Take a look at part 6 of our series for some tips on what to look out for with your precipitate to optimize filtration processes. I’ll look at precipitate creep, and give you some hints for effective washing, and when to use filter aids.
In our last few blogs in the Paper Filtration 101 series, I concentrated on the filter paper: how you can improve filtration protocols with some practical considerations and tips and tricks for paper folding and funnel choices. But there are also some useful points to consider relating to the precipitate.
This blog focuses on the sample precipitate and the effect it has on some of the choices you might make to optimize filtration. In particular, I’ll talk about using filter aids, precipitate creep—what it is and what you can do about it—as well as some advice on washing.
Dealing with difficult precipitate
Certain types of precipitate might initially filter rapidly but then settle into a semi-impermeable jelly on the filter paper. It then becomes increasingly difficult to penetrate through this jelly-like layer with each wash.
Sometimes, this effect is due to hydrated aggregates that are separated/free floating in suspension, but then collapse into a film on the paper.
It can be possible to overcome this issue through washing by decanting, but this type of precipitate often has an affinity for glassware. So, it’s usually easier if you get the precipitate into a filter paper cone as early as possible for washing in situ.
Difficult-to-filter samples like these can substantially add to filtration time, especially when handling multiple samples. So, it’s worth taking advantage of all time-saving options.
Filter aids to aid filtration
Using filter aids is one option for minimizing flow rate issues with challenging suspensions like aggregated jelly-like layers. These filter aids are inert materials, a typical example being ashless filter paper clippings, though other materials like diatomaceous earth or perlite are also used.
In the case of cellulose filter aids, these can either be added directly to the suspension to be filtered or used to create a thick retentive prefilter layer prepared in situ from mechanically disintegrated ashless filter paper clippings.
These types of filter aid are widely used to enhance filtration speed. The more open structure of the filter mass also contributes to these time-savings (Fig 1 ).
Fig 1. Effect of having a filter aid as a pre-coat and in suspension on filtration ease and efficiency.
What about precipitate creep?
In some cases, precipitate can creep up the sides of the filter paper cone and become stuck. Any downstream quantitative analysis you perform might not then take this into account, affecting the accuracy of results.
If there is already a tendency for precipitate creep, the effect can be made worse in some situations:
- Surface tension effects might encourage precipitate creep if the folds of the filter paper are not flattened properly against the funnel.
- During the transfer of precipitate to the filter paper cone, or as a result of washing steps, films of liquid can sometimes rise too high above the level of liquid in the filter paper cone, resulting in the loss of traces of precipitate.
So, making sure that the filter paper cone is flat against the funnel and keeping an eye on the liquid level in the funnel are two ways to help minimize precipitate creep.
Cleaning up the wash
There are a few factors that affect washing in any filtration step:
- Solubility of a precipitate (and/or it’s tendency to decompose).
- Specific surface area of the precipitate particles.
- Adsorption of impurities during precipitate formation.
- Diminishing effectiveness of each successive wash.
The surface area of the precipitate can affect the rate of filtration. Suspensions with a higher proportion of larger particulate matter filter more slowly than those with smaller particles and are more likely to clog the filter paper. Sometimes, impurities can be adsorbed to the surface of a precipitate and become weakly bound, increasing the surface area and affecting the rate of filtration.
It’s useful to match the filter paper grade to the sample precipitate and perform extra washing steps to maximize filtration efficiency.
In general, completing numerous small washes is the most efficient tactic, making sure that the precipitate is left to drain thoroughly through the filter media after each wash. Though there might be a decrease in effectiveness with each, it’s still useful to perform several successive washes for complete filtration.
The filter paper works most efficiently when pliable and ‘wettable’, which gives it the best fit possible in the funnel. Using a comparatively low wet-strength paper and carrying out a preliminary wetting step can help achieve this good fit.
However, there are circumstances where a higher strength hardened filter paper is more appropriate. For example, when there are heavy, bulky precipitates involved, there is a need to remove the recovered precipitate by scraping, or hot, corrosive suspensions are being filtered.
It’s worth keeping an eye on jets of water during your washing. They can easily damage the filter paper, with high pressure jets piercing holes and affecting the filtration accuracy and efficiency. Managing the power of the water stream and directing the flow at an angle, rather than perpendicular to the surface, can help minimize any damage to the filter paper.
During washing, it can also help to move the stream smoothly to minimize splashing. Where desirable, it’s also possible to maneuver the stream such that you bring the bulk of precipitate into suspension again. Though take care to avoid overfilling the filter paper cone at the same time!
For more help and advice on filtration workflows, explore the other blogs in our Filtration 101 series , or contact GE Healthcare Life Sciences Support . You can also use our filter selector tool to help you find the right filter for your needs.