Method chemical

Chemical methods are based on the fact that ki a is determined in a material system, in which parallel to the absorption process a chemical reaction occurs which is so fast that it proceeds at the phase boundary (see Section 4.9). Under such circumstances fet is a reaction kineticaUy determined constant, which permits a = ki a/ki to be evaluated from the absorption measurements by chemical means. 

Chemical methods for the detection and estimation of phosphorus in a compound are many and varied, but they usually require the phosphorus to be first obtained in the form of inorganic orthophosphate. For this, a prior oxidation and/or hydrolysis may have to be carried out. Insoluble phosphates are dissolved in mineral acids, with boiling if necessary. In some cases, fusion with alkaline fluxes (e.g. boric acid and sodium carbonate at lOOO C) is needed, followed by water or acid extraction of the soluble phosphates produced. Cation exchange resins may sometimes be used to convert insoluble into soluble phosphate salts. Numerous extraction and clean-up procedures have been prescribed for determination of the P content of compounds of bio origin. Pre-treatment of the sample by chemical methods is sometimes a necessary procedure before examination by physical methods (e.g. spectrophotometty), can be undertaken. 

Detailed analytical procedures have been evolved for the estimation of phosphorus in such diverse materials as drinking water, sea water, food products, plants, concrete, ores and rocks, oils, metals, pesticides and fertilizers. Each of these materials has its own special analytical requirements. In the case of fertilizers, for example, it is usual to estimate (a) water-soluble phosphate which relates to available phosphorus and (b) citrate-insoluble phosphate which is a measure of the phosphorus which plants cannot utilise. In the case of seawater, both dissolved P and particulate P content may be estimated. 

Chemical methods for the determination of hydroxyl groups in polymers are based on acetylation [1], phthalation [2] and reaction with phenyl isocyanate [3] or, if two adjacent hydroxy groups are present in the polymers by reaction, with potassium periodate [1]. 

The reactions on which these methods are based are as follows  

In all these methods, an excess of standardised reagent is added and, at the end of the reaction, unconsumed reagent is estimated. The concentration of hydroxyl groups can then be calculated from the amount of reagent consumed. 

1a Chemical Methods Reaction of Metals with Water 

That eh is the intermediate species and not the H atom has been verified by adding NzO and methanol to water then, N2, not H2, is the principal product. Alkali and alkaline earth metals above Na in the electrochemical series will also generate eh on dissolution in water. Moreover the H/D isotope effect in water containing 50% D is consistent with the reaction 2eh—H2 + 20H (Anbar and Meyerstein, 1966 Hart and Anbar, 1970). 

Although it is very hard to observe the absorption spectrum of eh when metal is dissolved in water because of its high reactivity, some attempts were made in water and ice (Jortner and Stein, 1955 Benett et al., 1964, 1967). Furthermore ESR (electron spin resonance) studies revealed that the trapped or solvated electron in ice interacts with six equivalent protons, thus ruling out H20-. 

Some experiments indicate that the cathode reaction may be due to eh at least, it is an open question (Walker, 1966, 1967 Hills and Kinnibrugh, 1966). Walker (1966), using platinum cathode, found that N20 reduces the H20 by about 65% but further addition of 0.1 M methanol did not change the N2 /H2 ratio. The result may be explained by the cathodic reaction 

When an aqueous solution containing an irreducible cation M+ is electrolyzed, H2 evolves at the cathode with the overall reaction Haq+ + e(cathode) — (l/2)H2(gas). The detailed mechanism of this reaction is somewhat ambiguous, as it could be attributed either to absorbed H atoms or absorbed H2+ ions. According to Walker (1966, 1967), the basic cathodic reaction is (6.II) followed by (6.1) to give H2. There are several possibilities for reaction (6.II) (Walker, 1968) (1) direct electron donation by the cathodic metal to water, (2) electron liberation from the diffuse double layer, and (3) neutralization of the irreducible cation M+ (e.g., Na+) at the cathode, followed by the reaction of the neutral atom with water  

