Single standard processing in flow injection on concentration ...M.C. U. Arafijo et al. Single...

Journal of Automatic Chemistry, Vol. 19, No. 5 (September-October 1997) pp. 157-164

Single standard calibration and dataprocessing in flow injection titration basedon concentration gradients

M. C. U. Ara6jo, A. V. Santos,R. S. HonoratoDepartamento de Quimica, CCEN, Universidade Federal da Paraiba, C.P. 5093,58051-970, Joo Pessoa, PB, Brazil

and C. PasquiniInstituto de Quimica, Universidade Estadual de Campinas

This paper describes use ofgradients of concentration generated injqow injection (FI) systems to perform determinations based onpoints where the concentration of titrant and analyte are atstoichiometric ratio. Two procedures were developed. In oneprocedure the titrant is injected in a FI manifold and merges withthe sample which is continuously pumped towards the detector. Inthe other procedure the sample is injected and merged with thetitrant which is continuously pumped. Both techniques make use ofconcentration gradients of the sample or titrant generated in FImanifolds that contain a mixing chamber. This gradient iscalibrated employing only one standard solution (usually thetitrant) in order to convert any detector signal, obtained in theelapsed time after injection, to instantaneous concentration values.The ]tow system is microcomputer controlled and data are treatedto locate points where the concentration oftitrant and analyte are atthe stoichiometric ratio. These points arefound in abrupt changesof the signal x concentration curves obtained in the presence of thereaction. The method has been evaluated for determination ofFe(II) and acetic acid by spectrophotometric and conductimetricdetection, respectively. Results show a mean relative standarddeviation lower than 1%, an average accuracy of 1% and a highsampling processing capability (40 to 60 samples per hour).

Introduction

When performed manually, titrimetric methods arecumbersome and slow. To overcome these drawbacks,automatic batch and continuous flow systems have beendeveloped. Development of batch automatic titratorsstarted more than 80 years ago [1] and this continues[2]. Automatic batch titration, however, requires indivi-dual flasks to contain each sample and, despite efforts toincrease the speed of determination, the overall samplingprocessing is considered slow. Furthermore, batch proce-dures require mechanical units to deliver the titrant andto homogenize the titrant and sample mixture and theseunits are expensive.

The inconveniences of batch classical titration procedurehave been minimized, employing continuous flowmethods, since the classical work by Blaedel and Laessig[3]. Following this first approach, some interesting mod-ifications have been proposed to the flow methodology[4-7]. In those systems, at least one point can beobtained when the concentration of the titrant and

analyte are at the stoichiometric ratio of the reaction.Usually this point is reached by keeping the flow rate ofone of the solutions (sample or titrant) constant andmaking the other change linearly [3-5] or by keepingthe flow rate of sample constant and by exploiting linearconcentration gradients of titrant generated by an ex-ternal gradient chamber [6] or by coulometry [7]. Unlikeconventional titration which uses the concept of equiva-lent quantity, the flow approach employs the concept ofequivalent concentration.

With the advent of flow injection [8], a new flowprocedure (FI titration) was developed [9]. A peak-likegradient of the sample is generated in a carrier flowcontaining a titrant. Sharp signal changes indicate twoinstantaneous points where the concentration of thetitrant is at a stoichiometric ratio to that of the analytein the sample. The time interval elapsed between bothpoints is linearly related, under certain conditions, to thelogarithm of the sample concentration. Calibration,employing various standardized sample solutions, isnecessary to establish this relationship. To increase thesensitivity of such systems, a flow reversal FI has alsobeen proposed [10].

Another FI technique (single-point titration [11]) hasdemonstrated that a linear relationship exists betweenthe peak height and the concentration of an acid or basewhen a potentiometric sensor, like a glass electrode, isemployed. For spectrophotometric sensors, the samesituation was observed when neutralization reactionswere employed and followed with the aid of acid-baseindicators [12].

