4.3
Filtration
Filtration is a physical separation process that involves passing a suspension through a porous medium to separate solids from fluids. During filtrati…
Filtration is the physical separation of solids from a fluid using a filtration medium.
The filtration medium is chosen depending on the purpose of filtration, quantity, and particle size of the solid.
In general, filtration efficiency increases with particle size.
In simple filtration, the precipitate is first allowed to settle, and the mother liquor is decanted down a glass rod into a funnel fitted with filter paper.
The precipitate is then transferred to the filter paper using a rubber policeman.
Ashless filter paper can be ignited to recover the precipitate, when weighing is required.
Alternatively, a sintered-glass funnel fixed to a Buchner flask, which has a sidearm that can be connected to a vacuum pump, is used to trap the precipitate.
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Q1: What is filtration and how does it separate solids from liquids?
Filtration is a physical separation process that passes a suspension through a porous medium to separate solids from fluids. Solids collect on the porous medium while liquids, called the filtrate, pass through. The filtration medium is selected based on the filtration purpose, quantity, and nature of the precipitate to ensure effective separation.
Q2: How does particle size affect filtration efficiency?
Filtration efficiency increases with particle size. Larger particles are retained more effectively on the filtration medium, while smaller particles may pass through more easily. The choice of filtration medium depends on the particle size of the solid being separated to optimize efficiency.
Q3: What are the key properties of a suitable filtering medium?
A suitable filtering medium must be inert, mechanically strong, nonabsorbent toward dissolved materials, and permissive toward rapid filtration. These criteria ensure the medium does not react with the sample, maintains structural integrity during use, and allows efficient liquid passage while retaining solids.
Q4: Why is ashless filter paper preferred in gravimetric analysis?
Ashless filter paper is preferred because regular filter paper is hygroscopic and can absorb moisture, affecting mass measurements. Ashless filter paper can be ignited to recover the precipitate without leaving residue, making it ideal for gravimetric procedures that require accurate weighing of collected solids.
Q5: What is the difference between simple filtration and suction filtration?
Simple filtration uses gravity to pass liquid through filter paper in a funnel, with the precipitate settling first and mother liquor decanted. Suction filtration uses a Buchner flask connected to a vacuum pump with a sintered-glass funnel to trap the precipitate more rapidly and efficiently.
Q6: How do you recover and weigh solids collected in a crucible during suction filtration?
The solid is transferred to a glass or silica crucible containing a porous glass disc, then fitted into a Buchner flask for suction filtration. After filtration, the crucible is dried and weighed directly. The weight difference of the crucible before and after filtration gives the mass of the collected solid.
Q7: What is the purpose of using a rubber policeman during filtration?
A rubber policeman is used to dislodge any solid that adheres to the glass rod or beaker during the filtration process. This ensures complete transfer of the precipitate to the filter paper, preventing loss of sample and improving the accuracy of separation and subsequent gravimetric analysis. Filtration often precedes precipitation and co-precipitation procedures.