Conversion of conductivity to molar concentration

Is it just a coincidence that the conductivity of pure water is 0.055 and the inv of the molecular weight is 55?

Wayne

IIRR 0.055uS/cm for pure water represents really pure water, usually known as conductivity water. Regular distilled/de-ionised water has about one hundred times the conductivity due to dissolved atmospheric CO2, which forms carbonic acid in solutuion, ionising to H+ and HCO3- ions.

In conductivity water, the conducting ions are just the H+ and OH- ions provided by the ionic dissociation of water itself, at a concentratio of about 10E-7 molar at room temperature. Regular distilled water has a pH of 5 due to the dissolved carbonic acid, and thus a roughly one-hundred-fold higher concentration of H+ ions, which are very mobile and dominate the measured conductivity.

Hope this helps.

----------- Bill Sloman, Nijmegen

I am trying to run this online Pourbaix charting program

I am using Cu with pure water (well close to 18.3MR or 0.055uS). According to the last message is seems that for Molality I select

10E-7?

Could anyone help with this?

WayneL

Molality is defined as the number of moles per kg of solvent. If you mean calculating the molality of the water, it is simply the inverse of the molecular weight (in kg) of water. It comes to roughly 55. You can take it from here.

--
Dieter Britz,   Kemisk Institut, Aarhus Universitet, Danmark.

You are trying to generate a redox potential versus pH plot for a specific concentration of a specific metallic ion in solution.

The molarity (concentration) of the ion in the solution is part of the data you need to provide before the program can makes its prediction.

If you couldn't work this out from the web-site in the first place, you are probably trying to run before you can walk .....

------------- Bill Sloman, Nijmegen

I am conducting a series of test using 80uS water and therefore need to calc the molarity of it. Pourbaix diagrams are new addition to my field and I am trying to gain an understanding of them. If you could be so kind and shine some light on my dilemma I would be grateful?

WayneL

Yes.

For water of that purity, yes, the molality of both H+ and OH- would be 1.0E-07. For the less pure water, see the other poster.

--
Dieter Britz,   Kemisk Institut, Aarhus Universitet, Danmark.