Pure materials

FIGURE 5-1 Constant-current and tuning circuits for coulometric analysis. (From Marinenko and Taylor. ) 

In a titrimetric standardization the solution to be standardized is compared directly or indirectly against a standard substance. Some substances can be weighed accurately, but do not give a solution of known concentration. Others cannot be weighed accurately, but give stable solutions. A primary standard is a pure substance (element or compound) that is stable enough to be stored indefinitely without decomposition, can be weighed accurately without special precautions when exposed to 

FIGURE 5-2 Elements obtainable with a purity of at least 99.9 % (shaded) and at least 99,99% (unshaded), from ASTM. -) 

From the viewpoint of primary standards for ordinary analysis, acceptable purity depends on the substance. Many contain at least 99.9-1-% of the major component, for example, potassium hydrogen phthalate, potassium chloride, or calcium carbonate but cholesterol of high purity is 99.4%, and bilirubin 99%. 

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Most electronic valence transitions shift to longer wavelengths at higher pressures drat is, the gap between the highest occupied orbital and lowest unoccupied orbital tends to decrease upon compression. The rates of shift usually are larger (1) for pure materials than for solutes in a solvent and (2) for stronger (more allowed) transitions. However, these correlations are not quantitative, and many transitions shift in the opposite... 

Checking the Purification. The purity of the dry re-crystallised material must now be determined, as it is possible that repeated recrystallisation may be necessary to obtain the pure material. The purity is therefore checked by a melting-point determination, and the recrystallisation must be repeated until a sharp melting-point is obtained. Should the compound have no well-defined melting-point e.g.y the salt of an organic acid or base), it must be analysed for one suitable component element, until its analysis agrees closely with that theoretically required. 

If the aec.-octyl alcoliol is pure the yield of pure material is almost quantitative. 

This weight of zinc powder assumes 100 per cent, purity an equivalent amount of less pure material may be used (see Section I V,83, Note 2). 

Clearly, there is a need for techniques which provide access to enantiomerically pure compounds. There are a number of methods by which this goal can be achieved . One can start from naturally occurring enantiomerically pure compounds (the chiral pool). Alternatively, racemic mixtures can be separated via kinetic resolutions or via conversion into diastereomers which can be separated by crystallisation. Finally, enantiomerically pure compounds can be obtained through asymmetric synthesis. One possibility is the use of chiral auxiliaries derived from the chiral pool. The most elegant metliod, however, is enantioselective catalysis. In this method only a catalytic quantity of enantiomerically pure material suffices to convert achiral starting materials into, ideally, enantiomerically pure products. This approach has found application in a large number of organic... 

It was dissolved in 75 ml of pentane and the solution was cooled to about -30°C with swirling. Some oil precipitated NHR spectroscopy showed the presence of only a very small amount of the allene. The supernatant yellow liquid was decanted from the oil and, after some pentane had been removed by evacuation, the solution was cooled below -40°C. The pale yellow crystals (m.p. 52°C) were filtered off on a sintered-glass funnel (note 2). From the mother liquor an additional small amount of product was obtained, bringing the yield of NMR-pure material to 62%. 

Enantiomeric excess (Section 7 4) Difference between the per centage of the major enantiomer present in a mixture and the percentage of its mirror image An optically pure material has an enantiomenc excess of 100% A racemic mixture has an enantiomeric excess of zero... 

To prepare the standard pH buffer solutions recommended by the National Bureau of Standards (U.S.), the indicated weights of the pure materials in Table 8.15 should be dissolved in water of specific conductivity not greater than 5 micromhos. The tartrate, phthalate, and phosphates can be dried for 2 h at 100°C before use. Potassium tetroxalate and calcium hydroxide need not be dried. Fresh-looking crystals of borax should be used. Before use, excess solid potassium hydrogen tartrate and calcium hydroxide must be removed. Buffer solutions pH 6 or above should be stored in plastic containers and should be protected from carbon doxide with soda-lime traps. The solutions should be replaced within 2 to 3 weeks, or sooner if formation of mold is noticed. A crystal of thymol may be added as a preservative. 

Plasticizer Range Alcohols. Commercial products from the family of 6—11 carbon alcohols that make up the plasticizer range are available both as commercially pure single carbon chain materials and as complex isomeric mixtures. Commercial descriptions of plasticizer range alcohols are rather confusing, but in general a commercially pure material is called "-anol," and the mixtures are called "-yl alcohol" or "iso...yl alcohol." For example, 2-ethyIhexanol [104-76-7] and 4-methyl-2-pentanol [108-11-2] are single materials whereas isooctyl alcohol [68526-83-0] is a complex mixture of branched hexanols and heptanols. Another commercial product contains linear alcohols of mixed 6-, 8-, and 10-carbon chains. 

Commercially pure (< 99.997%) helium is shipped directiy from helium-purification plants located near the natural-gas supply to bulk users and secondary distribution points throughout the world. Commercially pure argon is produced at many large air-separation plants and is transported to bulk users up to several hundred kilometers away by tmck, by railcar, and occasionally by dedicated gas pipeline (see Pipelines). Normally, only cmde grades of neon, krypton, and xenon are produced at air-separation plants. These are shipped to a central purification faciUty from which the pure materials, as well as smaller quantities and special grades of helium and argon, are then distributed. Radon is not distributed commercially. 

