Monosegmented flow titrator

Analytica Chimica Acta 438 (2001) 67–74

Monosegmented flow titrator Emerson Vidal de Aquino, Jarbas J.R. Rohwedder, Celio Pasquini∗ Instituto de Qu´ımica, Universidade Estadual de Campinas, CP: 6154 CEP: 13083-970 Campinas SP, Brazil Received 11 July 2000; received in revised form 26 October 2000; accepted 26 October 2000

Abstract An automatic titrator based on the monosegmented flow approach is described. The flow titrator is unique in fulfils the definition of IUPAC for the titration technique. This means that, the flow analyser produces a complete titration curve and is capable using the equivalence point concept for determination of the titrant concentration without the use of any calibration, usually employed in similar systems. The titrant is added to the sample monosegment by using a syringe driven by a step motor. The titrant is added with a precision of ±0.5 ␮l. The main advantage of the proposed flow titrator is in the fact that, if necessary, only one aliquot (100–200 ␮l) of the sample need be employed to make a complete titration with a suitable titrant solution. This feature has been demonstrated by titrating Fe(II) and H2 O2 solutions with KMnO4 . However, a complete titration is not the fastest way to find the end point. A successive approximations algorithm is described which allows the end point to be achieved in, at most, eight steps. The novelty of this approach is that the sample solution does not need to be discarded when titrant addition do not exceed the end point. The proposed flow titrator has been evaluated for spectrophotometric, indicator based, titrations of strong and weak acids and for titration of hydrogen peroxide in commercial products using KMnO4 . The results show that the system can, on average, perform one titration each 2 min, when employing the successive approximations algorithm. The precision and accuracy for HCl or acetic acid titrations (concentration range 0.0025–0.100 mol l−1 ), using phenolphthalein as indicator, and H2 O2 , using the self indicating KMnO4 titrant, was from 0.5 to 2%. Because the proposed system resembles a real titrator in all aspects, all detection techniques employed for conventional titrations can be, in principle, also employed with this flow manifold. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Monosegmented flow analysis; Flow titration; End point location

1. Introduction The titration procedure has been adapted to flow analysis since the pioneering work of Bladell and Laessig [1,2], who described a system for continuous and variable addition of the titrant to a constant flow of the titrand. In this work, a mixing chamber, containing a magnetic stirrer, was employed to ensure the mixing of the solutions and a potentiometric sensor ∗

Corresponding author.

was employed for identification of the titrant flow rate capable of reaching the stoichiometric ratio with a constant flow of titrant. The original system has been modified, keeping the same idea of finding the end point through the determination of the stoichiometric flow ratio between titrant and titrand solution [2,3]. Fleet and Ho described a system where the concentration gradient of the titrant was generated externally in a small gradient chamber [4]. The titrant gradient has also been generated by using coulometry with the inherent advantages of this technique [5].

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 1 2 7 2 - 1

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After the introduction of flow injection (FI) analysis by Ruzicka and Hansen [6], the concept of controlled dispersion was also employed to mimic a titration in flow systems. The FI approach is based on the logarithm dependence of the two pseudo end points found in the ascendant and descendent portions of the dispersion generated gradient of the injected sample. The relationship between the time between the two events and the concentration of the injected sample is known by employing a series of standard solutions in a calibration process. Another work, by Astrom, employing the FI technique, showed that the peak height is also proportional to the titrand concentration [7]. Again, calibration is necessary to find the relationship between titrand and the analytical parameter (peak height). Recently, some works can be found in the literature reporting on tentative of elimination this calibration, made by the use of various standard solutions, when the titration is employed with flow systems. The gradient calibration technique [6] can be used to convert the signal obtained in a FI system to a titration curve that resembles the conventional one and the flask concentration of the titrant solution can be employed directly to find out the titrand concentration employing stoichiometric calculus [8]. Coulometric titrations have also been used in FI systems with the advantage of in situ generation of the titrant, eliminating the need of solution standardisation and overcoming problems with reagent degradation [9–11]. Korn and co-workers proposed a binary sampling system [12] where the titrant and titrand solution were aspirated through the same pumping channel and mixed in an non-segmented stream. The volumetric ratio between titrant and titrand solutions necessary to achieve the titration end point was found by looking at the colour change of an acid–base indicator and by employing an binary searching algorithm [13]. A flow-batch hybrid system in which the sample and reagent are aspirated into a mixing chamber has been also proposed recently [14]. The one-dimensional Fibonacci optimization algorithm has been employed for fast end point location. Stoichiometry can be used for determination of analyte concentration. However, complete titration curves were not obtained. The monosegmented flow approach (MSFA) [15], where the sample is introduced in the flow manifold as a single segment, has been employed to effect

