Reef Chemistry Question of the Day #81 Conductivity of Pure Water

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #81

We are all familiar with using TDS to measure the functioning of RO/DI systems.

Generally, TDS is a measure of charged ions in the water, and these are detected by conductivity (a TDS meter is a conductivity meter). That is, the charged ions moved in the water when an external electric field is applied, and a TDS meter detects this movement.

It turns out, however, that absolutely pure fresh water has some conductivity that can be detected with a suitable conductivity meter.

By contrast, there isn't any significant conductivity in most organic solvents (e.g., hexane).

Why is there any conductivity at all in pure fresh water?

Good luck!

































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[5Bucks]

Even perfectly pure water self ionizes allowing for some conductivity.

 

[Steven@]

Oxygen in the water

 

[beaslbob]

H2O ionized into H+ and OH- ions even with nothing else present.

 

[Cory]

I'm going to say because Co2 gets inside and pure water has ph below 7 a bit. And co2 isn't charged?

 

[Randy Holmes-Farley]

And the answer is...Autoionization (as 5bucks and beaslbob got exactly right!)

Water (H₂O) has a tendency to spontaneously split apart into H⁺ and OH^-:

H₂O <----> H⁺ + OH^-

That reaction reaches an equilibrium virtually instantly in any aqueous solution.

Because of the presence of these ions, water conducts electricity.
For reference, the conductivity of totally pure water is 0.055 mS/cm. It doesn't really make sense as a unit of measure, but if one converted that into ppm TDS, it would be roughly half that, or about 0.02 ppm TDS.


Aside: Often the purity of very pure water is presented in units of resistance, or Mohms (megaohms). In that case, totally pure water has a resistance of 18 Mohms, which is just 1/conductivity in mS/cm. Also, for curiosity, the highest known conductivity of an aqueous solution is that for 31% nitric acid, at 865 mS/cm, which is equivalent to more than 500,000 ppm TDS.

Happy Reefing.

 

[Randy Holmes-Farley]

I'm going to say because Co2 gets inside and pure water has ph below 7 a bit. And co2 isn't charged?

Carbon dioxide will enter water and ionize into H⁺ and bicarbonate and carbonate, causing increased conductivity, up to 1-2 ppm TDS, but I was assuming it wasn't present.

 

[beaslbob]

beaslbob got exactly right!
...

Something my wife will not believe.

 

[Damon]

And the answer is...Autoionization (as 5bucks and beaslbob got exactly right!)

Water (H₂O) has a tendency to spontaneously split apart into H⁺ and OH^-:

H₂O <----> H⁺ + OH^-

That reaction reaches an equilibrium virtually instantly in any aqueous solution.

Because of the presence of these ions, water conducts electricity.
For reference, the conductivity of totally pure water is 0.055 mS/cm. It doesn't really make sense as a unit of measure, but if one converted that into ppm TDS, it would be roughly half that, or about 0.02 ppm TDS.


Aside: Often the purity of very pure water is presented in units of resistance, or Mohms (megaohms). In that case, totally pure water has a resistance of 18 Mohms, which is just 1/conductivity in mS/cm. Also, for curiosity, the highest known conductivity of an aqueous solution is that for 31% nitric acid, at 865 mS/cm, which is equivalent to more than 500,000 ppm TDS.

Happy Reefing.

Wow randy! Lol, i could have used you maybe 10 to 20 times with the "does conduct electricity" comment. I have argued that for years it seems to some toeheads, and always "no, no, no, your nuts!".. Lol, guess I'm not so crazy after all.. Lol well, i am crazy.. But not on this one!

Bookmarked thread (for the idiots I run into later)!

 

[Randy Holmes-Farley]

:lol:

Glad you found it useful.

 

[SantaMonicaHelp]

fascinating information... when did science figure all this out? the last 100 years?

 

[Chameleon]

I thought it went to H3O+ and OH-
balanced: 2H2O <-> H3O+ and OH-

 

[Randy Holmes-Farley]

Yes, quite right.
H⁺ is most often shown as H₃O⁺ in water. In terms of conductivity, however, the whole H₃O⁺ is not moving. Just the H+, and even that is not moving like a normal ion, but moving by attaching to an H₂O on one side and a new H⁺ pops off the other side.

For that reason, H⁺ and OH^- have unusually high conductivity.

 

[Chameleon]

Yes, quite right.
H⁺ is most often shown as H₃O⁺ in water. In terms of conductivity, however, the whole H₃O⁺ is not moving. Just the H+, and even that is not moving like a normal ion, but moving by attaching to an H₂O on one side and a new H⁺ pops off the other side.

For that reason, H⁺ and OH^- have unusually high conductivity.

cool us biochemists always leave it up to yall chemist to do the real chemistry

 

[Cory]

Carbon dioxide will enter water and ionize into H⁺ and bicarbonate and carbonate, causing increased conductivity, up to 1-2 ppm TDS, but I was assuming it wasn't present.

So I was right but slightly wrong

 

[Randy Holmes-Farley]

fascinating information... when did science figure all this out? the last 100 years?

Not sure exactly, but likely between 50 and 100 years ago.

 

[Randy Holmes-Farley]

So I was right but slightly wrong

Or wrong with a hint of truth.