Automated titrator

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. 

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... 

A detailed study of the determination of the anionic surfactant SDBS by automated titration against hyamine using an autotitrator has been undertaken. The following conclusions and observations can be made. 

For higher accuracy in the low polymer concentration range, two different methods were used. In the case of PAA, potentiometric titrations of solutions of PAA were performed with 0.01 N NaOH using a Brinkman model, Westbury, NY, automated titrator. Blank tests indicated no interfering species. Known amounts of PAA were used to prepare a calibration curve immediately after titration of the samples containing unknown amounts of polymer. The starting point of the titration was pH 4.0, and the end point was reached near pH 8. Total volumes of 75 or 100 cc were used for the titrations, and the ionic strength was controlled at 0.01 M NaCl. 

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. 

Whether a constant speed or a variable speed motor drive is used depends in part on the sophistication of the instrumentation and the nature of the anticipatory and detection section of the instrumentation. Because automated titration instrumentation frequently is needed for a specific problem, a great number of custom-fabricated assemblies have been developed. 

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. 

Potentiometry. The dissociation constants of 4-alkylamido-2-hydroxy-benzoic acids in the presence of Brij 35 micelles were measured at 25°C and 0.10 M ionic strength (NaNO ). The ligand (0.002 M) was titrated with 1 M NaOH using a 655-Multi-Dosimat automated titrator (Metrohm), equipped with a 605-pH-meter and a 614-Impulsomat unit. 

If the protein-ligand interaction is endothermic, more power (peal s ) is required relative to the reference cell. The power that is required, over baseline, comprises the raw data output of the ITC equipment. If the reaction is exothermic, less power is required, which is recorded as a downward deflection in output (Fig. 3.2). The overall interaction between a protein (enzyme or receptor) and a ligand (substrate, inhibitor, neurotransmitter, hormone, or drug) is carried out in a sequence of automated titrations. At each injection step, the power is recorded as a function of time. Each subsequent injection in the series is made after the power function returns to baseline. The output, therefore, forms an S-shaped curve, mirroring the progression of binding of the interacting species from initial... 

D.P. Arnold, M.R Russell, J.D. Petty, D.R. Sweatman, Automated titrations using a discontinuous programmed flow analyzer, Anal. Chem. 67 (1989) 2109. 

The biocatalyst mass obtained was 90.01 g, with 23% humidity, determined by the Karl Fischer automated titration model D18, Mettler. 

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. 

K. K. Stewart and A. G. Rosenfeld, Automated Titrations The Use of Automated Multiple Flow Injection Analysis for the Titration of Discrete Samples. J. Autom. Chem., 3 (1981) 30. 

FIGURE 23-19 High-performance automated titrator. Accommodates up to six buret drives, has user-programmable software, permits high sample throughput with an automatic sample turntable (not shown). The unit can be adapted for several different types of titrations. (Copyrighl 2006 Mettler-Toledo, Inc.)... 

Automatic devices cause required acts to be performed at given points in an operation without human intervention. For instance, an automatic titrator records a titration curve or simply stops a titration at an endpoint by mechanical or electrical means (such as a relay) instead of manually. Automated devices, on the other hand, replace human manipulative effort by mechanical and instrumental devices regulated by feedback of information, so, the apparatus is self-monitoring or selfbalancing. An automated titrator may be intended to maintain a sample at some preselected (set point) state— for example, at pH = 8. To do this, the pH of the solution is sensed and compared to a set point of pH = 8, and acid or base is added continuously so as to keep the sample pH at the set point. This type of automated titrator is called a pH-stat [2]. 

In today s lab, pyridine-free Karl Fischer reagents are used. The titration has been adapted to specialized automated titrators made by a variety of instrument manufacturers. The volumetric titration is monitored by two platinum electrodes held at a constant potential. The endpoint is reached when excess reagent results in a constant current. This test method has gained in popularity over LOD testing due to its accuracy, specificity, and smaller sample sizes. 

Other example is the determination of fuel parameters including the diene value and mercaptan sulfur. First, the robot weighs the sample, refluxes it in the presence of maleic anhydride, and extracts the analytes into an aqueous phase, which is poured into the titration vessel, where the robot inserts the photometric or potentiometric probe. For the determination of mercaptan sulfur, the robot also weighs the sample and removes sulfide by precipitation with a CdS04 solution and liquid-liquid extraction. Once the fuel is sulfide free, which is checked by using a photometric probe, it is poured into the titration vessel by the robot arm, which also plunges an Ag electrode prior to addition of the titrant (an AgNOs solution). In both cases, the automated titrator acts as a module of the robotic station and is operated by the robotic arm. 

The quality of the water will be controlled by means of pressure and flow meters and by a hardness tester. The hardness tester consists of an automated titration apparatus. The reservoir, with combined titre and indicator solution, should be refilled regularly. 

CD is a moderately sensitive method in the far-UV spectral region, requiring in the range of 0.01 mM solutions. Modem CD instmments can be purchased with thermoelectric cell holders for thermal scans and with automated titrator syringe pumps for chemical denaturant titrations. Also, the instmments will be capable of data averaging and on-line data acquisition. 

The measurement of hydrogen peroxide within slurries is often done with redox titration methods (manual or automated). Numerous commercial automated titration... 

The advantage of automated titration over manual titration can be significant. As previously mentioned, the subjective nature of endpoint determination in redox titrations leads to relatively high human-to-human variability and, with automation, samples to be titrated can be loaded in carousel configuration and thus the automated instrument will offer a significant reduction in man-hours required of manual titration. 

A computer-controlled automated titration unit with a remote controlled burette of the above specifications and photometric (UV) endpoint detection may be used (Williams and Jenkinson, 1982). Amperometric endpoint detection is also possible (Grasshoff, 1981). 

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. 

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. 

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