absorbance at 515 nm, 0.250 mg C Z O P C. Absorbance difference, A minus B
5
0.6
m
P
U
0.4
0.2
0.0
3.8. The pH 3.68 formate buffer afforded a means of conducting the pertinent reactions near the pH of maximum sensitivity to oxalate and still yielded a reproducible “apparent” pH of 4.8 upon the addition of DMF. Thus, the pH 3.68 buffer was incorporated as the means of pH control. A buffer of slightly lower pH could be used but the slightly increased sensitivity that would accrue is offset by diminished reproducibility due to decreased buffer capacity and the attendant need for increased ionic strength buffer. Effect of Volume Control by Water Addition. Very little effect on diminishment in absorbance was found when the reaction volume was varied by the addition of deionized water. Approximate reaction volume control was retained in the recommended procedure only to maximize the precision of the method. Effect of Uranyl Nitrate Reagent Concentration. This study and the succeeding one were performed to evaluate further the best U:PAR ratio to use at the approximately 5 micromoles of total uranium plus PAR level dictated by a previous study. Buffer solution and DMF were included in the reaction solutions used in this study. Reagent blanks
I
I
I
I
I
I
I
I
were determined in an identical manner except that no oxalate was included. Figure 5 indicates that about 0.20 ml was the optimum amount of 0.0100Muranyl nitrate reagent to use for the concentrations of PAR and oxalate employed. Effect of PAR Reagent Concentration. The effect of varying PAR reagent was evaluated using a fixed concentration of uranyl nitrate and oxalate in buffered D M F medium. Figure 6 reveals an increased sensitivity to oxalate with increasing PAR reagent until about 4 ml has been added, beyond which no further diminishment in absorbance accrues. Based on absorbance considerations and the results of this study and previous ones, a 1:2 ratio of uranium:PAR and 5.25 micromoles of total uranium plus PAR in the final 50-ml solution were considered optimum for the determination of oxalate at the 5-ppm level. Combined Reagent Study. A study was performed to determine the feasibility of combining all reagents prior to adding the oxalate sample in order to simplify the procedure and to improve the reproducibility of the results. The data obtained for oxalate determination using a combined reagent based on quantities arrived at in the other studies was quite
I
Figure 6. Effect of PAR reagent concentration A . Absorbance at 515 nm, 4.00 X 10+M uranium, no
oxalate B. Absorbance at 515 nm, 0.250 mg C Z O P C. Absorbance difference, A minus B
rnl 1.00 m M PAR Reagent
802
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
9 0*32 0.2 8
Table I. Effect of Foreign Ions
t
Foreign ion Mga+ Zn2+ CdZ+ ~13+ Zr Mn2f
0.24
Amount“ permitted, ppm 12 2 4 4
4 8 3 21 8 2 5 3 82 2 9
(+
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Figure 7. Application of continuous variations method at 515 M1
Absorbance diminishment at 515 nm, 2.50 pmoles total of uranium plus oxalate, us. reagent blank containing no oxalate
reproducible as indicated in the succeeding precision study. Thus, the master reagent described in the reagents section was included in the recommended procedure. Precision Study. The master reagent and recommended procedure were evaluated with regard to the precision attainable at a final solution concentration of 5 ppm of oxalate. Ten standards containing exactly 0.2500 mg of Cz04a-were subjected to the recommended procedure along with three blanks. The mean blank absorbance was used to calculate diminishment in absorbance values for each oxalate standard. The mean diminishment in absorbance was 0.427 and the per cent relative standard deviation was calculated to be 0.59. Conformity to Beer’s Law. A calibration curve for the range 0 to 5 ppm oxalate was constructed. Conformity to Beer’s law was exhibited by the plot of diminishment in absorbance us. oxalate concentration over the range 0 to 3 ppm and negative deviation from Beer’s law was observed beyond 3 ppm but the curve was still useful to about 5 ppm. Effect of Foreign Ions. A summary of the effects of a large number of foreign ions appears in Table I. In each case sufficient foreign ion solution was combined with standard oxalate in a 50-ml volumetric flask to yield from 0 to 200 ppm of foreign ion and 4.00 ppm of c204’- in the final solution. The recommended procedure was then applied to each solution. A 2 relative error in the determination of oxalate was considered permissible. The ions appearing in Table I can be tolerated in the amounts listed. Mole Ratio Method. This method (IO) was applied to establish the stoichiometric ratios of uranium :PAR and uranium :oxalate involved in this procedure. Two studies were performed to determine the uranium:PAR ratio. The first study was made using a final fixed analytical concentration of 4.00 X 10-5M uranium and variable PAR concentration. (10) J. H. Yoe and A. L. Jones, IND. ENG.CHEM.,ANAL.ED., 16, 111(1944).