titration in flow systems. The advantages explored so far include rapid and effective mixing that can be produced by the bolus flow pattern established by the monosegmented flow [16–20]. However, full exploitation of the advantages of the monosegmented system has not yet been reported because the sample is discarded, even when the end point is not achieved. Also, titrations made by way of a whole titration curve have not been reported. The overall procedure, although it does not employ calibration standards, should not be classified as a true titration, following the IUPAC recommendations [21], because in many steps towards the end point, only part of the sample is reacted before being discarded. The use of titration approach, made in flowing medium, is essential to cover the requirements for process control, where sampling to a discrete automated titrator would impart sample contamination and slow down the sample throughput. However, to turn a flow titration methodology amenable to process control it should not relay on calibration, but employ the conventional concept making use of only one standard solution. An overall evaluation of titration flow approaches described so far reveals that, despite all the efforts, none fulfils the IUPAC recommendations to classify the technique as a true titration, since all the systems described thus far employ more than one sample aliquot and cannot process the complete titration in a single sampling volume. The difficulties found in processing a true titration in a flow system apparently come from the necessity to achieve complete mixing between the titrant and titrand solutions and in the problems of controlling the flow system in order to effect more than one operation of titrant addition to the same sample aliquot. While mean, the proposed system turns the tritation methodology more suitable for process control as calibration is not necessary and an algorithm, to rapidly predict the end point, can be employed. This paper demonstrates that, these two operations can be effected using the monosegmented flow approach and sample position identification, which is easily achievable in such monosegmented flow systems [22]. The resulting system can perform a complete titration in a single monosegment of the sample and thus meets the IUPAC definition for a titration in all its aspects, even though made in a flow system.

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2. Experimental 2.1. Reagents, solutions and samples All reagents employed are of analytical grade and the solutions were prepared and standardised following the recommended procedures for acid and basic solutions. The NaOH standard solutions were prepared from a saturated solution to avoid the presence of carbonate. Commercial hydrogen peroxide samples were acquired from local dealers. Most of the samples were diluted 100-folds before being titrated. The samples presented as viscous suspensions were diluted 250-folds. KMnO4 solutions were prepared in fresh boiled water, filtered through glass wool and standardised by titration of known amount of sodium oxalate. Distilled and deionised water was employed throughout. 2.2. Flow manifold Fig. 1 depicts the flow manifold employed to perform the titrations in a monosegmented sample. The system is very simple and employs a peristaltic pump (Ismatec, IPC-5) that presents a RS-232 digital interface. A double proportional injector was employed to introduce the sample monosegment in air carrier stream, free of CO2 and flowing at 6.5 ml min−1 . The air stream is made free of carbon dioxide by pumping it through a small (10 cm long, 1 cm i.d.) plastic column containing ascarite. The

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path from the injector to the detector was made with a 50 cm long PTFE tube with a 1.8 mm i.d. diameter. Titrant addition was performed by a home made burette made with a linear actuator (RS-318-711) and a high precision 2.5 ml gas tight glass syringe (Hamilton, TLL). The step motor of the device is controlled by the microcomputer and its volumetric precision has been determined by weighting the mass of water delivered after a defined number of steps sent to the motor. The precision is of ±0.5 ␮l. Automatic refill of the syringe is provided by a three way solenoid valve connected to the outlet of the device, allowing access to the stock solution kept in a external flask. The capacity of the syringe is enough to perform about 25 titrations, supposing a titrant consumption of about 100 ␮l per titration. The flow manifold contains three optical switches, placed on the reaction/mixture tubing, employed to locate the monosegment. This location is made by sensing the transition between the liquid and gas (air) refractive index, through an appropriate electrical circuit which converts the light intensity change to a TTL digital signal capable of being accessed by the microcomputer [22]. A simple spectrophotometric detector was employed in this work. This is composed of a tungsten light source, an interference filter (isolating the appropriate wavelength) and a silicon photodiode. For the titrations employing KMnO4 a light emitting diode (LED, 560 nm) was used instead of the tungsten lamp. The necessary pre-conditioning of the signal