Although intrinsically based on the exploitation concen-tration gradients generated by changing the flow rate ofthe sample or titrant [3-5], by external gradient chamber[6] by coulometry [7] or by FI systems [9-12], all of theseflow titrations rely on calibration, performed with the useup to six standardized sample solutions, to relate theanalytical parameter to the sample concentration. There-fore, the direct use of stoichiometry is not found in flowtechniques.

The binary searching approach has recently been appliedto flow titrations [13]. This method makes use of variablevolumetric ratios between the titrant and sample solu-tion. Ratios are selected by the binary algorithm. About10 searching steps are sufficient to estimate, with areasonable precision, the stoichiometric ratio betweentitrant and analyte. The concentration of the titrantsolution is employed directly and no calibration is neces-sary. However, this method applies only to systems wherean indicator can be used by the binary searching algo-rithm to decide the next step in the searching process.

0142-0453/97 $12.00 (C) 1997 Taylor & Francis Ltd157

M. C. U. Arafijo et al. Single standard calibration and data processing in flow injection titration based on concentration gradients

Therefore, self-indicating titrations, as conductimetrictitrations, have not been performed.

In this paper, a new flow titration methodology isproposed. The methodology uses the concentrationgradient pattern generated by FI systems and is basedon the FI standard addition methodology previouslydescribed [14-16]. In the flow manifold, the concentra-tion of one of the solutions (sample or titrant) is kept at itssteady state during the whole process, by pumping itcontinuously. The other solution is injected and its con-centration is determined by the gradient calibrationtechnique 14-16].

The methodology described makes use of StoichiometricRatio Points (SRP), which are instantaneous concentra-tion points located between regions where excess oftitrant and analyte occurs. At these points, the instanta-neous concentration of the injected solution is at thestoichiometric ratio in relation to that of the continuouspumped solution. SRPs are found experimentally whenabrupt changes are observed: these experimental pointsare called SREPs (Stoichiometric Ratio ExperimentalPoints). The SREPs are found in curves when titrationsignals (absorbance or conductance, for example) areplotted against the concentration of the injected speci-men, replacing the time points by the values of theheights of the calibration profile at that time. Thesevalues are proportional to the concentration of theinjected specimen at that time and in absence of reaction.This procedure ensures that the changing property isevaluated against the instantaneous concentration of theinjected specimen in a similar fashion to conventionaltitrations which are evaluated for end point determina-tion in a curve of the property versus added amount oftitrant.

H20

NaOH

!,,’,,,,’,,,I2.3

t,0 crn

P

H20

ml.min"t

2.2

2.4KM.O

.2so :’"

P

(B) s[ w

300 p,L J cm-6.0 cm Io, cm1,0 cmlOOcmFigure 1. FI manifolds employed by the SIT methodology. (A)manifoldfor conductimetric determination of acetic acid in vinegarand (B), manifoldfor spectrophotometric determination ofFe(II).I, injection point; P, peristaltic pump; M, mixing chamber; W,waste line; S, spectrophotometric flow cell and C, conductimetricflow cell. For TIT, the titrant solutions are replaced, in bothmanifolds (A) and (B), by sample solutions and the volumeinjected refers to that of the titrant solution.

The FI titration methodology was evaluated using spec-trophotometric detection for determination of Fe(II),employing KMnO4 as titrant, and for determination oftotal acidity in vinegar, employing a conductimetricdetection and NaOH as titrant.

Experimental

Reagents, samples and standard solutions

0"1 M stock solutions of KMnO4, NaOH, and Fe(II)were prepared and standardized employing conventionalmethodology [17]. Acid solutions employed in theiron(II) titrations were H3PO4, 0"03M and H2SO4,0"01 M. Other solutions were prepared by suitable dilu-tion of the stock solutions.

Samples of iron ore and alloys were treated and dissolvedusing well-known procedures [17]. Vinegar samples wereanalysed directly from the flask without any previoustreatment. The reagents employed were of analyticalgrade and fresh distilled water free of CO2 was alwaysutilized.

