Hacker News Reader

Comment by quackerhacker | Hacker News Reader

Makes me wonder what Brita is actually filtering out.

OK, I learned this in A level (high school) chemistry, but I can't seem to find a wikipedia article about it.

They use clay beads, which are an aluminium mesh with H and OH groups weakly bonded to it (a weak ionic bond like this is called a "Ligand bond"[1]).

Firstly, it filters out bacteria and "bits" because of the porosity of the mesh (works like pushing current through a complicated network).

However, additionally it filters out non-H-or-OH ions from the water, which have a stronger Ligand bond with the Al mesh. (Obviously you eventually run out of H and OH ions in the network, as they're all replaced, which is when you need to change your filter.)

As such, we used clay-filtered tap water as 'distilled' water in all our experiments, because the worst it contains is bacteria and dissolved salts. I can't speak for the Coca Cola though, that's weird. I'm sure organic molecules would be small enough to get between the beads but too heavy/not ionic enough to bond to the mesh, or something.

[1] https://en.wikipedia.org/wiki/Ligand

The standard Brita pitcher filter is a mix of granular activated carbon (from coconuts), and the ion exchange resin you talk about.

Thanks, I didn't know that! What does the activated carbon do?

It's chemically inert, but has a very fine porous structure (thus 'activated'), so it works as a mechanical filter that adsorbs anything large enough, like bacteria and even very large molecules.

Activated carbon, also known as activated charcoal, was the useful ingredient in "universal antidotes". Although the mixture (see link) is no longer used, activated charcoal still is.

http://www.medilexicon.com/medicaldictionary.php?t=4805

It's chemically inert [...] so it works as a mechanical filter that adsorbs [...]

Because adsorption is a consequence of surface energy and weak bonding, I believe it's considered to be a chemical process, not a mechanical one.

Right, the distinction is fine here.

Adsorption is due to weak bonds that form between molecules of a surface and a fluid. AFAIK this bond is not a 'new' chemical bond since it does not break / replace existing chemical bonds in both the adsorbed molecule and surface molecules.

Rather, in the case of carbon at least, the 'bonds' are the van der Waals forces.

The coloring agent, presumably.

Which is caramel, presumably

looks to be "caramel color" [1] whatever that is

http://www.bevreview.com/wp-content/image_kosherforpassoverc...

Looks like it's caramel

https://en.wikipedia.org/wiki/Caramel_color

That is, sugars that underwent Caramelization https://en.wikipedia.org/wiki/Caramelization

Reply on news.ycombinator.com