Some chemical methods (Bouveng and Lindberg, 1960), which have been essential to structural analyses of carbohydrates for many years will be briefly described. Although widely used before, these methods have largely been replaced by enzymatic and spectro- 

These are mainly titration methods and they can be highly accurate. They are generally reserved for simple reactions in solution where either only the reactant or 

Question. The catalysed decomposition of hydrogen peroxide, H2O2, is easily followed by titrating 10.0 cm3 samples with 0.0100 mol dm-3 KMn04 at various times. 

Calculate the [H202] at the various times, and show that these values lie on a smooth curve when plotted against time. 

Nanomaterials are also prepared by chemical vapor deposition (CVD) or chemical vapor condensation (CVC). In these processes, a chemical precursor is converted to the gas phase and then it undergoes decomposition to generate the nanoparticles. These products are then subjected to transport in a carrier gas and collected on a cold substrate, from where they are scraped and collected. The CVC method may be used to produce a variety of powders and fibers of metals, compounds, or composites. The CVD method has been employed to synthesize several ceramic metals, intermetallics, and composite materials. 

Semiconductor clusters have traditionally been prepared by the use of colloids, micelles, polymers, crystalline hosts, and glasses. The clusters prepared by these methods have poorly-defined surfaces and a broad size distribution, which is detrimental to the properties of the semiconductor materials. The synthesis of monodisperse clusters with very well-defined surfaces is still a challenge to synthetic chemists. However, some recent approaches used to overcome these problems are (i) synthesis of the clusters within a porous host lattice (such as a zeolite) acting as a template and (ii) controlled fusion of clusters. 

Babushkin, A.I. Lyamkin, F. Tepper, Y. Biryukov, A. Vorozhtsov and V. Zarko, Energetic Materials , U. Teipel (ed.) 2005, Wiley-VCH, Weinheim, Germany. 

In a typical sol-gel synthesis, the metal or the compounds of main group elements undergo hydrolysis and condensation reactions giving gel materials with extended three-dimensional structures. As shown in Equation 5.32 for silicon, addition of an acid or base catalyst to a solution of an alkoxysilane reagent such as tetrame-thoxysilane (TMOS), water and methanol leads to the hydrolysis of the Si-OMe bonds to form Si-OH functional groups. 

Subsequent elimination of water from two such Si-OH groups eventually gives an extended silica gel matrix (known as a xerogel when dry). Because hydrolysis 

The hydrated electron can be produced chemically from the hydrogen atom 

Thus when hydrogen atoms, generated by a discharge in hydrogen gas, are passed into an alkaline, aqueous solution, reactions characteristic of the hydrated electron are observed [15]. The conversion of hydrogen atoms 

Production of hydrogen in the system would then be via the reaction 

Hughes and Roach [21] and Shaede and Walker [22, 23] designed experiments to try to identify the precursor of hydrogen. 

Shaede and Walker studied the reaction of sodium amalgam with water in the presence of dinitrogen monoxide and methanol at various pH values. Radiolysis experiments had already established that dinitrogen monoxide is a very efficient, specific electron scavenger 

The application of chemical methods as attempts to determine the structure of coal has a long history insofar as coal has been known to be reactive to a variety of chemicals agents (Chapter 12). Such attempts were, of course, originally applied with the object of degrading coal to smaller molecules that would not only have a greater degree of utility but also be much easier to identify. 

The oxidation of coal using reagents other than oxygen has been extensively studied and, the common oxidants such as nitric acid, permanganate, dichromate, various peroxides, as well as hypochlorites have all been applied to the oxidation of coal (Hayatsu et al., 1982 Speight, 1987). 

In the early years of coal science, one of the prime motives for investigating the oxidation of coal was the production of chemicals from coal. More recently, the oxidation of coal using specific oxidants has become a prime means by which structural entities in coal have been identified. This has, of course, led to postulates of the molecular structure of coal. 

