Automated continuous titration

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. 

W.J. Blaedel, R.H. Laessig, Continuous automated buretless titrator with direct readout, Anal. Chem. 36 (1964) 1617. 

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. 

An Ion Chromatograph has been successfully automated by interfacing it to an automatic sampler (7). Continuous unattended analysis was possible, the actual number of samples analyzed being limited by the ionic capacity of the suppressor column. The automated Ion Chromatograph was used to analyze soluble sulfates, ammonia and alkyl amines in stack and automobile exhqust samples. Excellent agreement between IC and automated barium chloroanilate titration for sulfate was obtained with a relative standard deviation less than 5%. 

The determination of ascorbic acid in foods is based, in part, on its ability to be oxidized or to act as a reducing agent. The most common method for determination of vitamin C in foods is the visual titration of the reduced form with 2,6-dichloroindophenol (DCIP) (4-7). Variations in this procedure include the use of a potentiometric titration (6), or a photometric adaptation (S) to reduce the diflSculty of visually determining the endpoint in a colored extract. The major criticisms of this technique are that only the reduced vitamin, and not the total vitamin C content of the food, is measured, and that there can be interference from other reducing agents, such as sulfhydryl compounds, reductones, and reduced metals (Fe, Sn, Cu), often present in foods. The DCIP assay can be modified to minimize the effects of the interfering basic substances, but the measurement is still only of the reduced form. Egberg et al. (9) adapted the photometric DCIP assay to an automated procedure for continuous analysis of vitamin C in food extracts. 

Protein Modification. Acylation was performed at room temperature by adding dropwise the anhydride to the protein solution (2 mg/ml). To reach the maximum level of modification, three equivalents of anhydride per lysine were needed. The pH was kept at 9 during the reaction by a controlled and continuous addition of 0.5 M NaOH using an automated pH-stat titration device. The reaction was considered to be complete when the pH of the reaction medium remained constant. To remove salts and excess reagents, the protein solution was dialyzed exhaustively against water and then lyophilized. 

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]. 

The dynamic (working) range of an automatic or automated instrumental technique must obviously fit the working range of concentrations to which it is being applied. In a continuous or automatic titration, for example, the dynamic range and span are governed by the sample size (the volume of the sample and the concentration of desired species in it), the size of the buret, and the concentration of the titrant. The presence of a second titratable species (interference) in the system reduces the usable span by the amount of the second species, since the titration will measure both species. 

Elliot Automation in Houston. Additional inputs to the computer were the calorific value and relative density of the gas. These were measured continuously by a GB-2000 instrument from Precision Measurement Incorporated (PMI). This instrument is a combination of the Therm-Titrator and a new PMI instrument for measuring relative density. Although the flow computer was provided with a temperature input, the temperature sensor had not yet been installed when the data presented in this paper were collected. The temperature of the gas as recorded on a circular chart at the site varied from 83° to 87°F. An average temperature of 85°F was programmed into the flow computer. From all of these continuous iiqiuts, the calculated instantaneous energy flow in the line was displayed on a strip chart recorder. The flow computer used the relative density and NX-19 procedure to correct for supercompressibility. 

Kinetic titration curves are constructed by plotting the relative analytical signal (absorbance, fluorescence, potential, or temperature) as a function of the volume of titrant added. If the two parameters are linearly related, then the titration curve will consist of two linear segments that can be extrapolated to intersect at the endpoint, as shown in Figure 4. Current instrumentation permits the use of automatic or semiautomatic devices capable of delivering the titrant and continuously monitoring the signal obtained. The time elapsed between the start of the titration and the endpoint is known as the pseudoinduction period and is proportional to the inhibitor concentration. These automated procedures are quite rapid and reproducible. 

The colorimetric Nesslerization method uses alkaline phenol and hypochlorite to react with ammonia to form indophenol blue in an amount proportional to the ammonia concentration. The blue color is intensified with sodium nitroprusside, and the concentration is measured using a calibrated colorimeter. The titration of basic ammonia is accomplished with standard sulfuric acid using a mixed indicator. Determination of ammonia may also be based upon the indophenol reaction adapted to automated gas-segmented continuous flow analysis. Potentiometric determination of ammonia is performed by ion-selective ammonia electrodes. 

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