TDS, EC, and Charge Balance

TDS (or EC) together with the charge balance error CBE are useful parameters to check the plausibility of a water sample. Both, TDS and CBE are in some sense complementary:

(1) TDS = Σ cations + Σ anions
(2) CBE = (Σ cations – Σ anions) / (Σ cations + Σ anions) × 100

Instead of TDS one can also use the electrical conductivity (EC) because they are (approximately) proportional to each other:

(3) EC (µS/cm)   ≈   b × TDS (mg/L) with b = 1.4 … 1.8

aqion calculates all three parameters: CBE, EC, and TDS. Now compare the measured EC with the calculated EC (or the measured TDS with the calculated), and you get helpful information about the water sample:

  CBE negative CBE positive
EC measured > EC calculated deficiency of cations deficiency of anions
EC measured < EC calculated surplus of anions surplus of cations

Exactly the same holds if EC is replaced by TDS in the table.

What is the optimal parameter for charge-balance adjustment?

The above table helps to select the optimal parameter (cation or anion) for charge-balance adjustment. In principal, the following cations and anions are available: 1

cations: Ca, Mg, Na, K, Fe(2), Fe(3), Al, Mn, NH4+
anions: DIC, Cl, Sulfate, Nitrate

In this way, the above table can be converted into the following recommendation for the parameter selection:

  CBE negative CBE positive
EC measured > EC calculated Ca, Mg, Na, K, Fe, Al, Mn, NH4+ DIC, Cl, Sulfate, Nitrate
EC measured < EC calculated DIC, Cl, Sulfate, Nitrate Ca, Mg, Na, K, Fe, Al, Mn, NH4+

What happens if you act against this recommendation, i.e. if you select a cation/anion whilst an anion/cation is proposed? Well, in such a case the discrepancy between the measured and calculated EC (or TDS) will increase.

The above considerations are implemented into aqion in form of an interactive Guidance for charge-balance adjustment.


  1. The parameter list contains only those elements that are de facto present in the actual input water (with a minimum concentration of 1 µmol/L). For example, if the input water does not contain any Al, then Al is absent in the parameter list. 

[last modified: 2013-12-25]