Quantitative separation

From Figure 1. ll we see that a quantitative separation of Cu + from Pb + and Cd + can be accomplished if the aqueous phase is buffered to a pli of less than... 

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and teUurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 W with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxylamine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and deterrnination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

These species are also unusual iu that they are extremely hydrophobic anions which form very strong conjugate acids. This unique combination of features leads to a number of potential uses such as the extraction of organic compounds from extremely dilute solutions and the isolation of metal cations, including the quantitative separation of radionucUdes, eg, Cs (192). 

In this work, a method based on the reduction potential of ascorbic acid was developed for the sensitive detennination of trace of this compound. In this method ascorbic acid was added on the Cr(VI) solution to reduced that to Cr(III). Cr(III) produced in solution was quantitatively separated from the remainder of Cr(VI). The conditions were optimized for efficient extraction of Cr(III). The extracted Cr(III) was finally mineralized with nitric acid and sensitively analyzed by electro-thermal atomic absorption spectrometry. The determinations were carried out on a Varian AA-220 atomic absolution equipped with a GTA-110 graphite atomizer. The results obtained by this method were compared with those obtained by the other reported methods and it was cleared that the proposed method is more precise and able to determine the trace of ascorbic acid. Table shows the results obtained from the determination of ascorbic acid in two real samples by the proposed method and the spectrometric method based on reduction of Fe(III). 

Since, however, K2 is comparatively large, the solvent effect is relatively small this is why in the quantitative separation of barium sulphate, precipitation may be carried out in slightly acid solution in order to obtain a more easily filterable precipitate and to reduce co-precipitation (Section 11.5). 

The solubility of most inorganic compounds is reduced by the addition of organic solvents, such as methanol, ethanol, propan-l-ol, acetone, etc. For example, the addition of about 20 per cent by volume of ethanol renders the solubility of lead sulphate practically negligible, thus permitting quantitative separation. Similarly calcium sulphate separates quantitatively from 50percent ethanol. Other examples of the influence of solvents will be found in Chapter 11. 

If the ion exchange column is loaded with several ions of similar charge, B, C, etc., elution curves may be obtained for each ion by the use of appropriate eluants. If the elution curves are sufficiently far apart, as in Fig. 7.2, a quantitative separation is possible only an incomplete separation is obtained if the elution curves overlap. Ideally the curves should approach a Gaussian (normal) distribution (Section 4.9) and excessive departure from this distribution may indicate faulty technique and/or column operating conditions. 

QUANTITATIVE SEPARATIONS BASED UPON PRECIPITATION METHODS... 

An interesting application of these results is to the direct quantitative separation of copper and cadmium. The copper is first deposited in acid solution the solution is then made slightly alkaline with pure aqueous sodium hydroxide, potassium cyanide is added until the initial precipitate just re-dissolves, and the cadmium is deposited electrolytically. 

The traditional methods for measuring the swelling degree [100] are, as a rule, limited due to the difficulties in quantitative separation of the swollen gel from the outer solution because of extremely low strength of the former. These difficulties can be avoided by measuring the dimensions of a regular shape sample directly in an excess of liquid [19, 101,102], The other example is the modified volumetric method recently developed by us especially for SAH [103],... 

In order to achieve a quantitative separation of the factors responsible for the temperature coefficient of the intrinsic viscosity, K should first be established as a function of temperature by carrying out measurements in -solvents having s covering the temperature range of interest. The expansion factor may then be obtained from the intrinsic viscosity measured at the temperature T in the given solvent. If Cm occurring in Eq. (10) were independent of the temperature, — should then plot linearly with 1/T, However,... 

The particle size and size distribution of adsorbents for preparative purposes are higher and wider, respectively, compared to analytical ones. In addition, the adsorbent layer is much thicker and effectively overloaded with the compoimds. These items make resolution difficult, which must even be better than for quantitative separations as discussed in Section 5.1. These facts necessitate an excellent and superior strategy to hud the best separation, i.e., the mobile phase with the best selectivity (see Chapter 4). It was also shown that plates with a thickness gradient, called Uniplate-T taper plate [5], could improve resolution in the lower-Mp range. 

Vitamin Bg and related compoimds (Figure 10.2) were quantitatively separated by preparative TLC on silica gel H. After elution, the pyridoxic acid lactone method was employed for fluorimetric determination of the concentration of the vitamin forms involved [8]. Table 10.2 shows Revalues obtained for various forms of vitamin Bg, using several solvent systems. The solvent selected, ethyl acetate/pyridine/water (2 1 2, v/v), gave excellent separation of pyridoxamine, pyridoxic acid, and pyri-doxine together with pyridoxal. 

