Titrators, automatic

This chapter presents an overview of commercial and non-commercial titrators excluding flow systems or continuous titrators, which were dealt with In Chapter 7. 

Although the number of direct instrumental methods of analysis Is growing steadily, titrations are still common practice in routine analyses carried out In many laboratories, probably as result of the lack of sufficiently selective Instrumental methods and of the greater sensitivity of automatic titrations. Manual titrations, on the other hand, are time-consuming and the accuracy of the results obtained depends to a great extent on the operator s skill. This Is not the case with automatic titrations, of proven efficiency and accuracy [1-3]. 

A titration usually involves two distinct aspects, namely the control of the performance of the actual titration and the calculation procedure(s). The most relevant advances In both are listed chronologically in Tables 13.1 and 13.2. 

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]. 

Titrators are wet-chemistry analytical Instruments comparable In complexity to current process gas chromatographs. They are normally expensive and cannot be run for long periods unattended, though their reliability has reached a point where maintenance efforts are chiefly concerned with the sample system and with ensuring that an adequate supply of reagent Is on hand. 

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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. 

The following data were collected with an automatic titrator during the titration of a monoprotic weak acid with a strong base. Prepare normal, first-derivative, second-derivative, and Gran plot titration curves for this data, and locate the equivalence point for each. 

Phosphoms content usually is measured by a double end point titration method in which a 1.0-g sample is dissolved in a hot HNO —H2SO4—HCIO4 mixture. The pH is adjusted to 2.5 with NaOH, and the resulting H PO is titrated with 0.5-A[ NaOH, using an automatic titrator. The titer between the first and second end points is used to quantify the phosphoms as H PO. ... 

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). 

Assay of beryUium metal and beryUium compounds is usuaUy accompHshed by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryUium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryUium content of the sample is calculated from the titration volume. Standards containing known beryUium concentrations must be analyzed along with the samples, as complexation of beryUium by fluoride is not quantitative. Titration rate and hold times ate critical therefore use of an automatic titrator is recommended. Other fluotide-complexing elements such as aluminum, sUicon, zirconium, hafnium, uranium, thorium, and rate earth elements must be absent, or must be corrected for if present in smaU amounts. Copper-beryUium and nickel—beryUium aUoys can be analyzed by titration if the beryUium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

In piston burettes, the delivery of the liquid is controlled by movement of a tightly fitting plunger within a graduated tube of uniform bore. They are particularly useful when the piston is coupled to a motor drive, and in this form serve as the basis of automatic titrators. These instruments can provide automatic plotting of titration curves, and provision is made for a variable rate of delivery as the end point is approached so that there is no danger of overshooting the end point. 

As has been indicated, if suitable automatic titrators are used, then the derivative curve may be plotted directly and there is no need to undertake the calculations described above. 

A 3 litre bioreactor with a working volume of 2 litre is inoculated with the three shaking flasks. The pH is maintained at 5.5 by automatic titration with 5mol l 1 NH4OH and the temperature is held at 37°C. 

FIG. 11 Titration plot of alkanesulfonates. Sample 60 wt % of Hostapur SAS 60, monosulfonates fraction contents ca. 140 mg/100 ml (10% MeOH) solution to be titrated 10 ml, 5 ml buffer pH 3 (Merck), 5 ml MeOH, diluted to 100 ml with water titrant 0.004 mol/l TEGOtrant A 100 (l,3-didecyl-2-methyl-imidazolium chloride, Metrohm 6.2317.000) titrator Titrino 716 DMS with automatic titrator 727 and propellant stirrer titration mode dynamic end point titration (DET), high-sense electrode Metrohm 6.0504.15Q, reference electrode Ag/AgCl Metrohm 6.0733.100, EP = end point. 

Physical methods for endpoint detection have been suggested. Hellsten [226] proposed an instrumental turbidimetric method to determine the endpoint, which does not need indicators. Since chloroform is emulsified by the anionic surfactant, changes in the optical density can be followed by a colorimeter thus detecting the endpoint when the emulsion breaks. Another turbidimetric method based on commercially available automatic titrators has also been proposed [227],... 

A plot of the pH of the analyte solution against the volume of titrant added during a titration is called a pH curve. The shape of the pH curve in Fig. 11.4 is typical of titrations in which a strong acid is added to a strong base. Initially, the pH falls slowly. Then, at the stoichiometric point, there is a sudden decrease in pH through 7. At this point, an indicator changes color or an automatic titrator responds electronically to the sudden change in pH. Titrations typically end at this point. However, if we were to continue the titration, we would find that the pH... 

A simple, reliable, and fast method of determining the pH of a solution and of monitoring a titration is with a pH meter, which uses a special electrode to measure H 0+ concentration. An automatic titrator monitors the pH of the analyte solution continuously. It detects the stoichiometric point by responding to the characteristic rapid change in pH (Fig. 11.9). Another common technique is to use an indicator to detect the stoichiometric point. An acid-base indicator is a water-soluble organic dye with a color that depends on the pH. The sudden change in pH... 

