Impact of pH on Derivatization of Nitrophenol Compounds

Our goal of evaluating the Normalized French Environmental Method 12673 required testing the method’s ability to derivatize nitrophenols and alkylphenols in addition to the chlorophenols the method was designed for. Unfortunately, nitrophenols proved challenging to derivatize at low concentrations. In an effort to improve the derivatization of nitrophenols, we experimented with the pH of the reaction mixture.

The original method requires 5 mL of 1 molar potassium carbonate (0.7 g solid potassium carbonate) to be added to 50 mL of water sample, followed by the addition of 1 mL acetic anhydride, then extraction with hexanes. In an attempt to improve the conversion of nitrophenols to acetic acid esters, the amount of potassium carbonate being added to the solution was increased, or replaced with sodium hydroxide. Our hope was that increasing the basicity of the solution would create a better environment for the leaving group, promoting the nucleophilic attack process.

A small experiment was designed in order to evaluate the effect of pH on derivatization of nitrophenol compounds. This experiment included both increasing and decreasing the pH of the derivatization reaction in order to observe the relative response from compounds, which were analyzed on the Rxi-5Sil MS column.

blog 2 - ph 9 11.66 13.5Figure 1: Three overlaid chromatogram showing peaks for (1) 2-nitrophenyl acetate, (2) 3-nitrophenyl acetate, and (3) 4-nitrophenyl acetate. Each overlaid chromatogram represents a derivatization in a different pH solution.

Only nitro-containing phenols were determined in this experiment. Nitrophenyl acetates performed significantly better than dinitrophenyl acetates, and were chosen to reflect to effect of pH. The pH 11.66 derivatization performed best for two of the three nitrophenyl acetates, followed by pH 9, and pH 13.5. The increasing pH, therefore, did not enhance the derivatization process. There appears to be a point at which the pH can impede the derivatization process; the increased pH was also observed to inhibit the derivatization of the alkyl and chlorophenol compounds.

blog 2 - tableTable 1: Responses of chloro- and alkyl phenyl acetate compounds after derivatization in pH 11.6 and pH 13.44 conditions.

The increased pH had a negative effect on the derivatization of these compounds. Despite being a higher concentration, the responses of the compounds derivatized at pH 13.44 are significantly lower than the responses of those derivatized at pH 11.66.

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