Kinetics of Oxalate Attack on Ferrozine-Iron( 11) Complex and Regeneration of Ligand in Determination of Iron(11) V. V. S. Eswara Dutt and Horacio A. Mottola* Chemistry Department, Oklahoma State University, Sfillwater, Okla. 74074
The kinetics of the reaction of tris[3-(2-pyridyl)-5,6-bis(4sulfophenyl)-l,2,4-triazine]iron(lI), the iron(l1) complex of ferrozine, with oxalate ion in a buffered medium (acetate buffer, pH 5.50, ionic strength 0.30 M NaCI) is reported. The rate law and rate constants (over a range of oxalate and complex ion concentrations) have been determined at 45-70 ‘C. This reaction is proposed for ferrozine regeneration in repetitive determinations of iron( II) in flow-through systems. The analytical implications of this regeneration and its use in kinetically assisted equilibrium-based determinations of iron(II) with either ferrozine or 1,lO-phenanthroline are presented. If 1,lO-phenanthroline is the chromogenic ligand, citrate should be used instead of oxalate.
We have recently reported conditions for fast, repetitive determinations of iron(I1) with ferrozine, 3-(2-pyridyl)-5,6bis(4-sulfophenyl)-l,2,4-triazine( 1 ) . These determinations are based on injection of the sample containing the iron(I1) into a continuously circulated reagent mixture. The analyte concentration is directly proportional to the height of the resultant transient signal. Sample injection takes place directly into the detection chamber and the transient signal is the result of two consecutive processes: 1)a fast chemical reaction producing the monitored species, and 2) the washing out of the generated signal as a result of the imposed flow. This novel approach aids in speeding u p the determination; but gradual buildup of background signal (using absorptiometric monitoring), produces a continuous baseline shift that limits the photometric scale useful for the determination. Although the baseline shift can be corrected by use of a double-beam system with circulation of the reagent mixture [plus accumulated product(s)] through a reference cell before the “sample” cell or by means of minicomputer manipulation of the data, this does not eliminate the decrease of the useful photometric scale. Destruction of the monitored species and regeneration of the main reagent, by a process imposed after signal measurement but before the diluted “plug” of solution containing the species reenters the detection chamber, should eliminate the baseline drift and considerably increase the number of samples amenable to determination with a fixed amount of reagent. This is of particular interest when using ferrozine, or other reagents of the 1,lO-phenanthroline family because, although very sensitive and somewhat selective, these are rather expensive reagents. In this paper we report some kinetic investigations of oxalate ion attack on the iron(I1)ferrozine comlex and the incorporation of an “in situ” reagent regeneration step based on this reaction, which eliminates the baseline drift in procedures with reagent recirculation. This regeneration permits the reuse of the spent reagent in determination of iron(I1) with ferrozine or 1,lO-phenanthroline. The essential features of the regeneration scheme are the use of the auxiliary ligand (oxalate or citrate) and a heatingcooling step into the flow-through loop between the reservoir and flow-through cell.
EXPERIMENTAL Apparatus. The modular spectrophotometric unit used has been described previously (2).A schematic of the experimental set up is given in Figure 1.The flask used to contain the reservoir solution was covered with asbestos tape and painted black to minimize heat loss and photolysis of the iron(II1) oxalate complex. The reservoir solution was heated with a 1000-watt Glas-Col heating mantle controlled with a “Powerstat” voltage regulator. A Lauda/Brinkmann Model K2/R circulator was used to maintain temperature in the reaction cell during kinetic studies and to circulate water through the cooling coil in repetitive determinations. Tygon tubing of 0.0655-inch i.d. and 0.1945-inch 0.d. was used for solution transport. Reagents. Stock solutions (4.00 X M) of FerroZine (Hach Chemical Company, Ames, Iowa) and 1,lO-phenanthroline (Eastman Kodak) were prepared in deionized-distilled water. The desired p H (5.00-5.50) was maintained with a 2.0 M solution of sodium acetate. The desired auxiliary ligand concentration was provided by solutions of potassium citrate or sodium oxalate, both 0.25 M. Dilute solutions of iron(I1) in sulfuric acid (0.05 M) were daily prepared by dilution of a 0.100 M stock solution standardized against dichromate. Reservoir solutions were made by mixing: 75 ml of 4.00 X lo-’ M ferrozine solution, 50 ml of 2.0 M sodium acetate, 25 ml of 0.25 M sodium oxalate, and 50 ml of water. Procedure for Repetitive Determinations. The reservoir solution (kept a t 95 “C) was circulated through the photometric cell a t a flow rate of 80 ml/min. Stirring was continuous in both the reservoir flask and the photometric cell. The iron-containing sample was introduced by means of the previously described injection system ( I ) and the transient signal was recorded in a Sargent SRG strip chart recorder. The amount of iron was evaluated by reference to a working curve ( I ) .
