the last traces of acid). The chelates were developed as white spots on a pink background by spraying first with 0.01% NiS04 in methanol, exposing the sheet to ammonia vapor, and spraying with 0.1% dirnethyglyoxime in ethanol.
RESULTS AND DISCUSSION Elution was initially attempted with formic acid on the resin in the chloride form as received. Poor separation was achieved because of the low affinity of the chelants and formate for the resin os. the chloride. Conversion of the resin to the formate yielded better separations. Table I indicates three solvents were required to separate the components. Solvent I (0.3M formic acid) resolved HEEDTA, HEIDA, and HEEDDA-DHEG. The last two chelants, being weaker, eluted with the solvent front. Solvent I1 (1.5M formic acid) resolved EDTA and NTA by partial elution, the hydroxy derivatives eluting with the solvent front. DTPA
tailed bad.ly and could not be resolved. Addition of 0.3M HCl to t h e formic acid (Solvent 111) eliminated the tailing and resolved DTPA from EDTA and NTA which eluted together. EDTA and NTA can be resolved from DTPA in Solvent, I11 and resolved from one another in Solvent 11. The above TLC system yields rapid, simple identification of the most common amino-acid type chelants. Additional specificity is gained with the chelant spray reagents.
LITERATURE CITED (1) Shoji Toyoshima and Yukio Nishimoto, Yakugaku Zasshi, 85, 235-239 (1965). (2j J. L. Savin and J. L. Sudmeier. Anal. Chem. 40, 418 (1968). (7,) D. G. Hill-Cottingham, J. Chromatogr., 8, 261, (1962). ( 4 ) C. Davies eta/.,J. Chromatogr., 18, 47-52 (1965).
RECEIVEDfor review June 26, 1974. Accepted October 3 , 1974.
Novel Approach to Reaction-Rate Based Determinations by Use of Transient Redox Effects V. V. S. Eswara Dutt and Horacio A. Mottola Department of Chemistry, Oklahoma State University, Stillwa ter, Okla. 74074
Industrial quality control and pollution studies. inevita bly, encounter situations where a large number of samples of similar nature have to be processed for the determination of a single species whose level of concentration is variable. Obviously, in such situations, the use of pj-e-calculated concentrations of reagents for each sample iricreases the operating cost and time. The use of the same reaction mixture for repetitive determinations in aliquots r,f the same or different samples can obviate this problem with the additional convenience that multiple reagent halidling is eliminated. A novel approach to fast, continuous kinetic-based determinations of a variety of chemical spet,ies using a flowthrough cell system and transient oxidation-reduction signals is present here. All necessary reagents contained in a single reservoir are continuously circulated, a t constant flow, through the cell into which an aliquot of the sample containing the species to be determined is quickly injected. The sought-for species participates in a very rapid redox reaction which dramatically perturbs the concentration of the monitored species. A subsequent, but slower reaction either regenerates the monitored species to its original concentration level or brings about :rl situation in which the original signal level is reinstated. This sequence of signal perturbations results in a recorded peak whose height is proportional to the amount of sought-for species in the sample. Because of the high level of concentrations of some of the chemicals involved in the reservoir solution and the nature of the systems studied up to now, signal monitoring seems to be confined + d absorbance of radiant energy, although some other specialized cases may lead to a situation amenable to a similar treatment by use of other monitoring procedures. In summary, the development of reaction rate (kinetic) methods based on the characteristics outlined above requires rapid, initial “indzcator reaction” resulting in a drastic, almost instantaneous change in signal level, products generated by the subsequent reactions (other than the monitored species) absorbing a t wavelength(s) different
from those of the monitored species (or the reagent mixture as an entity), and restitution of the initial signal level by a subsequent reaction(s). Fast “indicator reactions” also minimize errors due to changes in flow rates. I t should be noted that suitable adjustment of catalyst (if involved) and/or reagent concentrations, as well as other parameters affecting rates, can provide reaction rates of the desired values without sacrificing sensitivity. Recently we came across a series of reactions which have these characteristics and show promise for future developments along these lines. The oxidation of tris( 1,lO-phenanthroline)iron(II), ferroin, by chromium(V1) in presence of oxalic acid as a promoting activator ( I ), is one of them. The oxidation of ferroin is normally slow and rather minute concentrations of oxalic acid markedly accelerate the rate. The use of high concentrations of oxalic acid, on the other hand, considerably increases the rate of oxidation of ferroin by mass action, but after a few moments the ferroin is regenerated ( I ) . This is because a single species, viz., oxalic acid, acts first as an accelerator for ferroin oxidation and then is a reducing agent of ferriin when the concentration of this species has built up a level which results in the predominance of the ferroin regeneration step. The reactions involved may be represented as follows: CrW)
