what is the purpose of impregnating filter paper with colloidion solution?

Dear Student,
Ultrafilter papers are prepared by the impregnating it with a colloidal solution. In this way, the original pore size of filter paper gets reduced and colloidal​​​​ particles can be retained whereas impurities pass through.Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles. Colloidal particles can pass through ordinary filter paper because the pores are too large. However, the pores of filter paper can be reduced in size by impregnating with collodion solution to stop the flow of colloidal particles. The usual collodion is a 4% solution of nitrocellulose in a mixture of alcohol and ether. An ultra-filter paper may be prepared by soaking the filter paper in a collodion solution, hardening by formaldehyde and then finally drying it. Thus, by using ultra-filter paper, the colloidal particles are separated from rest of the materials.
Regards.

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