The electroneutrality principle is a very useful principle and one of the foundations of water chemistry. This principle states that the sum of positive and negative charges within the water must balance to zero. This principle is also very useful when working with groundwater samples. Below are two good examples of how AquaChem water quality analysis software uses this principle to help you verify the quality and accuracy of your water quality sample results.
Major ions are often one of the first things sampled as part of routine water quality monitoring. However, sampling is expensive and occasionally one of the major ions is not analyzed within a given sample.
For example, let’s say we received the sample results below, in which bicarbonate could not be reported because the field technician forgot to acidify the sample, making any such laboratory analysis invalid due to possible calcite precipitation within the sample bottle.
We can use AquaChem to quickly back-calculate the missing ion. It does so using the electroneutrality principle to determine the ion balance by assuming the total delta between the cation and anion sum is caused by the missing analyte.
AquaChem will also conveniently give you the option of saving this calculated value into the database.
AquaChem also adds a comment to this value indicating that it is estimated only, and was not directly measured.
One of the first questions those collecting water quality samples should ask is “how is the quality of the sample results?”. One simple check that helps ensure sample accuracy and completeness is the major ions check.
Let’s say instead we *had* measured bicarbonate in the previous sample and perhaps instead of calculating bicarbonate to see if it was within an “expected” or “typical” range, we wished to know whether we had correctly sampled all relevant major ions and/or that the laboratory had done a good job analyzing this suite of parameters.
We can double-check this by calculating the ion balance for the sample. AquaChem automatically calculated the electroneutrality for every sample based on the cation and anion results for the sample.
In this case, let us say the lab reported a HCO3- concentration of 83.2 mg/L. If we add this to our AquaChem database and flip to the calculated section of the Sample tab, we can see that the electroneutrality has been calculated as -1.1%. According to Appelo and Postma (1994), electroneutrality differences of up to 2% are nearly inevitable but differences of more than 5% mean that sampling and analytical procedures should be reviewed as this may indicate that there is another (perhaps rarer and unanalyzed) major ion in the water, or (more likely), that the sample was subjected to either sampling or analytical procedural errors.
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References
Appelo, C.A.J. and Postma, D, 1994. Geochemistry, groundwater, and pollution, Balkema, page 17.