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Objectives This tutorial aims to familiarize you with the following principles of crucible filtration:
Filtration is a laboratory technique that separates solid matter from liquid matter in a mixture of the two. The separation is achieved by passing the mixture through a membrane with small holes, known as pores. The liquid components of the mixture flow freely through the membrane while the solid matter cannot pass through the pores and is trapped on the surface of the membrane. On the molecular level, the molecules in the liquid are not arranged into a preset shape and are free to flow one by one through the pores of the membrane. The molecules in the solid, however, are clustered together into larger particles with a constrained size and shape. As a result, they cannot fit into the membrane's pores. The most common types of membranes used in the laboratory are filter paper and sintered glass crucibles. In filter paper, spaces between cellulose fibers form a matrix of pores. In sintered glass crucibles, the same effect is achieved by glass fibers that are randomly arranged within an amorphous (randomly organized on the microscopic level) disk of solid glass.
Manufacturing techniques allow the pore sizes in both filter paper and sintered glass crucibles to be varied. The filter can be as coarse as a coffee filter, which allows the occasional coffee ground to pass through, or as fine as a molecular filter used in biochemical applications, which traps proteins that are 0.0005 microns in size.
Since the membrane's pores are usually small and slow down the throughput of liquid, some form of force is necessary to move the liquid through the membrane. In the simplest kind of filtration, this force is simply gravity: the solid/liquid mixture is poured on top of the membrane and the liquid is pulled through the filter by its own weight. Alternatively, vacuum filtration is performed, where low air pressure on the outflow side of the filter is used to pull the liquid through. Question 1 Would a membrane with large pores or one with smaller pores be more likely to necessitate a vacuum filtration? Would a more viscous or a less viscous liquid phase in a mixture be more likely to require a vacuum filtration? Preparative Vs. Quantitative Filtration Filtrations can be performed preparatively or quantitatively. In a preparative filtration, it is impossible to separate (recover) every single particle of a solid from a liquid mixture. These filtrations are suitable for qualitative analysis, where the identity of a substance, rather than its precise amount, is being determined. For example, when analyzing a water sample for the presence of heavy metals by selective precipitation, preparative filtration suffices to isolate a particular metal ion precipitate.
Quantitative filtrations are performed when the amount of a solid sample in a mixture needs to be known to a very high level of accuracy. This is the case in your present experiment, where your task is to quantitatively determine the lead concentration in a water sample by accurately weighing a filtered precipitate of lead sulfate. When to Use a Sintered Glass Crucible in a Filtration If a quantitative analysis is performed on a solid that is filtered from a mixture with a liquid, the filtration must be carried out with a sintered glass crucible. Two requirements must be fulfilled for such an analysis. First of all, the filtration needs to be quantitative, trapping all of the solid particles on the membrane. Secondly, after separation but prior to analytical weighing, it is necessary to thoroughly dry the solid, usually by exposing it to elevated temperature in a laboratory oven. The high temperature causes the trace liquid particles in the solid to evaporate, leaving behind a highly pure sample of solid that is ready to be weighed accurately.
A filter paper is not suitable for this purpose since it is not heat-stable and will begin to decompose in the oven. A tempting way to circumvent this problem would be to scrape the wet solid from the filter paper into a tared Pyrex beaker and dry the solid in the beaker. However, it is impossible to quantitatively transfer the wet solid from the filter paper to the beaker (some will invariably remain on the filter paper even with a carefully executed transfer). Thus, the filtration, drying and weighing must be completed in a single vessel, and this is where the sintered glass crucible comes in. The sintered glass crucible, with its small pore size, is suitable for quantitative filtration. Its Pyrex glass construction makes it heat-stable up to 550°C. Thus, it is the ideal membrane for quantitative filtration.
A sintered glass crucible looks like a funnel that has a thick flat membrane of opaque glass placed across the bottom end. This disk is called a frit, and contains millions of microscopic pores that allow liquid to flow through but quantitatively trap solid particles. A vacuum filtration is necessary when using sintered glass crucibles since the membrane is substantially thicker than a filter paper, and its pores are very small. The Macromedia Flash walk-through presented below will show you how to properly use a sintered glass crucible in a filtration. Question 2 In the tutorial above, you are instructed not to touch the sintered glass crucible with your bare hands after it has been weighed for the first time. If you were to disregard those instructions and leave oil behind on the crucible, would the calculated mass of your collected filtered end product be too large or too small? Summary This tutorial on quantitative filtration with a sintered glass crucible has presented the following topics:
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