Solid supernatant concentration method

Supernatant Concentration Methods from Solid Material... 

Solvent determination involves screening different solvents for product solubility and then titrating the best solvent to find the minimum solvent whole-broth ratio. One method for screening a range of solvents is to add 1 vol of each solvent to 2 vol of whole broth. Samples are mixed for at least 1 h and then centrifuged or filtered to remove the solids. Product concentration in the supernatant IS measured, and the solvent with the highest product solubility is selected (Note 12)... 

Perfetti et al. (131) described a method for the determination of ethoxyquin in milk. Milk solids were precipitated by adding acetonitrile, and the water-acetonitrile supernatant was washed with hexane to remove fat. The addition of NaCl caused the water-acetonitrile solution to separate into an aqueous phase and an acetonitrile phase, thus separating ethoxyquin from most water-soluble impurities. A large volume of water was then added to the acetonitrile layer, and ethoxyquin was partitioned into hexane and removed at reduced pressure. The residue was dissolved in the mobile phase and analyzed on a 250-mm X 4.6-mm-ID. Ultrasphere ODS column using fluorescence detection with excitation of 230 nm, and emission of 418 nm, respectively. A mixture of water and acetonitrile with a diethylamine-acetic acid buffer was the mobile phase. Recoveries from milk samples fortified at 1, 5, and 10 ng/g averaged 78%, with a coefficient of variation of 5.0%. Low concentrations (less than 1 ng/g) of apparent ethoxyquin were detected in commercial milk samples analyzed by this method. 

Experimentally, the investigation of adsorption from solution is much simpler than that of gas adsorption. A known mass of adsorbent solid is shaken with a known volume of solution at a given temperature until there is no further change in the concentration of the supernatant solution. This concentration can be determined by a variety of methods involving chemical or radiochemical analysis, colorimetry, refractive index, etc. The experimental data are usually expressed in terms of an apparent adsorption isotherm in which the amount of solute adsorbed at a given temperature per unit mass of adsorbent - as calculated from the decrease (or increase) of solution concentration - is plotted against the equilibrium concentration. 

A possible way to increase the accuracy of this immersion approach is to use the slurry method and to analyse a weighed sample of the slurry in the bottom of the test-tube, instead of analysing the supernatant (Nunn etal., 1981). One then simply makes use of Equation (5.49), the operational expression of the relative surface excess of the solute with respect to the solvent. Here n1 and n2 are the total amounts of solute and solvent in the sample of slurry (either adsorbed or in solution) and c[ and c their concentrations in the solution. If one uses a liquid-solid ratio large enough to avoid any measurable change in concentration on adsorption, then c and c are simply the concentrations in the starting solution. The measurement is accurate provided the quantitative analysis of the slurry, which involves measuring the total amounts of 2 and 1... 

These are by far the most quantitative methods for the assessment of dispersing power. Known amounts of solids (in grams) with a surface area A (m g ) are equilibrated at a constant temperature with dispersant solutions of various concentrations, Cj. The bottles containing the various dispersions are rotated for several hours until equilibrium is reached, after which the particles are removed from the dispersant solution by centrifugation and/or filtration through Millipore filters. The dispersant concentration in the supernatant liquid, C2, is analytically determined by a suitable technique that can measure low concentrations. 

In this chapter, a summary of the methods that can be applied to assess the structure of the solid/Uquid interface will first be provided, followed by details of assessing sedimentation, flocculation, and Ostwald ripening. In the latter cases (flocculation and Oswald ripening), information is needed on the particle size distribution, and several techniques are available to obtain this from diluted systems. It is essential to dilute the concentrated suspension with its own dispersion medium in order not to affect the state of the dispersion during examination. The dispersion medium can be obtained by centrifuging the suspension, when the supernatant liquid will be produced at the top of the centrifuge tube in the case... 

Besides direct surface analysis methods, indirect solution depletion methods can be applied to study protein adsorption. Solution depletion methods use solids of known large surface area (usually dispersed solids) that are placed into a solution of known protein concentration. After incubation, the concentration of the protein in the supernatant is measured. From the depletion, the adsorbed protein amount can be calculated. 

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