Coulometric Generation of Titanium(111) in Aqueous Ethylenediaminetetraacetic Acid Solution Pierre Bourg,' Michel Asfruc,* and Jean Bonastre Laboratoire de Chimie Analytique, lnstitut Universitaire de Recherche Scientifique, Universite de Pau et des Pays de I'Adour, 640 16 Pau, France
Many applications of coulometric constant current titrations to organic analysis are available. They are mainly concerned with the use of oxidants or specific reactions. The control of electrochemical generation of strong reductants has been very rarely studied, and it is difficult to find information concerning selective coulometric reduction titrations. The main strong reductants which can be used for such purposes belong t o water-soluble redox couples: U(III), Cr(II), V(II), Ti(II1) . . . . Electrochemical generation of U(II1) and its application to coulometric titrations has been studied ( 1 , 2). T o our knowledge, however, U(II1) has not been applied to organic reductions. On the contrary, studies about the electrochemical generation of Cr(I1) and its application to coulometric organic titrations have been extensive (3-5). I t is a very powerful reductant in aqueous solution and is of general use; as a consequence, the selectivity of its action is very low (6) and other reductants would be necessary for selective titrations. Electrochemical generation of V(I1) does not seem to be very promising, but a comprehensive investigation is yet to be done. Electrochemical generation of Ti(II1) on the contrary has been investigated (7-14). Somewhat contradictory conclusions have been reached about the choice of the acidity of the solution and about the nature of the metal electrode; some investigators (7-9) have reported that the use of a metal with a high hydrogen overvoltage is necessary; others (10-12) propose to lower Ti(1V) overvoltage with strongly acid solutions, using platinum on which hydrogen overvoltage is, however, very small. The general and difficult use of Ti(II1) solution as a titrant in classical organic titrations prompted us to investigate its electrolytic generation in media where organic selective reductions are possible, and where its reducing power is intermediate between those of Cr(I1) and of a very smooth reductant already studied Fe(I1) ( 1 5 ) .
IId; IIIb,c; IV; VII), the slope of the dc polarographic wave as evaluated by ( E314 - E 114) in Table I, is low and these media were eliminated. Working with saturated electrolytic solutions is not easy and Ia and Va were eliminated as inconvenient. In tartaric acid media, Ti(1V) solutions are of low stability (Va,b,c). In the presence of a high hydrogen ion concentration, the electrochemical reduction of Ti(1V) is no more selective (IIIa). Parallel reactions must occur in KSCN media, as it is not possible to regenerate all Ti(1V) by homogeneous chemical oxidation from the electrogenerated Ti(II1) solution (IIa,b,c,d). Solutions containing EDTA are convenient; we choose VIc for current coulometric purposes, because of the high reducing power and experimental convenience (stability, high solubility of Ti(1V)). A cyclic voltammetric study demonstrated that first, the reduction oxidation mechanism is simple (Figure 1) and very fast (the potential difference between oxidation and reduction peaks is 59 mV). Second, the overall electrochemical reduction involves a one-to-one ratio between Ti(1V) and EDTA: in a series of
I
V
Figure 1. Voitammogram of 3.6 X 1 0 - 3 F Ti(lV)
Electrolyte composition, Vlc; hanging mercury drop electrode: potential sweep, 1.3 volt sec-'; reference, saturated calomel electrode
EXPERIMENTAL Throughout this work a classical (Tacussel) cell was used. Polarographic and cyclic voltammetric experiments were performed with Tacussel potentiostats ( P R T 500) and signal generators. The coulometric titrations and the plotting of current density-potential curves were obtained with a Beckman Electroscan 30. All chemicals used were reagent grade.
RESULTS Electrolytic Generation of Ti(II1). Electrochemical Ti(II1) was developed study of the reduction Ti(1V) e using different electrolytic solutions. Media where a fast overall reduction occurs are more convenient for coulometric purposes when electrochemical methods are intended for end-point detection. In some electrolytic solutions (Ib,c;
+
-
E"
Figure 2. Current density-potential curves for a platinum electrode
Present address, Cerabati, 60700 Pont Ste Maxence, France. Author to whom reprint requests should be addressed. 538
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Platinum area, 112 cm2; reference, SCE; solution composition: curve ( a ) electrolyte Vlc, 3.6 X lO-*f Ti(IV), curve ( b )electrolyte alone
~
~~~
Table I . Reversibility Check of Ti(1V) Polarographic Reduction tlectrolyte and composition
Reference
I.-CaCl, a): saturated solution b): 5 F , HC10.12 F c): 1 F , HC10.2 F 11.-KSCN a): 0.1 F b): 0.1 F,HClpH 1.68 c): 1.0 F , HClpH 1.70 d): 0.1 F , fiClpH2.25 III.-H,S04 a): 12 F b): 6 F c): 3 F IV.-Citric acid 1 F , H,SO, 0.6 F V.-Tartaric acid a): saturated solution, H2S040.6 F b): 1 F , H,SO, 0.6 F c): 1 F , HC1O.l F VI.-EDTA a): pH 1 to 2.5 b): CH,COO- and C10,pH0.5 to 7 c): 0.05 F , CH,CO,Na0.8 F HNO, 0.6 F pH 3.9 VI1.-HC1 variable concentration a A second ill defined irreversible wave appears.