Equipment

Flow determinationswere monitored by a Radelkis-OK-102/1 conductivity meter or by a Micronal-B342-II

visible spectrophotometer. The conductimetric flow cellis very simple and was made of an acrylic block(1 x x 2 cm) with a 1-mm-diameter hole. Two stain-less-steel screws were attached perpendicularly to thehole and at 180. The exposed end surfaces of the screwsconstitute the two electrodes of the flow cell. The home-made spectrophotometric flow cell [18] was a 1-cm-long,2mm i.d. glass tube placed in the optical path of thespectrophotometer. Light reaches the tube perpendicu-larly. In the flow cells, the fluids flow from the inferior tothe superior part of the tube helping to avoid air bubblestrapping inside the cell.

A peristaltic pump (Milan-206, SP-Brazil) was used;Tygon pumping tubes were employed with polyethylenetubing of 0’75 mm i.d. to assemble the FI manifolds. Anautomatic sample injection device [19] was used tointroduce the samples in the carrier fluids. The mixingchamber (figure 1) is a mini-bottle with a l’0cm basediameter, 0"5cm high and about ml inner volume.Stirring was magnetic.

A microcomputer, compatible with the IBM-AT 486,was employed to control the injection device and for dataacquisition using a versatile interttce [20]. The interfaceconverts the analogue signal from the detectors to a 12 bitresolution digital signal and actuates the solenoids micro-valves of the injection device [19].

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The control, data acquisition and data treatment pro-gram was developed in QuickBasic 4.5. The software is ,..menu driven and provides the procedures to calibrate theconcentration gradient, to locate the Stoichiometric

ORatio Experimental Point, SREPs and to calculate the oanalyte concentration.

Flow manifoldsFigure depicts the two FI manifolds employed. Theconnections from the point where the water carrierstream merges with the titrant to the mixing chamberand from there to the flow cell, were made as short aspossible. In this way, the dispersion from the mergingpoint to the detector is determined by the mixingchamber.

Two FI titration procedures were developed: SampleInjection Titration-SIT, and Titrant Injection Titra-tion-TIT. As shown in figure 1, the manifolds wereused to perform a SIT in which a discrete volume ofsample is injected and merged with the titrant that iscontinuously pumped towards the detector. The injectedsample is dispersed, mainly by convection, in the mixingchamber, over a constant steady-state concentration ofthe titrant. In the TIT, a discrete volume of titrant isinjected and merged with the sample which is, by thistime, continuously pumped towards the detector throughthe tube previously occupied by the titrant (KMnO4 orNaOH in the manifolds of figure 1).

Gradient calibration

The gradient calibration technique previously described14-16] is used to find the instantaneous concentration of

a solution injected in the flow manifold, in absence ofreaction, by associating the time elapsed from the time ofinjection to a dilution-like factor. This tctor defines therelationship of the concentration of the original solutionwith the instantaneous concentration at the detectionpoint. It means, for example, that after gradient calibra-tion, the concentration of the injected solution passingthe detection point at any instant, in absence of reaction,is known.

Gradient calibration procedure and equations

Both titration procedures proposed are based on theconcentration profile established in the FI manifold afterthe introduction of a discrete volume of sample (in SIT)or titrant (in TIT). Therefore, the first step necessary toboth procedures is the calibration of this concentrationgradient. This procedure is made, in the absence of anyreaction, by using only one solution. Any solutioncontaining a specimen capable of being sensed by thedetector and producing a response linearly related withits concentration can be used at this stage. However, forsimplicity, the titrant solution is usually employed. Theleft-hand side of figure 2 shows the signals obtained in thecalibration process for the conductimetric detection.Initially, the titrant (NaOH in figure 2) is continuouslypumped through its tubing until a steady-state conducti-metric signal is obtained. At this point, all of the othertubes in the FI Inanifold contain water or conditioning

(A) (a) (C)

H(tb)tc)

(b)S(td)

!i s(t.)-",:

: :__: :: i.__

time

Figure 2. (A) Gradient calibration signals; (B) and (C), signalsfor two consecutive (lower concentration on the left) conductimetricdeterminations of acetic acid in vinegar by the SIT. Hss, detectorsignal obtained by pumping the titrant (NaOH) through its lineuntil a steady-state is obtained; I, injection times; H(t), height ofthe gradient calibration signal; S(t) height of the titration signalOnlyfour H(t) and S(t) points (from the 600-800 values actuallyobtained) are shown.

