Determination of Reaction Stoichiometries by Flow Injection Analysis A Laboratory Exercise Angel ~ j o s ,M. Dolwes Luque de Castm, anl Miguei Vakircel University of C6rdoba. Cbrdoba, Spain Flow lnjprtion Analysis, FIA, is a nonsegmented, continuous flow technique (1.2) that has enjoyed great development since its birth in 1975 ( 3 . 4 ) . Its great success can he ascribed to i ~ qsimplicity, reproducibility, high sampling frequency, versatility,and,especially,toadrastic reduction in theusual manipulation operations involved in conventional analysis methods. 'Taking advantage of these features we have developed an easy and simple photometric method intended for calculation of com~lex-formationand redox reaction stoichiometries. This method is based on a closed-loop configuration recently proposed by the authors (5) and may be included in the laboratory curriculum for chemistry undergraduates. In addition to illustrating the experimental procedure used for calculation of stoichiometries and kinetic apparent rate constants, this method incorporates the FIA technique, of doubtless pedagogical interest. Experimental Reagents Aqueous solutions 1.8 X 10-3 Min iron(I1)and o-phenanthroline. Acetate/hydroxylaminehuffer (10 g of sodium acetate and 10 g of hydroaylammonium chloride dissolved in 100 mL of water). Aqueous solutions 5.0 X 10W M in potassium dichromate and ascorbic acid at pH 2.0 and 3.5. Equipment A Pye Unicam SP6-500 spectrophotometer connected to a Radiometer REC-80 recorder was used as detector. A Gilson Minipuls-2 peristaltic pump, a Hellma 178.12QS flow cell (inner volume 18pL), three Teeator L-100-1 injection valves and a Tecstor TM I1 chemifold were also used. Manifold The manifold employed is shown in Figure 1.The carrier is aspirated by pump P. Valves V, and Vb perform the simultaneous injection (merging zones mode ( 1 , 2 ) ) of the reaction ingredients, which merge simultaneously at point R, where the reaction starts. Valve S is also an injection valve, but it has been adapted to select channels 1 and 2 to feed channel 3. When it selects channel 2, channel 1 is wastea and the reacting plug is elwed and kept constantly circulating hy means of pump P' along a loop that contains the detector. Whenever the plug passes through the detector, a transient signal (FIA peak) is obtained in the recorder, until, after recording n peaks, physical (homogenization of the plug throughout the liquid volume enclosed in the loop) and chemical equilibrium (end point of the reaction) are attained. The shape of these recordings is also depicted in Figure 1. In addition to the absorbance at equilibrium (A.), this configuration allows monitoring the reaction at different times (At) with a single detector. Procedure Since the volume enclosed in the loop is kept constant, the Joh (continuous variations) (6) and Yoe and Jones (molar ratio) (7) methods may be applied to equimal solutions of both reagents involved in the reaction by just modifying the volumes of the reagents injected through V. and Vb. Thus, in order to apply the molar-ratio method, one of these volumes is kept constant while the other is changed; for the continuous variations method the V. and Vb volumes are changed simultaneously, keeping constant the sum of hoth. The ahsorbance values At and A- for each experiment are suitably collected.
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Journal of Chemical Education
Figure 1. FIA configurntion fw calculation of stoichlomelrles and usual shape absorbance-time recordings for (a) Fe(ll)/o-phenlkollnesystem. (b) ascorbic acidldichromate system; P.P' denote peristaltic pumps. D, detector and R. recorder. of the
Iron (M)/o-phenanthrollne System This reaction yields a colored chelate with maximum absorption a t 515 bm. A sodium acetatehydroxylamine mixture is used as carrier to ensure hoth a slightly acidic p H and the occurrence of iron in oxidation state (ll).~o-Phenhthrolineand iron(1l) solutionsare injected throughvalves V,and Vb, respectively. T h e complex~formationis practically instantaneous; therefore, when the plug attains the loop the onlv nrocess takine place in i t is the chelate dis. imnortant . persion throu&out the carrre; enclosed in the loop. T h e t -. v ~ i c ashaue l of these recordines amears in Fieure la. The ~ o b k e t h o dhas been aiplikd by keeping V. Vb = 383.7 uL. Since the FIA configuration used provides a great amount of information, the values of the absorbance of the maxima and minima have been collected for each experiment and plotted against the molar fraction of Fe(I1) (l~eZ+l/(l~e~+1+ 1~1))(L= o-phenanthroline). Since we have used equimol solutions, this ratio can be expressed as the VL)) As can be observed from injected volume (VFJ(V& the plots in Figure 2a, the value of this ratio at the stoichiometric point is 0.25 in all cases, which implies that the stoichiometrv is 1:3 (cationhieand). T o a p d y the ~ o and e J k e s method the injected volume of the lieand was kept constant (Vr = 269.4 uL) while that of e the cation was changed from 30.0 137 p ~ : ~ habsorbance values found in each experiment were plotted against the VFe/VLratio(Fig. 2bJ, yieldingavalueof0.33at thestoichiometric point, consistent with the result found by the method describkd above.