(16, 17)
(16, 18, 19)
( 16,20,21)
E3/4
-
EI/4'mV
E,,2, V / X E
56 65 ill defined 56 58 58 ill defined 60 105 130 90 59
-0.12 4.23 -43.5 -0.46 -0.45 (5) -0.40 (5) --0.5 -0.12 -0.19 (5)Q -0.22" -0.27 -0.24 (5)
60 60 58 from 55 to 63
-0.27 (5) -0.44 -0.22
variable -0.37
57 ill defined
-4.9
Table 11. Efficiency Check for Coulometric Titration of Aromatic Nitroso Compounds by Ti(II1) Number
Compound
-1
Figure 3.
Current density-potential curves for a mercury electrode
Mercury pool area. 50.3 cm2; reference, SCE; solution composition: curve (a) same conditions as in Figure 2(a), curve ( b )same conditions as in Figure 3 ( a )but 3.6 X F Ti(lV), curve (c)electrolyte alone
Figure 4.
Current efficiency for electrolytic preparation of Ti(lll)
Curve ( a ) same conditions as in Figure 2(a), curve ( b )same conditions as in Figure 3 ( a )
Sample
of
weighed,
titra-
Result,
hiean
mole x 105
tions"
mole x 105
variation,
p-Nitroso1.150 i 0.002 12 1.141 i 0.005 -0.8 aniline #-Nitroso1.029 f 0.003 6 1.027 i 0.005 -0.1 phenol Chloro-2 1.000 i 0.007 5 1.002 + 0.005 +0.2 nitroso-4phenol Methyl-31.036 i 0.007 5 1.033 i 0.007 -0.3 nitroso-4phenol Dimethyl-2,6- 1.026 i 0.007 4 1.029 f 0.007 +0.3 nitroso-4phenol Dimethyl-Z,5- 1.013 i 0.007 5 1.001 i 0.005 -1.2 nitroso-4phenol a Experimental conditions. Solution: VI, (200 ml) + 0.036F Ti(1V). Mercury pool cathode: 50.3 cm2. Platinum anode isolated in the anodic compartment by a fritted disk. Potentiometric end point detection with a micro mercury pool electrode (0.5 cmz) and a commercial SCE. Current density about 0.8 mA cm-2. Electrolysis time: about 100 seconds. drogen ions: the influence of the acidity of the solution is a shift of the peak potential of -0.118 volt per p H unit (2.5 < p H < 5). These results confirms the redox reaction proposed by Pecsok (21):
experiments throughout which the formality of Ti(IV), F T ~ , TiOY" + 2 H' + 2 e _r TiY- + H 2 0 is the same and t h a t of EDTA ( F y ) changes, the cathodic peak current is not dependent of F Y if F y is greater than T h e dissociation of the complexes was not noticeable in F T ~on ; the contrary, it varies linearly with Fy if F Y is this study. smaller than F ~ i . The formation of the complex ion T i OY2- by dissolution Third, the overall electrochemical reaction involves a of Tic14 in the electrolytic solution containing EDTA is one-to-two ratio between the numbers of electrons and hyslow (two days at room temperature, half a n hour at 80 "C). ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
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TiOY2- solutions may then be stored for a long time (over a month) without any special care. Current efficiency for electrolytic generation of Ti(II1) may be evaluated by the Lingane Method ( 1 1 ) from the current density potential graphs plotted in Figures 2 and 3 for platinum and mercury electrodes, respectively. A very good current efficiency (>0.996) is obtained with the mercury electrode throughout the current density region available with our apparatus (Figure 4, curve a ) but with the platinum it is never as good (Figure 4,curve b ) and falls down sharply when the current density is high, because of competition of hydrogen ion reduction. Titration Efficiency. The homogenous reoxidation of electrogenerated Ti(II1) to Ti(1V) by a variety of organic or inorganic oxidants is very fast and quantitative. It is therefore possible to carry out a series of coulometric titrations with the same Ti(1V) electrolytic solution in the same vessel. For example, we report here experimental results on the titration of samples of aromatic nitroso compounds (Table 11) performed successively in the same electrolytic solution. Titration efficiency is better than 99% for all the compounds but the last. The lack of purity of the products available probably accounts for these results. Application of these results to selective coulometric titrations of organic compounds are presented elsewhere (22, 23).
