Issues related to the testing of p-nitrobenzoic acid
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Dear experts, one of the testing methods for nitrobenzoic acid is as follows: Take about 2.5 g of the nitrobenzoic acid sample, place it in a beaker, add 25 ml of 1.25N sodium hydroxide solution, and dissolve it completely using ultrasound. Then observe the solution; it should be colorless and transparent, without turning red. As a result, we took samples of the product after drying it and tested them; when dissolved in an alkaline solution, the solution turned red, and the customer stated that it was not up to standard. I want to ask why it turns red? What is the mechanism behind the reddening? Is it incomplete reaction? Are there still impurities in the product? Is the material not washed clean enough?
Thank you to all the experts for participating in the discussion. Thank you!
This post was last edited by qqgd on 2026-1-22 09:43. p-Nitrobenzoic acid is generally synthesized through the oxidation of p-nitrotoluene; if the quality inspection fails, it indicates the presence of impurities. Impurity components are related to your synthesis process and purification methods, and they may also be associated with the solvents used. It is recommended to conduct impurity analysis on the product, using chromatography or spectroscopy to detect the specific potential impurities. Qualitative tests alone cannot determine the specific type and properties of impurities.
Substances that cause sodium hydroxide to turn red generally include phenolphthalein, as well as alkylanthraquinones
Thank you, moderator! I also think it is because phenols are formed.