+
+
Dichromate/As&ic Acid System In this case i t is the disappearance of dichromate that is monitored (A = 352 nm). both reaeents beine simultaneouslv injected through valve's V, and kb.~ i s t i l r e dwater a t thk same p H as the reagents is used as carrier. T h e experiments are carried out a t two pH's (2.0 and 3.5) resulting in different reaction rates. Similar to the above system, the different values of absorbance for each experiment were plotted as a function of the volume ratio vDiCi(vpic VA,,) (&tinuous-variation method) and VkrJVuic (molar-ratio method) a t both pH's. I n
+
Rate Constant Obtalned for the DlchromatelAscorblcAcld System at DiHerent Concentration Ratios
o:zo o i o
0'50 [FC'.] I [L]
Fogure 2. Plats tor calculatm of the ItOiChiomeWy 01 me Fe(ll)/pphenanthroline synsm. Measurements pertormed. (A) a1 equillorium. (B) at the lira minimum. (C) at me second minimum. (Dl at the third maximum. (a) Absorbance agalnsl IFe2+b(lFe2+l 1~1)(Job method) and (b) abmbance against (Yoe and Jones method). IFe2+1/t~1
+
every case, these ratios indicate that the dichromate/ascorbic acid stoichiometric ratio is 1 3 ; consequently, the reaction in which six electrons are exchanged iB CrzO?-
+ 3 C6HsO6 + 8 H' * 2 Cr3+ + 3 C6H606+ 7 Hz0
Since the recordings are indicative of the evolution of the reactions (of its kinetics), they may he used to calculate the experimental rate constant, ko, for~theoverall reaction from the expression, log (A, - A,) = log (A, - Ao) - kot/2.303, where At is the maximum absorbance of the peak obtained a t time t. By plotting log (A, - At) as a function o f t for each experiment, straight lines of slope -ko/2.303 are obtained, whichindicates that the reaction is firstorder with respect to dichromate (ascorbic acid is in excess).
Conclusions The determination of stoichiometries with the aid of the FIA configuration suggested here is of chemical and pedarorical interest. On one hand. the VIA methodoloplv offers major advantages over conventional analytical methods: simplicity, rapidity, low cost, and scarce manipulation. On the other hand, since the principle behind the FIA technique is kinetic (the transient sienal is obtained when neither physical nor chemical equil&rium has been attained) and since we verified that any FIA peak is useful to calculate (6, 7), we perstoichiometries by c ~ n v ~ n t i o nmethods al formed several kinetic determinations of stoichiometries to demonstrate that the attainment of equilibrium is unnecessary. The FIA confieuration used here. emnloved to imolement multidetection with a single detector a great amount of information and is hiehlv versatile as reeards the type of measurements for whicb it can be used.';rhus, i t allows carwine out measurements either a t the maxima or at the minima, depending on whether the samples utilized are diluted orconcentrated. As thesirnalat eauilibriumcnnalso heen known, kinetic parameters can be readily calculated, as demonstrated above. Literature Clted 111 Ruzieka. J.: Hensen. E. H."Flow lnirction Adusis": Wilcu: New Ymk. 1981
From the slopes of these lines can be obtained the value of ko for different IC6H~061/ICr2012-I concentration ratios (see table). Predictably, the constants are larger a t pH 2.0 and ~-I increase with increasing ICsHaOsI/ I C ~ ~ O Tratio.
Volume 63 Number 6 June 1986
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