NH4+ M06a2WOa2Si032F-
Br1-
clodc103-
Br03103104SOF SOazSzOa2-
c1EDTA2S20s2NOaNOzCZH302Tartrate Citrate OCN-
SCNCrz012-
voa-
NiS04(NH4)zS04. 6Hz0 NHaCl NazMo04.2Hz0 Na2W04.2Hz0 Na2Si03.9Hz0 NaF KBr KI KClOn KClOi KBr03 KIOa
KIOi Na2S03 NazS04 NazSz03.5Hz0 NaCl NaZCl aOsN2 KzSz08 NaN03 KNOz NaCzH3O2. 3Hz0 HGH406 NaC6H707 KOCN Na2HP04 KSCN K2Cr2O7
2 107 4
40 57 8
114 200 31 200
83 130 182 200 200 67 200 21 200 200
45 138 41 32 51 18
180 8
2 NazS 9H20 20 CNKCN 48 All solutions contained 4.00 ppm oxalate, absorbance measured at 515 nm. NaV03
S 2-
The second study utilized a fixed final analytical concentration of 6.00 x 10-5M PAR and variable uranium concentration. Mole ratio values of 0.87 and 0.95 for the uranium:PAR interaction were obtained from studies one and two, respectively, indicating a 1 :1 stoichiometry. One study was performed to determine the uranium :oxalate ratio, using k e d final analytical concentrations of 3.5 x 10M5M uranium, 7.0 X 10-5MPAR, and variable oxalate concentrations. The resulting diminishment in absorbance us. oxalate concentration curve was quite rounded, with no sharp break, allowing only imprecise extrapolations yielding a value of 0.75 for the uranium :oxalate ratio. Method of Continuous Variations. This method (11-13) was applied in order to confirm further the uranium:oxalate (11) P.Job, Ann. Chim., 9, 113 (1928). (12) Ibid., 6, 97 (1936). (13) W. C. Vosburgh’and G. R. Cooper, J . Amer. Chem. Soc., 63, 437 (1941). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
803
Table 11. Application of Method Set No. 1
2 3 4
Oxalate, ppm Known Found” 4.00 4.12 2.00 2.07 1 .OO 0.400
1.02 0.414
Re1 std dev, 7Z 0.17 0.40 0.59 3.38
4 Each value represents the mean of three determinations. The relative standard deviations are based on these determinations.
interaction ratio. A fixed final combined analytical concentration of 5.00 X 10+M uranium-plus-oxalate was used and diminishment in absorbance measurements were made at 515 at 540 nm US. deionized water. The continuous variations curve for the data obtained at 515 nm is shown in Figure 7. The appropriate extrapolations of this curve yield a value of 0.47 mole fraction oxalate for the curve maximum. A value of 0.45 was obtained from the 540-nm curve. These values correspond closely to a 1:l ratio for the uranium:oxalate interaction. APPLICATION OF METHOD
The practical applicability of the recommended procedure was evaluated by “spiking” natural water samples obtained from a drilled well more than 600 feet deep. Various amounts of oxalate stock solutions were combined with 5-ml water samples in 50-ml volumetric flasks and allowed to stand for 30 minutes to allow the oxalate to equilibrate with sample constituents. The recommended procedure was then applied to samples with oxalate concentrations of 4.00, 2.00, 1.00, and 0.400 ppm oxalate in the final solutions. Three samples were measured at each concentration level and compared to a calibration curve prepared by subjecting standard oxalate solutions to the recommended procedure. The results are summarized in Table 11. DISCUSSION
A new indirect spectrophotometric method for the determination of trace quantities of oxalate has been developed utilizing the oxalate interaction with uranium present as the uranium-4-(2-pyridylazo)resorcinol complex. The sensitivity and selectivity of the method are comparable to those of existing methods and its operational simplicity is unique among methods for oxalate. A single multicomponent reagent is simply combined with the sample, diluted to volume, the diminishment in absorbance measured, and the appropriate comparison made with standards or a calibration curve. The diminishment in absorbance at 515 nm is linear with increased oxalate in the range from 0 to about 3 ppm in the final solution. Based on the extrapolations of the mole ratio curves obtained in the studies of uranyl nitrate reagent concentration us. PAR reagent concentrations, it is concluded that a 1 :1 complex of uranium:PAR is formed under the conditions of this method. Further, it is believed that the species primarily responsible for the absorbance maximum at 515 nm is UOzPAR+, based on the work of Sommer, Ivanov, and Novotna (9). These authors have concluded that a unipositive species is formed between pH 3 and 5.5 and a neutral complex is formed between pH 5.5 and 8.5. The mole ratio studies of the oxalate interaction with the uranium-PAR complex were inconclusive in determining 804