FIGURE 11.9 A commercially available automatic titrator. The stoichiometric point of the titration is detected by the sudden change in pH that occurs in its vicinity the pH is monitored electronically. The pH curve can be plotted as the reaction proceeds, as shown on the monitor screen. 

FIGURE 3 Effect of the amount of cholesterol on the particle size. Phosphatidylcholine/cholesterol liposomes were prepared by the octyl glucoside dilution technique. The begin concentration of the mixed micelles was 150 mM octyl glucoside and 10 mM phosphatidylcholine in 10 mM tris(hydroxymethyl)aminomethane and 0.9% NaCl, pH 7.4. Dilution was performed with an automatic titration unit at a dilution rate (= dilution factor, relative to the initial volume, per unit of time) of 0.026 sec"l ( a and ) or 0.69 sec l ( and o). Mean diameters after dilution and ) and after filtration ( L and q) are repi sented. (Adapted from Jiskoot et al, 1986a.)... 

Cotlove, E. and Nishi, H. H. Automatic titration with direct read-out of chloride concentration. Clin. 

After 24 h of reaction, the catalytic bed was retrieved and sieved to separate the catalyst from the diluent. The used catalyst particles were placed in a Soxhlet apparatus, washed with n-hexane for 8 hours and then dried overnight at 393 K. Their carbon content was determined by automatic titration of the CO2 formed by burning the washed sample, in a Strohlein Coulomat 702 apparatus. 

Instruments specially constructed for automatic titration or autoanalysis in general will be treated in Part C. 

By connecting the PRS 12 Alpha Printer, complete documentation of titration results, stored methods or other user input is available. The SAC 80 sample changer can be connected for fully automatic titrations or pH measurements on 20 samples, while the sample weights can be keyed into the TTT 85 titrator. 

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 [28] has also described a semi-automatic titration for high-precision determination of chlorine in seawater, where it has been used for the potentiometric determination of total halides (silver electrode) and alkalinity (glass electrode), and for the photometric titration of total alkaline-earth metals. Several titrations can be effected simultaneously. 

Anfalt and Jagner [57] measured total fluoride ion concentration by means of a single-crystal fluoride selective electrode (Orion, model 94-09). Samples of seawater were adjusted to pH 6.6 with hydrochloric acid and were titrated with 0.01 M sodium fluoride with use of the semi-automatic titrator described by Jagner [28]. Equations for the graphical or computer treatment of the results are given. Calibration of the electrode for single-point potentiometric measurements at different seawater salinities is discussed. 

Lebel [224] has described an automated chelometric method for the determination of sulfate in seawater. This method utilises the potentiometric end-point method for back titration of excess barium against EDTA following precipitation of sulfate as barium sulfate. An amalgamated silver electrode was used in conjunction with a calomel reference electrode in an automatic titration assembly consisting of a 2.5 ml autoburette and a pH meter coupled to a recorder. Recovery of added sulfate was between 99 and 101%, and standard deviations of successive analyses were less than 0.5 of the mean. 

The titration process has been automated so that batches of samples can be titrated non-manually and the data processed and reported via printouts and screens. One such instrument is the Metrohm 670 titroprocessor. This incorporates a built-in control unit and sample changer so that up to nine samples can be automatically titrated. The 670 titroprocessor offers incremental titrations with variable or constant-volume steps (dynamic or monotonic titration). The measured value transfer in these titrations is either drift controlled (equilibrium titration) or effected after a fixed waiting time pK determinations and fixed end points (e.g. for specified standard procedures) are naturally included. End-point titrations can also be carried out. 

Electronic devices such as automatic titrators and digital burets may be used in place of the traditional glass buret and manual titration. Such devices provide electronic control over the addition of titrant and thus, with proper calibration, are accurate, high-precision devices. These will be discussed in Section 4.9. 

Bettina Pfeiffenberger of SACHEM Inc. uses an automatic titrator to titrate concentrations of TMAH to ensu re that the products meet customers specifications. 

There are also various devices that are commonly used for titrations in place of the glass burets previously described. A digital buret, for example, is an electronically controlled bottle-top dispenser that delivers 0.01-mL increments from a reagent bottle containing the titrant. There are also automatic titrators, such... 

FIGURE 4.18 An automatic titrator. A pump draws the titrant from the reagent bottle on the left and fills the reservoir in the back of the unit. Pressing a key on the keypad in the foreground delivers titrant to the titration flask on the right as the solution is automatically stirred. The volume delivered is displayed on a digital readout. 

Automatic titrators have been invented that are based on these principles. A sharp change in a measured potential can be used as an electrical signal to activate a solenoid and stop a titration. 

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... 

Fig. 5.2a shows examples of the results obtained on the dissolution of 8-AI203. In batch experiments where pH is kept constant with an automatic titrator, the concentration of AI(III)(aq) (resulting from the dissolution) is plotted as a function of time. The linear dissolution kinetics observed for every pH is compatible with a process whose rate is controlled by a surface reaction. The rate of dissolution is obtained from the slope of the plots. 

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