Titration methods thermometric

Twenty weakly acidic drugs, including niclosamide, were determined by a nonaqueous catalytic thermometric titration method. Catalysis of the anionic polymerization of acetonitrile was used for endpoint indication. The solvent used was a mixture of acetonitrile and dimethylformamide or pyridine, and the titrant was sodium methoxide, potassium hydroxide, tertiary butanol, or tertiary butanol-sodium nitrite. Recoveries, limits of detection and relative standard deviations were tabulated [31]. 

Numerous descriptions of methods of analysis of the acidic functional groups of humic substances have been published (e.g., Stevenson and Butler, 1969 Schnitzer and Khan, 1972 Stevenson, 1982). The published methods include direct titrations, discontinuous titrations, indirect titrations, indirect titrations coupled with either a distillation or ultrafiltration step, thermometric titration methods, nonaqueous titrations, irreversible reactions of acidic hydrogens with various organic and inorganic reagents, and so on. The two most commonly described methods are the barium hydroxide... 

See J.Chem.Educ.S 91 ) %%. Several other oxidants have been used for the titration, see G.G.Rao, V.N.Rao Zyfna/.CAcm. 147(1955)338. J.Barek A.Berka, Anal Utt. 8(1975)57. A thermometric titration method using Ce(IV) is described in Analyst, 112(1987)507. The kinetics and mechanism of the reaction were studied in K.C.Rajanna et al, Indian.J.Chem.nA( 919y66. The kinetics and mechanism of the oxidation by permanganate are reported in ZPI s.Chem. Iefpz/g.263(1981)622. 

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... 

J)Other analytical methods which include, among many, a thermometric method (104), a high frequency titration (105), and a colored indicator method (106). 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

A thermometric method of evaluating the fuming acid consists in adding excess of standard 80 per cent, sulphuric acid and titrating back with the fuming acid the temperature of the solution rises with each addition of the fuming acid until the water content of the standard is exhausted.3... 

For solutions containing sulphuric acid only, direct titration with standard alkali, and measurement of the specific gravity, are possible as methods of estimation, provided that the process in either case is, if necessary, preceded by suitable dilution (see p. 165). Thermometric methods have also been suggested, depending on the rise in temperature when the acid is mixed with water, or when titrated with barium chloride solution.4 The water content of the concentrated acid may be determined by similar titration with oleum which has been standardised thermometrically by 80 per cent, sulphuric acid (see p. 147 ).5... 

Ionization constants have been determined for numerous simple carbohydrates (10,13, i5, 25, 45), as well as for cellulose (32, 43), wheat starch (43), and alginate (43). Selected carbohydrates with their corresponding pK values are presented in Table I. The analytical methods involved in these determinations include conductimetry, potentiometric titration, thermometric titration, and polarimetry. Polarimetry was used by Smolenski and co-workers (45) to calculate a first and a second ionization constant for sucrose at 18°C (Ki = 3X 10"13 K2 = 3 X 10"14). 

ACIDIMETRY. An analytical method for determining the quantity of arid in a given sample by titration against a standard solution of a base, or, more broadly, a method of analysis by titration where the end point is recognized by a change in pH (hydrogen ion concentration). See also Analysis (Chemical) pH (Hydrogen Ion Concentration) Titration (Potentiometric) and Titration (Thermometric). 

A typical thermometric enthalpy titration curve is shown in Figure 6. The well defined endpoint provided a convenient method for quantitating sulfidic sulfur. 

Thermometric methods (not to be confused with thermometric titrations) in which temperature-time curves are obtained at various intervals. Heat evolution or absorption occurs continuously and preferably at a constant rate. The amount of heat supplied per unit time is not measured directly but may be calculated as a fraction of the total heat of melting of the substance. 

In thermal methods of analysis, either temperature change is measured or the temperature is manipulated to produce the measured parameter. Thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) are the three major methods that use temperature change as the independent variable. Thermometric titration (TT) and direct-injection enthalpimetry (DIE) use temperature as the dependent variable. These five methods will be discussed primarily from an analytical point of view. Each method has its unique characteristics and capabilities for that reason, the major aspects of each method are considered individually. 

Thermometric titrations (TT) and direct-injection enthalpimetry (DIE) are both calorimetric techniques the heat evolved or absorbed serves as an indicator of the progress of the reaction. Nowadays, TT and DIE are used for routine analysis and in fundamental research involving the chemical equilibrium, reaction kinetics, and thermochemistry of processes not readily studied by other methods. 

