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Washing a precipitate chemistry12/21/2023 ![]() ![]() When dialysis is complete, the solution in the cassette is removed through an injection port. The external solution is stirred using a stir bar, and usually replaced several time during dialysis. A foam buoy, used as a stand in the photo, serves as a float so that the unit remains suspended in the external solution. The cassette is inverted and submerged in a beaker that contains the external solution. Four ports, one of which is labeled, provide a means for injecting the sample between the dialysis membranes. Two sheets of the membrane are separated by a gasket and held in place by the plastic frame. The dialysis membrane in this unit has a molecular weight cut-off of 10000 g/mol. During kidney dialysis, metabolic waste products, such as urea, uric acid, and creatinine, are removed from blood by passing it over a dialysis membrane. Dialysis frequently is used to purify proteins, hormones, and enzymes. While small species freely pass through the membrane, larger species are unable to pass. If there is a difference in a species’ concentration on the membrane’s two sides, the resulting concentration gradient provides a driving force for its diffusion across the membrane. The sample is injected into the dialysis membrane, which is sealed tightly by a gasket, and the unit is placed in a container filled with a solution with a composition different from the sample. Shows an example of a commercially available dialysis cassette. A dialysis membrane usually is made using cellulose and fashioned into tubing, bags, or cassettes. The filter unit shown here has a capacity of 150 mL.ĭialysis is another example of a separation technique in which size is used to separate the analyte and the interferent. To store the filtrate, the top half of the unit is removed and a cap placed on the lower unit. ![]() The sample is added to the upper unit and vacuum suction is used to draw the filtrate through the membrane and into the lower unit. (c) A disposable filter system with a 0.22 μm cellulose acetate membrane filter. The filtrate is collected in a test tube or other suitable container. Samples are placed in the syringe and pushed through the filter. In the photo on the left, the filter is attached to a syringe. The photo on the right shows the membrane filter in its casing. Upon spinning the unit at \(2000 \times g-5000 \times g\), the filtrate collects in the bottom reservoir and the retentate remains in the upper reservoir. The sample is placed in the upper reservoir and the unit is placed in a centrifuge. The membrane has a nominal molecular weight cut-off of \(1 \times 10^6\) g/mol. (a) A centrifugal filter for concentrating and desalting macromolecular solutions. Examples of three types of membrane filters for separating analytes and interferents. For applications of gravity filtration and suction filtration in gravimetric methods of analysis, see Chapter 8. Provides information about three types of membrane filters. ![]() A membrane filter is the method of choice for particulates that are too small to be retained by filter paper. Gravity filtration and suction filtration using filter paper are techniques with which you should already be familiar. The solution that passes through the filter is called the filtrate, and the material retained by the filter is the retentate. In a filtration we separate a particulate interferent from soluble analytes using a filter with a pore size that will retain the interferent. Examples of size-based separations include filtration, dialysis, and size-exclusion. To accomplish the separation we use a porous medium through which only the analyte or the interferent can pass. Size is the simplest physical property we can exploit in a separation. ![]()
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