However, the diversity of the oxidants renders the oxidation of coal very complex because the experimental parameters can vary widely (Speight, 1987). The diversity in the structural types in coal (which vary not only with rank but also within the same rank) causes many problems associated with studies of the oxidative degradation of coal. For example, optimal conditions of time, temperature, and ratio of oxidant to coal can only be determined when several experiments are performed for each oxidant. Furthermore, the presence of the mineral matter must also be considered to be an integral part of coal oxidation since mineral constituents may change the chemistry of the oxidation. If coal is pretreated with hydrochloric acid to remove mineral matter prior to oxidation, the actual oxidation reaction may be more facile. 

At this point, it needs to be stated that the studies of the oxidation of coal have resulted in valuable contributions toward understanding the nature of (a) the polynuclear aromatic systems, (b) the aliphatic systems, and (c) the nature of the heteroatomic systems (Hayatsu et al., 1982 Speight, 1987 Winans et al., 1988). However, considerable gaps still remain in the knowledge of coal structure. 

There are also surprisingly few examples where chemical reactions have been used to form emulsions by a nucleation and growth process. All that is required, in 

In the case of PDMS droplets, TEOS is replaced by dimethyldiethoxysilane (DMDEOS), and the base-catalysed hydrolysis is carried out in water, since PDMS is soluble in ethanol. The basic chemistry is set out below. 

Obey and Vincent carried out extensive H and Si NMR studies on the PDMS ( silicone oil) phase that formed. They showed that the product was predominantly the cyclic D4 species (where D is a -Me2SiO- repeat unit), although some short, linear homologues of the form 0-(Me2Si0)x-Me2Si0- are also formed (x 6). Because of their anionic nature, these linear species would appear to accumulate at the silicone oil/water interface, and hence give rise to charge stabilisation of the droplets. The electrophoretic mobility of the (dialysed) droplets. 

The fact that the emulsions are free of added surfactant or polymeric stabilisers is of some significance in that the droplet/solution interface is in this case a truly fluid one, whereas those bearing adsorbed stabilisers are invariably viscoelastic, exhibiting Gibbs-Marangoni effects. This should be of interest to those wishing to carry out experiments to test the various hydrodynamic theories of liquid droplets (e.g. diffusion, sedimentation, viscosity, electrophoresis), compared to solid parti- 

Gilbert, Emulsion Polymerisation , Academic Press, London, 1995. 

Breaking up the large complex compounds by simple chemical reactions into smaller, more manageable molecules, has developed as a result of the difficulties 

Most of the techniques described here cannot establish whether a metal carbonyl cluster is present on the surface of a zeolite particle or within the intracrystalline cages. Chemical treatments have been used to determine whether the clusters are inside or outside, for example by the use of probe molecules that are too large to fit in the pores and those that do fit. 

Ousters adsorbed on the outside surfaces of zeolites can often be easily extracted with neutral solvents or with salt solutions which remove the cluster ions by cation metathesis (ion exchange). Comparison of the infrared spectra of the extracted spedes and those of known spedes helps identify the encaged spedes. When treatment of a sample with such solutions fails to remove sorbed dusters, they are inferred to be trapped within the cages. The inference is supported when the same clusters adsorbed on the surface of a large pored material such as an amorphous metal oxide are removed by extraction. 

For example, [Ir6(CO)jsp in the NaX zeolite could not be extracted with [PPN][Q] in tetr ydrofunm solution, [4, 60] whereas this duster on the surface of MgO was completely extracted. [37] 

Ousters on the outade surface of a zeolite can sometimes be distinguished from those within the cages by reactions with probe molecules of various sizes, whereby some are small enough to enter the pores and some are too large to enter. The probes most commonly used are phosphines, as they react with many metal carbonyls. The products are typically characterized by infrared spectroscopy. 

