Charge balance
The sum of positive charges equals the sum of negative charges in natural waters. If this were not the case, you would get an electrical shock each time you touched water. The ‘electroneutrality’ or ‘charge-balance’ equation:
Eq. 1. åzmc = åzma
in which mc and ma are the molalities of cationic and anionic species and z is the ionic valence, can be used to indicate the accuracy of a water sample1. Major ions comprise most of the ionic content of a water sample, and are usually used on their own when assessing charge balance errors. For example, if major cation and anion concentration values (milligrams per litre) are substituted into Eq. 1 as:
Eq. 2. (Na+) + 2(Mg2+) + 2(Ca2+) + (K+) = (Cl–) + (HCO3–) + 2(CO32-) + 2(SO42-)
the right-hand side of the equation should be roughly equal to the left-hand side. Charge balance errors are most often reported as percentages e.g. by substituting Eq. 2 into Eq. 3:
Eq. 3. E = åzmc – åzma / åzmc + åzma * 100
In most cases, charge balance errors of <5% are considered acceptable. Charge balance errors of greater than 5% usually indicate that either an analytical error has occurred, or that an ionic species found in significant concentrations has been excluded from the analysis.
- Freeze, R.A. and Cherry, J.A. 1979. Groundwater, Prentice-Hall Inc., Englewood Cliffs, N.J., pp. 604.