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
the ratio of the oxalate:uranium interaction. The study in which the oxalate and PAR concentrations were held constant and the uranium concentration varied, must be essentially invalidated because of the shifting absorbance base of unknown magnitude arising from the ever-changing uranium concentrations. This obviously had the effect of varying the absorbance accruing from the uranium-PAR complex throughout the study. The second mole ratio study in which the uranium and PAR concentrations were held constant while varying the oxalate concentration yielded a mole ratio value of 1.33 for the oxa1ate:uranium interaction. This value was obtained from a curve that appeared to be sufficiently smooth to allow a good extrapolation. However, it must be noted that the break in the curve was not sufficiently sharp to yield a precise mole ratio value of unquestioned validity. The continuous variations curves obtained by plotting diminishment in absorbance us. mole fraction of oxalate are considered to be the most convincing evidence regarding the nature of the oxalate-uranium interaction. Precise extrapolations of the curves were possible at both the wavelengths employed, yielding results of 0.47 and 0.45 mole fraction of oxalate, indicating a 1 :1 oxalate :uranium interaction in each case. It is particularly significant that essentially the same extrapolation values were obtained at each of the two wavelengths, indicating a good probability that only one uraniumPAR complex is being attacked by the oxalate. This infers that the application of the continuous variations treatment is valid in this case. On the basis of this evidence, it is the authors’ conclusion that oxalate interacts with uranium in a 1 :1 ratio under the conditions of this method. It was discovered early in this work that the reduction in the uranium-PAR complex absorbance at 515 nm was accompanied by a corresponding increase in absorbance at 390 nm. This was attributed to the appearance in solution of increased amounts of uncomplexed PAR by virtue of the removal of uranium from the uranium-PAR complex by oxalate. It was believed that oxalate formed a stronger complex with uranium than did PAR. Further evidence that the increased absorbance at 390 nm was due to “free” PAR was the fact that when PAR was placed in the medium employed in this method, excluding uranium, an absorbance band centered at 390 nm was observed. On the basis of the foregoing information, the following reactions are proposed as the predominant ones operating under the conditions of this method: PAR F? PAR-
+ H+
+ UOzz+ F? UOZPAR+ UOzPAR+ + CzOa’-~3 UOzCzO4 + PAR-
PAR-
(1) (11) (111)
where PAR- represents
This proposed scheme is supported by the following considerations: (a) the dissociation constant expressed as p C , for reaction I has been evaluated as 5.9 in 30 % vjv D M F (9), meaning that appreciable PAR- would be available at pH 4.8 in 30% v/v DMF (conditions of this method). At the same time, the limited availability of PAR- due to only the partial dissociation of PAR explains the relatively high ratio of analytical concentrations of PAR :uranium needed in this method ; (b) reaction I1 is consistent with the work of Sommer and coworkers mentioned previously in which a unipositive species
was reported to exist between pH 3 and 5.5, accounting for the absorbance maximum at 515 nm; (c) the pK, values for oxalic acid are 1.23 and 4.19 for the first and second dissociations, respectively; therefore oxalate would exist largely as C20d2at pH 4.8; (d) the complexation of the uranyl ion is probably bidentate with respect to PAR- and oxalate; and (e) the PAR- regenerated in reaction 111 could then “re-enter” reaction I to explain the increased absorbance at 390 nm presumed to be due to an increase in the PAR concentration as a result of the oxalate interaction with the U02PAR+complex.
In conclusion, it is believed that a valuable new method for the determination of oxalate has been added to the analytical capability for anion analysis. The method should be especially helpful to researchers in the area of water resources research. RECEIVED for review July 14, 1971. Accepted September 19, 1971. This work was supported by the Office of Water Resources Research, Department of Interior, under Grant OWRR NO.A-014-MO.
