[Borek]

If you are adding NaOH you are most likely already creating carbonate buffer. If you add the buffer in the minute quantities at the very beginning you may have things under control from the start.

[ahmed_saleh]

Borek,
I'm not sure that I have got your point... Would you clarify please? Thanks

[Borek]

in general we try to avoid any actions that may seriously affect the product specifications or working plans.

If you are adding NaOH to keep pH at 7, and your solution becomes contaminated with carbon dioxide, it contains carbonate buffer (CO2/HCO3-). In effect it doesn't matter (much) if you add bicarbonate to prepare buffer by yourself, or NaOH to keep pH at requested level - in both cases final effect (final composition of the solution) is the same.

[ahmed_saleh]

See lease the Conclusions from the above answers & my questions based on them:

Conclusions:

1- Due to CO2 abosrbtion & available some amount of NaOH: buffer NaHCO3/H2CO3 can be formed with pH close to 6.4 (not desired for our case).

2- If adding bicarbonate, at the beginning when sending the batch to storage : buffer of Na2CO3/H2CO3 can be formed with pH ~ 7.4 (desired for our case).

Questions:

1-When adding Na2CO3: Would the formed H2CO3 convert some of this bicarbonate to become NaHCO3?

2- What would be the pH value after adding Na2CO3? This question is because, as I know, at pH<9-10 you do not have carbonate ion,  but rather bicarbonate ion (HCO3-) !!

3-If the facts in questions no.1 & 2 take place: What buffer would have the prevailing effect: NaHCO3/H2CO3 with pH ~6.4 or Na2CO3/H2CO3 with pH ~7.4 ?

Thanks for your response & assistance

[Borek]

1-When adding Na2CO3: Would the formed H2CO3 convert some of this bicarbonate to become NaHCO3?

You mean H₂CO₃ formed by dissolving CO₂? Yes.

2- What would be the pH value after adding Na2CO3? This question is because, as I know, at pH<9-10 you do not have carbonate ion,  but rather bicarbonate ion (HCO3-) !!

It will depend on the concentration, you may use my BATE (pH calcualtor, see my signature) to calculate by yourself.

3-If the facts in questions no.1 & 2 take place: What buffer would have the prevailing effect: NaHCO3/H2CO3 with pH ~6.4 or Na2CO3/H2CO3 with pH ~7.4 ?

No such thing as CO₃^2-/H₂CO₃ buffer. It is either CO₃^2-/HCO₃^- or HCO₃^-/H₂CO₃. Which pair will be at work depends on the initial concentrations of carbonates and amount of CO₂ dissolved.