Titrations, automation

NaCl solution. In one set of experiments, the slurry was titrated with 0.1 normal NaOH solution in one cm increments to a pH of about 10. The samples of this set are referred to as the "slurry" samples (titration of the calcined zeolite - NaCl solution slurry). In another set of experiments, the calcined zeolite - NaCl solution slurry was filtered, the filter cake washed with about 100 cnP distilled water, and the combined filtrates were titrated with 0.1 N NaOH solution, again to a pH of about 10. The samples of this latter set are referred to as the "filtrate" samples (the zeolite being removed by filtration prior to titration of the filtrate). In addition to the manual titrations, automated potentiometric titration curves were obtained with a Metrohm E636 Titroprocessor, which has an estimated pH measurement accuracy of 0.001 pH unit and an estimated volumetric addition accuracy of 0.001 cm ... 

Titrations may be automated using a pump to deliver the titrant at a constant flow rate, and a solenoid valve to control the flow (Figure 9.5). The volume of titrant delivered is determined by multiplying the flow rate by the elapsed time. Automated titrations offer the additional advantage of using a microcomputer for data storage and analysis. 

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

Contaminant by-products depend upon process routes to the product, so maximum impurity specifications may vary, eg, for CHA produced by aniline hydrogenation versus that made by cyclohexanol amination. Capillary column chromatography has improved resolution and quantitation of contaminants beyond the more fliUy described packed column methods (61) used historically to define specification standards. Wet chemical titrimetry for water by Kad Eisher or amine number by acid titration have changed Httle except for thein automation. Colorimetric methods remain based on APHA standards. 

A number of simple, standard methods have been developed for the analysis of ammonium compounds, several of which have been adapted to automated or instmmental methods. Ammonium content is most easily deterrnined by adding excess sodium hydroxide to a solution of the salt. Liberated ammonia is then distilled into standard sulfuric acid and the excess acid titrated. Other methods include colorimetry (2) and the use of a specific ion electrode (3). 

The use of "fixed" automation, automation designed to perform a specific task, is already widespread ia the analytical laboratory as exemplified by autosamplers and microprocessors for sample processiag and instmment control (see also Automated instrumentation) (1). The laboratory robot origiaated ia devices coastmcted to perform specific and generally repetitive mechanical tasks ia the laboratory. Examples of automatioa employing robotics iaclude automatic titrators, sample preparatioa devices, and autoanalyzers. These devices have a place within the quality control (qv) laboratory, because they can be optimized for a specific repetitive task. AppHcation of fixed automation within the analytical research function, however, is limited. These devices can only perform the specific tasks for which they were designed (2). 

Another step in laboratory automation to be achieved is the conversion of standard chemical procedures such as titrations or thermal gravimetric analysis, into unit laboratory operations. A procedure could then be selected from these laboratory operations by an expert system and translated by the system to produce a set of iastmctions for a robot. The robot should be able to obey specific iastmctions, such as taking a specified sample aliquot and titrating it using a specified reagent. 

The methods dependent upon measurement of an electrical property, and those based upon determination of the extent to which radiation is absorbed or upon assessment of the intensity of emitted radiation, all require the use of a suitable instrument, e.g. polarograph, spectrophotometer, etc., and in consequence such methods are referred to as instrumental methods . Instrumental methods are usually much faster than purely chemical procedures, they are normally applicable at concentrations far too small to be amenable to determination by classical methods, and they find wide application in industry. In most cases a microcomputer can be interfaced to the instrument so that absorption curves, polarograms, titration curves, etc., can be plotted automatically, and in fact, by the incorporation of appropriate servo-mechanisms, the whole analytical process may, in suitable cases, be completely automated. 

A number of commercial titrators are available in which the electrical measuring unit is coupled to a chart recorder to produce directly a titration curve, and by linking the delivery of titrant from the burette to the movement of the recorder chart, an auto-titrator is produced. It is possible to stop the delivery of the titrant when the indicator electrode attains the potential corresponding to the equivalence point of the particular titration this is a feature of some importance when a number of repetitive titrations have to be performed. Many such instruments are controlled by a microprocessor so that the whole titration procedure is, to a large extent, automated. In addition to the normal titration curve, such instruments will also plot the first-derivative curve (AE/AV), the second-derivative curve (A2 E/AV2), and will provide a Gran s plot (Section 15.18). 

Tam et al. [37-47] developed an impressive generalized method for the determination of ionization constants and molar absorptivity curves of individual species, using diode-array UV spectrophotometry, coupled to an automated pH titrator. Species selection was effected by target factor analysis. Multiprotic compounds with overlapping pK s have been investigated binary mixtures of ionizable compounds have been considered assessment of inicroconstants have been reported. 

Avdeef and Bucher [24] investigated the use of universal buffers in potentiomet-ric titrations. Recently, such a buffer system, formulated with several of the Good components, has been designed specifically for robotic applications, where automated pH control in 96-well microtiter plates is required, with minimal interference to the UV measurement [48]. This universal buffer has a nearly perfectly linear pH response to additions of standard titrant in the pH 3-10 region [8, 48]. 

Glomme, A., Marz, J., Dressman, J. B. Comparison of a miniaturized shake-flask solubility method with automated potentiometric acid/base titrations and calculated solubilities. /. Pharm. Set. 2005, 94, 1-15. 