Fig. 1. Manifold of the monosegmented flow titrator. C, ascarite colum; P, peristaltic pump; T1-2 , miniature electromechanical three way valves; M, linear actuator with step motor; S, gas-tight syringe (2.5 ml); V, titrand injection port; OS1-3 , optical switches; D, spectrophotometric detector; A, titration coil (PTFE, 50 cm long, 1.8 mm i.d.). Typical air flow rate = 6.5 ml min−1 .

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produced by the photodetector was made by an analogue circuit, as previously described [22]. Light was delivered perpendicularly to the PTFE tube by the optical bundle or by the LED and collected by the photosensor, placed on the opposite side. 2.3. Analyser control Control of the flow titrator was made through the use of a IBM compatible microcomputer having a general purpose analogue/digital interface (PCL-711S, Advantech) and running software written in Visual Basic 4.5. The software controls the peristaltic pump, allowing it to reverse its pumping direction and to stop it, when necessary. The software also allows detection of the transition of the refractive index the between air and liquid segments. Therefore, the location of this segment is easy achievable. The absorbance measurements are made by the software and the results of the titration can be stored for further data treatment. Data treatment was performed using Origin 5.0 graphic software. 2.4. Titration procedure A titration cycle is started by turning the three way valve (T1 ) on and pumping water and air for 10 s in order to wash out the system from the previous sample. The washing cycle is finished by pumping air for 5 s to remove any excess liquid from the manifold. A reproducible volume of sample (100–200 ␮l), present in the sample loop of the injector, is introduced in the manifold and carried by the CO2 -free air stream, defining a monosegment. When the sample reaches the optical switch (OS2 ) the addition of the titrant is made by opening T2 and sending the suitable number of steps to the syringe driven motor. The monosegment is carried towards the detector through the path where the mixture between the solutions occurs. When the segment reaches OS3 the absorbance measurement is made. Depending on the absorbance measurement, the program decides if the end point was or was not achieved. If the end point was not reached, now the pump is reversed and the monosegment containing the partially titrated sample is returned to the titrant addition point. The optical switches OS1 and OS2 are used together to identify the sample position for new titrant addition. The titration proceeds following

the algorithm selected by the user. If the end point was found, the monosegment is discarded and a new washing cycle is initiated before new sample introduction. Alternatively, the user can make a titration by collecting any pre-selected number of points, in order to produce a complete titration curve. Finally, a hybrid procedure can be employed where in the end point can be approached by using four or five steps of a successive approximations algorithm. The end point is then estimated and the titration curve can be produced from three or four points obtained before and after the end point. 2.5. Determination of the titrand volume The titrand volume can be achieved by simple gravimetric calibration of the volume delivered by the sample loop placed in the injector, employing a procedure similar to the conventional calibration of a volumetric pipette. The mass of ten aliquots of water, at constant temperature, is determined after collection in a pre-weighted flask. The density of water employed for volume determination. This procedure is effective when the transfer of the sample volume to the titration tubing is ensured by the absence of points (junctions and connections of the valves) capable of retaining part of the sample in a place that will be not reached by the monosegment during the titration. However, if retention is significant, titration of a strong acid can be processed to find the true volume delivered for titration. This correction was not necessary considering the hydrophobic material employed for the manifold and the diluted aqueous solutions of the samples. A typical gravimetric calibration for a nominal 200 ␮l loop resulted in an average volume and standard deviation of 201.3 ± 0.8 ␮l. This value is employed in the stoichiometric calculations. 2.6. Successive approximation searching algorithm for acid–base indicator supported spectrophotometric titrations This algorithm has been used before to accelerate end point location in a flow titration [13] and it works as in analogue-to-digital conversion electronic devices [23]. The basic difference between the procedure proposed in this work is that the sample does not need to be discarded if the end point is not

