Ion Exchange: Hydroxyl interferes in chloride test?

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Borisfa

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I did a test of the water that present 2.1 mg/L of chloride and 226.9 mg/L of bicarbonate. I passed this water in a ion exchange resin column (basic strong anionic) for bicarbonate removal. The bicabonate content reduced to 126.5 mg/L, but the chloride content rose to 48.0 mg/L. This type of resin changes anions for hydroxyl. You believe hidroxyl could have been an interference in the test and have been presented as chloride instead of hydroxyl?

Thanks,

Fabiano
 
Bicarbonate was measured by Standards Method (SMEWW - 2320 B ).

This method actually measures alkalinity ans cautions that when one interprets the results in terms of carbonate and bicarbonate that

...ion concentrations in the strictest sense are not
represented in the results, which may differ significantly from
actual concentrations especially at pH > 10.

This casts doubt on the bicarbonate observations. It would be better if you told us the results of the before and after titrations in mL of acid used to the various end points.

Your basic exchanger is replacing HCO3- with OH- e.g.

NaHCO3 ---> NaOH

so the pH is going to go up (unless you are also going through a column that replaces cations with H+). If the pH goes up the P alkalinity will increase but the M alkalinity should not change. If you use the formulas is 2320 B for estimating actual ion content you will get a decrease in bicarbonate and an increase in carbonate and hydroxyl ions.

I would also expect the column to remove chloride ions and replace them with OH- ions

NaCl --> NaOH

so the apparent increase in chloride is a mystery. Perhaps the high pH is interfering with the chloride test.
 
@ajdelange, pH goes to about 11 and I am using only one column (anionic). I will try take the results before and after titrations.
 
The Lab is looking for gross data for the titrations.

As pH increase to 11, there is an equilibrium equation change for carbonate side. So, what you expect happen with alcalinity (reduce less then for bicarbonete reduced, because some bicarbonete redution could be associeted by shift of species)? Could bicarbonete concentration be wrong?

HCO3- + OH- --> CO3-2 + H2O

Thanks,

Fabiano da Mata
 
If the sample's original pH is less than 8.3 (P alkalinity = 0) then most of the carbonic acid species will be bicarbonate ion and the pre-treatment alkalinity tells you how many moles of bicarbonate are present. For example, if you put 2 mmol NaHCO3 into distilled water there will be 2 mmol HCO3- ions, the pH will be 8.3, the P alkalinity will be 0 and the M alkalinity 2 mEq/L. If the bicarbonate are all replaced by (OH)- you will have 2 mmol/L OH- instead of 2 mmol/L bicarbonate and the pH would be 14 -(- log(2E-3)) = 11.3. The M alkalinity (the amount of acid required to neutralize all the (OH)-) is still 2 mmol/L but the P alkalinity, required to neutralize to pH 8.3 is
1000*(10^(11.3 - 14) - 10^(8.3 - 14)) = 1.993 (almost the same as the M alkalinity).

At pH 11 nearly all the carbonic acid species are carbonate but as all anions (bicarbonate and carbonate) have been replaced by (OH)- there is no bicarbonate left in the solution.

Now if you add 1 mmol NaCl and 1 mmol NaHCO3 to 1 L of water you should wind up with 1 mmol of (OH)- from replacement of Cl- ions and 1 mmol of (OH)- ions from replacement of HCO3- ions. You would, again, have a total of 2 mmol/L (OH-), a pH of 11.3 and P and M alkalinities that are almost equal and a chloride content of 0 or a very low value.

What was the original sample pH? How are you testing for chloride?
 
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