Classical methods

Genuine chemical methods were preceded by techniques such as the touchstone,26 and also by the direct investigation of the material or object by the use of the senses. A material could be felt, it could be tasted, its colour could be observed, it could be smelt, and the noise it could be induced to make could be listened to. The use of such methods of organoleptic analysis in the work of the Georgian King, Vakhtang VI (1675-1737) have been described.27 Probably the first demand for a quantitative analytical method arose from a desire to estimate the purity of samples of gold. While 

In addition to his work on titrimetry, Gay-Lussac made important contributions in other areas of analytical chemistry. He studied the reactions of hydrogen sulphide in different media, thus laying the foundations for the classification of metals into different analytical groups, and in conjunction with Thenard, he developed the first truly practicable method for the determination of carbon and hydrogen in organic compounds.54 

The history of acid-alkali indicators from the time of Robert Boyle to the present has been reviewed in an article that also discusses test papers and biological stains.55 

Given a set of particles with positions r the positions a small time step Af later are given by a Taylor expansion. 

SIMULATIONS, TIME-DEPENDENT METHODS AND SOLVATION MODELS 

The velocities v, are the first derivatives of the positions with respect to time (dr/dt) at time ti, the accelerations a are the second derivatives (d r/d ) at time are the third derivatives etc. 

The positions a small time step At earlier are derived from eq. (16.28) by substituting At with - At. 

This is the Verlet algorithm for solving Newton s equation numerically. Notice that the term involving the change in acceleration (b) disappears, i.e. the equation is correct to third order in At. At the initial point the previous positions are not available, but may be estimated from a first-order approximation of eq. (16.29). 

The positions a small time step At earlier are derived from eq. (16.28) by substituting At with — At,------------------------------------------------------------------------------ 

This is the Verlet algorithm for solving Newton s equation numerically. Notice that the 

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G. D. Billing, Quanium-Classical Methods in Reaction and Molecular Dynamics, Lecture Notes in Chemistry, A. Lagana and A. Riganelli, eds,. Springer-Verlag, Berlin, 2000,... 

Billing, G. D. Quantum-Classical Methods. In Numerical Grid Methods and Their Application to Schrodinger s equation (C. Cerjan, eds.). Kluwer Academics Publishers (1993)... 

Haug, K., Metiu, H. A test of the possibility of calculating absorption spectra by mixed quantum-classical methods. J. Chem. Phys. 97 (1992) 4781-4791... 

Syntheses of a,)3-dihalogenoethers can be achieved in various ways the classical method (37), wherein a current of dry gaseous hydrochloric acid, is made to react in an equimolar mixture of ethanol and aldehyde at 20°C first to form the monochloroether (50% yield) and then by the action of bromine, the dibromoether (80 to 90% yield) can be used. The second and simpler method is the direct bromination of ethylvinylether in a chloroformic or dioxane solution if the product is used directly without purification,... 

Classical methods for separation and purifica tion include fractional distillation of liquids and re crystallization of solids and these two methods are routinely included in the early portions of laboratory courses in organic chemistry Because they are capa ble of being adapted to work on a large scale frac tional distillation and recrystallization are the preferred methods for purifying organic substances in the pharmaceutical and chemical industries... 

Analytical Procedures. Standard methods for analysis of food-grade adipic acid are described ia the Food Chemicals Codex (see Refs, ia Table 8). Classical methods are used for assay (titration), trace metals (As, heavy metals as Pb), and total ash. Water is determined by Kad-Fisher titration of a methanol solution of the acid. Determination of color ia methanol solution (APHA, Hazen equivalent, max. 10), as well as iron and other metals, are also described elsewhere (175). Other analyses frequendy are required for resia-grade acid. For example, hydrolyzable nitrogen (NH, amides, nitriles, etc) is determined by distillation of ammonia from an alkaline solution. Reducible nitrogen (nitrates and nitroorganics) may then be determined by adding DeVarda s alloy and continuing the distillation. Hydrocarbon oil contaminants may be determined by ir analysis of halocarbon extracts of alkaline solutions of the acid. 

Since PVA fiber as spun is soluble in water, it is necessary to improve the water resistance of the as-spun fiber (10). Heat treatment followed by acetalization is a classic method to provide high water resistance. 

Reaction of a metal lactate (such as silver lactate) with an alkyl haUde is a classic method of preparation of the ester, but it is too expensive to be of commercial relevance. Lactamide [2043-43-8] is another high yielding condensation product from lactic acid. It can be produced by aminolysis of dilactide or lactate ester such as methyl or ethyl lactate. 

Photopolymers and photothermoplasts are mentioned only in connection with holographic data storage (see Holography). The classical method of optical data storage in silver haUde films (photographic film, microfiche technique) is not discussed (see Photography). 

Transmission, Absorption, and Beer s Law. The majority of infrared spectrometry is stiU done by the classic method of transmission spectrometry the intensity of an infrared beam passing completely through a sample is measured. The standard description of how much radiation passes through the sample is that of Beet s law (or the Bouguer-Beer-Lambertlaw) ... 

Commercially, sulfonation is carried out by the classic method with sulfuric acid. Modem reactors are glass-lined older equipment was made from cast iron or coated with enamel Processes often use chlorosulfonic acid or sulfur trioxide to minimi2e the need of excess sulfuric acid. Improved analytical methods have contributed to the success of process optimi2ation (9—12). 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Acetylene has a low solubiHty in Hquid oxygen. Excessive concentrations can lead to separation of soHd acetylene and produce accumulations that, once initiated, can decompose violently, detonating other oxidizable materials. Acetylene is monitored routinely when individual hydrocarbons are determined by gas chromatography, but one of the wet classical methods may be more convenient. These use the unique reaction of acetylene with Ilosvay s reagent (monovalent copper solution). The resulting brick-red copper acetyHde may be estimated colorimetricaHy or volumetricaHy with good sensitivity (30). 