For simple titrations the equipment necessary is not particularly sophisticated a thermometer, a buret, and an insulated beaker (or even two Styrofoam cups, one sitting in the other) will do. However, if one wants to measure specific heats or the heat of reaction, better control is required. Modern automated thermometric titrators consist of a constant delivery pump for the titrant, a temperature control system for the titrant, an insulated cell, cahbration circuitry, electronic temperature sensing, and a data processing system. Most modem instraments are totally computerized, so different methods can be programmed and mn unattended. 

The method is quite useful, particularly if the rate of chemical reaction between A and B is slow. The final quantity of heat evolved is not a function of time, but a function of the concentration of sample. This is a distinct advantage over conventional volumetric analysis and, in some instances, thermometric titrations. As stated earlier, slow reactions give rise to errors in the endpoint determination in thermometric titrations. 

DIE may be used for the same applications as discussed for thermometric titrations, for example, for the volumetric analysis of materials, such as boric acid, which are virtually impossible to titrate using endpoint indicators or pH indicators. DIE can also be used in biological studies where the reaction rates may be slow. Eor example, proteins have been titrated with acid or base, antibodies have been titrated with antigen, and enzyme-coenzyme systems have been studied. DIE is used to determine kinetic parameters for slow reactions. The use of a large excess of one reactant (the titrant) favors the forward reaction (according to Le Chatelier s principle) even if the equilibrium constant is small, so equilibria may be studied using DIE that cannot be studied using other titrimetric methods. 

A word of caution should be mentioned about possible interferences, particularly if two simultaneous reactions take place. Since both reactions may generate heat, a direct error will be involved with injection enthalpimetry methods. However, with thermometric titrations, frequently consecutive reactions take place and can be determined consecutively without prior separation. This has been demonstrated in the titration of calcium and magnesium with oxalate. 

Psing a method of enthalpy dependent thermometric titrations , Jordan and Dumbaugh have measured the heats of ionization, at 26 C, of a number of acids. The heats and derived entropies of ionization of the halo-acetic acids are shown in Table 5. 

Modem automated thermometric titrators consist of a constant delivery pump for the titrant, a temperature control system for the titrant, an insulated cell, calibration circuitry, electronic temperature sensing, and a data processing system. Most modem instruments are totally computerized, so different methods can be programmed and run unattended. 

Stability constants for many olefin complexes are currently known/ A variety of experimental methods are used to determine these constants potentiometry, polarography, thermometric titration, and others/... 

When the time constants r and r" and T(t) are known, it is possible to determine the T (t) and T"(t) values consecutively, and thus P(t). The numerical differential correction method has also been applied to reproduce the thermokinetics in these calorimetric systems, in which time constant vary in time [258-264], such as the TAM 2977 titration microcalorimeter produced by Thermometric. These works extended the applications of the inverse filter method to linear systems with variable coefficients. In many cases [258-262], as in the multidomains method, as a basis of consideration the mathematical models used were particular forms of the general heat balance equation. 

Nearly all chemical sensors useful for liquid samples can be utiUzed to indicate titrations. Besides the preferred potentiometric, other electrochemical probes are also used, mainly amperometric and conductometric sensors. The so-called biamperometric titration works with simple wire pairs. Photometric and thermometric indication techniques are less common than electrochemical methods. Miniaturization does not play an important role for titration probes. Classical arrangements predominate to this day. Commercial titration instruments are only slowly starting to make use of the achievements of modern sensor technology. As an example, optodes have achieved a certain popularity in recent years for titration applications. 

Other titration designations have also been given in the literature. Some are named after the instrumental method used to detect the equivalence point. We can list, for example, conductometric, potentiometric, amperometric, spectrophotometric, and thermometric titrations. Others are named after the titrant nature. We speak then of iodometric, complexometric, and acidimetric titrations. 

Unfortunately, the equivalence point of a titration reaction is the least quantitative one, as we demonstrated in the preceding section. This is why logarithmic titrations are the most sensitive to the extent of the titration reaction. This is not the case for linear titrations. Let s first recall that linear titrations are those in which the dependent variable (the registered one) is directly proportional to the fraction titrated or to the concentration of the independent variable (in no case to the logarithm of its concentration or of its activity). Of course, the independent variable may be the activity rather than the concentration. Examples of linear titrations are spectrophoto-metric, amperometric, thermometric titrations, and so forth. This is the reason why Gran s method is interesting. Indeed, its principle involves the linearization of the logarithmic titration curves. 

---