Read Part 2 of this series on cellulose filter paper to explore how different properties can influence the filter paper choice for your application.
Paper filtration is an effective separation method, supporting various applications in industry and research. In the first part of this blog series, we covered a few initial considerations for efficient filtration of your sample.
In this part, we’ll go through several properties of cellulose filter papers that can affect your filtration efficacy and efficiency. Taking these properties into account should help you select the most appropriate filter paper for your application.
Filter paper purity
One piece of filter paper looks much like another. But the way they affect your sample might be quite different. In the lab, you cannot afford to use filter paper that introduces significant impurities into the filtrate or has any other chemical or physical impact.
Impurities can affect sensitive analytical techniques, such as flame photometry and atomic absorption. If you’re using these or similar techniques downstream of the filtration step, it makes sense to find and use the purest possible paper to minimize the chances of impurities affecting results.
Take Whatman filter papers, for example. These papers are all made from high-quality cotton linters, treated to maximize alpha cellulose and purity.
But there will always potentially be some trace impurities, no matter how high the quality of source material. So, what can we do about this? In most cases, with known and consistent levels of trace elements, it’s possible to apply a ‘blank’ correction in any analytical results, effectively cancelling out the impurities.
Should you remove organic impurities?
Non-carbohydrate organic impurities largely come from the cotton waxes present during the manufacturing process. As these are insoluble in water, removing them from filter paper requires some aggressive reagents, making the whole process a little tricky. Even then, some trace level might remain.
These aggressive reagents tend to oxidize, depolymerize, and degrade the cellulose. Hydroxyl groups, which usually help hold cellulose chains together, oxidize to aldehydes and carboxyl groups. As well as forming soluble carbohydrates, this process can weaken the filter paper.
So, there needs to be a pretty good reason to go to the trouble of removing these organic impurities.
Taking inorganic impurities into account
Filter papers can, depending on the raw materials and manufacturing processes, contain some inorganic impurities. Igniting filter paper at 900°C to leave just ash serves as a useful measure of general purity. This process burns off the cellulose and any volatile substances, leaving just the inorganic, non-volatile ones.
Qualitative filter papers have ash content in the region of 0.06%, which is good enough for general purpose filtration. In “ashless” paper, used for accurate residue measurements, processes employed by the manufacturer reduces the level of inorganic impurities. Quantitative “ashless” papers contain no more than 0.01% ash content, typically lower.
The impurities that do remain in ashless paper are usually inaccessible to normal reagents, and therefore don’t affect the filtration process. For example, there might be traces of complex silicate incorporated into the structure of the cellulose fiber itself or associated with a small proportion of carboxyl groups.
For even higher purity, acid hardened filter papers, treated to reduce inorganic impurities further, can maximize the accuracy of any residue measurement.
If your process involves measuring volatile inorganic impurities, the normal ashing process can burn off your element of interest. Maximizing recovery in these cases requires some care.
Storing and handling filter paper
Care should be taken during storage, to minimize the potential for filter papers to pick up impurities from their environment. The absorptive and hydrophilic nature of cellulose means that this type of contamination is surprisingly common and can influence sensitive measurements.
Common contaminants picked up through general storage and handling include:
- Chlorides, sulfates, mineral acids, and ammonia from volatile components in the atmosphere or lab environment
- Sodium salts, iron, and other metal oxides and salts from airborne dust
- Amino acids from human skin or perspiration
Some simple precautions can minimize these contaminants. Always storing filter papers in closed boxes will minimize contamination from airborne dust and other work in the lab. Also, handling filter papers with forceps, especially for critical trace analyses, will minimize the risk of transferring contaminants from skin or gloves.
Chemical resistance of cellulose paper
The relatively open structure of cellulose filter papers weakens when wet. Measuring and comparing this chemical resistance across papers involves determining their tensile strength when wetted with reagents of known strength.
Organic solvents tend not to make cellulose fibers swell as much as aqueous reagents, though this does vary by polarity. So if you’re using an organic solvent, an untreated filter will likely retain much of its strength and be sufficient for most applications.
Working with aqueous reagents can be a little trickier. For some applications, the wet strength of standard cellulose filters is sufficient. But if you need the filter paper to maintain its strength, for example, with a large mass of retained material, then you can benefit from selecting a type of filter paper with improved wet strength.
How to improve wet strength for filter paper
On top of adjustments and controls during the paper making process, there are three methods manufacturers can use to increase wet strength:
- Increasing paper thickness: Improves strength, but very thick paper is highly absorbent and can be difficult to wash
- Treating with mineral acid: Removes trace metals (improving purity), producing a tough paper with a hard surface, as found in Whatman Grade 50 and 540 series
- Adding a stable synthetic resin: Suited for non-critical applications as the resin can affect critical analyses
If you need a strong filter paper, selecting acid-hardened or resin wet-strengthened papers is likely to give you a final strength well within practical limits.