Ion Balance in Wastewater 1

[Bee5]

Hi

I have been struggling to balance ions in a solution with multiple cations and anions and a pH of 2. I have heard that at low pH values that this is expected, i.e. that the ion balance will not necessarily work at very low pH. Is there anyone with water chemistry experience that could confirm this?

 

[kkala49]

Although my water chemistry knowledge comes from University age, I think that balance of (ion concentration)*(ion electric charge) is valid, to result in an electrically neutral solution.
For instance, suppose a solution of BaCl2 0.1 M (0.208 mg/l), having received few drops of aq HCl to result in pH=2.1, that is (H+)=10^-2.1=0.007943 grion/l. Since (H+)*(OH-)=10^-14 (25 oC), (OH-)=10^-11.9=1.259*10^-12 grion/lt. Above balance is expressed as 2*(Ba++)+(H+)=(Cl-)+(OH-), that is 2*0.1+0.007943 = (Cl-)+1.259*10^-12. So (Cl-)=0.207943 grion/l, of which 0.20 grion/l from solved baCl2 and rest 0.007943 from aq HCl.
It is noted that (Ba++)*(OH)^2=0.1*(1.259*10^-12)^2=1.6*10^-25, much less than Ba(OH)2 solubility product = 5*10^-3 (~25oC). In similar calculations check for cation precipitation had better be made, especially when pH increases (gets more alkaline). Preliminary indications at <

http://www.chem.sc.edu/analytical/chem111/backup_080707/solubility.html>

and <

http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm>.

 

[kkala49]

Sorry, links mentioned in previous post do not work. Whole name of them is
"

http://www.chem.sc.edu/analytical/chem111/backup_080707/solubility.html"

"

http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm"

Hopefully they will work now, it seems better to include then in quotation marks than in < >

 

[Bee5]

Thank you for the explanation. It makes sense, but I took the approach of converting all cation and anion concentrations to milliequivalents per litre (meq/l). I ended up with a significantly greater millieqivalent value for anions, and hence my values do not balance (the difference is supposed to be <10%).

PS. I was unable to access either of the links you posted.

 

[bimr]

At low pH, the cations and anions will balance. The missing parameter is free mineral acidity, which is generally not reported with the analytical results. If you add free mineral acidity, the water will balance.

The ion balance is used to check analytical results. The ion balance should be very close to equal but may have slight discrepancies due to errors in analytical testing.

Total anions must be in balance with total cations. Therefore the sum of the concentrations of anions should equal the total concentration of cations and the ratio of total anions to total cations should be 1.

When you have carefully converted all the elements of your water analysis in meq/L units, the sum of anions should be the same as the sum of cations. The only exceptions to that rule are:

•A small difference due to imprecision in the analytical procedures is acceptable as long as the difference between total cations and total anions is less than 3 %.
•At high pH (> 8.2), e.g. in the presence of ammonia or after lime decarbonation, there will be hydroxide or carbonate ions. Hydroxide ions are usually not reported separately. Carbonate ions are not always reported. In such a case, you would have more cations than anions.
•At low pH (say < 6.8), the water may contain either free mineral acidity (very rare for natural water) or free carbon dioxide, both producing H ions wich are usually not reported separately.

http://dardel.info/IX/water_analysis.html

 

[kkala49]

For each ion, mgeqv/lt = ion charge X mgion/l (e.g. ion charge for Ba++ is 2). So relation for gions in post no 2 (and its simplified example) is equivalent to that for geqvs. All positive and negative ions have to be considered (even "hidden") in the "electrical" balance, including water dissociation.
I can access links in post no 3. They mentioned solubility rules and solubility products, not of primary importance to the subject.

 

[Bee5]

bimr, the water I am dealing with is wastewater, with a multitude of cations, and only a few anions. However, it seems like I have more anion meq/l in my balance. Does your statement about free mineral acidity apply to wastewater as well? Would the lab simply analyse for H+ ions?

 

[bimr]

Acidity is measured by titration. The linked paper provides an outline.

http://www.ecs.umass.edu/cee/reckhow/courses/572/572bk16/572BK16.html

http://www.google.com/url?sa=t&rct=...source=web&cd=20&ved=0CGUQFjAJOAo&url=http://

http://www.civil.ubc.ca/home/env_la...64HgCg&usg=AFQjCNHCiWexgw0uBO8yIGHH_nliYMDhxQ

Why is this important?

 

[Bee5]

At university we were taught that all the ions will balance, which was further reinforced by your post (post number 5). However, when dealing with real samples, I have not had much luck. I even have the "acidity as H+" values, and even with this, the balance doesn't work out.

Thanks anyway. I appreciate your effort.

 

[bimr]

Maybe your water analysis is incorrect.

We have always used the ion balance as a rough check on the quality of the water analysis. And if a parameter in the water analysis was missing, the value was determined by the subtracting the known values from the total.

In certain wastewaters, you may have parameters such as dissolved metals that may be present. Ferric chloride for example.

If your solution consists basically of acid, HCl for example, the cations would consist of H.

 

[Bee5]

Thank you so much. I know there should be a lot more H+ ions in the water than reported by the lab, so I will look into it. As a rough estimate, is it accurate to calculate the H+ ions from the pH of the wastewater?

 

[bimr]

Not sure that you can calculate from the H+ ions, titration is probably the best.