+
ferroin
t
o x a l i c acid
Cr(II1)
+
ferriin
1
+ oxalic acid
Experiments have been conducted to see whether this reagent regeneration cycle could be used for continuous determination of Cr(V1). A 250-ml aqueous reaction mixture containing 8.0 X 10-5M ferroin and 0.40M oxalic acid was constantly circulated through an absorption cell by means of a peristaltic pump. The outlet from the cell was fed continuously back to the reservoir. The reaction mixture was constantly stirred both in the reservoir and in the cell with the help of magnetic and air driven stirrers (2) respectively. Figure 1 shows the schematic of this setup. A typical signal profile is shown in Figure 2; the peak height is propor-
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 7 , NO. 2 , FEBRUARY 1975
357
I
I
B
1' I
'-
I
b Figure 1. Flow-through cell and flow-through loop for repetitive and
continuous injection determinations ( a ) Hypodermic syringe and sample injection port, ( b ) Magnetic stirring bars, ( c ) Peristaltic pump (Masterflex with SRC Model 7020 speed controller and 7014 pump head), ( d ) Reagent solution reservoir. (The flow-through cell is 13-mm 0.d. and 70 mm high, equipped with 5 12/80 for easy disassembly and cleaning. A flow rate of 15 ml/minute was used in the determinations illustrated in this note)
1
Figure 3. Signal profiles in the determination of vanadium(V), manganese(Vll),and cerium(lV)using the diphenylamine sulfonate indicator system Conditions for curves A, 8, and C: Diphenylamine sulfonate 0.025%, perchloric acid 1.5M, oxalic acid 0.25M. total reservoir volume 200 ml, concentration of vanadium(V) in flow-through cell: ( A ) 0.51 pg/ml, (8) 0.34 pg/ml, (0 (7.17 pg/ml. Conditions for curves D, E, F, G, H, and /:diphenylamine SUIfonate 0.01 %, sulfuric acid O.lM, total reservoir volume 200 ml, concentration o f manganese(Vii)in flow-through cell: ( 0 ) 0.21 pg/ml, ( E ) 0.14 pg/ml, ( F ) 0.07 bGg/ml; concentration of cerium(lV) in flow-through cell: ( 0 2.79 pg/ml. ( H ) 1.86 pg/ml, (0 0.93 pg/ml
B'
w
.Figure 4. Signal profiles in the determination of hydroquinone, isonia-
Figure 2. Signal profiles in the determination of chromium(V1)
zid, and chloropromaaine hydrochloride
Experimental conditions are as described in the text. Concentration of chromium(V1) in flow-through cell: ( A ) 1.38 pg/ml, ( 8 ) 1.15 pg/ml. ( C ) 0.92 wg/mi, ( D ) 0.69 pg/mi, ( E ) 0.46 pg/ml, ( f )0.23 pg/mi. Typical moments of sample injection indicated by arrows
Conditions for curves A, 8, and Cc cerium(iV) 8.0 X 10-4Min 0.1Msulfuric acid (monitored at 525 nrrr), total reservoir volume 100 ml, concentration of chloropromazine hydrochloride in flow-through cell: ( A ) 5 pg/ml, ( B ) 10 pg/ ml, (C) 15 pg/mi. Conditions for curves D, E, and F: vanadium(V) 2.0 X 10-3M, phosphoric acid 1.5M, osmic acid 0.002% (monitored at 425 nm), total reservoir volume 100 mi. Concentration of isoniazid in flow-through cell: ( D ) 1 pg/mi, ( E ) 2 pg/ml, ( F ) 3 pglmi. Conditions for curves G, H, and I: chromium(V1) 3.33 X 10-4M, sulfuric acid O.lOM, oxalic acid O.OlM, ferroin 1.00 X 10-4M (monitored at 510 nm) total reservoir volume 100 mi. concenvation of hydroquinone in flow-through cell: ( G ) 2.9 pg/ml. ( H ) 2.2 pglml, ( I ) 1.6 pg/ml
tional to chromium(V1) concentration. As many as 50 samples containing 0.90 ppm of chromium(V1) were manually injected in a time period of about 30 minutes and the chromium(V1) concentration was evaluated with 4.97% standard deviation. Better precision, of course, can be achieved by mechanically controlled sample injection. In the COD determination of water samples, where the final step necessitates the determination of unreacted chromium(VI), the adaptation of the repetitive method described here can offer convenience. The same indicator reaction could be utilized for the repetitive determination of a variety of reducing species. Consider, for instance, a mixture containing sulfuric acid (0.25M), large excess of chromium(VI), (1.0 X 10-3M), ferroin (2.0 X 10-4M) and a low concentration of oxalic acid (4.0 X 10m3M,large enough to promote the oxidation of ferroin but not sufficient enough to react with ferriin), functioning as the reservoir solution. In this situation, ferriin reacts with any reducing agent like ascorbic acid, uric acid, or hydroquinone to yield ferroin, which subsequently gets oxidized by the excess 358
chromium(V1) present in the system (Figure 3). The transiently formed ferroin concentration would be, then, directly proportional to the amount of reducing agent injected. Although this reagent-regeneration type of chemical system is not often encountered, these observations seem to open up interesting possibilities for the fast determination of a variety of chemical species. If a reagmt does not contribute to absorbance a t the wavelength chosen for monitoring, it would, for instance, be introduced in a large excess and the transient signals of even irreversible chemical reactions could be analytically used for species determination. Observations on some reactions belonging to this category are briefly discussed below.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, F E B R U A R Y 1975
