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 one), 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

The same applies 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 contrary to this recommendation, i.e. if you choose a cation/anion while an anion/cation is suggested? Well, in such a case the discrepancy between the measured and the calculated EC (or TDS) becomes larger.

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: 2020-12-13]