I'm having a recurrent problem with some aqueous solutions I'm preparing.
The last ones were a 0.02g/ml boric acid solution and a 0,08g/ml urea solution, all made with distilled water and stored inside clean volumetric flasks.
Just after preparing the solutions, they look quite clean, even looking them in close up I won't see anything but a few thin filaments floating in it, most of them around 0.1 mm in length and much thinner than a hair, plus one or two small grains of dust. And all of them are only visible (and still barely visible) thanks to the strong lensing effect of the volumetric flask.
My estimates are that the volume of those impurities is less then one billionth of the total solution volume in the beginning (as there are usually just a few dozens of filaments and they are absolutely microscopic)
However, the amount of impurities seems to grow with time, and it doesn't only include tiny filaments and dust grains.
I'd expect more small impurities to appear as I insert pipettes into the flasks to take the solutions from them, but with few uses a great deal of stuff appear into it and it doesn't make sense all of that came from the pipettes, as I always clean them before inserting inside the flask (and I don't dry them with paper to avoid fibers).
Some of these additional impurities look like small transparent plaques, and there are no impurities of that shape in the beginning (plus they're bigger than a grain of dust).
The solution never becomes so dirty that it microscopically change, but the amount of small and transparent impurities grow a lot and it's quite easy to see them floating on close inspection.
In one week their estimated volume proportion go from 1 ppb to 100 ppb, and when those plaques start appearing they easily go into the ppm order.
I'm suspecting of some sort of algae growing, but shouldn't the boric acid keep the solution sterile? Also, although urea serve as a N source, there's no phosphor in there to feed the algae. Only if there are trace amounts of phosphor that can feed a little bit of algae, but I find this improbable, all my compounds are high purity (> 99.9%).
What you think that might be? Are algae a common problem in labs? Or is that level of impurity normal somehow?
Ps.: My distilled water tank and my distiller has both been recently cleaned by me, they were thoroughly washed, disinfected and rinsed multiple times until I used it again, but the problem didn't stop.