RESULTS AND DISCUSSION Regeneration of main reagent from the monitored species can be accomplished by two means in cases involving metal complexation: a) reaction of the monitored metal complex species with an auxiliary ligand forming a stronger complex with the metal ion than the main reagent does, or b) conversion of the metal ion (free and/or complexed) to an oxidation state in which reaction with an auxiliary ligand is favored (thermodynamically or kinetically) over reaction with the main reagent. In both cases the new chemical species should make negligible contribution to the monitored parameter (e.g., absorbance at a given wavelength). The first approach requires critical adjustment in the concentration of auxiliary ligand and for the particular case of iron(II), under optimum conditions for complex formation, it is difficult to find a complexing agent strong enough to displace ferrozine or 1,lOphenanthroline. Consequently regeneration of type b) as described above was applied. I t is known that ferrozine and 1,lO-phenanthroline are resistant toward oxidation ( 3 , 4 )and to set free the ligand from the complex with iron(I1) it suffices to oxidize the Fe(I1) to Fe(II1) in the presence of an auxiliary ligand (citrate or oxalate) which by complexation lowers the corresponding formal potential and by mass action sequesters the iron(II1) in complexed form. In the ligand regeneration step proposed here, oxidation was accomplished simply by means of dissolved aerial oxygen present in the reservoir solution. No re-
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2 , FEBRUARY 1977
* 319
0
' 1
1
I
0,./
d
Figure 1. Flow-through system for repetitive determinations of iron(ll) with ferrozine, incorporating ferrozine regeneration steps (a) Sample injection port and flow-through cell. (b)Reagent solution reservoir and heating mantle. (c) Peristaltic pump (Masterflex with SRC Model 7020 speed controller and 7014 pump head). (d) Voltage regulator. (e) Air-cooled reflux condenser. (f) Cooling coil and cooling reservoir. (9) Cooling water from Lauda K-2/R circulator. (h) To Lauda K-2/R circulator. (i) Thermometer. (j) Flow integrator. (k) Debubbler
generation takes place if oxygen is previously displaced by bubbling nitrogen into the reservoir solution. The proposed regeneration step is preceded by the reaction (in the detection area) between solvated iron(I1) and the ferrozine reagent, L2-
+
Fe(H20)G2+ 3L2- -+FeL34-
+ 6H20
(1)
The imposed flow transports the colored species FeL34- to the so-called "reagent solution reservoir" (Figure 1)in which the combined action of dissolved oxygen, elevated temperature, and high oxalate concentration converts iron(I1) to iron(II1) which becomes complexed with oxalate. The net effect is the regeneration of ferrozine (free to react with injected iron again) by the overall reaction: FeL:j4-
+ 3 C ~ 0 4 ~+-I/402(g) + H+ F e ( C ~ 0 4 ) 3 ~+- 3L2- + l/zHzO
-
(2)
To learn about the succession of chemical events involved in the reaction illustrated by Equation 2, several aspects of the kinetics of the oxalate-ion attack on the iron(I1) complex of ferrozine were investigated. Conventional (graphical) treatment of the rate data (obtained over a range of oxalate and FeL34- concentrations) gave the following rate law, valid a t 45-70 "C and up to an oxalate concentration of approximately 1.7 X
M:
-d[FeL34-]ldt = h1[FeLa4-]
-
+ h~[FeL:3~-] [C2042-]
(3)
and the following rate-controlling steps: FeLs4-
ki
FeL22-
+ L2-
(4)
and
These findings agree with the general reactivity pattern reported for the reaction of the iron(I1)-ferrozine complex with hydroxide, cyanide, and peroxodisulfate ions and for similar reactions of other low-spin iron(I1) chelates ( 5 ) .Equation 4, the rate-controlling step in the absence of oxalate, is followed by rapid formation and oxidation of iron(I1) hydroxide. Equation 5 represents the overall rate-controlling reaction in 320
30
10
OXALATE
50
CONCN x 1 0 2 M
Figure 2. Effect of oxalate ion concentration on the value of the average Experimental conditions: first-order rate constant, k = k l k2[C204*-]. Ferrozine, 8.0 X M; iron(ll), 2.4 X M; sodium acetate, 0.10 M; ionic strength, 0.30 M NaCI; temperature 58 "C.Intercept with the y axis agrees very well with the independently determined value for k l of 7.0 0.2 x 10-4 s-1