The chief uses of chromatographic ad-sorption include (i) resolution of mixtures into their components (ii) 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) quantitative separation of one or more constituents from a complex mixture and (vii) identi-l ig., 46, 3. anj control of technical products. 

Several qualifying features for polymer extract analysis are summarised in Table 2.11. Quantitative separation of polymer and (thermolabile and/or volatile) additives without decomposition of the analyte(s) is difficult for thermoplasts, but even more difficult for... 

Chatterjee et al. [20] quantitatively separated primaquine from amodiquine by a selective precipitation method. A powdered sample containing primaquine and amodiaquine was dissolved in 0.01 N hydrochloric acid 4 N ammonia was added to precipitate amodiaquine. The mixture was filtered and the combined filtrate and washings containing primaquine was diluted with water and 0.1 N hydrochloric acid. The absorbance of this solution was measured at 282 nm versus a solvent blank. 

Levi, Hawkins and Hibbert6 have recently confirmed the structure for L. mesenteroides dextran proposed by Fowler, Buckland, Brauns and Hibbert.34 Completely methylated dextran in an over-all yield of 71% was obtained by three methylations with dimethyl sulfate and sodium hydroxide, followed by six modified Muskat methylations. The methylated dextran was hydrolyzed at 140° with methanolic hydrogen chloride, and the mixture of methylated glucosides, obtained in 95% yield, was quantitatively separated by fractional distillation. 2,3-Di-methyl-D-glucose was identified by means of the phenylhydrazide of the corresponding D-gluconic acid. 

All these problems could be convincingly overcome by application of the continuous operation mode [63 d)]. Most interestingly, this unusual continuous biphasic reaction mode enabled the quantitative separation of relatively high boiling products from the ionic catalyst solution under mild temperature conditions and without use of an additional organic extraction solvent. More details of this process are included in Chapter 8, Section 8.2.2.3. 

Most of the selective-etherification studies on polysaccharides have been made with cellulose, and nearly all of them have involved quantitative separation of the D-glucose derivatives formed on hydrolysis of the partially substituted celluloses. In view of their stability, ethers of polysaccharides are particularly suited to this approach. 

Often, it is not possible to extract one solute quantitatively without partial extraction of another. The ability to separate two solutes depends on the relative magnitudes of their distribution ratios. For solutes A and B, whose distribution ratios are DA and DB, the separation factor P is defined as the ratio DA/DB where D>DB. Table 4.2 shows the degrees of separation achievable with one extraction, assuming that Da = 102, for different values of DB and P. For an essentially quantitative separation P should be at least 105. 

Assuming that log P should be at least 5 for an essentially quantitative separation by a single extraction (see p. 57), ApH1/2 should be 5, 2.5 and 1.7 respectively for pairs of mono-, di- and trivalent metals. Selectivity by pH control is greatest, therefore, for trivalent metals and least for monovalent. This is reflected in the slopes of the curves which are determined by n and decrease in the order M3+ > M2+ > M+. 

If P is insufficiently large to enable a quantitative separation to be made by pH control alone, the addition of a masking agent which forms a water-soluble complex more strongly with one metal than the other will shift the extraction curve for the former to a higher pH range with a consequent increase in P, which is now given by... 

Separations of transition and non-transition metals by deposition of the former at a mercury-pool cathode can be used as a preliminary step in an analytical procedure. In such cases, the reduced metal cannot be weighed, although quantitative separations may be achieved. 

A method of analysis frequently requires the separation of the analyte from interfering materials present within the sample matrix. Many different procedures are used to effect such separations (Chapter 4), and the establishment of quantitative separations is a singularly important step in method development. Careful study and control of experimental conditions are needed and a large number of experiments may be required to establish the most suitable. 

Details of the first stereoselective hydrogenation in ionic liquids were published by the group of Chauvin [68], who reported the enantioselective hydrogenation of the enamide a-acetamidocinnamic acid in the biphasic system [BMIM][SbF6]/ iPrOH (ratio 3 8) catalyzed by [Rh(cod) (-)-diop ][PF6]. The reaction afforded (S)-N-acetylphenylalanine in 64% enantiomeric excess (ee) (Fig. 41.4). The product was easily and quantitatively separated and the ionic hquid could be recovered, while the loss of rhodium was less than 0.02%. 

A kinetic model was developed to describe the pH-dependent uncoupling activity of substituted phenols in bacterial photosynthetic membranes [2]. In this model, the overall uncoupling activity is quantitatively separated into the contribution of membrane concentration, which can be estimated by the Kmw, and of intrinsic activity. The intrinsic activity of an uncoupler is influenced not only by the hydrophobicity and acidity, but also by steric effects and by the charge distribution within the molecule [2]. 

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