For example. Rode et al. [64] found that the infrared bands characteristic of [Rh6(CO)i ] in a NaY zeolite were not affected by exposure of the samjde to n-hexyldiphenylphosphine, whidi has a critical diameter larger than that of the NaY apertures (about 7.4 A). The authors conduded that the clusters were located in the supercages of the zeolite, consistent with the inferences of earlier researchers. [30, 38] 

Acid degradative methods are not specific, the hydrolysis randomly generates a large amount of monomers, and later on the removal of acid poses problems and is not economical. Chemical treatment using strong acids (viz. 

For a simple system, it is only necessary to follow the extent (progress) of reaction by means of one type of measurement. This may be the concentration of one species or one other property dependent on concentration. The former would normally involve a chemical method of analysis with intermittent sampling, and the latter a physical method with an instrument that could continuously monitor the chosen characteristic of the system. We first consider a-situ and in-situ measurements. 

A large variety of tools, utilizing both chemical and physical methods, are available to the experimentalist for rate measurements. Some can be classified as ex-situ techniques, requiring the removal and analysis of an aliquot of the reacting mixture. Other, in-situ, methods rely on instantaneous measurements of the state of the reacting system without disturbance by sample collection. 

The titration of an acid with a base, or vice versa, and the precipitation of an ion in an insoluble compound are examples of chemical methods of analysis used to determine the concentration of a species in a liquid sample removed from a reactor. Such methods are often suitable for relatively slow reactions. This is because of the length of time that may be required for the analysis the mere collection of a sample does not stop further reaction from taking place, and a method of quenching the reaction may be required. For a BR, there is the associated difficulty of establishing the time t at which the concentration is actually measured. This is not a problem for steady-state operation of a flow reactor (CSTR or PFR). 

These normally utilize the low- and medium-temperature decomposition of inorganic aluminum salts and hydroxides, or metal-organic compounds of aluminum. Typical precursors include aluminum nitrate and aluminum hydroxides. Hydro-thermal conditions are often applied [8], but colloidal methods (sol-gel) have been extensively studied over the past three decades [9-11]. Recent efforts have been aimed at reducing the particle size of a-Al203, and decreasing the temperature of formation of a-Al203 from transient aluminas to 1000 °C [12]. Results similar to those in sol-gel can be achieved with the use of metal ion-polymer-based precursor solutions. Here, the precursor solution (e.g., nitrate salt) is mixed with a water-soluble polymer, which provides a matrix for the dispersion of cations [13]. 

The graft copolymerization of many monomers onto cellulose and onto cellulose derivatives has been carried out by different methods that can be generally classified into three major groups (i) chemical methods, (ii) radiation-induced grafting, and (iii) plasma-initiated grafting [11]. 

The alternative to measuring the physical attributes of paper is to measure the chemical properties of the paper. As with physical tests, care has to be taken that the sample of paper is representative of the entire sample, for example, in an old book the edges of a book block are often more degraded than the centre because the edge has absorbed air pollutants and is often of higher acidity. The popular methods will now be briefly described, readers should see fuller descriptions before attempting any of these methods. 

Although some papers are acidic when new, ageing generally increases acidity and pH can be used to monitor the progress of ageing. There are several methods for measuring the pH of paper including pH-sensitive dyes that can be placed onto the paper, and surface pH electrodes that can be placed directly on the surface of the dampened paper. However, the best method is to macerate paper in pure water and measure the pH of the aqueous extract after a period. There are several standard methods that are similar. Typically, they mix 1 g of paper with 50 mL water (hot or at room temperature) and measure pH after 1 hour. 

A related method involves measuring the solubility of the paper in 1% NaOH at 100°C after a period of time, e.g. 60 min. The solubility is proportional to the copper number (see below) that is itself proportional to the number of carbonyl groups in the paper, a measure of the oxidation and hydrolysis of the paper. This method has also been used on lignin-containing fibres of various kinds with some success. 