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reached, as the monosegment can be returned for a new titrant addition. This work employed an eight steps successive approximation (such as an analogue to digital converter of eight bits) which can offer an ideal precision of about 0.5% in the end point loca-

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tion. A maximum titrant volume of 150 ␮l for titrant volume is assumed. Therefore, searching for the end point starts with the addition of 75 ␮l of titrant to a 200 ␮l sample monosegment. The algorithm evolved is shown in Fig. 2. The expected resolution is

Fig. 2. Flow chart of the successive approximation algorithm for titration end point location. Vincr , initial value for the titrant volume (half of the maximum volume admitted for the titration); N max = maximum number of approximation steps; Vtit , added titrant volume not enough to surpass the end point; Vexc , added volume of titrant capable to exceed the end point; Vep , end point estimated volume.

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somewhat higher than the burette precision (about 0.58 ␮l). When using the successive approximation algorithm the volumetric precision of the titrant delivering system must be better than (Vmax /2n ), where Vmax is the maximum volume admitted for the titration and, n is the number of successive approximations to be used. In addition, the property used to indicate the excess or lack of titrant can produce a proportional response which can be used to decide how far the end point is after a titrant increment. Alternatively the user can select a lower resolution for the searching process. For example, if a five steps process is selected the end point volume can be estimated to be between Vtit and Vexc (see Fig. 2) with (Vexe − Vtit ) = 4.7 ␮l. In the proposed system, the user can stop the searching algorithm and restart the titration of a new aliquot of the sample adding Vtit and incrementing the titrant by (Vexe − Vtit )/n for each new addition, where n is the volumetric resolution with which the end point will be found. Observe that, this new sample aliquot does not need to be discarded until the end point is reached. The algorithm can also helps the optimisation of procedures employing a complete titration curve by anticipating the end point volume and allowing to estimate a lower number of fixed increments to constructed the titration curve only around the end point.

Fig. 3. Complete spectrophotometric titration curves obtained by the monosegmented flow system. Titrand: (146.0 ± 0.5) ␮l of Fe(II) solution in H2 SO4 1.8 mol l−1 , 8.546 × 10−3 (䊏); 4.274 × 10−3 (䊉) and 2.137 × 10−3 mol l−1 (䉱). Titrant: KMnO4 , 1.920 × 10−3 mol l−1 .

algorithm with four steps to estimate the end point volume. A titration around the final point was processed and the end point volume found by intercepting the two straight lines obtained before and after the end point.

3. Results and discussion 2.7. Spectrophotometric titration of Fe(II) and H2 O2 with KMnO4 In order to evaluate the proposed system in its ability to obtain a complete titration curve, standard solutions containing Fe(II) (as ferrous) were titrated with standard KMnO4 solution in 1.8 mol l−1 sulphuric acidic medium. A fixed number of equally spaced additions of titrant volume was selected and the titration proceeds as in a conventional procedure with the absorbance of the titrated solution being accessed after each titrant addition. The results provide a complete titration curves as shown in Fig. 3. It is important to mention that, the titration curves were obtained for only one sample aliquot of only 200 ␮l and that stoichiometry and the usual end point location method can be employed using the results obtained in the flow system. The determination of H2 O2 in commercial products was effected by using the successive approximations

The results for the acid–base titrations made in the proposed system reveal a precision and accuracy for determination of acetic acid and hydrochloric acid in the range of 0.5–2% for an acid concentration range between 0.1 and 0.0025 mol l−1 , respectively. These titrations were made using phenolphthalein as indicator and NaOH as titrant employing the successive approximation algorithm described in the experimental section. The overall consumption of titrant and sample depends on how many steps of the titration lead to an excess of titrant (after which the sample solution is discarded). A well dimensioned titration system, which considers the expected average concentration of the samples, should employ, at most, four sample aliquots to perform the titration. Therefore, the maximum sample consumption will be at least two times lower than the consumption observed with the previously described system, for which eight aliquots are necessary [13].

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Table 1 Comparative results obtained for the determination of hydrogen peroxide in commercial productsa Sample nos.

