Microprocessor-controlled titrators

The Memotitrator can also be fitted with conventional optical detectors. Thus, a Perkln-Elmer 650-10S spectrofluorlmeter has allowed the development of the direct catalytic titration of EDTA based on its Inhibitory effect on the oxidation of 2-hydroxybenzaldehyde thlosemlcarbazone by hydrogen peroxide, catalysed by Iron(III), and the indirect determination of this and some alkaline-earth metal ions [61]. 

A special titration vessel with a side opening through which powdered solids are introduced with a weighing spoon Is used for Karl Fischer titrations. These have a distinct feature In that the solvent must be titrated before the sample Itself as It also contains water. To avoid dosing the solvent with every new sample, the vessel Is furnished with a stopcock which allows draining of the solution to make room for the next sample and for the Karl Fischer reagent to be added. 

Precipitation titrations usually benefit from the use of some protective dried gas (CO2, N2). The equalization of the pressure In the reservoir Is ac- 

Voltammetrlc end-point detection with the Memotltrator calls for the use of a dual platinum pin electrode and a polarized power supply. 

Metrohm markets microprocessor-controlled Instruments for routine coulo-metric (KF 652 Processor) and amperometrlc-voltammetrlc (EP/KF 678 Processor) analyses. These Instruments are frequently used for research purposes. Thus, the Mettler Memotltrator and the Metrohm Tltroprocessor 636/Rod Stirrer 622 were used by Johansson et al. [62] to demonstrate that potentiometric two-phase titrations can be carried out in an automatic fashion. The typical background noise from the electrodes can be reduced by Introducing hydrophobic anions or cations in the aqueous phase. These ions also affect the acid-base equilibrium by extracting the sample ions as ion-pairs into the organic phase, which allows the conditional acidity constant (apparent K ) to be manipulated to make selective titrations possible. 

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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. 

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 microprocessor-controlled titrator is discussed in chapter 13. 

According to the classification established by Pungor et al. [56], there are three levels of automatic tltrators defining as many degrees of automation, namely (a) hardware-controlled tltrators, (b) microprocessor-controlled titrators and (c) computer-controlled tltrators. 

Fig. 13.7 Scheme of the microprocessor-controlled titrator developed by Betterldge et al. (Reproduced from [63] with permission of the Royal Society of Chemistry). 

Although the DL 40 was capable of performing Karl Fischer water titrations and Mettler developed a separate microprocessor-controlled push-button operated DL 18 KF titrator, they also introduced as an all-purpose apparatus the improved DL 40 RC (see Fig. 5.11) with a dual titration head and with a modified software program to handle the new two-component titrants for Karl Fischer titration (see pp. 204-205). The instrument can also be expanded into an automatic series titrator by connecting the RT 40 sample transport for 16 samples and storage of 50 sample weights from a connected balance this series routine can be interrupted at any time after completion of the titration in progress. 

Although the E 636 allows Karl Fischer water determinations, as any other titration, a separate microprocessor-controlled 658 KF processor has been developed, and there is also the microprocessor-controlled 652 KF coulometer (see pp. 221-222). 

A Microprocessor-controlled Potentiometric Titrator. 2. An Infrared Spectrometer Interfaced to a Dedicated Microcomputer. 3. 

Simplified block diagram of a microprocessor-controlled potentiometric titrator. 

The microprocessor control also enables the instrument to be set to calculate pA"a values directly from the pH profile it obtains by titration of a sample. A sample changer can be incorporated so that several samples can be automatically titrated. 

In this era of automatic titrators, microprocessor-controlled thermal analysis, and definitive spectral techniques, one of the most powerful techniques, that is, optical microscopy, is frequently overlooked. The value of direct sample observation, preferably while it is exposed to different relative humidities, cannot be overstated. In the author s laboratory, a plexiglass chamber was constructed that can be placed on the stage of the microscope, through which air of known humidity can be circulated. This simple technique has been very useful in examining the swelling (or lack) of disintegrants and the influence of very hydrophilic excipients in combination with a moisture sensitive drug. ... 

Microprocessor- or computer-controlled titration equipment where the parameters required for any specific titration can be logged into the memory system. These include, in part ... 

Note that the duplicate results are 2.503% and 2.501% acetic acid. These are typical precisions for modern microprocessor-controlled potentiometric titrators. 

Note that, here again, the microprocessor-controlled potentiometric titration shows very good precision. 

The use of modern temperature detection methods in thermal titration has increased considerably in recent years, with several commercial instruments now available. Marini and Martin have recently reviewed this field (Marini and Martin, 1979) extensively so that only a brief discussion will be given here. We have developed a combined pH—thermal differential titration apparatus that is modelled after our earlier single-cell system (Berger et al., 1974 Marini et al., 1980). Figure 14 shows the essentials of the instrument. The unique part of this device is that it is under microprocessor control. The computer starts the titration, records the data, and speeds up or slows down the titration automatically if the curve is changing too rapidly. Data-correction programs adjust for response time and... 

Fig. 1.11 Automation of the first and third stages of the analytical process (Type 8 analyser). Scheme of microprocessor-controlled automatic potentiometric or photometric titrator.

Depending on the titrator apparatus, add the bromide-bromate solution manually or by microprocessor control in small increments from e buret. The endpoint of the titration is achieved when the potential reaches the pre-set value (see 9.4) and persists for more than 30 s. 

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). 

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). 

Commercially available Modem KF-Titrators are usually equipped with specifically designed titration vessels that are exclusively meant to check and prevent the contact with atmospheric moisture. Quite a few such devices are armed with microprocessors that will perform the requisite operations sequentially in a programmed manner automatically and may also dish out a print-out of the desired results including the percentage moisture content. In fact, these Modem KF-Titrators not only afford greater accuracy and precision in results but also offer much ease and convenience in routine analysis as compared to the classical techniques based on either caulometry or controlled current potentiometiy using two indicator electrodes. 

Titrations can be automated and controlled by a microprocessor. The titrant is delivered via an automatic burette and the end-point is detected potentiometrically... 

Earlier, the work of Zetlmeisl and Laurence [22] was described. With their instrument, the current decayed exponentially during the titration. Control of the current via an algorithm can be done with the computer. An example of this is found in the work of Earle and Fletcher [64]. Their titrator was based on the Intel 8008, an early 8-bit microprocessor. For acid-base titrations, the applied current was reduced linearly with the difference in pH, ApH, between the measured value and the endpoint pH. An algorithm compared ApH with a set of rate functions specified in units of mA/pH. The magnitude of the current was then computed by multiplying ApH by the rate function. This process was repeated every 65.536 ms. The coulombs passed were computed for each of these time intervals and summed until the endpoint was reached. The result was then... 

Potentiometric titrations can be easily automated and controlled by microprocessors or computers. The extent of automation is such that, in the same vessel, multiple titrants may be used sequentially, providing for the determination of several analytes in one sample. Sample changers carrying up to and beyond 60 samples can be attached with ease, vastly decreasing the technician time to produce multiple sample results. 

The performance of the titration can be controlled In a variety of ways (see Table 13.1) by use of empirical equations for the calculation of AV from preceding titration data points by use of microprocessors to control volumetric equipment (e.g. in photometric, potentlometrlc, coulometrlc titrations) or expand the scope of a given technique by use of robot stations In Implementing laborious manual methods or In handling toxic or hazardous substances etc. End-point detection Is usually based on E/A.V maxima and on first or second derivatives In the case of microprocessor- and microcomputer-controlled processes, respectively. Table 13.2 lists a chronological selection of calculation methods applied to titration curves [46]. 

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