Homogeneous Liquid-Liquid Extraction Method Extraction of Iron(ll1) Thenoyltrifluoroacetonate by Propylene Carbonate Katsuo Murata, Yu Yokoyama, and Shigero Ikeda
Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka, Japan A new homogeneous liquid-liquid extraction method by propylene carbonate was devised, which is characterized by immediate formation of the complex upon achieving a single homogeneous liquid phase at elevated temperature. Two distinct and separable phases appear upon cooling to room temperature. This method was applied to the extraction of iron(ll1) thenoyltrifluoroacetonate, which i s known to extract at a slow rate. Iron(ll1) was rapidly and completely extracted by using this proposed method, in contrast to the several hours required for complete extraction by conventional liquid-liquid extraction at room temperature. The nature of the mixed solvent, the chemical state of thenoyltrifluoroacetone (TTA) in some solvents, and the formation of iron(ll1) thenoyltrifluoroacetonate were also investigated in order to obtain information about the extraction behavior in comparison with the usual liquid-liquid extraction. THELIQUID-LIQUID EXTRACTION method has been extensively applied to studies of chemical equilibria, the separation of different elements, and the synthesis of inorganic compounds. Some problems do, however, remain in solvent extractione.g., the slow or incomplete extractions. The authors have devised a new homogeneous liquid-liquid extraction method and obtained satisfactory results in the extraction of molybdenum (VI) with a simple procedure ( I ) . This method is based on the high solubility of organic solvent in water at higher temperature and is characterized by immediate formation of the complex upon attaining a state of homogeneous solution consisting of water and the organic solvent during the procedure. At room temperature the two phases are present heterogeneously, but at the elevated temperature they change into a homogeneous solution, which separates into the two phases again upon cooling. During these sequential procedures, the species in the aqueous phase transfers into the organic phase-i.e., the extraction is achieved. This method of equilibration by achieving a homogeneous state is different from the common mechanical shaking method. Molecules of the organic solvent rather freely enter into the aqueous solution. Consequently, the water structure of the aqueous media and the environment of solute species will be (1) K. Murata and S. Ikeda, Bunseki Kagaku, 18, 1137 (1969).
altered remarkably by participation of the organic solvent molecules. This “unshielding” of the environment may change the extractability ; such a condition is not satisfied fully in the conventional extraction method. One of the most suitable organic solvents for the homogeneous liquid-liquid extraction is propylene carbonate (4-methyl-l,3-dioxolane-2-one), which has found recent use in solvent extractions (2, 3) and in electrochemical studies (4-6). It has a high dielectric constant (65 at 25 “C), low vapor pressure, and high boiling point. Especially noteworthy is the characteristic property of infinite solubility in water at temperatures higher than 73 “C. In the present paper, the method is applied for extracting ferric thenoyltrifluoroacetonate, which is known as a slow extraction system (7) (12 hours are required to establish equilibrium during extraction of this complex), and the behavior in the extraction is investigated. EXPERIMENTAL Reagent and Apparatus. Ferric perchlorate solution was prepared by first dissolving metal iron in hot perchloric acid, then evaporating almost to dryness with addition of several drops of hydrogen peroxide to prevent incomplete oxidation, and finally dissolving in dilute perchloric acid. Propylene carbonate (PrC03) was distilled under reduced pressure; bp 92 “C at 4.5 mm (8). 2-Thenoyltrifluoroacetone (TTA) was obtained from Wako Pure Chemical Inc. and used without further purification. The proton magnetic resonance measurements were made with a Varian T-60 or a Varian A-60 spectrometer operating
(2) B. G. Stephens and H. A. Suddeth, ANAL.CHEM.,39, 1478 (1967). (3) K.Murata and S. Ikeda, J. Inorg. Nucl. Chem., 32, 267 (1970). (4) R. E. Meredith and C. W. Tobias, J . Electrochem. SOC.,108, 286 (1961). (5) L. S. Marcoux, K. B. Prater, B. G . Prater, and R. N. Adams, ANAL.CHEM., 37, 1446 (1965). (6) R. F. Nelson and R. N. Adams, J. Electroanal. Chem., 13, 184 (1967). (7) Poskanzer and B. M. Foreman, Jr., J. Inorg. Nucl. Chem., 16, 323 (1961). (8) P. L. Kronick and R. M. Fuoss, J. Amer. Chem. SOC.,77, 6114 (1955). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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