Info Wanted On Reverse Osmosis (r.o.) Water

[ArauraDiscus]

ok, but thats considering you don't add anything to it at all and use pure ro water and let it sit by itsef for days... Thats an ambigious expectation.

 

[Bignose]

Well, whether it is 7.0 or 6.998, it definitely isn't 4 or 5

It takes some pretty beefy equipment to get a reading to 3 significant figures, all the stuff I've ever used in the lab only reports tenths, and those probes were usually at least a hundred dollars. I can only imagine how expensive a probe that measures thousandths must be.

I'd be surprised if anyone really bothers. From what I can gather the theoretical "pure" water value is actually incorrect as pure water contains nothing to measure with and in order to read what the pH involves introducing some of what will be measured, so it is no longer pure. Secondly, it seems that the value is derived by theoretical maths, rather than by actual measurement.

This really depends, I think, on what you really want to mean by "pure" Water itself will disassociate into acidic and basic parts:

H2O <--> H+ + OH-

(just as an aside, this really should be written as 2H2O <--> H3O+ + OH- but usually that extra H2O is not carried around in the chemical equaitons, since it doesn't really change anything. Consequently pH should really be pH3O, but I don't think that after pH has been made common enough that it going to change now.)

Water will always dissociate, that is it's equilibrium condition, I know of nothing that can prevent water from disassociating. So, if you mean "pure" as only H2O, then yes, there is no pH, since there in no H+, but water on its complete own will disassociate and will make some H+, so I personally consider pure water so have some disassociation and hence will have a pH. I don't know completely, but I always was under the impression that pure water's pH of 7.0 was based on observations from experiments, not based on theoretical calculations. Like I said in the previous post, it really is only recently that pH meters accurate enough to have 3 significant figures after the decimal point have even existed, so it probably is a rather recent observation that the pH of pure water isn't 7.0, but 6.998. Finally, that equilibrium is a strong function of temperature, so a change of probably a few degrees and the pH is probably 7.0 again. Through a few degrees the other way and it is probably 6.991 or something even farther away from 7.0, too.

 

[ArauraDiscus]

Those are just andys opinions, I've yet to see his proof.

Water is the universal solvent, therefore it must have the potential to have a nuetral Ph and not only does it have the potential, it very often reaches a neutral Ph.

 

[angelmouse]

the r/o water that comes out of my unit has ph 7-7.2 and gh/kh-0-1. i use 80% r/o and 20% tap to get my water how i want it, it ends up ph-7.4 and kh-4 gh-6.
In my breeding tank the ph stays at 7.4 and wont drop but in my main tank with a higher fishload and co2 it drops to ph 6.5-6.8
The % depends on the ph of my tap water. only really needed for marines or softwater fish if you have hard tapwater.
Angel

 

[andywg]

Those are just andys opinions, I've yet to see his proof.

Water is the universal solvent, therefore it must have the potential to have a nuetral Ph and not only does it have the potential, it very often reaches a neutral Ph.

Read the talk page on the wiki pH page, that's where I am getting most of my info from. However, I am not disputing whether water has a neutral pH (which it appears is 6.998), I am stating what I have read regarding whether pure water has any pH at all. what I found was:

Pure water contains no hydrogen ions (or any ions at all as no dissociation takes place. By strict definition, pure water cannot have a pH value. It is not even posible to measure the pH of pure water, because, as I noted above, the measurement process itself introduces all of the measurable ions, completely invalidating any result.

It is then contested that:

Water does inherently dissociate. Pure water, under 'standard' conditions, dissociates from H2O into hydrated H+ and OH- to the extent of 1x10-7 moles/liter, also called pH 7 or neutral pH, the pH where there is no excess of acid (H+) or base (OH-).

However, I am unsure of what 'standard' conditions are and whether that is the real world infringing on the theoretical world.

 

[Bignose]

Standard conditions should be 20 or 25 degrees C, and 1 atmosphere (or maybe 1 bar) of pressure. There is a little variability in the definition since 20 and 25 degrees C are pretty close (in terms of measuring a substance's physical properties, not in terms of the weather outside. 5 degrees is close since they measure physical properties of substances over ranges of thousands of degrees sometimes). And 1 atm and 1 bar of pressure are also almost identical.

I don't know why the author of your second quote there would think that pure water has no pH. Again, like I said and your third quote said, water will always disassociate. That is just the state nature prefers. Maybe it is possible to prevent this from happening, but I know of no such way. I also don't know why anyone would want to stop water from disassociating other than to create a case where, technically, since there would be no H+ ions, there would be no pH. Even if you had only 2 water molecules, most of the time they would each be H2O, but there is a certain amount of time that they would exist as H3O+ and OH-. That is the nature of equilibrium. The molecules will always be changing back and forth. So, given a volume of water, with no additives, there will always be some molecules in that jar that are H2O (the vast majority), H3O+ and OH-.