inake it useful, especially in laboratories with limit,ed instrumentation. This method can be used for a preliminary separation of manganese in other determinations provided the conditions for stoichiometric precipitation are met. ACKNOWLEDGMENT
The authors are indebt’ed to Robert E. Bosshart’ for technical assistance.
LITERATURE CITED
(1) Comprehensive Analytical Chemistry,
C. E. Wilson and D. W. Wilson, ed.,
1701. IB, p. 349, Elsevier, New York,
1960. (2) Feigl, F., “Spot Tests,’’ YOl. I, p. 167, Elsevier, Sew York, 1954. (3) Feldman, F. J., Christian, G. D., Anal. Chim. Acta 33, 266 (1965). (4) Feldman, F. J., Christian, G. D., J. Electroanal. Chem., in press. ( 5 ) Xewberry, C. L., Christian, G. D., J . Assoc. O$ic. Agr. Chemists 48, 322 (1965).
(6) Willard, H. H., Diehl, H., “Advanced Quantitative Analysis,” p. 175. \-an Sostrand, Xew York, 1943. FREDRIC J. FELDMAN GARYI>. CHRISTIAN Division Of Biochemistry
Walter Reed Armv Institute of Research Walter Reed A r m i LIedical Center Washington, D. C. 20012 Mention of trade names of equipment does not constitute an endorsement by the United States Government, but is used for purposes of completeness of scientific detail.
Extraction of Anions Using Triphenylmethane Dyes SIR: Triphenylmethane dyes (10) are strongly colored anion acceptors which form complexes extracted by organic solvents. Krasnov, Kashirina, and Yatsimirskii have studied the physical chemistry of these extraction systems using a small concentration of dye and a large concentration of anion (4-9). The dye complexes were extracted into relatively polar solvents such as chloroform and isoamyl alcohol. Yang (12, 13) surveyed the use of these dyes as extractants of metal ion complexes such as tetrachlorogallium(1V) from strongly acidic systems. A review with more than 100 references has been published ( 1 ) . A method for the determination of nitrate by extraction of the crystal violet complex into chlorobenzene ha> been woiked out by Yamamoto and coworkers (1I ) . Iwasaki Utsumi, and Kang used methylene blue and 1,2-dichloroethane to extract perchlorate ( 3 ) . Fritz, Abbink, and Campbell ( 2 ) studied the extraction of perchlorate into n-butyronitrile using ferrous l,l0-phenanthroline as a cationic extractant. I n no instance has the interference of anionic detergents been considered a? a practical limitation to these methods. I n this work, the best conditions for selectively extracting perchlorate, periodate, and alkylbenzenesulfonate anions are explored. EXPERIMENTAL
Apparatus. A Beckman D B spectrophotometer was used in all absorbance measurements. Extraction apparatus consisted of 60-ml. separatory funnels equipped with stopcocks of Teflon. Reagents. Benzene and other solvents were analytical reagent grade. Crystal violet chloride, C. I. 42555, 96% dye content, was a product of Allied Chemical Corp. Crystal violet solution was prepared by dissolving 40.9 mg. of the dye in 100 ml. of distilled water, filtering, and extracting twice using 20 ml. of benzene. Sodium lauryl sulfate, mol. wt. 288.4,
was twice recrystallized from ethyl alcohol and dried. Stock solutions were prepared by dissolving carefully weighed quantities of the dried sodium or potassium salts of the anions in 0.1M sodium chloride. The solutions were approximately Ill. Smaller concentrations were prepared by dilution of the standard stock solutions using sodium chloride. Procedure. Ten milliliters of the sample, which should be lO-jJ4 in extractable anion, and 0 . l M in sodium chloride, were pipetted into a 60-ml. separatory funnel. Exactly 2.00 ml. of crystal violet solution were added, and 10.00 ml. of benzene. The funnel was stoppered, and inverted, not shaken, for 1 minute. Vigorous shaking causes haze formation in the benzene layer. The layers were allowed to separate, and the benzene layer was measured at 615 mp in a 1-cm. absorption cell against a benzene blank. d blank was measured using sodium chloride solution, and was subtracted from the sample measurements. The quantity of extracted anion was obtained by referring t o a standard calibration curve. Samples which contain concentrated chloride brines or seawater may be diluted to an ionic strength of 0.1 molal. Samples which contain complex salt mixtures may be referred to a synthetic solution blank which approximates the composition of the mixture. No satisfactory method has been found to resolve the mutual interference of alkylbenzenesulfonate anions with perchlorate or periodate anions. This should be considered when natural waters or commercial salt preparations are to be analyzed. It is advisable to prepare a large quantity of the dye solution, and to recheck the calibration curves frequently because of changes in the dye solution with aging. RESULTS AND DISCUSSION
