Sciencemadness Discussion Board

Increase Sintered Glass ("Frit") Pore Size

Petit Homme - 15-9-2025 at 02:13

Hello,

I have a column with a sintered glass disk ("Frit") at the bottom. It's supposed to be the smallest pore size but despite having never been used not a drop of distilled water (alone) gets through with 99% vacuum applied.

I remember hearing or reading about someone who used hot alkaline solution (like either hot/molten NaOH or a hot NaOH solution) to increase the pore size of such filters (in addition to it being a savage solution to unclog clogged frits).

If possible to do so in a simple efficient way I'd like to make my column functional and "unclog" the neither-really-clogged-by-use-nor-ever-used-at-all column to make it the equivalent of a column with medium to large pore size frit (without damaging the unsintered glass parts, to a reasonable degree, if possible).

Has anyone heard about this?

Does the normal glass get badly hurt in the process or is it mainly the frit taking the hit?

Does anyone have experience with this who could outline the best way to go about doing so to achieve the aforementioned desired result?

[Edited on 15-9-2025 by Petit Homme]

Sulaiman - 15-9-2025 at 03:11

I doubt that a column would have a frit so fine that water can not pass,
so I guess that ;
. you only put a little water on the frit,
allowing air to pass atound the water
.. the surface is hydrophobic, eg oily
... the filter is blocked... can air pass through with vacuum ?

PS If you really have 99% vacuum (7.6mm Hg or 1kPa absolute)
then your water will freeze :)

[Edited on 15-9-2025 by Sulaiman]

Keras - 15-9-2025 at 04:24

Quote: Originally posted by Sulaiman  

PS If you really have 99% vacuum (7.6mm Hg or 1kPa absolute)
then your water will freeze :)
[Edited on 15-9-2025 by Sulaiman]


Rather boil.
But it seems to me very unlikely that a frit even grade 5 blocks water. 4 does not (I have one I use quite often).
There’s probably something clogging. You can try two things:

1. Pass a high boiling solvent such as diethylene glycol butyl ether through it. If there are solid things like paraffin or such clogging the pores, they should melt away.

2. Use (with care) piranha solution.

[Edited on 15-9-2025 by Keras]

Petit Homme - 15-9-2025 at 04:27

Quote: Originally posted by Sulaiman  
I doubt that a column would have a frit so fine that water can not pass,
so I guess that ;
. you only put a little water on the frit,
allowing air to pass atound the water
.. the surface is hydrophobic, eg oily
... the filter is blocked... can air pass through with vacuum ?

PS If you really have 99% vacuum (7.6mm Hg or 1kPa absolute)
then your water will freeze :)

[Edited on 15-9-2025 by Sulaiman]



The vacuum is on the receiver side. I can be wrong, but wouldn't it evaporate rather than freeze, though, at room temperature?

My understanding is that the filter is blocked. I don't see why it would be oily (it at least gives no trace of that in the water). Is there any rationale for that, as a common glass manufacturing step involved in the manufacturing of glass columns with a sintered glass disk?

I filled the entire column with water, so no air is passing through (except, perhaps, minute quantities of air, initially) from what may have been in the pores.

I don't really have a reliable way to measure if minute quantities of air can pass through (I could leave it under vacuum and observe the evolution on the pressure gauge, but apart from a fair flow of air through the frit, which I don't think I'd have with a smallest pore size frit, there is no simple/obvious way I could tell a tiny leak comes from the frit rather than from many other sources of leaks of air in mintue amounts, such as joints).

My intention (and focus) is on finding a simple/quick/efficient solution to transform what I have into a column with a medium/fast flow/pore size, and I'm thus quite interested to hear about people having experience with the NaOH method.

jackchem2001 - 15-9-2025 at 04:28

You could put some concentrated alkali solution (molten alkali sounds like a terrible idea) above the frit and pull a mild vacuum. Use the resistance to flow to see how progressed the alkali corrosion is

Petit Homme - 15-9-2025 at 04:43

Quote: Originally posted by Keras  
Quote: Originally posted by Sulaiman  

PS If you really have 99% vacuum (7.6mm Hg or 1kPa absolute)
then your water will freeze :)
[Edited on 15-9-2025 by Sulaiman]


Rather boil.
But it seems to me very unlikely that a frit even grade 5 blocks water. 4 does not (I have one I use quite often).
There’s probably something clogging. You can try two things:

1. Pass a high boiling solvent such as diethylene glycol butyl ether through it. If there are solid things like paraffin or such clogging the pores, they should melt away.

