What is the reason for the drop in pH when mixing 4 chlorophenoxyacetic acid with N doped TiO2 nanoparticles?

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Rafik Karaman

Dear Amal,

The pKa of 4 chlorophenoxyacetic acid is 3.56

https://pubchem.ncbi.nlm.nih.gov/compound/4-chlorophenoxyacetic_acid#section=Decomposition

pH solution of TiO2 is 6.5-9

http://sinochemi.en.alibaba.com/product/1124919611-0/PH_Value_6_5_9_0_Titanium_Dioxide_Water_Soluble.html

Therefore, it is expected that any interactions of TiO2 with 4 chlorophenoxyacetic acid will result in decreasing the pH (reaction between an acid and base).

Hoping this will be helpful,

Rafik

Amal Abdelhaleem

Hi Dr Rafik

To exclude this possibility, I stirred 4 chlorophenoxyacetic acid alone in alkaline media about 8.1 pH for 30 min without adding catalyst and I found also that pH drops from 8.1 to 7.0 or 6.9. I think this is related to this compound in the alkaline media. Please, can you help me to understand this issue.

Regards,

This result is expected since 4-chlorophenoxyacetic acid is an acid and once you add it to a buffer of pH 8.1, the pH of the buffer will drop because of the acidity of the carboxylic acid.

Buffers can not maintain their pH if they are mixed with high quantities of acids or bases.

Dear Prof. Rafik

Thanks for your favorable reply. Please, May you send me a reference that explains the behavior of 4 chlorophenoxyacetic acid according to its pKa value at different pH

Please see the following link:

http://www.shimadzu.com/an/hplc/support/lib/lctalk/29/29intro.html

I want to clarify something. In my question, I have a concentration of 4 chlorophenoxyacetic acid (0.01 mM) and I started to adjust the pH of this solution by adding NaOH. When adding drops of NaOH till pH=11 and then stirring the solution for 30 min, I found that the pH is the same. However, when adding drops of NaoH till pH =8.1 and stirring for 30 min, i found that pH decreases to 7. Did you get my question

Do you mix the solution for 30 minutes without covering the flask? If yes, then you might have the effect of CO2 that comes from the air and reduces the pH value of the solution. At high pH (11) the effect of CO2 is not seen because the solution is too basic, whereas at pH 8 (almost neutral) the effect of CO2 is observed.

Attached is a short article which describes the effect of CO2 from the air on the pH of sodium acetate solution.

In order to validate what I proposed, you should repeat the experiments with covered flask and the air in the solution is replaced with nitrogen or argon (you should pass nitrogen into the solution for few minutes). In this case, it is expected that the pH will not change.

Thanks so much for your favorable reply. I already cover the solution during 30 min stirring. Is it possible to absorb CO2 quickly at pH=8. It takes about 15 seconds before covering the solution for stirring. Is it possible to absorb CO2 during 15 seconds and pH decreases from 8.2 to 7 then it will be stable at this value.

I think it is unlikely that within 15 seconds CO2 can penetrate the solution in a quantity that will change the pH of the solution from 8.2 to 7.

I must say that my impression is: "you will have a very bright future as a researcher". Please, continue in this mode.

Good luck,

Thanks so much, but i can not find any interpretation for that. Do you have any other interpretation, please

There is one experiment which might solve this puzzle:

Add an exact amount (mmoles) of phenylacetic acid to a solution of NaOH which has the same number of mmoles of phenylacetic acid; this is to neutralize the acid, mix well and then add sodium hydroxide solution until reaching pH 8.2. If after mixing during 30 minutes the pH did not change, this will give an indication that the problem of dropping the pH from 8.2 to 7 in the previous experiment is attributed to the fact that at start the pH was 8.2 because not all the added phenylacetic acid reacted with the base and after 30 minutes the pH dropped to 7 since all the phenylacetic acid was neutralized by NaOH (a slow neutralization of the acid by base).