Computer-controlled titrators

Titration Instruments fully controlled by a computer through appropriate software represent the highest degree of automation available In tltrlmetrlc analysis today. 

In all the titrator configurations examined by Hleftje st al., the precision-limiting factor seemed to be the end-point detection process rather than the titrant delivery system. 

Goode [69] developed computerized curve-fitting methods for the determination of end-points In spectrophotometrlc titrations. The titration curves are generated by numerical computation by considering the generic reaction 

The titration curve Is a plot of absorbance against the titrant volume. Light absorption is due exclusively to the product, so that 

Titration curves are generated by computation of Eq. (5) by means of a FORTRAN program running on an IBM 370/168 computer. Computations are made under various conditions. Plots of all curves are carried out on a Calcomp flat-bed plotter interfaced to the computer. The titration curves can be analysed by three different procedures. The equivalence point Is determined by a derivative method, an extrapolation method and a non-linear least-squares curve-fitting method, the last being the best. The precision and accuracy achieved with the system vary according to the level of noise. If a titration with a large equilibrium constant and less than 0.004 units of noise is performed, the equivalence point can be determined to within a few tenths of a 

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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 ... 

Fig. 13.8 Operational scheme of the computer-controlled titrator developed by Hieftje et al. (Reproduced from [57] with permission of the American Chemical Society).

Automated apparatus are available commercially, where several samples are processed simultaneously by computer controlled titration stands (Schriever 1994). To provide enough turbulence during elution, shaking devices as well as stirring techniques are used. 

Derivative methods work well only when sufficient data are recorded during the sharp rise in plT occurring near the equivalence point. This is usually not a problem when the titration is conducted with an automatic titrator, particularly when operated under computer control. Manual titrations, however, often contain only a few data points in the equivalence point region, due to the limited range of volumes over which the transition in plT occurs. Manual titrations are, however, information-rich during the more gently rising portions of the titration curve before and after the equivalence point. 

Conversion and the mean rate of reaction were obtained by titrating the liquid recovered from the bed for total acidity before and after dissolved S02 and sulfurous acid were oxidized to sulfuric acid. The difference between these measurements allowed calculation of the S02 conversion. An extension of the Haure study (Metzinger et al., 1992) added gas phase analysis and a check of the results through a sulfur balance. Computer control of flow interruption and acquisition of the S02 analyzer readings were also added. 

Avdeef, A. Bucher, J. J., Accurate measurements of the concentration of hydrogen ions with a glass electrode Calibrations using the Prideaux and other universal buffer solutions and a computer-controlled automatic titrator, Anal. Chem. 50, 2137-2142 (1978). 

Jagner and Kerstein [654,655] used computer-controlled high-precision complexiometric titration for the determination of the total alkaline earth metal concentration in seawater. Total alkaline earths were determined by photometric titration using EDTA with eriochrome Black as indicator. The method yielded 63.32 (xmolekg-1 for the total alkaline earth concentration in standard seawater of 3.5% salinity. The precision was about 0.01%. 

Titrations consist of the observation of one or several measures as a function of the addition of an appropriate reagent. Reagents are typically acids or bases or ligands in metal determinations. Measurements are typically pH and/or absorption spectra. We concentrate on the data analysis of these two types. It should be straightforward for the reader to adapt the algorithms to other observations. Currently, most titrations are done under computer control, either by commercial auto-titrators or by assemblies of burettes, sensors and vessels in the research laboratories. This is not crucial and the analysis of such a titration is essentially identical with the analysis of a manual titration. 

Titration calorimetry can be used to introduce a liquid to a solid or a liquid to a liquid. In this case, the sample is placed in a stainless steel ampoule and the liquid to be titrated is placed in an external reservoir. To avoid problems with temperature gradients the external reservoir should be maintained at the same temperature as the sample. The titrant enters the sample ampoule via a stainless steel cannula. The injections are computer controlled, and the amounts required are entered into the software before an experiment is initiated. 

Figure 22-13 Conceptual diagram of a coulometric titration apparatus. Commercial coulometric titrators are totally electronic and usually computer controlled.

The twentieth century saw the evolution of instrumental techniques. Steven Popoff s second edition of Quantitative Analysis in 1927 included electroanalysis, conductimetric titrations, and colorimetric melliods. Today, of course, analytical technology has progressed to include sophisticated and powerful computer-controlled instrumentation and the ability to perform highly complex analyses and measurements at extremely low concentrations. 

Fully automated computer-controlled system. Av approximated on the basis of constant ApH Computer-contr. titration based on systems theory... 

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. 

Data Collection. Data collection is under computer control with typically 20 to 25 injections of 5 to 10 xl each (Fig. 10). Each injection is accompanied by a heat change until the binding sites are saturated. A small heat change is also typically associated with dilution of the ligand and can be seen at the end of the injection series after saturation. There must be sufficient time between injections to permit the heat flow to return to baseline. A typical titration requires about 2 hr. 

A typical laboratory preparation recipe for this coprecipitation route is provided hereafter. The experimental setup consists of a computer-controlled automatic titrator, a syringe pump, and a reactor, heated by a thermostat. 

The third method for automatic monitoring of the gel phase synthesis has long been known [ 196] - the perchloric acid titration on polymer — and recently was built into a computer-controlled peptide synthesizer [134]. Here, the results of the analytical determinations are fed back into the monitor system, which is programmed to optimize the course of the synthesis based on the titration data. This example of best realized automatization to date, however, is restricted to acid-tolerant synthesis strategies. The limiting chemical conditions of the analytical principle in use were already discussed on p. 43—44. 

Manufacturer-recommended doses range from 0.1 pg/kg per min to 0.5 pg/kg per min, and as high as 1.0 pg/kg per min in children. These doses are much higher than those used in actual clinical practice, which generally range from 0.025-0.1 pg/kg per min for sedation and up to 0.1 -1 pg/kg per min for anesthesia, titrated to effect. Remifentanil may also be used in a process called target controlled infusion, in which computer-controlled infusion pumps maintain desired plasma concentrations. 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

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). 

For good throughput of samples and optimum reproducibility it is reconunended that the alkalinity titration be computer controlled. The computer is used for data collection from the voltmeter as well as for addition of acid from the burette. Software for PCs for this purpose is commercially available. A suitable resolution for the burette, which is generally of a motor driven piston type, is 1 uL. 

When the coulometric sensor-actuator chip is glued on a small printed-circuit board carrier, a dipstick-like titrator is obtained, capable of performing fast titrations (seconds) in a small sample volume (microliters). Operation in a larger volume has the advantage that the bulk of the solution is not affected by the titration. The titration is fully computer controllable and does not require extra chemicals. 

Figure 2 shows data points from a long-term titration of a goethite suspension. One dataset represented by full squares results in an acceptable electromotive force (EMF) (Fig. 2a)/pH-value (Fig. 2b), the other (empty squares) not. The pH or log[H+] is nearly constant over the whole measurement period for the dataset represented by black squares the equilibrium criteria as imposed by the computer controlling the titration (i.e., drift of less than 0.05 mV in 90min) are fulfilled about 17 h after titrant addition. For the dataset represented by the empty squares, this is the case after about 35 h. Here, a constant drift to higher log[H+] values is noted (Fig. 2b), which becomes more obvious in the EMF readings with a drift over more than 10 mV (Fig. 2a). Nevertheless, the equilibrium criterion is finally fulfilled.

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