Titrators, automatic components

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. 

A 10 mM ionic strength universal buffer mixture, consisting of Good zwitterio-nic buffers, [174] and other components (but free of phosphate and boric acid), is used in the pION apparatus [116,556], The 5-pKa mixture produces a linear response to the addition of base titrant in the pH 3-10 interval, as indicated in Fig. 7.53. The robotic system uses the universal buffer solution for all applications, automatically adjusting the pH with the addition of a standardized KOH solution. The robotic system uses a built-in titrator to standardize the pH mapping operation. 

There are two general ways by which the titration can take place. One is the volumetric method, in which the titrant is added to the sample via an automatic titrator. In this case, the titrant is either a mixture of all of the reactants above (a composite titrant) or an iodine solution (other components already in the... 

There are two types of Karl Fisher titrations volumetric and coulometric. Volumetric titration is used to determine relatively large amounts of water (1 to 100. ig) and can be performed using the single- or two-component system. Most commercially available titrators make use of the one-component titrant, which can be purchased in two strengths 2 mg of water per milliliter of titrant and the 5 mg of water per milliliter of titrant. The choice of concentration is dependent on the amount of water in the sample and any sample size limitations. In both cases, the sample is typically dissolved in a methanol solution. The iodine/SCVpyridine (imidazole) required for the reaction is titrated into the sample solution either manually or automatically. The reaction endpoint is generally detected bivoltametrically. 

Reference to Table 4.1 indicates that olefins can be determined by the electrochemical generation in situ of halogens. Bromine is effective for both olefins and sulfur compounds and is the basis for an automatic coulometric titrator for continuous analysis of petroleum streams.17 The basic principle of this instrument is a potentiometric sensing system that monitors bromine concentration in a continuously introduced sample stream. The bromine in the solution reacts with the sample components and causes a decrease in the concentration of bromine. When this decrease is sensed by the potentiometric detection electrodes, the electrolysis current producing bromine adjusts itself to maintain the bromine concentration. Because the sample is introduced at a constant rate, the electrolysis current becomes directly proportional to the concentration of the sample component. Thus, the instrument records the electrolysis current as concentration of sample component and provides a continuous monitor for olefins or sulfur in petroleum streams. 

Both the automatic coulometric titration of petroleum streams and the continuous monitoring of pesticides and sulfur-halogen compounds indicate that the coulometric titrator method is amenable to the automatic maintenance of the concentration of a component in a solution system. A manual version of this approach has been used to study the kinetics of hydrogenation of olefins as well as to determine the rate of hydrolysis of esters.12 The latter system is a pH-stat that is based on the principles of coulometric titrations. Equations (4.9)-(4.11) indicate how this approach is applied to the evaluation of the rate constants for ester hydrolysis. A similar approach could be used to develop procedures for kinetic studies that involve most of the electrochemical intermediates summarized in Table 4.1. The coulometric titration method provides a convenient means to extend the range of systems that can be subjected to kinetic study in solution. 

Homeostasis. Anyone who tries to regulate a chemical reaction system by a multitude of valves or switches (Figure 5) soon becomes frustrated with the instability of his experimental system and appreciative of automatic control devices (servo systems). For example, for the external control of preselected pH and C02 activity, an automatic titrator (pH — stat) can be used to dose continuously and automatically the quantity of C02 which is necessary to maintain constant pH. Feedback is an essential feature of such a control system there are essentially two major components, a controlling system (error detector) and a controlled system (13) (Figure Id). 

The second step of the analytical process — the components analysis or black box — can be automated by using automatic devices. The samples automatically change the automation required for this step, decreasing the time needed for the analytical process. This has been used in radiochemical analysis for operator protection. Now, automatic devices are often used for RXF, AAS, and ESCA methods, as well as for chromatographic techniques, such as HPLC, to increase the speed of the analytical process. By using automatic titration, the reliability and quality of the analytical information increases, the objectivity increases and the time needed decreases dramatically. 

The most important component of an automated potentiometric titrator is the potentiometer. In simpler titrimeters, ordinary burettes are used to deliver the titrant and are equipped with automatic valves. Usually piston (syringe) burettes that allow delivery of the titrant with predetermined constant or variable speed are employed. Most automatic titrators have built-in electronic switches to perform various duties and include one or more derivatizing circuits enabling the location of the endpoint. The automated potentiometric titrators available in the market may be divided into four groups (1) automatic titration curve recording instruments (2) preset endpoint titrators, in which the flow of the titrant is stopped when the potential of the indicator electrode reaches a preselected value (3) first- or... 

For the quantitative analysis of a certain component in mixed gases, an automatic titration technique for evolved gases from TA connected with the process of continuous titration was developed [58, 70- 72, 94], This technique is known as thermo-gas titration (TGT). Detected curves (TGT curve) and derivative curves (DTGT curve) can be obtained. The TG-DTA-TGT coupled simultaneous technique and relevant equipment were also established in the 1980s [70]. This technique is based on the direct determination of CO2 and SO, (SO,) and on the indirect determination of HjO. A schematic diagram of the TG-DTG-DTA-TGT-DTGT coupled simultaneous apparatus [71] is shown in Figure 2.42. 

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