Sodium hydride is insoluble in organic solvents but soluble in fused salt mixtures and fused hydroxides such as NaOH. It oxidizes in dry air and hydrolyzes rapidly in moist air. The pure material reacts violently with water ... 

The oxidation reactor effluent and methanol ate sent to the esterification reactor, which operates at up to 250°C and a pressure sufficient to maintain the Hquid phase. This latter is about 2500 kPa (25 atm). The oxidation products are converted to methyl -toluate and dimethyl terephthalate without a catalyst. Excess methanol is suppHed, and steam and vaporized methanol ate removed and enter a methanol recovery column. The esterification products flow to a cmde ester column, which separates the toluate from the terephthalate. The overhead stream of methyl -toluate is returned to the oxidation reactor, and the bottoms stream of dimethyl terephthalate goes to a primary distillation. The distillate is dissolved in methanol, crystallized, and sohd dimethyl terephthalate is recovered. The dimethyl terephthalate can then be either recrystallized or distilled to yield the highly pure material needed for the polyesterification reaction. 

Absorption. As a separation technique, absorption (qv), also called extractive distillation, starts with an energy deficit because the process mixes in a pure material (solvent) and then separates it again. This process is nevertheless quite common because it shares most of the advantages of distillation. Additionally, because it separates by molecular type, it can be tailored to obtain a high a. The following ratios are suggested for equal costs (7) ... 

Eig. 1. Schematic pressure—temperature diagram for a pure material showing the supercritical fluid region, where is the pure component critical point... 

Performance. Alkaline manganese-dioxide batteries have relatively high energy density, as can be seen from Table 2. This results in part from the use of highly pure materials, formed into electrodes of near optimum density. Moreover, the cells are able to function well with a rather small amount of electrolyte. The result is a cell having relatively high capacity at a fairly reasonable cost. 

Chemically pure calcium carbide [75-20-7] is a colorless solid however, the pure material can be prepared only by very special techniques. 

Distillation. Most fatty acids are distilled to produce high quaHty products having exceUent color and a low level of impurities. Distillation removes odor bodies and low boiling unsaponifiable material in a light ends or heads fraction, and higher boiling material such as polymerized material, triglycerides, color bodies, and heavy decomposition products are removed as a bottoms or pitch fraction. The middle fractions sometimes can be used as is, or they can be fractionated (separated) into relatively pure materials such as lauric, myristic, palmitic, and stearic acids. 

As commercially pure materials, the ethyleneamines exhibit good temperature stabiUty, but at elevated temperatures noticeable product breakdown may result in the formation of ammonia and lower and higher mol wt species. This degradation becomes mote pronounced at higher temperature and over longer time periods. Certain contaminants, such as mineral acids, can lower the onset temperature for rapid thermal decomposition. The manufacturer should be contacted and thermal stabiUty testing conducted whenever ethyleneamines ate mixed with other materials. 

The bulk density of powdered diatomite varies from 112 to 320 kg/m. The tme specific gravity of diatomite is 2.1 to 2.2, the same as for opaline sihca, or opal (1). The thermal conductivity of bulk quantities of diatomite is low but increases with higher percentages of impurities and a higher density. The fusion point depends on the purity but averages about 1430°C for pure material, which is slightly less than for pure siUca. The addition of chemical agents, such as soda ash, reduces the fusion point. 

Multiblock Copolymers. Replacement of conventional vulcanized mbber is the main appHcation for the polar polyurethane, polyester, and polyamide block copolymers. Like styrenic block copolymers, they can be molded or extmded using equipment designed for processing thermoplastics. Melt temperatures during processing are between 175 and 225°C, and predrying is requited scrap is reusable. They are mostiy used as essentially pure materials, although some work on blends with various thermoplastics such as plasticized and unplasticized PVC and also ABS and polycarbonate (14,18,67—69) has been reported. Plasticizers intended for use with PVC have also been blended with polyester block copolymers (67). 

P rtl IMol r Properties. The properties of individual components in a mixture or solution play an important role in solution thermodynamics. These properties, which represent molar derivatives of such extensive quantities as Gibbs free energy and entropy, are called partial molar properties. For example, in a Hquid mixture of ethanol and water, the partial molar volume of ethanol and the partial molar volume of water have values that are, in general, quite different from the volumes of pure ethanol and pure water at the same temperature and pressure (21). If the mixture is an ideal solution, the partial molar volume of a component in solution is the same as the molar volume of the pure material at the same temperature and pressure. 

The empirical representation of the PVT surface for pure materials is treated later in this section. We first present general equations for evaluation of reduced properties from such representations. 

Although the Pitzer correlations are based on data for pure materials, they may also be used for the calculation of mixture properties. A set of recipes is required relating the parameters T, Pc, and (0 for a mixture to the pure-species values and to composition. One such set is given by Eqs. (2-80) through (2-82) in Sec. 2, which define pseudopa-rameters, so called because the defined values of T, Pc, and (0 have no physical significance for the mixture. 

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