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Jono

Charge balance errors

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Because GWB reports charge balance errors in faradays it makes it a little difficult to quantify the magnitude of error if you are used to using simple CBE percentage errors. The example I have just run gives an error of -0.0001346 faradays and a percentage error of -0.36%. Is the relationship between the two methods linear? If an analysis result returns a balance error of less than 5% it is normally considered useable, what would be the equivalent acceptable error in faradays? I know a similar question was asked early in 2008 but some clarification would be helpful.

 

Cheers

Jono

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Because GWB reports charge balance errors in faradays it makes it a little difficult to quantify the magnitude of error if you are used to using simple CBE percentage errors. The example I have just run gives an error of -0.0001346 faradays and a percentage error of -0.36%. Is the relationship between the two methods linear? If an analysis result returns a balance error of less than 5% it is normally considered useable, what would be the equivalent acceptable error in faradays? I know a similar question was asked early in 2008 but some clarification would be helpful.

 

Cheers

Jono

 

Hi Jono:

 

Faradays and equivalents are actually closely related:

 

  • An equivalent is the amount of substance that will react with one mole of electrons (1 eq Na+ = 1 mol Na+, 1 eq Ca++ = .5 mol Ca++)
  • A Faraday represents the amount of charge (in Coulombs) per mole of electrons.

 

Therefore, 1 equivalent is often the same as 1 Faraday (or 1000 meq = 1 Faraday). The primary difference is that the more traditional cation-anion balance methods contrast anion and cation concentrations on a 'per kg solvent' or 'per kg solution' basis (usually meq/kg or meq/L). So for more concentrated solutions, or for systems where solvent mass is not 1 kg, you'll need to adjust units appropriately.

 

When comparing charge balance differences reported by GWB vs. other methods, also keep in mind that the methodologies used to calculate anion and cation concentrations can vary significantly.

 

I hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

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