LITERATURE CITED J. J. Lingane, Anal. Chim. Acta, 50, 1 (1970). G. C. Farrington and J. J. Lingane, Anal. Chim. Acta, 60, 175 (1972). A. J. Bard and A. G. Petropoulos, Anal. Chim. Acta, 27, 44 (1962). C. Sheytanov and N. Neshkova, Anal. Chim. Acta, 52,455 (1970). D. A. Aikens and Sister Maria Carlita, Anal. Chem., 37, 459 (1965). R. L. Williams and C. W. Sill, Anal. Chem., 46, 791 (1974). P. Arthur and J. F. Donahue, Anal. Chem., 24, 1612 (1952). (8)H. V.Malmstadt and C. B. Roberts, Anal. Chem., 27, 741 (1955). (9) H. V.Malmstadt and C. 6.Roberts, Anal. Chern., 28, 1412 (1956). (10) J. J. Lingane and R. T. Iwamoto, Anal. Chim. Acta, 13, 465 (1955). (11) J. J. Lingane and J. H. Kennedy, Anal. Chim. Acta, 15, 465 (1956). (12) J. H. Kennedy and J. J. Lingane, Anal. Chim. Acta, 18, 240 (1958). (13) L. B. Agasyan. E. R. Nikolaeva. and P. K. Agasyan, Zhur. fiz. Khim.. 22, (1) (2) (3) (4) (5) (6) (7)
904 (1967). (14) N. I. Stenina, P. K. Agasyan. G. A. Verenstveig, Zhur. Fiz. Khim., 22, 91 (1967). (15) W. i. Awad, S. S. M. Hassan and M. T. M. Zaki, Anal. Chem., 44, 911 (1972). (16) L. Meites. “Handbook of Analytical Chemistry.” McGraw-Hill. New York, N.Y., 1963. (17) M. Kalousek, Collect. Czech. Chem. Commun., 11, 592 (1939). (18) Muira. Jap. Anal., 8, 5 (1959). (19) P. Grenier and L. Meites, Anal. Chim. Acta, 14, 482 (1956). (20) T. Dono, K. Morinaba, T. Nomura, Bull.’Nagoya lnst. Techno/., 8, 165 (1955). (21) R. L. Pecsok and E. F. Maverick, J. Amer. Chem. SOC., 76, 358 (1954). (22) P. Bourg, Thesis, Pau. 1973. (23) M. Astruc, J. Bonastre. and P. Bourg, Analusis, submitted for publication.
RECEIVEDfor review July 16, 1974. Accepted October 29, 1974.
Profiles of the Polynuclear Aromatic Fraction from Engine Oils Obtained by Capillary-Column Gas-Liquid Chromatography and Nitrogen-Selective Detection M. L. Lee, K. D. Bartle,’ and M. V. Novotny2 Department of Chemistry, Indiana University, Bloomington, Ind. 4740 1
Examination of used engine oils for the presence of certain characteristic compounds is of considerable importance in forensic science. Small amounts of used motorvehicle engine oils may be transferred to victims of “hitand-run’’ accidents, spilled a t the scenes of crimes, or carried on stolen engine parts and servicing equipment ( I ) . Polynuclear aromatic hydrocarbons (PAH) are known to be quite characteristic products of all combustion processes. Consequently, they build up in engine oils in a manner strongly dependent on the type of vehicle, its condition, and its combustion parameters. Thus, sufficiently detailed analyses of PAH in the investigated oil samples may often provide the means of “fingerprinting” for forensic purposes. Because of the extreme complexity of PAH mixtures (2) obtained under various combustion conditions, a lack of separation efficiency ( 3 ) is a major problem in PAH determination. Analyses of used oils for forensic “fingerprinting” have been attempted by both spectral methods ( I ) and liquid chromatography (1, 4 ) , but the low resolution available provided results of only limited value. On the other hand, methods for identifying oil spills in
sea water from characteristic profiles obtained by high-resolution GLC and a sulfur-sensitive (flame-photometric) detector have been described by Adlard et al. ( 5 ) . We have recently outlined (6) a similar approach for screening air pollutant PAH in urban and industrial areas and have found that the profiles of polynuclears are unique to the area of sampling. In this publication, we report the extension of such studies to “fingerprinting” PAH fractions in used engine oils. A simple method described here consists of the extraction of engine oils and their partition between cyclohexane and nitromethane. An aliquot of the nitromethane fraction (containing mostly PAH) is then sampled and analyzed by a high-efficiency glass capillary column by means of a precolumn concentration method (7). High-resolution “fingerprints” were observed for different oil samples. The diagnostic power of this rapid method is further aided by the use of a novel detecter (8, 9) which records, selectively, nitrogen-containing compounds present in the PAH fraction, and thus creates a complementary type of characteristic profile.
On leave from the Department of Physical Chemistry, Universit of Leeds, Leeds, England. YAuthor to whom a l l correspondence should he directed.
Fifty-microliter samples of oil taken from the o i l pan of four different automobiles and a 50-pl sample of an unused commercial brand oil were each partitioned between 10 ml of cyclohexane and
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
