Coal Geology

Geochemistry quick tips: Cation Anion Balance or Charge Balance Error (C.B.E)!

Are you lost during a geochemical investigation? Well, sometimes I do get lost. Sometimes I feel like I need more water quality parameters to answer some of the geochemical questions. I never thought about doing just a simple “cation - anion” balance before until I really had to for one unique situation where I had to think about some metal that might be present in the water to produce low pH in the system.

A fundamental property of an aqueous solution is that they are electrically neutral! So, total number of equivalents for the positive constituents must equal to total number of equivalents of negative constituents.

A “cation - anion” balance will never give you 100% accuracy simply because it is virtually impossible to analyze every single ions in the water. In most cases it is also not required. The major cations and anions in ground or surface water almost always make up >95% of the ion balance.

A very good exercise on simple water quality analysis can be found HERE.

Well, we will anyway do our own math using a real water quality analysis!

C.B.E (Charge Balance Error or Cation - Anion Balance)=((Tot Cation meq/L - Total Anion meq/L) / (Total Cation meq/L + Total Anion meq/L))*100%

Below is an example of actual water quality analysis from a coal mine drainage. Note that they reported 4.34% CCE in the laboratory analysis. Now let’s do our analysis.

(Click on the Image for FULL VIEW)

To do our own calculation, we need to convert each anion to meq/L unit. I am attaching a spreadsheet that converts all anions and cations to meq/L.

Below is the screen shot of the actual spreadsheet I used to do the calculation. You should download the spreadsheet and check the calculation yourself. You can also use the spreadsheet to do your own Cation - Anion Balance calculations just by changing values for “measured concentrations”. Remember that you may need to add or subtract few cations or anions based on your particular water analysis.

Download - Cation - Anion Balance Calculation Worksheet

(Click on the Image for FULL VIEW)

So, we get
Cation - Anion Balance = -2.9%

Now “negative” number for “cation - anion balance” means:

• Excess anions
• or, Lack of cations

While “positive” number for “cation - anion balance” means:

• Excess cations
• or, Lack of anions

A reasonable balance for routine water quality analysis is generally considered to be less than 5%. If you have a higher number, you should reconsider further constituents for analysis.

There are several reasons that may also create an electrical imbalance such as:

1. Laboratory error.
2. Sampling procedure.
3. Use of unfiltered water sample - this is another thing we often neglect to check. Unfiltered water samples contain particulate matter that dissolves into the sample when we add add small acid as preservative.
4. Sometimes some of the constituent may precipitate in the sample container.

Note: Also make sure that you use only “dissolved” constituents to do water “cation - anion” balance calculations.
The Cation Anion Balance or Charge Balance Error should always be included in any water quality analysis. Sometimes they can point out to some obvious errors.

So, never underestimate the importance of Charge - Balance Error (C.B.E) or Cation - Anion Balance.