(non-reactive) solutions. The height of this signal isdenoted as Hss and it embodies the effect of the dilutionsuffered by the pumped solution due to confluence withother solutions found in its path to the detector.

The constant concentration (mol. 1-1) of the continu-ously pumped solution (C) passing the detection point, isgiven by:

C-q

C (1)qtotal

where C is the original solution concentration, q is itspumping flow rate and qtotal is the total flow rate.

For a linear response the values of q and qtotal are directlyproportional to Hss and Hmax, respectively. Hma, denotesthe steady-state signal that would be obtained if thesolution was pumped through all the tubing in the FImanifold. As will be shown below, the Hmax value doesnot need to be measured. Therefore, the value ofC can bealso given by the expression:

Hssc= .c (2)HrnaxThe same titrant solution is then injected in the manifold,carried by water, merged with the line containing thesame solution and with the other pumping lines, passedthrough the mixing chamber, and the peak-like concen-tration profile, observed in figure 2a, is obtained over thesteady-state signal (Hss).The data acquisition software is triggered when thesolution is injected in the FI manifold and stores anumber between 600 and 800 point H(t) (a time depen-dent signal value, taken above the steady-state plateau tothe peak profile signal). The index refers to the pointcollected and can be associated with the time elapsedafter injection. Data collection starts at injection and isspaced for a fixed time interval of 110ms. The height,Hss, and the profile points, H(t), are stored in order to beused in the construction of the titration curves. These

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data are also employed to find out the time dependentfactors that convert the time elapsed from the instant ofthe injection of a solution in the manifold, to the instan-taneous concentration of the specimens, present in thatsolution, at the detection point. For any solution injectedin a given manifold, the instantaneous concentration(mol. 1-1) at the detector, C(t), can be found by:

c(t) c

where C is the original concentration of the injectedsolution and g(t) is the time dependent factor found by:

g(t)Hmax (4)

If necessary, a solution of titrant at lower concentrationthan the one injected during the determination, can beused to generate a lower Hss (steady-state signal) in orderto keep the calibration signals in the linear range of thedetector response.

Because the dispersion is caused by convection inside themixing chamber, the g(t) and C(t) values are indepen-dent of the specimen being injected [8]. Therefore, thegradient calibration can be used by both the proposedmethodologies as long as the titrant (in SIT) or sample(in TIT) is flowing through the same tube used to findthe Hss value during the gradient calibration procedure.

(A)I-- region I region "[]"

P

SRPR SRP

""!: "/: T

0

time

FI titration procedures

The system is generally ready to perform a SIT just afterthe gradient calibration procedure is finished (which doesnot require more than 2min), because the titrant isalready being pumped through its tube at this stage.To perform a TIT procedure, the sample must replacethe titrant or any solution in the tubing previously usedin the gradient calibration and be pumped for a timeinterval long enough to wash out any solution previouslypresent in that tube.

Samples or titrant solutions (depending on the titrationprocedure chosen) are injected under computer controland the resulting signals, now obtained in the presence ofthe reaction between the analyte and the titrant, aresampled in the same time intervals employed in thegradient calibration procedure. The right side of figure2 (b and c) shows two typical SIT conductance (S(t))xtime curves obtained by injecting vinegar samples andreacting them with a continuously pumped sodiumhydroxide solution.

