Sciencemadness Discussion Board

Reduction in fintrate volume after subsequent filterings

RogueRose - 10-4-2018 at 03:01

So I noticed something which I didn't expect and I'm wondering if this is unique to the compound I'm working with or if this is something that is normal across most compounds. I'm working with MgCO3 that has been made from MgCl2 + NaHCO3 ( MgCl2 + 2NaHCO3 -> MgCO3 + 2NaCl) with about 2L (~170-190F) of H2O per mole of MgCl2 to ensure the bicarbonate is completely dissolved.

Everything precipitated nicely and the solution was mixed thoroughly with a high speed hand blender to make sure any lumps were broken up. The result was fine particulate with no small clumps of unreacted compounds.

So I used my buchner funnel w/ vacuum (should have invested in one earlier probably best investment yet!) to filter everything and it resulted in 8 "cakes" or "pucks" of the same size. I them added about 3L of hot water per cake, mixed with blender and filtered. This time I got just under 5 cakes of the same size and after the final washing I had just over 4 cakes.

It seemed that all the cakes were the same size and weight and I can't figure out why the difference in volume, especially when each time I blended/stirred it so thoroughly and I pulled vacuum on each cake to the same amount (about 1 drip every 3-4 seconds). There should have been no compound undissolved in the solution other than the carbonate and the salt shouldn't have been that high of a concentration the first pass but it should have been basically negligible on the final pass.

I've repeated this 2x with 2 processes of making MgCO3 and both times had the same result with a loss of volume after each filtration, especially first. The only thing I can think of that could account for this is the NaCl or that the MgCO3 somehow compacted much better after each blending/stirring.

One thing I did notice was after each washing, the solution (water/MgCO3) would settle much faster and "deeper" the less salt there was in the solution. This makes sense as the water is less dense.

Has anyone else experienced this and know what is causing it? I know I'm not loosing more than a few grams each filtration (less than ~.1%).

DrP - 10-4-2018 at 03:36

It is my guess that you could be loosing the fraction with the finest particle sizes with each wash. As the filter clogs up each time it retains particles that would normally go through.... If you take this mass and redisperse it then each time you do this you will loose more of the finer particulates each time until the filter has been clogged with product enough to retain it.

That's my 'guess' anyway.

RogueRose - 10-4-2018 at 08:32

Quote: Originally posted by DrP  
It is my guess that you could be loosing the fraction with the finest particle sizes with each wash. As the filter clogs up each time it retains particles that would normally go through.... If you take this mass and redisperse it then each time you do this you will loose more of the finer particulates each time until the filter has been clogged with product enough to retain it.

That's my 'guess' anyway.


I don't think it is any loss of filtrate as the water is crystal clear and I used the same filter paper for the whole batch - it came off the bottom of the cake totally clean after each funnel filled up. There would be a little file particulate that passes through on the first funnel full of liquid (to make a cake, I passed about 4L of liquid in a 500ml funnel, I just poured more in as it filtered down) I'd pour that 500ml back through and the fines were then caught by the carbonate in the filter, leaving totally clear water.

Boffis - 10-4-2018 at 08:43

Another effect is the "aging" of precipitates. I recently made some piperinic acid as I have described else where, when I acidified the sodium salt solution the first time I ended up with a sulphur yellow blancmange like ppt that filled the beaker. It proved difficult to filter and wash on the filter so after drying and weighing the cake I dispersed it into the same beaker full of boil water, simmered it for a while and then left it to cool. The following day the precipitate had settled to a layer only 1 cm thick, it filtered and washed easily but the dry weight of the cake was only about 15% less. My interpretation of this is that the "cooking" process allows larger equant crystals to grow which are denser an pack into a denser solid cake too. I have come across a similar effect with barium sulphate precipitates too where your only have to leave then to stand for a few days with periodic stirring and the precipitate improves in terms of density and filterability.

happyfooddance - 10-4-2018 at 09:54

I think what can be learned from this is that there are many reasons that this could be happening, and without a better analysis (weighing the dried cakes is one way, titration of a portion of the wet ones is another), no one including yourself can do anything better than guess.

Also, save all supernatant liquid until you are done with your experiment. Not much can be recovered from the sink/waste bucket.