Osmosis and Reverse Osmosis

Diluted sample solutions can be concentrated by both osmosis and reverse osmosis. The concentration process is based on a pressure gradient over a membrane, which rejects the analyte molecules. High molecular weight substances are already rejected by ultrafiltration membranes. But the typical application of reverse osmosis is the separation of low molecular weight substances from aqueous solutions to purify the water or to concentrate the substances which are to be determined.

To concentrate transition and heavy metal ions from dilute aqueous solutions by osmosis the sample solution is separated from a high ionic strength solution by a membrane which is permeable only to the water. An osmotic pressure is built up, which then propels the water into the acceptor (filtrate) solution and concentrates the donor solution. In a thin-layer flow-cell, which is similar to the cell shown in Figure 5A and has a mechanically supported separation membrane (reverse osmosis membrane of cellu lose triacetate) preconcentration factors of up to 8-10 for copper(n), cadmium(n), manganese(n), nickel(n) and zinc(n) can be achieved in a countercurrent flow regime.

To implement reverse osmosis, an outer pressure (Figure 2) is applied to propel the water through the membrane. An interesting practical aspect is the possibility of reducing the necessary outer pressure by an osmotic pressure difference which has the same direction. This is implemented by high ionic strength acceptor solutions.

The concentration factor E (eqn [32]) increases considerably with increasing reflection factors (eqn [29])). Highly diluted sample solutions can be concentrated to values that can be determined with the available determination methods. The sample solution can be concentrated up to the precipitation of the solute. Then an additional filter layer is used, which can be exchanged and directly analysed, e.g. by X-ray fluorescence spectrometry. Transition metals could be analysed in drinking water up to the microgram per litre level. Organic contaminants, e.g. chlorobenzene and phenols in alkalized sample solutions, could also be concentrated by reverse osmosis. After this precon-centration traces of the contaminants could be analysed by LC and GC after their redissolution.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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