The copper number of paper may be obtained using copper(II) solution, some of which is reduced to Cu(I) oxide as the carbonyl groups are oxidised to carboxylic acid. There are several variants of the test, one of which involves treating the produced Cu(I) oxide with phosphomolybdic acid. The reduced molybdenum is then titrated with potassium permanganate. 

Phenylhydrazine is able to react with carbonyl groups to give coloured compounds. Paper sheets can be reacted with this reagent or modifications of it (e.g. 4-nitrophenylhydrazine) and the colour produced can be measured by reflectance spectroscopy. The colour may also be measured by dissolving the paper and measuring the absorbance of the solution. Lignin interferes badly with this test. 

However, physical methods suffer from microscopic-level phase separation [155] between the two constituents. This occurs due to the removal of solvent molecules which not only cause the aggregation of nanocrystals to form super-crystals but also lead to partial crystallization of regioregular polymeric chains. In such type of hybrids, it is difficult to realize the intimacy between QCNs and CP at nanometer scale. The loose networks of QCNs scatter the electrons in electronic devices which in turn leads to poor device efficiency, e.g., in solar cells, it is responsible for low photoconversion efficiency due to deterioration of the efficiency at charge generation, separation, and transport steps. In order to overcome above limitations, efforts have been made to improve the intimacy between the phases by adopting chemical methods that involve means for realizing chemical linking between two partners. 

The chemical methods involve chemical linking of functional groups present over QCNs surface with the complementary groups over polymeric backbone. There are three ways by which QCNs can chemically tether the CP viz. ligand exchange, direct grafting ( onto and from approach), and in-situ growth of QCNs [156,157]. 

TOPO capped cdse Pyridine capped cdse 

Cdse capped with amine functionaiized P3HT 

The above chemical coupling leads to uniform dispersion (as inferred from TEM images) of QCNs within CP matrix [162] leading to improved charge transfer/transport and enhancement of optoelectronic performance, e.g., light-to-electricity conversion efficiency in hybrid solar cells. 

On the other hand, estimates of the metal dispersion have been achieved by use of EIREM. Thus, such parameters can be inferred from the particle size distribution by assuming a certain metal particle geometry (equations relating this to N, and Nt being available), structure and orientation with respect to the support. Application of such an approach becomes extremely useful in cases when such a parameter cannot be readily obtained by chemical methods as a consequence of relatively strong spillover phenomena, such as that which occurs for systems supported on ceria-related materials.  

The quantity of molecules selectively chemisorbed by the metallic component of the catalyst may be determined by what are commonly described as static (volumetric) or dynamic (flow) methods. The former is performed at reduced pressure and involves allowing the system to reach equilibrium between the adsorbed and gaseous states. The later group of methods, which involves pulsed chemisorption, are generally performed at atmospheric pressure and the equilibrium between adsorbed and gas states is not achieved (or maintained). 

F re 2.1 Adsorption isotherms where the monolayer is obtained (LHS) directly from the isotherm or (RHS) from application of the double isotherm method. 

As there is a difference between the heat of adsorption of this weakly bound adsorbate on the support and the more strongly bound irreversible adsorption on the metal, it is possible to determine monolayer capacity of the metal by noting the value of coverage at which the heat of adsorption abruptly decreases to give a constant value. Heats of adsorption may be obtained from isochore type measurements where variations in pressure are measured while varying the temperature after exposing the catalyst to different initial amounts of adsorbate. 

