Manual titration

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. 

Potentiometric titration procedures with sodium methoxide have been reported for non-sulfur-containing organotin compounds in solvent extracts of polymers, and for phenolic antioxidants with sodium isopropox-ide in pyridine medium [21]. Organotin compounds in solvent extracts of PVC can be determined by potentiometric and manual titration procedures [487,488]. 

In normal manual titration practice, increments of titrant are added while the pH, pI or voltage values are measured after each incremental addition and registered as a function of L The increments are taken smaller and added more frequently in the area where the end-point is expected and should be accurately detected. 

Considering the related Mettler DL 40 and DL 40 RC MemoTitrators, the DL 40 can be used for ten different volumetric and potentiometric methods titration to a pre-selected absolute (EPA) or relative (EPR) end-point equilibrium titration (EQU)-recording titration (REC)-incremental titration (INC) Karl Fischer water determination (KF)-controlled dispensing (DOSE) pH and pX measurements (pX/E) multi-level titrations and back-titrations with automatic calculation (CALC) and manual titration (MAN)-automatic calibration of electrodes (CAL). 

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. 

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. 

Figure 11-6a shows experimental results for the manual titration of a hexaprotic weak acid, H A, with NaOH. Because the compound is difficult to purify, only a tiny amount was available for titration. Just 1.430 mg was dissolved in 1.00 mL of water and titrated with microliter quantities of 0.065 92 M NaOH, delivered with a Hamilton syringe. 

Other types of microanalytical apparatus, such as chromatography, have replaced manual titration methods, particularly where a procedure can be tooled for making tens of thousands of similar determinations automatically. 

The most commonly used technique for the determination of hydrogen peroxide in an industrial environment is the classical manual titration of a bath sample with potassium permanganate after acidification with sulphuric acid. In the past, numerous attempts have been made to perform the determination by means of an instrumental technique however, disadvantages were always encountered, which, up to now, have hindered the breakthrough of an alternative method. 

An automatic or manual titration apparatus may be used. In the latter case, the burette is graduated in U.U05-mL increments and the pH meter is provided with a wide reaching scale and glass-calomel electrodes. After each test the reaction vessel is evacuated by suction and washed several times with water, the washings being removed each time by suction,... 

Automatic titration device with a 25 0.02 mL burette and a pH meter giving a reading to 0.01 are suitable (or manual titration burette (25 0.02 mL... 

Manual Titration Follow the same procedure as with Automatic Titration, but keep the pH at 7.0 with 0.02 N Sodium Hydroxide Solution from a 25-mL buret, demarked in 0.02-mL units. 

NaCl solution. In one set of experiments, the slurry was titrated with 0.1 normal NaOH solution in one cm increments to a pH of about 10. The samples of this set are referred to as the "slurry" samples (titration of the calcined zeolite - NaCl solution slurry). In another set of experiments, the calcined zeolite - NaCl solution slurry was filtered, the filter cake washed with about 100 cnP distilled water, and the combined filtrates were titrated with 0.1 N NaOH solution, again to a pH of about 10. The samples of this latter set are referred to as the "filtrate" samples (the zeolite being removed by filtration prior to titration of the filtrate). In addition to the manual titrations, automated potentiometric titration curves were obtained with a Metrohm E636 Titroprocessor, which has an estimated pH measurement accuracy of 0.001 pH unit and an estimated volumetric addition accuracy of 0.001 cm ... 

Figure 1-12. Assembly of a pH meter electrode and stirring apparatus used to measure pH during the manual titration of an amino acid.

These can be used for general purposes, but they have more specialist appUcations. The narrow mouth and sloping shoulders reduce losses on stirring/swirling and evaporation and make them easier to seal. The absence of a lip does not favour quantitative transfer useful in manual titrations (p. 145) and recrystallizations (p. 96). Volume markings are approximate. 

Most frequently, using a robotic station to develop an entire analytical process is unwarranted. In the mid-1980s, when robotic technology first reached the analytical laboratory, robots were more of a novelty than a useful tool. At that time, conventional manual titrations and similarly easy tasks were entrusted to robotic stations. At present, however, well-established criteria exist to ensure correct use of the potential of robotic technology. 

