Sciencemadness Discussion Board

how slow is diffusion in liquids ?

Sulaiman - 2-1-2017 at 07:07

Background: I have started another batch of EtOH as feedstock for distillation experiments,
The main batch is on the left, 1kg unrefined sugar, 50g rasins, 20g yeast nutrient, 2g GV4 yeast, originally 5.6l total.
7l_2l.jpg - 330kB
The brown colour is from the cane sugar.

I intend to add more sugar (refined sucrose) in stages as it is consumed by fermentation,
so I put 1 kg sucrose in a 2l flask, with boiled tap water to dissolve it at room temperature, 1.4 litres total volume,
I then topped off the 2l flask with liquid from the now 1-day fermented batch.

The dense saturated sucrose solution at the bottom and the fermenting layer at the top formed two well defined layers.
I expected the two water-based layers to diffuse into each other,
and the high sugar concentration would stop the yeast working.

After one day the layers are quite distinct but with the top c1cm of the lower layer showing signs of diffusion, but not a lot,
and a thin darker layer at the bottom of the top layer. (sediment ?)
The upper layer is still bubbling away, quite vigorously.

TwoLayers.jpg - 255kB

The upper 'fermentation' layer being separated from the lower 'feedstock' solution means that the yeast is not exposed to high sugar levels.
This may be a method of achieving maximum ABV.
As I only want the sugar in the 2l flask to add to the main batch
I shall not be persuing this possibility,
but I mention it in case any frequent brewers may want to investigate.

Question : Why have the two layers not diffused into each other ?

even gentle 'swirling' of the flask caused no observable change.

[Edited on 2-1-2017 by Sulaiman]

Tsjerk - 2-1-2017 at 07:08

As fast as the molecules diffusing.

Edit; Ok, I can see the actual question now, I only saw the question "how slow is diffusion in liquid"

[Edited on 2-1-2017 by Tsjerk]

Fulmen - 2-1-2017 at 07:38

It can be slow all right. I have collected brine from the bottom of a lake, perhaps 80-100m deep. It's a remnant of the last ice age, a lake of salt water at the bottom of a fresh water lake. And 10'000 years later it still hasn't disappeared.

Sulaiman - 2-1-2017 at 07:49

I shall not wait for results then :P

How and why did you take the sample, and did you find anything interresting ?

[Edited on 2-1-2017 by Sulaiman]

careysub - 2-1-2017 at 08:40

Quote: Originally posted by Fulmen  
It can be slow all right. I have collected brine from the bottom of a lake, perhaps 80-100m deep. It's a remnant of the last ice age, a lake of salt water at the bottom of a fresh water lake. And 10'000 years later it still hasn't disappeared.

Here is a paper studying the times for diffusion to erase the stratification in five saline lakes, and the calculated effective diffusion coefficient (ranges from 0.6 to 22 10^-9 m^2 s^-1). The lakes studied had calculated times for 95% removal of the initial salinity difference from 5,060 years to 34,800 years.

And here is a paper on sucrose diffusion specifically that finds diffusion coefficients in ranging from about 10^-9 m^2 s^-1 (very low sucrose concentration) to 10^-12 (66.6% sucrose by weight) to 10^-17 (for 90% sucrose by weight). This last would be for something like a partially dehydrated honey with a glassy structure.

So if you dump a layer of sugar into a container, if it makes a very concentrated solution at the bottom it might have a very slow diffusion rate.

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Fulmen - 2-1-2017 at 10:27

We were out collecting samples from a drinking water reservoir, so we did it simply because it was there and we had the equipment. A dingy, a hand winch and a standard water sampler is all you need. Pretty amazing to see something like that, few are aware of such phenomenons and even fewer have seen it with their own eyes.

Sulaiman - 2-1-2017 at 14:09

careysub, thanks for the links, to be honest - information overload!
I ended up skimming through to the conclusions :(
my only excuse is that I have a lazy brain. :D
nevertheless very interesting.

I get more of an idea now why Magpie quotes Nicoderm

"The single most important condition for a successful synthesis is good mixing"

(one day I may have enough experience to know it)
I had always assumed, for no reason, that diffusion was much faster than it is ... by many orders of magnitude :o
... still learning

Fulmen ... I bet you wish you'd kept samples now (that I mention it)
did you taste the ancient brine ?

j_sum1 - 2-1-2017 at 15:32

There is a nice little comparative experiment that you can do on this.

Mix up a phenolphthalein solution and separate into two beakers. Drop a Pasteur pipette into each an allow some time for the liquid to become still. Mix up equal concentrations of KOH and NaOH and add an equal volume of each to the pasteur pipettes. You can watch the diffusion as it happens spreading from the tip of the pipettes. And of course the KOH is slower.

AJKOER - 2-1-2017 at 16:53

I would try placing your compounds in thick clear glass and allowing direct solar light exposure.

Basically, a passive approach to adding energy to the system.

The thick glass is to limit harmful effects of light and still may not work for the likes of fermentation experiments.

Sulaiman - 2-1-2017 at 17:46

I have to experiment in my bedroom at the moment, so with the added heat of fermentation,
keeping the temperature below 25 C (a target that I set based on minimal research) sometimes needs intervention.
(turning off the heat and opening the window :)

I deliberately shaded the fermentation - just because I thought sunlight might be 'bad' (I don't know)
I assume that it is just for thermal input, not photo-chemical ?

I am only fermenting for chemistry purposes, I hope to get enough EtOH from this batch to learn the art of distilling.
After that I will probably dry the azeotrope
(of course I can get it to azeotropic .. nearly/maybe, with the wind behind me).

I don't drink now so fermentations will be few and far between
but I used to consider myself quite good at home made wines
elderberry, and gooseberry were my favourites.

P.S. sugar-wash to EtOH is really cheap chemistry :D

[Edited on 3-1-2017 by Sulaiman]

Fulmen - 3-1-2017 at 00:57

Sulamain: Smelling it was more than enough. Not exactly tap water quality...