Pulse chemisorption (Fig. 2.2) involves exposing the reduced catalyst to a number of pulses of adsorbate gas from a calibrated loop which are passed over the catalyst in a flow of inert carrier at atmospheric pressure. Early studies employed only a single pulse with a recommended volume of ca. twice the volume likely to be consumed. The quantity of gas remaining (i.e. not adsorbed) from the pulse is measured by TCD and in a typical experiment the first two or three pulses would be completely consumed while only fractions of the subsequent peaks would be taken up by the sample. Eventually, the area of the peaks detected reaches a constant level indicating that the sample has reached monolayer capacity. The volume of gas adsorbed is calculated from the sum of the areas of peaks fully consumed plus the areas of the partially consumed peaks by equating the area of a peak eluted where no gas was taken up with 

Several other studies revealed alternative biological effects of sulfoximines. For example. Mock found 3 to be a transition-state analogue of carboxypeptidase 

and Tsujihara demonstrated the antifungal activity of sulfoximine 4 [5]. In a collaboration with Weinhold [6] we discovered an unusual cleavage pattern of pseudotripeptide 5 when it was treated with peptidase A and attributed this effect to the presence of the central acylated P-carboxysulfoximidoyl unit, which we had studied separately previously [7]. 

The amide-like linkage between the sulfoximine nitrogen and the natural a-amino acid of 5 revealed interesting properties, which were investigated spectroscopically and theoretically in collaboration with Raabe and Fleischhauer [8], 

During the optimization of the reaction conditions it was noticed that at higher temperatures the imination proceeded even in the absence of the metal. Subse- 

The metal-free imination, as well as the Rh and Ag catalyses, can be applied to the conversion of sulfides into sulfilimines such as 22 and 23 [24]. Unfortunately, however, double iminations leading to sulfondiimides (N-protected forms of 24) did not occur, and compounds such as 25 [27] still require the intermediacy of sulfondiimide 24. Thus, its preparation was achieved by treatment of the corresponding sulfide with a mixture of ammonia and t-butyl hypochlorite [28]. 

Equilibrium pressure of adsoiptive Equilibrium pressure of adsorptive 

As there is a difference between the heat of adsorption of this weakly bound adsorbate on the support and the more strongly bound irreversible  

The use of H2-O2 titration is a method of enhancing the sensitivity relative to a standard hydrogen chemisorption measurement which involves the use of hydrogen to titrate a chemisorbed layer of oxygen.63 The enhanced sensitivity results from an increased hydrogen consumption as the stoichiometry is raised to 3 H 1 Pt 

The uncertainty involving ascribing a specific value to the O Rh stoichiometry, at least for poorer dispersed catalysts, has implications when employing the hydrogen-oxygen titration method63 to determine dispersion where a value of x is required in determining the overall reaction stoichiometry  

It has already been mentioned in Section 2.3.5 that a rational synthesis of even the simplest fuUerene makes high demands on the preparation of a suitable precursor, which has to be closed to the carbon cage in a subsequent pyrolytic reaction. This problem becomes all the more salient in attempts on designing a strategy for the rational synthesis of multishell fullerenes it is not only the curvature of the carbon skeleton that must be considered here, but also the supramolecular pattern of concentric fullerenes of suitable sizes. Hence, for the time being, a directed synthesis of these multilayered species has not yet succeeded. 

Furthermore succeeds, with the support of hydrogen, the reductive transformation of supercritical carbon dioxide into nano-onions (CO2 -r2H2 C-F2H20). A platinum catalyst [Pt(T] -C,S-Ci2H8)(PEt3)2] is employed here which presumably releases [Pt(PEt3)2] as reactive species. The resultant carbon onions exhibit a partially spiral structure, or they form aggregates enclosed in a common shell. 

The chlorination of titanium carbide can as well lead to the formation of onionlike carbon stractures. The resulting titanium(IV) carbide is removed from the reaction mixture by distillation. The carbon blend obtained has a large specific surface of about 1400 m g and contains, among others, carbon onions with a diameter of 15-35 nm. Eurther products include considerable amounts of amorphous carbon and small quantities of nanotubes. 

