Automated titration system

Early on, it was anticipated that many repetitive calibrations and EMF measurements would be carried out in the evaluation of a large quantity of electrodes. The first microcomputer-based automated titration system utilizing high level software (CONVERS) hastened these studies (10), as did a more recently constructed minicomputer system (Figure 1). Typical results are shown in Figure 2, where a set of five protriptyline CWEs were calibrated simultaneously (11). Graphic side-by-side comparison of different electrode calibrations was also useful in establishing structure-selectivity relationships. 

Potentiometric titration curves normally are represented by a plot of the indicator-electrode potential as a function of volume of titrant, as indicated in Fig. 4.2. However, there are some advantages if the data are plotted as the first derivative of the indicator potential with respect to volume of titrant (or even as the second derivative). Such titration curves also are indicated in Figure 4.2, and illustrate that a more definite endpoint indication is provided by both differential curves than by the integrated form of the titration curve. Furthermore, titration by repetitive constant-volume increments allows the endpoint to be determined without a plot of the titration curve the endpoint coincides with the condition when the differential potentiometric response per volume increment is a maximum. Likewise, the endpoint can be determined by using the second derivative the latter has distinct advantages in that there is some indication of the approach of the endpoint as the second derivative approaches a positive maximum just prior to the equivalence point before passing through zero. Such a second-derivative response is particularly attractive for automated titration systems that stop at the equivalence point. 

Automated titration system For example, Metrohm SM-Titrino 702, with subdivision of 0.005 mL connected to a combined Ag/AgO electrode. It is also possible to plot the titration curve by measuring the potential ( ) with a digital voltmeter using two silver wires as electrodes one placed in the burette Le., in dUute AgNOs solution) maintaining a constant potential and the other wire is immersed in the seawater sample (as the AgQ electrode). This electrode must be cleaned from time to time. 

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... 

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. 

In addition to the fully automated 670 system, Metrohm also supply simpler units with more limited facilities which nevertheless are suitable for more simple titrations. Thus the model 682 titroprocessor is recommended for routine titrations with automatic equivalence pointer cognition or to preset end points. The 686 titroprocessor is a lower-cost version of the above instrument, again with automatic equivalence point recognition and titration to preset end points. 

An interesting method is single-point titration, in which a certain amount of reagent is added to the sample and the ISE potential is measured in the resultant solution [6, 7, 28,62, 64, 66, 77). The authors claim that the precision is similar to that of a classical titration (a relative standard deviation of 0.1 to 1.3%). Time is saved and the method is readily applicable to automated measuring systems. 

The titration intervals can be set individually via the instrument software. The compound is solved in the measurement cell at a concentration of 10-100 xM, whereby the pH adjustment by automated titration of the system has an accuracy of 0,02 pH units. Tam suggests in their report titration steps of 0.1 pH units and to perform 25-35 pH readings and absorption spectra measurements during each titration. 

Another test commonly performed in the QC laboratory is the assay. This test is used to determine the purity of an active substance or the amount of an active ingredient present in a dosage form. The information is used to support the manufacturer s claim on the label. Analytical techniques such as chromatography are typically used. Common methods for testing assays are UV spectroscopy, titration, and HPLC. In older methods, automated UV systems or column chromatography may be employed. For recently developed assay methods, HPLC is the technique usually chosen because its specificity, and older techniques such as automated UV and column chromatography have become obsolete. However, some examples of testing with these techniques do exist in the USP. 

The electrometrical methods have also experienced a high development rate because of the possibility of determining the analytes from solutions without a prior separation step. Altemouse202 said, "The introduction of electrical methods allowed precise quantitation." The introduction of the automated buret and automated registration system improved the reliability of titration using electrodes (ISME, redox electrodes, biosensors). 

The distinctions established by IUPAC are clear-cut. Thus, the speed of titrant addition is always constant in an automatic titrator, whereas it Is adjusted by a feedback system according to the nearnesa of the equivalence point in an automated titrator. However, some workers [11,12] acknowledge the accuracy of these definitions but consider them too stringent. Very often, the term automatic Is used to refer to systems with and without feedback Indistinctly. In any case, whenever the concept automatic process Is referred to In this book, It will be meant In its widest connotation, namely that Involving partial or complete elimination of human Intervention not related to Instrumentation. 

An alternative to the hydrogen peroxide/sulfuric acid titration with alkali is distillation of sulfur dioxide from the acidified sample into excess iodine solution. A proportion of the iodine, equivalent to the amount of sulfur dioxide, is reduced to iodide, and the residual iodine is titrated with sodium thiosulfate using starch as indicator. This more rapid method is suitable for meat products where other volatile sulfur compounds do not interfere, and it has been developed for use with automated distillation systems. 

In the classical Kjeldahl method, the proteins are digested (wet oxidized) in sulfuric acid with a catalyst (mercury and selenium tablets now succeed by the much safer potassium and copper sulfate tablets). An acid solution of ammonium sulfate is formed which is then diluted in water. The solution is made alkaline with sodium hydroxide and heated to distil off ammonia into excess standard acid sulfuric acid. The excess acid is back-titrated with standard sodium hydroxide to determine the amount of ammonia. It is more usual now to use boric acid in which to collect the ammonia and titrate with standard hydrochloric acid. The Kjeldahl procedure has been partially automated in systems such as the Kjeltec Analyzer. Total protein can be calculated as nitrogen content x 6.38. 

A typical setup of a working system for manual determinations of oxygen as given in Fig. 4-5 is convenient for standard oxygen titrations applying visual (starch) endpoint detection and may be converted into an automated titration unit by adding a detector and a computer plus interface. 

Another example of an MCFIA system is shown in Figure 4.6. This MCFIA system was designed to implement an automated titration procedure [36]. The system was applied to samples with a wide range of acid concentration such as vinegar and lemon, orange, pineapple, maracock, and acajou juices. 

Automated titrations are important in producing rapid, reproducible results in commercial and research laboratories. Samples may be prepared and loaded using mechanical pipets or direct weighing and dissolution methods. Titrant is added to the sample titrand solution using peristaltic pumps, or burets driven by pressure or piston systems. The addition is very reproducible after accurate calibration. The progress of the titration is most often followed by potentiometric measurements, as outlined above (see also Topic H2). 

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. 

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]. 

A variant of the Karl-Fischer water determination was described [40], By heating the drug substance, the contained water was transferred into a titration cell by a carrier gas. The automated system consisted of an oven sample processor and a coulometer. 

Potentiometric titrations are readily automated by using a motor-driven syringe or an automatic burette coupled to a chart recorder or digital printout system. This is described in more detail in Chapter 12. A micro-processor-controlled titrator is discussed in Chapter 13. 

Drug dissociation constants are experimentally determined by manual or automated potentiometric titration or by spectrophotometric methods.40 Current methods allow determination of pXa values with drug concentrations as low as 10 to 100 pM. For highly insoluble compounds (concentration <1 to 10 pM), the Yesuda-Shedlovsky method41 is commonly used where organic cosolvents (i.e., methanol) are employed to improve solubility. The method takes three or more titrations at different cosolvent concentrations, and the result is then extrapolated to pure aqueous system. The dissociation constant can also be determined with less accuracy from the pH-solubility profile using the following modification of Henderson-Hasselbach equation ... 

The advantages of the bipolar pulse technique include speed (discrete measurements at a rate as high as 30 kHz), accuracy, and signal-to-noise ratio. The system has been employed as a detector in automated conductometric titrations and in stopped-flow mixing systems with excellent results. 

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