charge balance error

[hyedy]

hi, all of the experts

i got some water samples and used  basic function of the phreeqc, 'solution' tab, just simply put chemical composition data of sample into it and run it. ( i attatched part of my raw data)
The result  showed very large (negative)  percent error for the sample. 
so i set Na+ as the charge balance, and run it again, percent error showed almost 0%.

is it okay to set the charge balance component that i want to choose? and i also want to know the difference between 'electrical balance(eq)' and 'percent error'.
 I found that when i run phreeqc with the original data (without charge balance) , 'electrical balance(eq)' showed very small value, while the 'percent error' showed negatively large value. ( I inquired the raw data to analyst first, but he said there is no problem with the raw data. )

[dlparkhurst]

Using the database phreeqc.dat, the first solution gives the following output:

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                  Electrical balance (eq)  =  -1.048e-03
 Percent error, 100*(Cat-|An|)/(Cat+|An|)  = -19.97

The formula for the percent error is given  "Percent error, 100*(Cat-|An|)/(Cat+|An|)", where Cat is the sum of cations (z(i)*moles(i)) and An is the absolute value of the sum of the anions (|z(i)|*moles(i)|. The "Electrical balance (eq) is simply Cat-|An|.

The electrical balance and percent error are used as an indication of the accuracy of the analysis. In your solution 20 percent imbalance is poor. One possibility is that you are not defining the units of N(5) correctly. The default gram formula weight used for the conversion depends on the SOLUTION_MASTER_SPECIES data block of the database that you use. Phreeqc.dat uses the gram formula weight of N for the conversion of ppm nitrate to moles because that is the common reporting unit for many labs in the U.S. (as N). Perhaps your analysis is reported "as NO3", in which case you should use "ppm as NO3" in the second line of SOLUTION_SPREAD for N(5). If you are using PhreeqcI, you change this with the Unit/Options tab of the SOLUTION_SPREAD screen. Making this change does improve the charge balance for your solutions.

If the analyst has done a charge-balance calculation and thinks the analysis is accurate, ask for his calculation and compare it to the PHREEQC calculation.