Several dyes and solvents were tested to find a system in which the blank from 0.1M sodium chloride was small, the extraction of anions selective, and few interferences from other anions were present. The dyes crystal violet, methyl violet, basic fuchsin, and malachite green oxalate were investigated in
combination with ethyl acetate, diethyl ether, chloroform, isoamyl alcohol, nitrobenzene, carbon tetrachloride, oxylene, toluene, benzene, and heptane. The polar solvents such as isoamyl alcohol and ethyl acetate gave high blank values, larger than 0.05 absorbance units. This is especially true in the case of malachite green, which has an unsubstituted benzene ring in the molecule, making the molecule more easily extracted into nonpolar solvents. The best solvent proved to be benzene, and the best dye crystal violet. The other dyes were less selective in extracting anions. The magnitude of the blank in 0.1M sodium chloride was 0.053 absorbance unit using the stated procedure. It was of interest to study the effect of ionic strength on the extraction of the crystal violet dye. I n sodium chloride solutions, the blank increases until about 0.55M, then decreases slightly. At concentrations above 0.6Jf, a skin forms a t the interface between the benzene phase and the sodium chloride solution. On the other hand, when the ionic strength is adjusted using m a g n e h m sulfate, the blank remains quite low, approximately 0.025 absorbance unit, until an ionic strength of 0.55M is reached, a t which the absorbance increases slightly. The effect of ionic strength on the extraction of crystal violet is presented in Figure 1. From this phenomenon, we conclude that ionic strength or salting-out effects are not the cause of the extraction. While magnesium sulfate gives a smaller blank at low ionic strengths, sodium chloride was chosen as the supporting medium because seawater and most brines contain relatively large quantities of chloride anion. A number of anions were investigated, for possible interferences. I n general polyvalent anions are not extracted a t concentrations of approximately 10-2M. Halates did not interfere a t a concentration of 10-3Jf, and iodate was tolerated as high as 10-251. Nitrate, cyanate, bicarbonate, acetate, and benzoate are not extracted. Thiocyanate interfered VOL. 38, NO. 6, MAY 1966
791
IONIC STRENGTH
Figure 1 . Extraction of crystal violet with increasing ionic strength Curve A (NaCI) shows effect of chloride ion concentration. Curve B (MgSOa) shows that adjustment of ionic strength using nonextracted
M O L A R CONCENTRATION, X 106
magnesium sulfate does not have any effect until an ionic strength of 0.55 molal is reached
a t concentrations higher than l O - 4 M . Sulfite and cyanide interfere by decolorizing or reacting with the dye. Iodide interferes a t a level of lO-4,TfI but the other halogens do not extract appreciably. Large anions such as alkylbenzenesulfonates and tetraphenylboron extract very well. The other interfering anions could be extracted as crystal violet complexes, but could not be converted into analytical extraction methods because the degree of extraction a t higher concentrations was small, typically 0.2 to 0.3 absorbance unit. Tetraphenylboron is extracted at concentrations as low as lO-5M, but the reproducibility is poor and the color fades a t the rate of approximately 0.05 absorbance unit per minute. The extraction of alkylbenzenesulfonate anions was demonstrated using sodium lauryl sulfate. Beer’s law is followed below 10-5hl. Other anions which extract well a t this concentration level are perchlorate and periodate. Typical calibration curves are presented in Figure 2. In Figure 2, the calibration curves do not extrapolate to zero absorbance after subtraction of the blank. I n the case of perchlorate and sodium lauryl sulfate, a slightly high intercept is obtained. This might be due to a synergistic effect of these anions on the extraction of crystal violet. I n the case of periodate, a negative intercept is obtained. One explanation is the greater extraction of the blank below 0.3 x lO-5M periodate. The spectrum of the dye in water and the dye-perchlorate complex in benzene is presented in Figure 3. The spectra of the other extracted anion complexes in benzene are similar. The extraction of certain anions by crystal violet is a complex phenomenon. Not only is an equilibrium between ion pairs and dissociated species present, but the distribution ratio of the ion pairs between the benzene and aqueous