2. Use (with care) piranha solution.

[Edited on 15-9-2025 by Keras]



Thank you for contributing.

The solutions you mention seem good to address (mostly) the possibility of organics / non-polar stuff clogging the pores, but as the column has never been used I don't know why that would likely be the case, per my limited understanding. I rather think it might be a manufacturing defect.

In any case, I want a solution that does what the title of this topic describes, i.e. to increase the pore size of the sintered glass disk. If NaOH is a good solution to achieve this, then I believe it would solve the problem, no matter what the problem is. First because it increases the pore size and I want a reasonably fast flow (and I don't know of another solution that would increase the pore size). Second because I don't need to understand what is clogging the disk so much as to transform it into a functional column with medium to average flow.

Since the column is new (was never used), whatever is the reason for the water not passing through, I think it's rather unlikely there is anything I would need to worry about after hot NaOH and then distilled water flushing through the frit, hence not needing to understand what is clogging it as much as to simply increase the pore size no matter what the reason is for the water currently not passing through, with the use of NaOH if it indeed would work and I receive good advice on it, preferably by someone who has successful experience with the method.

Petit Homme - 15-9-2025 at 04:46

Quote: Originally posted by jackchem2001  
You could put some concentrated alkali solution (molten alkali sounds like a terrible idea) above the frit and pull a mild vacuum. Use the resistance to flow to see how progressed the alkali corrosion is


That's the kind of thing I was thinking about going for. Do you have practical experience with this? Do you know that a concentrated aqueous solution of NaOH at say 70C, for instance, efficiently increases the pore size when flushed through a frit?

bnull - 15-9-2025 at 05:28

Fill a beaker with water, dip the lower end of the column until the tip is about halfway down the beaker, then apply vacuum on the upper end. If the column is clogged, there's a huge chance of stuff being on the top side of the frit. Larger diameter, open to atmosphere, no tap to protect etc.

Whatever you have that uses a glass frit, if it is clogged, try sucking water from both sides. There are doors that open inward and doors that open outward. The same applies to frit.

Texium - 15-9-2025 at 07:23

I would take bnull's suggestion before trying anything with alkali that can't be undone. In fact, I would caution against trying an alkali method at all, and would suggest you instead return the column to wherever you bought it if it's truly defective.

I have no practical experience using alkali on frits because I was always warned that it was a terrible idea to clean a frit with strong alkali, more likely to cause the frit to become loose and fall out of its seat, or clog the frit even more by fusing the holes shut. Proceed at your own risk if you go that way, and know that you will have no chance of returning it after you start down that path.

fx-991ex - 15-9-2025 at 07:34

with the alkali what i think will happen is once a passage/hole is cleared/cleaned/open, all the solution will go through this one enlarging it and just making a big hole rendering the frit totally useless.

The sln will follow the path of least resistance, which will be the first passage/hole to open, it will keep going this path enlargin it, not making new ones.

Id try cleaning with solvent, then piranna, or a mild cold NaOH sln or just trash it and get a new one if this dont work.

Usually piranha do the trick, i mix it on the frit and it get warm and is very effective(and dangerous).

[Edited on 15-9-2025 by fx-991ex]

Sulaiman - 15-9-2025 at 16:13

I've not had a frit in a column blocked, but I have had fritted filters blocked or contaminated.
I would use dilute peroxymonosulfuric acid, aka Caro's acid, like diluted piranha solution, using dilute sulphuric acid.
because it often works and it will not harm the frit,
patience is required, eg half fill your column with Caro's acid, clamp, leave for hours to days.
(with a vessel to collect the acid below the column)
maybe a little genuine piranha solution as mentioned above will work better.

if that fails I'd try a little conc. nitric acid
(which, incidentally, makes glass slightly hydrophobic)
and lastly dilute NaOH, which may ruin your frit
(and make it hydrophilic)

PS. OOPS!...I miscalculated/misguessed earlier, it requires 99.4% vacuum to guarantee ice
(triple point of water 611.657 Pa @ 0.01oC)

Once the frit is clean then you could , I suppose, run hot sodium hydroxide solution through the frit if you still want to enlarge the pore size.
If clean/working, but too fine pore size, then I see two choices:
. try to enlarge the pore size then buy a new column if you fail, or
.. buy a new fritted column and have the choice of pore sizes in the future.
..,...............
I'm fairly sure that I have some nos, still in original cardboard tubes (>10yrs. old) quickfit columns with fritted disks,
I've never had a use for them,
I'll find them and test them when I get home (about 2 weeks maybe)

[Edited on 16-9-2025 by Sulaiman]