FI titration comportment

Because it is possible to measure the analyte and titrantconcentrations at the detection point on the SIT proce-dure then, in the absence of any reaction, the concen-tration x time curves in figure 3(a) should be obtained.The peak-like curve S is related to the analyte and curveT to the titrant. Four time intervals in the SIT procedureare important: the elapsed time when the concentrationgradient of the analyte increases and there is an excess ofthe titrant (tl to t2); when there is an excess of the

Figure 3. (A) The hypothetical concentration x time for theSIT. (B) SIT curvesfor acetic acid determination. The peak-likecurve S is related to lhe analyte and curve T to the titrant. Thesample is injected at to, achieves the detection point at it, maximumconcentration oint P) at t3 and quits the detection point at t5.The SRPR (at t2) is the stoichiometric point related to risingregion (region I) and SRP (at t4) related to the falling region(region II).

analyte, the concentration gradient of the analyte in-creases and reaches the maximum (t to t3); when theanalyte concentration gradient starts to decrease, but theconcentration is still in excess (t, to t4) and, finally, whenthe titrant returns to being in excess and the zone samplequits the detection point (t4 to t5).

Because the concentration of the injected solution firstincreases region I in (figure 3a), reaches a peak (P infigure 3a and decreases (region II in figure 3a), two SRPpoints are obtained: one related to the region which isrising, SRPI?, where the excess of the titrant is followed byan excess of analyte; and the other is related to the fallingregion, SRP, where excess of analyte is followed by anexcess of titrant. The curve related to rising region showsan abrupt change in concentration x time profile and isnot useful for the analytical procedure, because the SRPcannot be located with reasonable precision. On theother hand, the curve related to the fallingregion clearly shows a smooth change in the con-centration x time profile and is useful for the analyticalprocedure, because the SRP can be located with goodprecision.

160


1600

1400

1200

1000

800

600

(A)

region II

0 100 H(ts"200--__ 300 400 500 600H(t)

3000

f25002000

1500

lOOOreg

5oo IregionI1:- . ’"-._ pSREP -’.SREP,,"i\ region IIo0 200’,H(ts 400H(ts 600 800 1000 120(

H(t)

Figure 4. (A) S(t)xH(t) (both in the arbitrary units ofconductance) curve for conductimetric determination of acetic acidin vinegar (sample 1 in table 2) by SIT, showing the two straightlines usedfor SREP and H(ts) determination. (B) S(t) x H(t)(both in the arbitrary units of absorbance) curves for spectro-photometric determination of Fe(II) (samples: SS-4 (dashedcurve) and SS-2 in Table 1) by the SIT. Regions I and II arethe portions of the signalfor the rising andfalling concentration ofthe injected sample gradient; respectively. P is the signal related tothe peak of the gradient calibration profile.

In the TIT procedure a similar pattern is obtained, butthe peak-like curve S is related to the titrant and curve Tis related to the analyte.

In both the SIT or TIT procedures, the concentration ofthe injected solution, analyte (in SIT) or titrant (inTIT), at the interval time of the t2 to t4 (figure 3a),must be higher than the concentration of the solutioncontinuously pumped, analyte (in SIT) or titrant (inTIT). If this condition is not fulfilled, the SRP]e and SRPwill not be reached and the SIT or TIT will be incom-plete.

In presence of a reaction, the curve reflects the change inthe property being observed due the formation of theproducts. For example, the reaction between the acetic

acid injected and NaOH continuously pumped causes theconductance of the resulting solution to decrease (tl to t2in figure 3b) as a consequence of the water formation,while the NaOH is in excess (tl to t figure 3a). When theSRPR is achieved (at t), the conductance is at its mini-mum and subsequent excess of acetic acid (t to t4, figure3a) causes a little change in the conductance (t to t4 infigure 3b). In the SRPF (at t4), the conductance begins toincrease (t4 to t5 in figure 3b) because the NaOHconcentration is in excess in relation to the acetic acidconcentration (t4 to t5 in figure 3a). The profiles shown infigures 2b and 2c are then obtained. Similar curves areobtained for the spectrophotometric determination ofFe(ii) by the SIT.

