Electrical balance and charge balance

[Yongqiang]

Hi PHREEQC Experts，

I want to know the difference between "Electrical balance" and "Charge balance" in the output section of solution species. Could anybody provide some help on this problem?

Regards,
Yongqiang

[dlparkhurst]

The term is used interchangeably. Two items are included in the output section "Electrical balance", given in equivalents, and percent error calculated by 100*(Cations-|Anions|)/(Cations+|Anions|) , where cations and anions are absolute values in equivalents.

The basic function CHARGE_BALANCE gives the same value as "Electrical balance" in the output section.

[Yongqiang]

Thank you, Parkhusrt. One more question, I don't understand the unit for electrical balance (eq)? Could you give me some guidance?

[davidblevy]

(This post would be more appropriately located under PHREEQC Basics rather than under redox equilibria). To evaluate charge balance we must be able to compare the total number of positive (+) and negative (-) charges, which theoretically should be equal. Equivalents will be the same as equivalents per liter (Eq/L) in PHREEQC, where Eq/L is the moles of charge/L. For example, a 0.10 M solution of sulfate (moles/L) would be 0.20 N (normality) or contain 0.20 moles of charge/L (eq/L).

To do this by hand when data are reported on a mass basis (mg/L), convert to an equivalent basis (meq/L) by dividing the mass concentration (mg/L) by the equivalent weight (atomic weight divided by the charge). The equivalent weight of Ca2+ is 20 mg/meq (40/2) and that for SO4 is 48 mg/meq (96/2).  For example, a CaSO4 solution containing 240 mg/L Ca2+ and 576 mg/L SO4 is perfectly balanced (240/20 = 12 meq/L of + charge) and 576 mg/L/48 = 12 meq/L of - charge). I hope this helps, David L.