[arie2044]

Hello To all members
I use alkaline solution to absorb H2S, CO2 & RSH.
Sometimes for the results of the alkali concentration we receive the results for free alkali as negative number.
Does any body can explain the meaning of the negative result?

Thank you in advance

[Mr Peanut]

Here I assume the test was done by titrating to a pH equivalence point. The negative result indicates that the final solution was more alkaline that the initial solution.

How could this be? Probably because the scrubber solution was concentrated through evaporation during the gas impingement. .

[arie2044]

Hello
Thank you for reply.
I think i didn't explain the problem. On titration of this alkaline solution there are 3 equivalent points (pH=8, pH=7, pH=3.4). very often the calculated results for free alkaline are negative.
What represents the negative amount in this case. if needed i can add the analytical method.
Thank to all

[Mr Peanut]

Yes, provide analytical data. Include initial molarity of alkaline solution (is it NaOH?).

Also, include the initial and final volume of solution in the scrubber.

[arie2044]

Hello nd thank you for the answer.
I tried to atach the analytic method as attachment, but it failed. so you can contact me on e-mail :
edok04@walla.com, then i'll transfer to you the file.

Thank you again

[Mr Peanut]

OK,

I see one possible bug. The set of equations in the three formulas assumes that ALL of the bicarbonate in the third break was originally present in the solution as EXCLUSIVELY Na₂CO₃. If there was some initial NaHCO₃ in the original solution, the assumption that the HCO₃- in the 3rd equation = the CO-- in the second would not hold. It is not uncommon for Na₂CO₃ in bulk form (say from a drum of raw material) to degrade to NaHCO₃ through the absorption of atmospheric CO₂ in the presence of moisture:

CO₂ + H₂O = H₂CO₃

Na₂CO₃ + H₂CO₃ =  2NaHCO₃


This can also happen during the test but your SOP calls for blanketing the titration with nitrogen, which prevents the interference.

[Mr Peanut]

OK AIRE;

The moderator explained how to get your pdfs into the post. I combined them and posted them below. I am hoping some of the others may have some insights.

So far, I see:

The method assumes:

• that all P alkalinity is from OH^-, CO₃^-- and S^--

• that all m alkalinity is from HCO₃^- and HS^-

• that all of the HCO₃^- and HS^- was originally present in the system as CO₃^-- and S^--.

When conditions 2, and 3 do not hold you will have P<M indicating a negative level of OH^-. Another source of m alkalinity is indicated.

For an enlightening experiment take a sample that is giving you a positive free alkali and add an additional 50g/l spike of NaHCO₃.

Interfering bicarbonates from your reagents is my best guess for the additional components of m alkalinity. CO2 absorption is a problem for NaOH.


Anyway, those are my thoughts. Perhaps some of the others have good ideas too.

[arie2044]

Hi Mark
Thank you very much. i got the point. but do you thing that i must to renew the NaOH sol at the point that free alkali=0, or i can use it even when free alkali shows negative results.
For some time i thought the negative results represents the release of CO2- from the solution during the titration, HCL 1N as titrant. and i see that this is your point too.

Thank you again

Arie

[Mr Peanut]

You should always check your absorber solution just before it is used by running the titration on it. This is called running a blank. It should have an m alkalinity that is small (preferably zero). Sometimes it is small enough that you can simply subtract the initial m alkalinity from the sample's final m alkalinity to get a more reliable result.

Running a blank is a good idea to consider for all analytical tests in the lab.
As the initial solution's m alkalinity creeps-up to where it is say 1 or 2 percent of the typical m alkalinity for the sample, you should replace it with fresh absorber. The exact level that you use as a maximum allowable initial alkalinity for the starting solution before you deem it unusable depends on the amount of error you are willing to tolerate. (It's rarely zero.)

The good thing about having a lab is that you can test your ideas about why a problem might be occurring and also test to determine how much error you are willing to tolerate, but remember you will always have some error in every test.

On the CO₂ formation, it occurs in all of your titrations during the m alkalinity part of the test, whether the m alkalinity is from degradation of the starting solution or from legitimate m alkalinity picked-up in the scrubbing process. (Provided some of it is caused by HCO₃, which it, in part, usually is).

When the amount of CO₂ is small you won't see the bubbles because it remains in the hydrated form: H₂CO₃, ie H₂O-CO₂, unless you overshoot the the m alkalinity end-point significantly.

 

[arie2044]

Hello to all

Thank you very much for the explanation.