Substances, chief

Properties White, crystalline, waxlike substance. Chief constituent of commercial purified spermaceti. Mp 50C, bp 360C, d 0.832, refr index 1.4398 (70C). Soluble in alcohol and ether insoluble in water. Combustible. 

The chief danger and main source of error in a combustion is that of moving the Bunsen forward a little too rapidly and so causing much of the substance to burn very rapidly, so that a flash-back occurs. This usually causes an explosion wave to travel back along the tube towards the purification train, some carbon dioxide and water vapour being carried with it. If these reach the packing of the purification train they will, of course, be absorbed there and the results of the estimation will necessarily be low. 

Small quantities of solids may be spread upon unglazed porcelain plates. The chief disadvantage of this method is the comparatively high cost of the porous plates, since they cannot be conveniently cleaned nor can the same area be used for different substances. However, a plate may be broken and used for small amounts of material. 

The chief uses of chromatographic adsorption include (i) resolution of mixtures into their components (Li) purification of substances (including technical products from their contaminants) (iii) determination of the homogeneity of chemical substances (iv) comparison of substances suspected of being identical (v) concentration of materials from dilute solutions (e.g., from a natural source) (vi) quantita tive separation of one or more constituents from a complex mixture and (vii) identi-1 ig- II, 16, 3. gcajjQij and control of technical products. For further details, the student is referred to specialised works on the subject. ... 

With samples that are difficult to dissolve, the first approach is usually to try digesting the sample with an acid or base. Table 7.2 lists the most commonly used acids and bases and summarizes their use. Digestion is commonly carried out in an open container, such as a beaker, using a hot plate as a source of heat. The chief advantage of this approach is its low cost as it requires no special equipment. Volatile reaction products, however, are lost, leading to a determinate error if analyte is included among the volatile substances. 

The present method is applicable with slight modifications to the preparation of both the ortho and para aminonaphthols and to many homologues, benzologues, and heterocyclic isologues of these substances. The chief feature of novelty is in the use of stannous chloride as an antioxidant in preparing and crystallizing the amine hydrochlorides. 

An exception may be made when there is good reason to believe that the chief effects of the different harmful substances are not in fact additive, but independent, such as a combined exposure to silica dust and lead dust. In the determination of whether or not the additive effects are excessive, the following formula should be used ... 

The chief characteristics of these substances are given in the following table —... 

In the following table the characters of the principal isomerides and other transformation products of the cinchona alkaloids are summarised and references are given to the chief papers dealing with them, and upon which the foregoing account is based. The capital letters in brackets printed after the names of the substances refer to the formulae and explanatory footnote on p. 449. 

In the analysis of oils and fats, where the quantity of fatty acid is the chief object of the deteiminalion, it is customary ttj hydrolyse the substance with a standard solution of aLoholic potash in place of aqueous potash, and to estimate the excess... 

The anhydrous trihalides are ionic, high melting, crystalline substances which, apart from the trifluorides are extremely deliquescent. As can be seen from Table 30.4, the coordination number of the Ln changes with the radii of the ions, from 9 for the trifluorides of the large lanthanides to 6 for the triiodides of the smaller lanthanides. Their chief importance has been as materials from which the pure metals can be prepared. 

Furthermore, intramolecular hydrogen bonding is one of the chief factors in determining the structure of such important biological substances as proteins, as discussed in Chapter 24. 

When coal is heated to a high temperature in the absence of air, it undergoes decomposition volatile products (coal gas and coal tar) distill away and a residue called coke remains. Coke is a valuable industrial material which finds its chief use in the reduction of iron ore (iron oxide) to iron for the manufacture of steel. Coke is essentially carbon that still contains the mineral substances that are present in all coals (and form the ash that results when coal or coke is burned). 

The chief advantage of colorimetric and spectrophotometric methods is that they provide a simple means for determining minute quantities of substances. The upper limit of colorimetric methods is, in general, the determination of constituents which are present in quantities of less than 1 or 2 per cent. 

Infrared absorption spectra can be employed for the identification of pure compounds or for the detection and identification of impurities. Most of the applications are concerned with organic compounds, primarily because water, the chief solvent for inorganic compounds, absorbs strongly beyond 1.5//m. Moreover, inorganic compounds often have broad absorption bands, whereas organic substances may give rise to numerous narrower bands. 

There are also methods based on the detection of aldehydic substances, for the typical odor and flavor of rancidity seem to be associated with the liberation of aldehydic materials during the oxidative deterioration. The Kreis test (18), perhaps the best known of these methods, consists of treating the fat with concentrated hydrochloric acid and a solution of phloroglucinol. The red color formed is attributed to a condensation product of epihydrin aldehyde with phloroglucinol. Historically, the chief difficulty with this method has been that fats which are not rancid will often give a positive Kreis test. It has been shown that if this test is quantitatively correlated with the induction period... 

Incompleteness of the reaction this is the chief criterion of chemical equilibria. A reversible reaction is never complete in any direction provided none of the products is allowed to escape from the system. Stated differently, in the equilibrium condition, the reactants and the products are all present simultaneously in the reaction vessel. If any of the substances were to vanish, its concentration would become zero and the value of the equilib-... 

Chief of Research Explosive Substances Department of Police in Dnepropetrovsk Region. 105a, Heroev Stalingrada St., Dnepropetrovsk 49033, Ukraine Tel. +38 0562 415798... 

---