+
*
the presence of oxalate. Subsequent reactions of FeLZ2- and FeL~(C204)~ with - C Z O ~and ~ - the oxidation step: Fe(C204)s4-
+ 3/402(g)+ H+ + F e ( C ~ 0 4 ) 3 ~+- IhH20
(6)
are very rapid in the temperature range studied and a t the oxygen concentration levels normally found in distilled water. Although in a nitrogen atmosphere (complete expulsion of oxygen), no decomposition of FeL34- was observed even a t 100 "C, approximately equal decomposition times were observed in solutions saturated with oxygen and those containing the concentration of dissolved oxygen normally found in distilled water. Bubbling of air into the reservoir solution (Figure 1) is however recommended, to compensate for the gradual depletion of dissolved oxygen due to the high temperature in the reservoir vessel. The estimated values for hl and h2 are 7.0 & 0.2 X s-l, and 1.30 f 0.20 X 10-1 1. rnol-l-s-l, at 58 "C, respectively. The estimated value for the empirical Arrhenius energy of activation, E,, is 23.3 kcal.mo1-I. A plot of the average first-order rate constants for the reaction vs. oxalate concentration (Figure 2) shows an increase in rate up to an oxalate concentration of about 1.70 X M. The recommended oxalate concentration for ferrozine regeneration is well in the region characterized by an almost pseudo-zero-order dependence on - preoxalate. Considering that a t a p H of 5.50, C Z O ~ is~ the dominant oxalate species, the first-order rate dependence up M (Figure 2) to an oxalate concentration of about 1.7 X requires some justification, particularly because of the bidentate nature of this ligand and the pseudo-zero-order dependence on oxalate above 1.7 X M. T o interpret these observations one possible mechanism would be that reaction 5 is actually the result of two consecutive steps, with oxalate ion acting as an unidentate ligand and forming an intermediate via a rapid prior equilibrium step:
ANALYTICAL CHEMISTRY, VOL. 49, NO. 2, FEBRUARY 1977
with the rate determining step following:
I
I
ooc-coo-
n
I
Table I. Effect of Temperature on the Decomposition of the Iron( 11)-Ferrozine Complexa 'OC
in which N N represents the coordination sites in L2-, the ferrozine ligand. At relatively low oxalate concentration, the concentration of the intermediate species and the overall rate are directly proportional to the oxalate concentration. At relatively high oxalate concentrations, a maximum amount of intermediate has been formed and a further increase of oxalate concentration does not increase the rate. Hence, the observed rate becomes the rate of ring completion according to reaction 8. For reagent regeneration in repetitive determinations, it becomes relevant to look a t the effect of temperature and auxiliary ligand(s) concentration in terms of the time needed for total decolorization of the FeLa4- complex. These are presented in Tables I and 11. From a practical viewpoint, a temperature of 95 "C in the reservoir vessel is a good compromise between the need of as high as possible a temperature to speed up the rate-controlling step and the water loss by evaporation. Of several ligands used, oxalate and citrate appear the most useful for the regeneration step. Ethylenediamine-N,N,N',N'-tetracetic acid, EDTA, and nitrilotriacetic acid, NTA, also aid in the decomposition of the complex, but results for iron(I1) were then low because of kinetic competition resulting in faster oxidation to iron(II1) by oxygen than complexation of iron(I1) by ferrozine. Phosphate cannot be used a t the experimental pH values because of precipitation of Fe(II1)phosphate. Fluoride can be used only at concentrations so high (about 0.25 M) that attack of glass surfaces is possible. Addition of oxalate or citrate to the system signiificantly aids in the decomposition of the monitored species (Table 11). At an overall concentration of 0.025 M oxalate or citrate, decomposition occurs in about 60 s at 95 "C. A further increase in concentration reduces the decomposition time (e.g. to about 45 s at 0.080 M, 95 "C) but also results in a twofold effect: loss in signal height due to a faster oxidation of iron(I1)to iron(II1) and slower formation of FeL34-, to the point th,st signal removal by the imposed flow becomes significant. If 1 , l O phenanthroline is used as the main ligand, citrate should be ued as the auxiliary ligand since ferroin forms insoluble ion pairs with oxalate. The concentration of citrate needed, however, is about three to four times the oxalate used in the regeneration of ferrozine. Also, the photochemical decomposition of the iron(II1)-citrate, probably because of the high citrate concentration, is more prominent than that of the iron(II1)-oxalate in ferrozine regeneration. The iron(I1)-ferrozinecomplex formed from stoichiometric amounts of iron and reagent completely decomposes in less than 60 s at 95 "C, even in the absence of the auxiliary ligand. As the ferrozine concentration is increased, decomposition is slowed down and a t 50-fold excess of ferrozine no significant decomposition is noted in 5 min, in the absence of the auxiliary ligand. The reservoir solution and the transport tubing as well as connecting parts should be protected from the laboratory diffused light. This is because in the presence of light, Fe(II1) oxalate and citrate (products in the regeneration step) undergo photodecomposition giving Fe(I1) and COz. The liberated Fe(I1) is free to combine with the regenerated ferrozine (or 1,lO-phenanthroline) and re-form the monitored species. The reservoir solution when prepared as described in the Experimental section was found to yield 100%recoveries for iron(I1) during five consecutive recycles. With a 200-ml res-
Temperature, "C
Time needed for complete decolorization
25
>2 h >10 min >10 min 120 s 100 s 60 s 50 s
50 70 80 90
95 100
a Experimental conditions: Ferrozine, 1.5 X lo-* M; sodium acetate, 0.50 M; sodium oxalate, 0.025 M; iron(II), 2.0 X M.
Table 11. Effect of Oxalate Concentration on the Decomposition of the Iron(I1)-Ferrozine Complexa Oxalate concn, M
Time needed for complete decolorization
...
>10 min
0.0080 0.0100
0.0150 0.0250 0.0400
200 s 120 s 90 s 60 s 50 s
0.0800
45 s
M; sodium Experimental conditions: Ferrozine, 1.5 X M; temperature, 95 "C. acetate, 0.50 M; iron(II), 2.0 X
ervoir solution containing 1.5 X M ferrozine and 0.030 M sodium oxalate, 200 samples (0.10 ml in volume) containing 6.0 X 10-4 M iron(I1) can be processed. If diffused light is excluded, the iron(II1) oxalate or citrate accumulated in the reservoir solution does not absorb at 562 nm (wavelength a t which the FeL34- complex is monitored) and no shift in the baseline signal was detected. Because of the time needed for regeneration, time intervals of about 60 s are needed between sample injections. Alternatively, and perhaps preferably, the reservoir can be continuously used at a sampling rate of 1sample/lO s for about 50 determinations/200 ml reservoir volume and then the solution heated for about 10 min at 95 "C and reused. Regeneration studies like those described in this paper with other metal complexes with reagents of the ferrozine and 1,lO-phenanthroline family are under consideration. Regeneration and recycling, whenever it is possible to implement them, results in a saving of operator time and cost of analysis.
ACKNOWLEDGMENT The assistance of Daniel Scheeler in some of the experimental work reported here is gratefully acknowledged. LITERATURE CITED (1) V. V. S. Eswara Dutt, A. Eskander-Hanna,and H. A. Mottola, Anal. Chem., 48. 1207 11976). (2) E. W.Ghlapowski and H. A. Mottola, Anal. Chim. Acta, 76, 319 (1975). (3) A. A. Schiit, "Analytical Applications of 1,lO-Phenanthroline and Related Compounds", Pergamon Press, New York, N.Y., 1969. (4) L. L. Stookey, Anal. Chem., 42, 779 (1970). (5) E. Roy Gardner, F. M. Mekhail, J. Burgess, and J. M. Rankin, J. Chem. SOC., Dalton Trans., 1340 (1973).
RECEIVEDfor review August 23,1976. Accepted October 25, 1976. This work was supported by the National Science Foundation (Grant CHE-76-03739). ANALYTICAL CHEMISTRY, VOL. 49,
NO. 2,
FEBRUARY 1977
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