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| Purification of Colloidal Sols |
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| The colloidal solutions prepared by various methods usually contain electrolytes and other soluble substances as impurities. These impurities if not removed can destabilize the sols. Impurities are removed by the following methods: |
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| 1) Dialysis |
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| 2) Ultra-filtration |
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| 3) Ultra-Centrifugation. |
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| Removal of soluble impurities from sols by the use of semipermeable membrane is known as dialysis. |
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| Solutes present in a true solution can pass through a semipermeable membrane such as parchment paper or cellophane. However, sol particles cannot pass through such membranes. When a bag made up of such a membrane is filled with the colloidal sol and then placed in fresh water, the soluble particles such as electrolytes pass through the membrane and go into the water leaving behind the colloidal sol. |
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| The movement of the ions across the membrane can be enhanced by applying electric potential across two electrodes. This is known as electrodialysis. |
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| fig 7.13 - An apparatus for electrodialysis |
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| The most important application of dialysis is in the purification of blood with the aid of an artificial kidney machine. The dialysis membrane permits excess ions and waste products like urea molecules to pass through and does not allow the colloidal particles of haemoglobin to pass through. |
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| In this method, colloidal sols are filtered through graded filters called ultra-filtrers. These filters allow the electrolytes to pass through but not the colloidal particles. These ultra-filters are made from ordinary filter papers by impregnating them with colloidal particles. Filtration is usually carried out by either by applying pressure or by the use of suction. |
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| Ultra Centrifugation is used to separate colloidal particles from the impurities by centrifuging the colloidal sol. Centrifugation is carried out in a centrifuging machine where the tube containing the colloidal sol is rotated at very high speeds. The colloidal particles settle down at the bottom of the tube and the impurities remain in the solution called centrifugate. The settled colloidal particles are then mixed with an appropriate dispersing medium to regenerate the sol. |
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