In Part 5 of our Paper filtration 101 series, I discuss some practical considerations for successful paper filtration, especially for quantitative analysis.
The filter paper: practical considerations
If you’ve read Parts 1-4 of our Paper Filtration 101 series, then you have had a great introduction to the basics!
In this blog, I’ll focus specifically on using cellulose filter papers, with some more practical considerations and advice that’s particularly relevant for those of you performing quantitative methods.
If you missed our previous posts on filter papers, you can always go back and read up on the properties of filter papers and the options for folding filter paper cones. If you’re specifically looking for guidance on filter papers for quantitative analysis, then read on.
What else should I consider for quantitative analysis?
In addition to thinking about the different filter paper options, their properties, the best method for folding a filter paper, and which funnel to use, there are a few last points to consider, including:
- Size of filter paper circle
- Preliminary wetting
- Filtering techniques
Each of these factors can contribute to the success or failure of quantitative analyses, so a little extra consideration here is time well spent.
Size of filter paper circle
The size of the filter paper circle is, unsurprisingly, one of the first and most vital considerations in quantitative procedures. Ash weight and volume of precipitate are the two factors at play here.
Ideally, the filtration surface area should balance against the overall ash weight per circle, if ash is of importance. The filter paper ought not to be so large as to project over the rim of the funnel or so small as to be overloaded with precipitate.
For quantitative work that involves ignition, or small amounts of sample, the smaller the circle the better. As a general rule, you should look to use a filter paper circle that’s just large enough so that the precipitate doesn’t occupy more than half the volume of the cone (preferably less).
Preliminary wetting of the filter paper circle
While it’s not often thought necessary, a preliminary wetting of the filter paper after placing it in the funnel is good practice and serves two purposes.
First, wetting can help with bedding the paper properly into the funnel, creating a good seal. I talked about achieving the perfect seal in Part 4 of this series.
Second, wetting will help remove any minute traces of inorganic material, such as chlorides or ammonium salts, that could affect quantitation. Wetting will also help remove any loose fibers created from handling and manipulation of the paper.
Where experimental conditions allow, bedding the paper into the funnel with a little distilled water is preferable. If you need to use ‘dry’ organic solvents, the paper isn’t wetted in the sense that it will adhere to the funnel, but it’s still good practice.
Filtering techniques: washing and decanting
The choice of filtration technique, particularly the method of transferring or withholding the precipitate from the paper, depends on the final destination and use of the precipitate.
Your objective might be to collect the precipitate on the paper for subsequent ignition, or it might merely be a case of separating the solid and liquid phases so that one or both can be analyzed at a later stage.
Once you have this overall objective, you can consider the properties of the precipitate itself, which will help you adjust your filtering technique.
If the precipitate tends to cling to glassware, it can help to transfer to the filter paper quickly and wash in situ. Bear in mind that this will likely mean adding to the overall filtration time.
When working with an easily separated precipitate, or separating liquids from solids, you can reduce filtration time and improve washing efficiency by decanting. This is particularly suited to coarse, crystalline, or curdy precipitates that settle rapidly, leaving a distinct layer of supernatant for decanting.
Under normal circumstances, only a small fraction of the precipitate finds its way to the paper in the early stages—with the bulk transferring by the time washing is complete.
Filtering techniques: more options
Other adjustments to your filtering technique might include filtering hot when possible. You might also have various useful pieces of equipment in your lab that could improve your workflow.
Glass rods, for example, are a simple but useful tool for decanting. They help break the surface tension between supernatant and glassware when pouring, reducing the risk of sample loss and therefore experimental error.