Conversion of the signal x time experimental curves to sig-nal x H(t) curves

The signal x concentration curves can be easily derivedby replacing each point, obtained at any time intervalafter injection, for its equivalent H(t) value obtained bythe gradient calibration procedure. The H(t) values aredirectly proportional to the concentration of the injectedspecimen. Figure 4 shows the effect of converting (inSIT) a S(t) x time to a S(t) x H(t) plot. The resultingplot (figure 4a) shows the behaviour of the signal as theanalyte (acetic acid) concentration changes inside themixing chamber feed with a constant sodium hydroxideconcentration, as imposed by the flow system.

Figure 4b shows two S(t) x H(t) plots obtained for thespectrophotometric determinations of iron(II) by theSIT. The resulting plot reflects the change of instanta-neous concentration of KMnO4, since the spectropho-tometer is monitoring the absorbance of the KMnO4 at525 nm.

Determination of stoichiometric ratio experimental points(s’s)The SREPs employed to calculate the analyte concentra-tion could, in principle, be located in the signal x timeexperimental curves obtained by any of the procedures(TIT or SIT) described above. However, these pointscan be located more precisely and the real behaviour ofthe analytical signal can be observed by using sig-nal x concentration curves. It is not so easy to locateSREPs in the original signal x time curve as the non-linear addition of the titrant or analyte with time,provided by the FI gradient, produces a very short linearportion after the SREP has been reached.

The SREP can be located in the S(t)x H(t) at thecrossing point of the two straight lines observed afterand before the abrupt change in the conductance, asshown in figure 4a, or in the absorbance as shown infigure 4b.

Analyte concentration determination

After the SREPs have been found from the data analysis,the analyte concentration can be determined. The follow-ing reaction between the analyte (A) and the titrant (T)is assumed:

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xA +yT - AxTIn the SREP, ts and the H(ts) associated with theSREP is found in the S(t) x H(t) curve and employed inthe following calculations.

Analyte concentration calculation for the SITFor a SIT, the instantaneous analyte concentration at theSRP (CA(ts)) will be, in the absence of reaction, andusing equations 3 and 4:

c (ts) Hma-----" CA (5)where C is the unknown analyte concentration in thesample solution.

The titrant concentration (in the absence of reaction) isconstant during the titration and it is calculated asfollows:

Hss (6)Hmaxwhere Cr is the original known concentration of thetitrant present in its flask. At the SRP, the followingrelationship is valid:

(7)Therefore, the original concentration of the analyte inthe injected solution (C) can be obtained from equa-tions 5, 6 and 7 and expressed by:

Hss.CAY" H(ts)

Analyte concentration calculation for the TITFor the TIT procedure, the analyte concentration (Ca) isconstant and is calculated as follows:

Itss (9)HmaxAt the SRP, the following relationship is valid:

cr(ts)The instantaneous titrant concentration at the SRP iscalculated by:

c (ts) (ts)Hma--- CA (11)Therefore the analyte concentration is:

.H(ts) .CrCA (12)Y//ssEquations 8 and 12 show that the Hmax value does notneed to be measured in order to establish the originalconcentration of the analyte (C).To perform the calculations, the program searches for theH(ts) values in the S(t) x H(t) curves. These values arealways found at the crossing points of straight linescoming from two regions where the titrant and theanalyte are in excess. The concentration of the titrant(Cr) needs to be known in order to find the analyteconcentration. The titrant solutions (KMnO4 or NaOH)were always standardized in this study.

Table 1. Results, expressed in mmol. dm-3,for the determinationof Fe(g) in synthetic samples(SS), iron alloys(IA) and ironores(IO) by the SIT procedure employing spectrophotometricdetection. KMn04 concentration 2"40 mmol. dm-

Sample Classical titration SIT*

SS-1 19.5 19.5 (-I--0.6%)SS-2 35.5 36-0 (--I-1.2%)SS-3 50.0 49.3 (-t-0.6%)SS-4 64.9 65.5 (4-1.1%)SS-5 90-2 90"3 (4-1"7%)

IA-IPT12A 27"3 26"8 (4-1"2%)IA-IPT29 276 26"6 (-t-0.9%)IA-IPT24 197 19"2 (4-1"2%)IO-IPT21 193 19"4 (4-1"4%)

* +Relative standard deviation for 10 replicates.