792
ANALYTICAL CHEMISTRY
Figure 2. Calibration curves for extraction of crystal violet anion complexes Curve A shows extraction of perchlorate anion from 0.0 to 1 O-KM concentration. Curves B and C show extraction of periodate and sodium lauryl sulfate within the same concentration range
I
WAVELENGTH, m r
Figure 3. Spectrum of crystal in violet water in 1 O-5M (3-month-old soh.) is shown in curve A Curve B shows spectrum of crystal violet perchlorate anion complex in benzene. The spectrum of the periodate and alkylbenzenesulfonote anion complex in benzene i s similar to Curve B
phase is subject to change. By multiple extractions, the distribution ratio of crystal violet perchlorate was estimated to be 1.4. Thus, not all of the anion is extracted in one equilibration. Furthermore, multiple extractions are impractical a t this low level, due both to the interferences of other anions and to the increased influence of the blank. Another disadvantage of the crystal violet extraction method is the short concentration range, only 0.1 to 1.0 X lO-5M. Advantages of the method include sensitivity, selectivity for the individual anions Perchlorate, periodate, and alkylbenzenesulfonate, freedom from interference by many other anions, and spectrophotometric absorption in a
wavelength region which is relatively free from interferences. The precision of the measurements is well within 0.005 absorbance unit. The calibration curves should be checked frequently because new solutions of crystal violet or aging of the solutions tends to change the slope of the calibration curve. This is true even though the concentration of the dye has been adjusted t o a standardized value by absorbance measurements of diluted dye samples. Although filtration of the dye solutions is advisable, extracted samples should not be filtered because a substantial quantity of the dye-anion complex is adsorbed onto the filtering medium, whether paper or glass.
The method is recommended for determining small concentrations of alkylbenzenesulfonate, perchlorate, or periodate in dilute brines or seawater, but not in mixtures of one or more of the anions. A test of Atlantic coastal seawater, diluted to 0.1M in chloride, showed an absorbance of 0.029 above the blank. It is more likely that this represents detergent than perchlorate. No satisfactory method for differentiating between anionic detergents and the other extracted ions has been developed for this method a t the present time. LITERATURE CITED
(1) Blyum, 1. A., Pavlova, N. N., Zauodsk. Lab. 29, 1407 (1963); C. A . 60, 34639 (1964).
(2) Fritz, J. S., Abbink, J. E., Campbell, P. A., ANAL.CHEM.36, 2123 (1964). (3) Iwasaki, I., Utsumi, S.,Kang, C., Bull. Chem. SOC.J a p a n 36, 325 (1963); C . A. 58, 131098 (1963). (4) Krasnov, K . S., Radiokhimiya 5 , 222 (1963); C. A . 6 0 , 4 8 6 5 ~(1964). (5) Krasnov, K. S., Kashirina, F. D., Ibid., 6(2), 191 (1964); C. A. 61, 4519c (1964). ( 6 ) Krasnov, K. S., Kashirina, F. D., Ibid., 6, 651 (1964); C. A. 62, 13921b f 196.5’). ( 7 ) Krasnov, K. S.,Kashirina, F. D., Yatsimirskii, K. B., Tr. Komis, PO Analit. Khim. Akad. Nauk SSSR, Inst. Geolchim. i Analit. Khim. 14, 59 (1963); C. A . 5 9 , 1 2 2 3 9 ~(1963). 18) Krasnov. K . S.. Yatsimirskii. K. B.. ‘ Kashirina,’ F. D.,’ Radiokhimiya 4, 148 (1962); C. A . 58 12008e (1963). (9) Zbid., p. 638; A . 58, 120089 (1963). (10) Venkataraman, K., “The Chemistry
6.
of Synthetic Dyes,” Vol. 11, p. 705, Academic Press, New York, 1952. (11) Yamamoto, Y . , Uchikawa, S., Akabori, K., Bull. Chem. SOC.Japan 37, 1718 (1964); C . A. 62, 4616f (1965). (12) Yang, Ping-Yu, Hua Hsueh Tung Pao 1964 ( l o ) , 606; C. A . 62, 11121e (1965). (13) Ibid., p. 628; C . A. 62, 11121f (1965).
C. E. HEDRICK BRUCEA. BERGER Department of Chemistry University of Pennsylvania Philadelphia, Pa. 19104 WORK supported under the National Science Foundation Undergraduate Research Participation Program NSF GE 6436. Division of Analytical Chemistry, Winter Meeting, ACS, Phoenix, A r k , January 1966.