A transient blue chromophore can be seen in the oxidation of sodium diphenylamine sulfonate by vanadium(V) also in the presence of oxalic acid. Injection of cerium(1V) or manganese(VI1) into a large concentration of diphenylamine sulfonate produces a similar transient signal. In this case, however, it is not yet clear if the disappearance of color is due to regeneration, destruction, or simply to the formation of a new chemical species. In any event, vanadium(V), cerium(IV), and maganese(VI1) can be determined a t the microgram level by repetitive injection and signal monitoring a t 550 nm. Oxidation of brucine by chromium(V1) is markedly accelerated in presence of oxalic acid and, once again, the colored bruciquinone can be only transiently seen in presence of high concentrations of oxalic acid. This can be used for the continuous determination of either chromium or brucine by signal monitoring a t 530 nm. Signals proportional t o brucine concentration have been recorded by injecting 2 to 15 ppm of brucine into a reaction mixture 0.25M in H2S04,0.10M in oxalic acid, and 5.50 X lOP4Min &(VI). Metal phthalocyanines, an important class of industrial pigments, have been considered inferior. redox indicators because of the very unstable colored free radical intermediates produced upon oxidation with cerium(1V) ( 3 ) . With the set-up described above, continuous quantitative determination of metal phthalocyanines is possible. Injection of 0.5 to 5.0 ppm of cobalt phthalocyanine into 0.001M Ce(1V)0.1M HZS04 solutions gave transient signals proportional to concentration. Common tranquilizers, uiz.,chloropromazine hydrochloride and promethazine hydrochloride, similarly can be determined by use of a strongly oxidizing mixture and by monitoring the absorbance of the transient semi-quinone formed. The most extensively used anti-tuberculosis drug “isoniazid” (isonicotinic acid hydrazide) can be assayed through a transient signal by utilizing the catalytic effect of osmium on the internal oxidation-reduction of vanadium(V)- “isoniazid complex” ( 4 ). A reaction mixture comprising osmic acid, phosphoric acid, and an excess of vanadium(V) acts as a reservoir, and injection of isoniazid sample to this results in the transient appearance of vanadium(V)-isoniazid com-
CORRECTION
plex which absorbs a t 425 nm. This is because, in the presence of osmium, the complex is highly unstable and rapidly decomposes to give vanadium( IV), nicotinic acid, and nitrogen. This procedure seems to be well suited for the quick assay of isoniazid content in pharmaceutical preparations. Use of a mixture of arsenic(II1) and iodide as a reaction reservoir permits fast and continuous determination of cerium(1V) and halates (such as bromate and periodate) by monitoring the transient generation and reduction of iodine (coupled with or without starch). The reactions are:
--
As(II1) + I, As(V) + I‘ + Ce(IV) Ce(II1) A I, (Reservoir mixture: As(II1) + I-) I-
Likewise, the bromate and periodate oxidation of organic dyes like a-naphthoflavone and p-ethoxychrysoidine, catalyzed by vanadium(V), can be applied for the repetitive determination of low concentrations of halates. Some signal profiles obtained in most of the above itemized cases are shown in Figures 3 and 4. I t should be mentioned that in all the above cases the determination times are in the order of seconds, and effects arising from the decrease of reagent concentration during continuous analysis could always be made negligible by employing high initial concentrations. On the basis of the observations reported above, it is believed that this new approach to non-equilibrium determinations can be extended to several other syst,ems, and studies along these lines are currently under consideration.
LITERATURE CITED (1) V. V. S. Eswara Dutt and H. A. Mottola, Anal. Chem., 46, 1090 (1974). (2) H. Hall, E. E. Simpson, and H. A . Mottola, Anal. Biochem., 45, 453 (1972). (3) J. N. Brazier and W. I. Stephen, Anal. Chim. Acta., 33, 625 (1965). (4) P. V. K. Rao and G. B. E. Rao, Analyst (London).96, 712 (1971).
RECEIVEDfor review August 15, 1974. Accepted November 8, 1974. This work is supported by Grant GP-38822X, from the National Science Foundation.
CORRECTION
Effect of Hydrogen Ion Concentration on the Determination of Lead by Solvent Extraction and Atomic Absorption Spectrophotometry
this paper by R. J , E~~~~~~and H, E, parker, Chem., 46, 1966 (1974), there is an error on page 1970 in the first column. The last two lines in the last full paragraph should read:
Improved Penicillin Selective Enzyme Electrode
In this paper by L. F. Cullen, J. F. Rusling, Arthur Schleifer, and G. J. Papariello, Anal. Chem., 46, 1955 (19741, there is a missing line of print on page 1958. Line 35 in One begin a new paragraph and read: The Type I11 electrode system was designed to conve-
phase increased. The G5Zn data, however, reflected no change in the amount of zinc in the organic phase.
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