Another method to generate onion-hke structures (besides nanotubes and irregular carbon particles) is the ultrasonication of chloroform, dichloromethane, or similar solvents containing halogens. The decomposition of the solvent takes place on hydrogen-terminated sihcon nanowires which, at the same time, also serve as a template to the formation of tubular structures. 

Under suitable conditions, the reaction of perfluorinated hydrocarbons like teflon (PTFE) or perfluoronaphthalene with alkali amalgam yields carbon nanostructures as weU. The reductive dehalogenation generates intermediate polyynes that subsequently condense to give the respective graphene structures. Normally, a rruxture of carbon nanotubes and onions is obtained. 

An in-depth discussion on the major in-situ polymerization processes is provided in Chapter 4. In addition, encapsulation using the mini emulsion process is discussed in Chapter 2, and interfacial polycondensations processes are described in Chapter 5. 

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Figure 3-7. Fugacity coefficients for a saturated mixture of propionic acid (1) and raethylisobutylketone (2). Calculations based on chemical method show large variations from ideal behavior.

In this section we will discuss only the analytical techniques that are in very general usage without presenting the older chemical methods. 

In all spent researches the received ratio signal / noise and sensitivity for revealed defects, distribution of material density and the weights of high density components (tungsten, lead, uranium) are well agreed with results of alternate physics-chemical methods of analyses... 

In most of the connnonly used ab initio quantum chemical methods [26], one fonns a set of configurations by placing N electrons into spin orbitals in a maimer that produces the spatial, spin and angular momentum syimnetry of the electronic state of interest. The correct wavefimction T is then written as a linear combination of tire mean-field configuration fimctions qj = example, to describe the... 

Quantum chemical methods, exemplified by CASSCF and other MCSCF methods, have now evolved to an extent where it is possible to routinely treat accurately the excited electronic states of molecules containing a number of atoms. Mixed nuclear dynamics, such as swarm of trajectory based surface hopping or Ehrenfest dynamics, or the Gaussian wavepacket based multiple spawning method, use an approximate representation of the nuclear wavepacket based on classical trajectories. They are thus able to use the infoiination from quantum chemistry calculations required for the propagation of the nuclei in the form of forces. These methods seem able to reproduce, at least qualitatively, the dynamics of non-adiabatic systems. Test calculations have now been run using duect dynamics, and these show that even a small number of trajectories is able to produce useful mechanistic infomiation about the photochemistry of a system. In some cases it is even possible to extract some quantitative information. 

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

The third method is of limited application and is used only in special cases, The second is the most accurate and rapid method, and is of considerable technical importance. The chemical method (described below), although less accurate than the polarimetric method, is of great value for the estimation of sugars in biological fluids. In fact, for such purposes, it is often to be preferred to the polarimetric method owing to the probable presence of other substances having high optical rotations. 

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

Chemical methods may be employed if the reagent attacks only one of the components. Thus quicklime may be employed for the removal of water in the preparation of absolute ethyl alcohol. Also aromatic and unsaturated hydrocarbons may be removed from mixtures with saturated hydrocarbons by sulphonation. 

Chromatography is based upon the selective adsorption from solution on the active surface of certain finely divided solids. Closely related substances exhibit different powers of adsorption, so that separations, which are extremely difficult by ordinary chemical methods, may be effected by this means. When, for example, a solution of leaf pigments... 

In connexion with the above chemical methods of separation, it is important to note that sufficient of the extracting reagent must be used to remove completely the component which it dissolves or with which it reacts. 

Several chemical methods have been devised for identifying the N terminal ammo acid They all take advantage of the fact that the N terminal ammo group is free and can act as a nucleophile The a ammo groups of all the other ammo acids are part of amide linkages are not free and are much less nucleophilic Sanger s method for N terminal residue analysis involves treating a peptide with 1 fluoro 2 4 dimtrobenzene which is very reactive toward nucleophilic aromatic substitution (Chapter 23)... 