Results and discussion

Results for the SITThe SIT results obtained for the concentration of syn-thetic samples of Fe(II) using SREPR were not goodbecause the curve related to rising region of the peak(region I in figure 4b) did not give a clear SREPR. Thiswas because during the tl to t2 stage (figure 3a), theFe(II) concentration rose due to an excess of KMnO4titrant and species of Mn of intermediate oxidation statebetween Mn(VII) and Mn(II) were formed, changingthe reaction stoichiometry. The falling region reflects theinverse and a clear SREP- was obtained (region H in thefigure 4b). During the stage t3 to t4 (figure 3), there wasan initial excess of iron(II) which decreased as it washedout from the mixing chamber until the concentration ofthe KMnO4 was in excess. With an excess of Fe(II),Mn(VII) is directly converted to Mn(II) (colourless),causing the absorbance at 525 nm to decrease linearlyand a stoichiometric reaction to be obtained.

Table shows the results obtained for the determinationFe(II), by SIT, of five synthetic samples (SS1-SS5),three alloys (IA-IPT12A, IA-IPT29 and IA-IPT24)and one iron ore (IO-IPT21) using SREPF. About 40samples can be processed per hour by the spectrophoto-metric SIT procedure and the results agree with theclassical titration.

The SIT procedure was also applied to conductimetricdetermination of acetic acid in vinegar. The resultsobtained for SREPR are not useful because no sharptransition in the conductance was obtained (see regionI in figure 4a). However, a very good precision wasobtained for the determination using SREPF (see table2). This was the result of a well-defined transition inconductance. The system is capable of processing 60samples per hour and the results also agree with thosefrom classical titrations.

Results for the TITThe TIT results obtained for the concentration ofsynthetic samples of Fe(II) using SREPF were alsoquestionable. The curve S(t) x H(t) related to the fallingregion did not present a clear SREPF. The same problem

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Table 2. Results, expressed in mmol. dm-3, obtained for thedetermination of acetic acid in vinegar samples by the SIT pro-cedure employing conductimetric detection. NaOH concentra-tion O" 100 mol. dm- 1.

Sample Classical titration SIT*

804 796 (+0.7%)79 790 (+0.6%)828 831 (-t-0.4%)748 751 (+/-0.7%)866 874 (+/-O.8%)

* +/-Relative standard deviation for 10 replicates.

was obtained in the attempts to find the SREPR in thedetermination Fe(II) by the SIT. On the other hand, inthe rising region of the gradient, the SREPR was achievedwith a gradual addition of KMnO4 over a fixed Fe(II)concentration of an Fe(II) solution which was continu-ously pumped. Therefore, at this stage, the Mn(VII) wasdirectly converted to Mn(II) and a sharp SREP wasobserved.

In the Fe(II) determination by the TIT using SREPn, anaverage relative standard deviation and average relativeerror greater than 3% and 2%, respectively, were ob-tained. The precision of these results are not good due toa high fluctuation found in H(ts) values. On the otherhand, in the Fe(II) determination by the SIT, theaverage relative standard deviation (c. 1%) is lowerthan that for the TIT procedure, because the SREPsemployed are found in the falling region of the concen-tration gradient profile, where the values for H(ts) areless affected by small fluctuations in time because theconcentration gradient is less abrupt.

