摘要:

IntroductionTitrimetric analysis techniques are widely used in laboratories and in the control of various industrial processes. The time-consuming and labour-intensive operations stemming from the batchwise mode of performing titrations, as well as the troublesome processes of collecting and plotting the data sometimes required for the titration curve, are some of the main drawbacks of titrimetric procedures. The above reasons could justify the use of titrations only when direct measurement does not yield acceptable results and why researchers are interested in the automation of the titration procedure.First, batch titration systems were partially1,2or totally3automated. However, as these systems required elaborate and expensive equipment, several workers then proposed continuous flow systems. Blaedel and Laessig4described a continuous titrator in which the sample is pumped at a constant flow rate and the reagent stream at a variable flow rate. After mixing, the resultant stream passes through the detector. By adjusting the speed of the reagent pump, the mixed stream can be made to attain the equivalence point composition.A gradient titration technique was developed by Fleet and Ho,5involving the semi-continuous monitoring of a sample stream with a reagent stream in which the concentration of the reagent increases linearly in the form of a gradient. Nagyet al.6developed the ‘triangle-programmed titration’ technique. In this methodology, the titrant concentration is first linearly increased and then decreased in a symmetrical way, thus obtaining two equivalence points.Continuous single-point titrations have been proposed by Astrom,7involving the use of a peristaltic pump; two channels were used so that equal buffer and sample flows were mixed. Measurements were made on the effluent after mixing in a conventional titration vessel. Ruzicka and co-workers8,9developed the flow injection titration (FIA titration) method, in which the time between the stoichiometric points for the rising and falling parts of the titration curve is proportional to the logarithm of the concentration.Valcárcelet al.10,11have used flow rate gradients to generate concentration gradients. They combined a fixed flow rate pump with a programmed flow rate pump that was used to increase gradually the titrant concentration in the flow system.A flow injection analysis technique based on stop flow coulometric titration was developed by Tayloret al.12This technique uses a gradient chamber, a reagent generation chamber, and a detector flow cell integrated into a single unit. By stopping the flow at a suitable delay time, following sample injection, the sample zone will be held inside the mixing chamber, allowing automatic dilution prior to titration. If the detection and the coulometric reagent generation are performed inside the mixing chamber, all sample material will react, as in conventional titrimetry.Araújoet al.13presented a flow injection titration based on a concentration gradient with a single standard calibration. In this flow manifold, the concentration of one of the solutions (sample or titrant) is kept at its steady state during the whole process, by pumping it continuously. The other solution is injected and its concentration determined by the gradient calibration technique.All the above-mentioned flow titration techniques require a calibration process for the attainment of the unknown concentration.A binary search strategy for the end point determination using a multicommutated flow system was proposed by Kornet al.14and promoted the development of unsegmented14or segmented15,16flow titrations. This method is based on variable volumetric ratios between the titrant and sample solution selected by a binary algorithm. Although these titrations do not require any calibration, they were only applied to systems in which an indicator14,16or other reference solutions15can be used by the binary searching algorithm to decide the next step in the searching process.In this work, a flow titration strategy based on the unsegmented sequential introduction into a reactor (mixing chamber) of increasing volumes of titrant and decreasing volumes of titrand is used to perform titrations without requiring a calibration step. This flow system enables one to attain complete titration curves similar to those of batch titration systems, allowing more data to be obtained during the titration process which could be used for several purposes,e.g., equilibrium studies. The software to control every step of the titration procedure, perform data acquisition and data processing was also developed.The equivalence point in a batch potentiometric titration is generally determined by finding the point of maximum slope of the titration curve. In many instances there are very sharp breaks in these curves and, therefore, no difficulty in finding the equivalence point. However, in other cases, the curves are hardly evaluated because the potential variation is small close to the equivalence point. It is then usual to use the Gran method17,18which renders the titration curves linear, thereby making possible the determination of the equivalence volume by using several points on the titration curve, instead of only the inflection point. The applicability of this end point location method is also evaluated in this work by using the automatic flow titrator presented.The model19used for the determination of the theoretical end point time for each titrand standard concentration was tested and proved to be suitable for the description of the analytical process.The flow system proposed allows one to simulate batch titration procedures without requiring a calibration step because the determination of the unknown titrand concentration is carried out by an iterative procedure, which, based on the equations of the theoretical model,19estimates the titrand concentration whose theoretical end point time corresponds to the experimental value.In order to evaluate the usefulness of the developed titration system, chloride determination in bottled natural waters was selected as it is a parameter determined in the quality control of waters and where direct potentiometry cannot usually be used. The results given by the proposed and reference methods showed a good agreement.

ISSN:0003-2654    

出版商:RSC    

年代:1999

数据来源: RSC