Use of Citrate-EDTA Masking for Selective Determination of Iron With (IO-Phenanthroline SIR: I n situ masking is an effective, convenient method for eliminating interferences in the colorimetric determination of iron with 1,lo-phenanthroline. For example, such metals as AI, N o , Sb, Sn, T h , Ti, U, W jand Zr can be masked with citric acid; Bi, Cd, Cr, and Zn with EDTA; Cu with mercaptoacetic acid; and T a with tartaric acid ( 1 ) . This paper describes the use of duo citrate-ED’T.1 masking for the selective determination of iron with 1,lO-phenanthroline. With duo citrateEDT-4 masking, the selectivity and versatility of the 1,lo-phenanthroline procedure is greatly enhanced such t h a t it can be used advantageously for the routine analyses of samples with unknown and widely varying compositions. Factors Affecting the Formation of the Iron (11)-1, 10 -Ph e n a n t h r o l i n e Complex in Citrate-EDTA Medium. The formation of t h e iron(I1)-1,lOphenanthroline complex is dependent on t h e concentrations of the 1,lOphenanthroline chromogen, the hydroxylamine reductant, and the citrate and EIlT,1 masking agents. The color developnient is also dependent on p H because changes in p H alter the effective concentrations of each of these reagents. This is illustrated by the following equilibria : Fe(I1,III)-EDTA
+ NHLOH H+ 1 2
1
c
-t
OH-
NH30H +
formation of the iron(I1)-1,lO-phenanthroline complex and the displacement of Equilibrium 1 to the right are directly proportional to the phenathroline and hydroxylamine concentrations and inversely proportional to the H2EDTA+ concentration. Equilibria 2, 3, and 4 show the effect of acidity on the hydroxylamine, H2EDTA-* and phenanthroline concentrations. Citrate, like EDTA, complexes iron(II1) and its effect on Equilibrium 1 is similar to t h a t of EDTA. With the citrate and EDTA levels arbitrarily maintained a t 2.5 mmoles and 1.25 mmoles, respectively, and the hydroxylamine and phenanthroline levels maintained at 2 mmoles and 0.10 mmole, respectively, complete reproducible color development is obtained at p H 5.0 to 6.5 in 25 minutes at room temperature. Above or below this p H range, the color development is not complete. I n addition to the 2.5 mmoles of citrate and 1.25 mmoles of EDTA used for masking, up to 2.5 mmoles of citrate or EDTA or 1.0 mmole of oxalate or tartrate do not affect the color development. At higher levels, however, complete color development is obtainable only with heating. Heating at 60’ C. for 15 minutes, cooling, then standing for 25 minutes, is satisfactory.
Phen
1 Fe(I1)-Phen
H+13/0= Phen.H+
where Phen is the unurotonated 1,lOphenanthroline molecuie and Phen . H + is the protonated 1,lO-phenanthroline ion. \There iron is predominantly in the + 3 oxidation state, both the rate of
+ H2EDTA-+ + NP
11
H+ 4
OH-
H4 EDTA
The 25-minute standing Deriod is necessary because the iro;(iI)-l,lO-phenanthroline complex dissociates at elevated temperatures, then reforms slowly at room temperature (Figure 1). The
data plotted in Figure 1 were obtained by measuring the absorbance of the iron(I1)-1 ,l@phenanthroline complex at various temperatures as it was warmed from 20’ to 50’ C. The solution was then cooled rapidly and the absorbance of the iron complex was measured a t 5-minute intervals. Effects of Diverse Ions. The effects of diverse ions were studied by analyzing synthetic samples containing 54.6 gg. of iron and varying concentrations of cations admitted generally as the nitrate or chloride salts, and anions admitted as the acid or as the alkali metal salts. A “t”test at the 95% confidence level was used to establish interference. For a single determination, the allowable limits were 1 0 . 0 0 5 absorbance unit or ~k0.60gg. of iron.
Table 1 . Effect of Diverse Metal Ions at Ion to Iron Molar Ratios above the Maximum Tolerance Ratios
Diverse ion
Cr(II1, VI) cum) .
I
Ni(I1)
Diverse ion to iron
molar ratio 200 12.5 25 50 100 200 50 200 25 50
Interference,
-
+--
c/o
5.2 1.6 1.8 4.1 6.1 2.7 2.7 5b
2.0 -12.5 a These data were obtained with 54.6 fig. (-0.001 mmole) of iron present. A difference of this magnitude was ob-
served when the sample absorbance was measured between 25 to 35 minutes after color development; however, the absorbance decreases fairly rapidly upon standing.
VOL. 30,
NO. 6, MAY 1966
793