The copolymer composition equation relates the r s to either the ratio [Eq. (7.15)] or the mole fraction [Eq. (7.18)] of the monomers in the feedstock and repeat units in the copolymer. To use this equation to evaluate rj and V2, the composition of a copolymer resulting from a feedstock of known composition must be measured. The composition of the feedstock itself must be known also, but we assume this poses no problems. The copolymer specimen must be obtained by proper sampling procedures, and purified of extraneous materials. Remember that monomers, initiators, and possibly solvents are involved in these reactions also, even though we have been focusing attention on the copolymer alone. The proportions of the two kinds of repeat unit in the copolymer is then determined by either chemical or physical methods. Elemental analysis has been the chemical method most widely used, although analysis for functional groups is also employed. 

Throughout the history of the development of fats and oils, many wet chemical methods have been developed to assess the quaUty of the raw materials and products. As sophisticated instmmentation develops, many of the wet methods are being replaced. Particular attention is being given to methods that eliminate the use of solvents which cause an environmental disposal problem. Many in-line sensors are also being developed to allow corrections of critical parameters to be made more quickly in the process. 

Fluorine in the atmosphere can be detected by chemical methods involving the displacement of halogens from haUdes. Dilute fluorine leaks are easily detected by passing a damp piece of starch iodide paper around the suspected area. The paper should be held with metal tongs or forceps to avoid contact with the gas stream and immediately darkens when fluorine is present. 

Eoamable compositions in which the pressure within the cells is increased relative to that of the surroundings have generally been called expandable formulations. Both chemical and physical processes are used to stabilize plastic foams from expandable formulations. There is no single name for the group of cellular plastics produced by the decompression processes. The various operations used to make cellular plastics by this principle are extmsion, injection mol ding, and compression molding. Either physical or chemical methods may be used to stabilize products of the decompression process. 

Determination of DNA Sequence Information. Almost all DNA sequence is determined by enzymatic methods (12) which exploit the properties of the enzyme DNA polymerase. Whereas a chemical method for DNA sequencing exists, its use has been supplanted for the most part in the initial deterrnination of a sequence. Chemical or Maxam-Gilbett sequencing (13) is mote often used for mapping functional sites on DNA fragments of known sequence. 

Enzyme Immunoassay. In EIA, antibody (or antigen) is labeled with (or conjugated to) an enzyme, and this reagent is used to complex and quantify the target antigen (or antibody) in a sample. Conjugation may utilize a variety of chemical methods. 

There are many chemical methods for generating radicals reported in the hterature that do not involve conventional initiators. Specific examples are included in References 64—79. Most of these radical-generating systems carmot broadly compete with the use of conventional initiators in industrial polymer apphcations owing to cost or efficiency considerations. However, some systems may be weU-suited for initiating specific radical reactions or polymerizations, eg, grafting of monomers to cellulose using ceric ion (80). 

Organic isocyanates are generally characterized with respect to NCO content. A number of spectroscopic and chemical methods for isocyanate deterrnination are available (96). 

Electrorefining. Electrolytic refining is a purification process in which an impure metal anode is dissolved electrochemicaHy in a solution of a salt of the metal to be refined, and then recovered as a pure cathodic deposit. Electrorefining is a more efficient purification process than other chemical methods because of its selectivity. In particular, for metals such as copper, silver, gold, and lead, which exhibit Htfle irreversibHity, the operating electrode potential is close to the reversible potential, and a sharp separation can be accompHshed, both at the anode where more noble metals do not dissolve and at the cathode where more active metals do not deposit. 

Other Meta.Is, Although most cobalt is refined by chemical methods, some is electrorefined. Lead and tin are fire refined, but a better removal of impurities is achieved by electrorefining. Very high purity lead is produced by an electrochemical process using a fluosiUcate electrolyte. A sulfate bath is used for purifying tin. Silver is produced mainly by electrorefining in a nitrate electrolyte, and gold is refined by chemical methods or by electrolysis in a chloride bath. 

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