Comparison between both procedures, analytical requirements andcare

Whether the SIT or TIT methodology should be usedneeds to be evaluated in terms of the specific application.If the quantity of sample is not a restrictive factor and ifin-line monitoring is wanted, the TIT procedure is moreeconomical as only small volumes of titrant are employedto perform each determination. On the other hand, theSIT procedure is more suitable when quantity is limited.In some cases, however, the procedure to be used may bedictated by the characteristics of chemical reactioninvolved in the titration. For example, the SIT procedureappears to be more suitable for the spectrophotometricdetermination of Fe(II), by using KMnO4 solution astitrant, because a good precision is obtained for theSREP found at the falling region of the gradient, wherethe reaction is performed with slow decrease of theanalyte concentration over that of the titrant solution.Side reactions are avoided by adding the Fe(II) solutionto the KMnO4 solution.

For both procedures, and when it is possible to choose,the SREP located at the falling region of the concentra-tion profile must be employed for final calculations.However, because of the dynamic characteristic of theflow titration system, the SREP should not be located inthe region of the falling gradient where very slow change

in the concentration is observed, because the change inthe signal may not be sharp enough to permit the preciselocalization of the SREP.

Routine determinations, made on samples whose concen-tration range is limited and previously known, could befurther optimized by forcing SREPs to be located in thebest region of the falling gradient. This region corre-sponds to the one where an almost linear change inconcentration is observed and where standard deviationsfor the SREPs lower than 1% are easily achieved. Thecontrol over this parameter is made, in FI systems, by anappropriate selection of the titrant concentration and ofthe injected volume.

An overall comparison of the results obtained by theproposed flow technique and the classical titration re-veals a relative accuracy of about 1%. The proposedprocedures are reliable only if the concentration gradientprofile, calibrated by the injection of the titrant, is equalto that produced by the sample (in SIT) or by the titrant(in TIT). The mixing chamber present in FI manifoldsprovides for this equality because the mass transport, thatrules the dispersion, is by convection rather than bydiffusion.

A directly proportional response of the detector in rela-tion to the titrant concentration is required for thegradient calibration procedure, and a fast chemicalreaction between the analyte and titrant must occur.Classical batch titration also benefits from the use of fastreactions, because the determination can be carried outquickly and accurately, although this is not essentialbecause the batch procedure is not as time restrictive asthe flow method.

Conclusion

The possibility of using the concentration gradients,generated by FI systems, to perform analytical proce-dures similar to titrations and based on stoichiometricconcentration ratios has been demonstrated. The instan-taneous concentration of the analyte or titrant at thedetection point was achieved employing the gradientcalibration technique [14-16]. The SIT and TIT proce-dures proposed are, along with the binary searchingmethod recently described [13], capable of employingstoichiometry to establish the analyte concentration in aflow system. Obviously, as in any automatic batchtitration procedure made with the titrant being addedat constant flow rate, the reaction between the analyteand titrant must be fast in order to be processed in theshort time interval during which the specimens resideinside the mixing chamber.

The most attractive aspect of the proposed methodologiesis the use of only one standard solution for calibration.The proposed methodologies are more suitable for con-tinuous automatic monitoring than other flow procedures[3-7, 10-12], which require the use of various standardsolutions for system calibration. Furthermore, the instan-taneous concentration of titrant and analyte that definesthe SRP is known. This is another characteristic thatdistinguishes the proposed flow methodology from others[3-7, 9-12] in which the concentration of titrant or

163


analyte that defines the SRP was not known or em-ployed.

The TIT works in a similar way to the flow rate gradienttitration methodologies previously proposed [3-5]. How-ever, the generation of the titrant gradient withoutchanging the flow rate of the peristaltic pump is anadvantage in terms of simplicity of the flow manifold,because it can be constructed with only one of thesedevices.

The methodologies proposed here require, as suggestedby IUPAC definition of titration [21], the use of only onetitrant whose concentration is well known and the stoi-chiometric ratio experimental points are located betweenregions where excess of titrant or analyte occurs. Theadvantages of classical titration procedures are retainedby the suggested procedures. The detector needs to bestable only during the gradient calibration procedurewhile the relationship between time and concentrationis being established. After this procedure has been com-pleted, small fluctuations will not significantly affect thelocation of SREPs.

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