H2 O2 (%, m/v) manual

1 2 3 4 5 6

2.878 2.660 3.101 4.103 5.688 8.305

± ± ± ± ± ±

0.003 0.007 0.007 0.011 0.010 0.051

H2 O2 (%, m/v) Monosegmented flow titrator 2.854 2.621 3.075 4.083 5.660 8.240

± ± ± ± ± ±

0.007 0.010 0.014 0.015 0.007 0.020

Relative error (%) 0.6 1.4 0.8 0.5 0.5 0.9

a

The results are shown with their standard deviation for 5 replicates. Samples 5 and 6 correspond to a product commercialised as viscous white suspension. Manual titrations employed 25.00 ml of sample while the flow procedure employed 146 ␮l.

Average sample consumption for the acid–base titrations made during this work was 800 (4 × 200) or 484 ␮l (for a 146 ␮l titrand volume). The titrations made in the flow system were designed to employ up to 150 ␮l of titrant to reach the end point. Therefore, for a monosegment of 200 ␮l, the maximum final volume will be 350 ␮l. Volumes of this order can be easily homogenised in the short path to the detector. Greater volumes can also be generated by adding the reacting solutions in a monosegment, with an additional mixing step. This can achieved by reversing the flow direction two or three times before the monosegment reaches the detector where the end point identification occurs. The time necessary to perform a titration using the successive approximations algorithm also depends on the relative concentration of the sample and titrant. On average, a titration can be performed in about 2 min. The results for titration of Fe(II) with KMnO4 provide complete spectrophotometric titration curves, as shown in Fig. 3. It is important to emphasise that, the titration curves were obtained for a single sample aliquot of only 146.0 ␮l and that stoichiometry and the usual end point location method was employed, using the results obtained with the flow system in the same way as in a conventional titration. 3.1. Determination of hydrogen peroxide in commercial products Six samples of H2 O2 had their peroxide content determined by titration with KMnO4 . The results, compared with conventional manual titration, are presented in Table 1. No significant difference could be detected between both sets of results at 95% confidence level.

Samples 5 and 6 refer to a commercial product in which the hydrogen peroxide is present in a viscous white suspension. These samples were diluted 250 times, resulting in a turbid solution. However, the automated titration could be performed without any loss in accuracy, even when performed in a such matrix. Therefore, the advantage, usually associated with the conventional titration procedure (lower dependence on the sample matrix) was also obtained by the flow approach herein described.

4. Conclusions The complete titration of a single sample aliquot could be achieved employing the proposed system. This appears to be the first time the IUPAC recommended titration process [18] could be realised in a flow system. This possibility arises by adopting the monosegmented flow approach, which ensures rapid and efficient mixing of titrant and titrand, and from the determination of the true volume of sample presented for titration. Considering the titration approach, one can ponder on reduced sample and titrant consumption and on the use of stoichiometry as fundamental for this technique. The technique allows greater tolerance on the stability of the detector employed to follow the titration or to observe the abrupt transition which indicates the end point. Furthermore, the flow approach allows for the use other techniques for end point determination, such as the successive approximation algorithm employed in this work. The flow system is closed, facilitating the control of the atmosphere to which the sample will be in contact while being titrated.

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Since the proposed system presents a behaviour that is very similar to that of conventional titration procedures, it is possible to predict the use of most of the detection and titration techniques with the monosegmented flow titrator.

Acknowledgements The authors are grateful to Dr. Carol H. Collins for manuscript revision. EVA is grateful to FAPESP for the M.Sc. fellowship (Proc. No. 98/01157-9). References [1] W.J. Blaedel, R.H. Laessig, Anal. Chem. 36 (1964) 1617. [2] W.J. Blaedel, R.H. Laessig, Anal. Chem. 37 (1965) 332. [3] J.M. Calatayud, P.C. Falcó, R.M. Abert, Analyst 112 (1987) 1063. [4] B. Fleet, A.Y.W. Ho, Anal. Chem. 46 (1974) 9. [5] G. Nagy, Z. Fehér, K. Tóth, E. Pungor, Anal. Chim. Acta 91 (1977) 7. [6] J. Ruzicka, E.H. Hansen, Anal. Chim. Acta 78 (1975) 145. [7] O. Astrom, Anal. Chim. Acta 105 (1979) 67.

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