The procedure is the same, only the pH is kept at 7.0 by manual titration with 0.02 N NaOH from a 25-mL burette, divided in 0.02 mL. 

Assume that this is the typical manual titration of a solution initially containing Fe with added titrant Ce". For simplicity, we will neglect dilution effects and assume both the Fe /Fe and Ce" /Ce couples behave nearly reversibly at a platinum microelectrode. Schematic i-E curves obtained at different values of/(where/= moles Ce added/moles Fe initially present) are shown in Figure 11.5.1. Initially (/= 0), the cell contains only Fe, and only the anodic wave for Fe oxidation is observed. During the titration (0 solution contains Fe ", Fe ", and Ce ", while after the equivalence point (/ >1) the solution contains Fe ", Ce ", and Ce. The titration curves for the different potentiometric and amperometric methods can be derived from these i-E curves. These are discussed in more detail in the first edition. We describe here representative examples of the more widely used end-point methods. 

The amperometric titration curves for coulometric titrations have somewhat different shapes than the ones for the manual titrations described, because usually one form of the couple exists in large excess. For example, in the titration of Fe " with electrogenerated Ce, Ce " will be present from the start at a concentration that is large compared to that of the Fe. Titrations for multicomponent systems can be treated in a similar manner. In all cases the curves can be derived by consideration of the i-E curves that arise during the titration. 

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]. 

The experimental apparatus for a potentiometric titration can be quite simple only a pH or millivolt meter, a beaker and magnetic stirrer, reference and indicator electrodes, and a burette for titrant delivery are really needed for manual titrations and point-by-point plotting. Automatic titrators are available that can deliver the titrant at a constant rate or in small incremental steps and stop delivery at a preset endpoint. The instrument delivers titrant until the potential difierence between the reference and indicator electrodes reaches a value predetermined by the analyst to be at, or very near, the equivalence point of the reaction. Alternatively, titrant can be delivered beyond the endpoint and the entire titration curve traced. Another approach to automatic potentiometric titration is to measure the amount of titrant required to maintain the indicator electrode at a constant potential. The titration curve is then a plot of volume of standard titrant added versus time, and is very useful, for example, for kinetic studies. The most extensive use of this approach has been in the biochemical area with so-called pH-stats—a combination of pH meter, electrodes, and automatic titrating equipment designed to maintain a constant pH. Many enzymes consume or release protons during an enzymatic reaction therefore, a plot of the volume of standard base (or acid) required to maintain a constant pH is a measure of the enzyme activity, the amount of enzyme present. 

Karl Fischer titration is specified for use in plasticizers. The method determines free and Itydration water in solid and liquid samples. In most cases, automatic titration is used and this is the main subject of standard but manual titration method is also included in the appendix. The sample containing a maximum of 100 mg of water is dissolved in a suitable solvent and titrated with Karl Fischer reagent which consists of iodine, sulfur dioxide, organic base, and solvent. The titration end-point is determined amperometrically. The ASTM method lists numerous substances in which the presence of water can be determined. There is also a list of substances which interfere with measurements. 

The simple equipment used for a typical titration is shown in Figure 1. A sample is measured into the flask from a pipette, or by weighing. The accuracy and precision of manual titrations using visual indicators is critically dependent on the use of correct experimental technique by the analyst. As in most analytical procedures, precise measurement of the amount of sample (or sample aliquot) is necessary, but it is most important in titrimetry if an accuracy of <0.2% is to be achieved. Thus, proper use of the pipette, burette, and balance, and a careful sample preparation procedure is crucial. Measurement devices of high quality, such as class A pipettes and burettes, should be used. 

Figure 1 Equipment used for a manual titration. (A) Transfer pipette (B) burette (C) information provided on volumetric glassware (D) conical or Erlenmeyer flask (normally 250 or 100 cm ) (E) burette reader. (Reproduced with permission from Belcher etal. (1970) and Rudolf Brand and Co. Working tMth Volumetric Instruments. Wertheim Brand.)...

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