Also, I would like to pay tribute to my former co-workers, especially those in the field of titration in non-aqueous media Dr. H. B. van der Heijde, Dr. N. van Meurs and Dr. M. Bos, of whom the last mentioned also deserves my additional appreciation not only for his work on electroanalysis in solvents of relatively low dielectric constants, but also for his development of automated and computerized methods of electroanalysis. Some other investigators, such as Dr. H. Donche (State University of Ghent, Belgium) and Dr. B. H. van der Schoot (Twente University of Technology, The Netherlands), most obligingly provided me with details of their recent work even before it had been published. My sincere thanks are also due to Mr. A. A. Deetman for his literature research on automation of electroanalysis and to his employers AKZO Zout Chemie, Hengelo, The Netherlands, for their kind permission to carry out this task. Further, I am indebted to many firms and their representatives in The... 

It must be realized that the constant current (-1) chosen virtually determines a constant titration velocity during the entire operation hence a high current shortens the titration time, which is acceptable at the start, but may endanger the establishment of equilibrium of the electrode potentials near the titration end-point in an automated potentiometric titration the latter is usually avoided by making the titration velocity inversely proportional to the first derivative, dE/dt. Now, as automation of coulometric titrations is an obvious step, preferably with computerization (see Part C), such a procedure can be achieved either by such an inversely proportional adjustment of the current value or by a corresponding proportional adjustment of an interruption frequency of the constant current once chosen. In this mode the method can be characterized as a potentiometric controlled-current coulometric titration. 

Automated titrations can be divided into discontinuous and continuous, the former representing a discrete sample analysis, as a batch titration is the usual laboratory technique and the latter a flow technique, which is used less frequently in the laboratory, e.g., in kinetic studies, but is of greater importance in plant and environment control. 

In routine analysis, often a one-dimensional so-called end-point titration can be automatically carried out up to a pre-set pH or potential value and with a previously chosen overall titration velocity in order to avoid overshoot, the inflection point should be sufficiently sharp and the titrant delivery must automatically diminish on the approach to that point in order to maintain equilibrium, and stop in time at the pre-set value. For instance, the Metrohm 526 end-point titrator changes both the dosing pulse length and its velocity by means of a pulse regulator in accordance with the course of the titration curve in fact, the instrument follows the titration two-dimensionally, but finally reports only a one-dimensional result. The Radiometer ETS 822 end-point titration system offers similar possibilities. However, automated titrations mostly represent examples of a two-dimensional so-called eqilibrium titration, where the titration velocity is inversely proportional to the steepness of the potentiometric titration curve hence the first derivative of the curve can usually also be recorded as a more accurate means of determining the inflection... 

The above so-called automated titrations still require manual sample introduction directly into the measuring cell60 in order to avoid this in series analysis, a few manufacturers added automatic samplers, which in laboratory practice require only the previous introduction of samples into a series of cups... 

In 1976, Radiometer61 presented for the first time a microprocessor-controlled titration system. Since then, the microprocessor has been used preferentially and as a fully integrated part (in line) in electroanalytical instruments as a replacement for the on-line microcomputer used before. Bos62 gave a comprehensive description of the set-up and newer developments with microprocessors in relation to microcomputers and indicated what they can do in laboratory automation. Many manufacturers are now offering versatile microprocessor-controlled titrators such as the Mettler DL 40 and DL 40 RC MemoTitrators, the Metrohm E 636 Titroprocessor and the Radiometer MTS 800 multi-titration system. Since Mettler were the first to introduce microprocessor-controlled titrators with their Model DK 25, which could be extended to a fully automated series analysis via the ST 80/ST 801 sample transport and lift together with the CT 21/CT211 identification system, we shall pay most attention to the new Mettler MemoTitrators, followed by additional remarks on the Metrohm and Radiometer apparatus. 

This wording may be considered as duplication, because one can hardly think of continuous titration without automation however, the intention is simply to stress its character as an alternative to automated discontinuous titrations. The principle of continuous titration can be illustrated best by Fig. 5.151 it applies to a steady stream of sample (C). Now, let us assume at first that the analyte concentration is on specification, i.e., it agrees with the analyte concentration of the standard (B). If, when one mixes the titrant (A) with the sample stream (C), the mass flow (equiv./s) of titrant precisely matches the mass flow of analyte, then the resulting mixture is on set-point. However, when the analyte concentration fluctuates, the fluctuations are registered by the sensor it is clear that the continuous measurement by mixing A and C is only occasionally interrupted by alternatively mixing A and B in order to check the titrant for its constancy. 

Flow techniques have become of considerable importance, not only in routine titrations but also in other analytical methods as automated analytical processes they all need to be under the control of a detector, often called a sensor and sometimes a biosensor. We can divide the techniques into the following ... 

The measurement of absorbance of light by a dyestuff-anionic surfactant complex, which has been extracted into an organic solvent is a key feature of many methods, and Sodergren has successfully used segmented flow colorimetry for an automated version of this procedure (2 ). An alternative is the two phase titration technique, pioneered by Herring (3) which uses dimidium... 

CHOICE OF FILTER FOR AUTOMATED PHOTOMETRIC TITRATION. At the end of a photometric titration using the above two indicators the colour of the chloroform phase changes from pink to blue. To choose a filter to detect this end point the visible spectra of the separated chloroform layers of surfactant titrations were recorded before, at and beyond the end point, see Figure 2. At 580 nm there was a greater change in absorbance than at 440 nm, thus the 580 nm filter was preferred. 

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