Three general methods exist for the resolution of enantiomers by Hquid chromatography (qv) (47,48). Conversion of the enantiomers to diastereomers and subsequent column chromatography on an achiral stationary phase with an achiral eluant represents a classical method of resolution (49). Diastereomeric derivatization is problematic in that conversion back to the desired enantiomers can result in partial racemization. For example, (lR,23, 5R)-menthol (R)-mandelate (31) is readily separated from its diastereomer but ester hydrolysis under numerous reaction conditions produces (R)-(-)-mandehc acid (32) which is contaminated with (3)-(+)-mandehc acid (33). 

Qualitative. The classic method for the quaUtative determination of silver ia solution is precipitation as silver chloride with dilute nitric acid and chloride ion. The silver chloride can be differentiated from lead or mercurous chlorides, which also may precipitate, by the fact that lead chloride is soluble ia hot water but not ia ammonium hydroxide, whereas mercurous chloride turns black ia ammonium hydroxide. Silver chloride dissolves ia ammonium hydroxide because of the formation of soluble silver—ammonia complexes. A number of selective spot tests (24) iaclude reactions with /)-dimethy1amino-henz1idenerhodanine, ceric ammonium nitrate, or bromopyrogaHol red [16574-43-9]. Silver is detected by x-ray fluorescence and arc-emission spectrometry. Two sensitive arc-emission lines for silver occur at 328.1 and 338.3 nm. 

Analytical Techniques. Sorbic acid and potassium sorbate are assayed titrimetricaHy (51). The quantitative analysis of sorbic acid in food or beverages, which may require solvent extraction or steam distillation (52,53), employs various techniques. The two classical methods are both spectrophotometric (54—56). In the ultraviolet method, the prepared sample is acidified and the sorbic acid is measured at 250 260 nm. In the colorimetric method, the sorbic acid in the prepared sample is oxidized and then reacts with thiobarbituric acid the complex is measured at - 530 nm. Chromatographic techniques are also used for the analysis of sorbic acid. High pressure Hquid chromatography with ultraviolet detection is used to separate and quantify sorbic acid from other ultraviolet-absorbing species (57—59). Sorbic acid in food extracts is deterrnined by gas chromatography with flame ionization detection (60—62). 

The classical method for the determination of vitamin K is based on the clotting time of a vitamin K-deficient chick. It is relatively easy to produce a hemorraghic state ia chicks (17). Vitamin K-deficient tats have also been used for this assay (18). Owiag to the development of modem chromatographic techniques, this method of analysis has been supplanted by other methodology. 

Infusion Method. Infusion is a classic method for top-fermented beers and is used for ad British types. The whole mash is heated graduady from mashing-in to mashing-off with holding times for the degradation of protein and starch. No part of the mash is boiled and the malt, therefore, must be well-modified to assure the breakdown of ad soluble substances. Because no boiling takes place there is no physical breakdown of the malt, and consequentiy infusion is not as effective as decoction despite the better protection of the enzymes. 

Reactions with Nitrogen Compounds. The reaction with ammonia is the classical method for preparing primary carbamates. Excess ammonia is used as an acid acceptor to remove the HCI formed (see Carbamic acid). 

Density. Measurement of the density of water by pycnometry is the classical method (30) for estabHshing deuterium concentrations in heavy water. Very precise measurements can be made by this method, provided the sample is prepared free of suspended or dissolved impurities and the concentration of oxygen-18 in water is about 0.2 mol %. However, in nearly all heavy water manufactured since 1950 in the United States, the... 

The synthesis of pyrazoles, indazoles and their derivatives generally follows classical methods, the two most important methods for practical purposes being the reaction between hydrazines and /3-difunctional compounds, and 1,3-dipolar cycloadditions (Section 4.04.3.1.2). Both procedures are well documented (64HC(20)l, 66AHC(6)327, 67HC(22)l) and thus the length of the sections in this part of the chapter reflects not only the number of publications dealing with a particular method but also its interest and novelty. 

In indazoles there are two possibilities for ring closure by creation of an N—C bond, depending on whether the bond is the N(2)—C(3) or the N(l)—C(7a) bond. Both are classical methods of indazole synthesis (types A and C (67HC(22)l)). An example of each class is shown in Scheme 50 (78S633). 

In the structure sections, labelled compounds have often been used to solve a spectroscopic problem involved in microwave (Section 4.04.1.3.2), nitrogen NMR (Section 4.04.1.3.5), IR (Section 4.04.1.3.7(i)) or mass spectrometry (Section 4.04.1.3.8). The synthesis usually involves non-radioactive compounds ( H, N) by classical methods that must be repeated several times in order to obtain good yields. 

Diaziridines also show slow nitrogen inversion, and carbon-substituted compounds can be resolved into enantiomers, which typically racemize slowly at room temperature (when Af-substituted with alkyl and/or hydrogen). For example, l-methyl-3-benzyl-3-methyl-diaziridine in tetrachloroethylene showed a half-life at 70 °C of 431 min (69AG(E)212). Preparative resolution has been done both by classical methods, using chiral partners in salts (77DOK(232)108l), and by chromatography on triacetyl cellulose (Section 5.08.2.3.1). 

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