Seeking advice on the analytical method for determining the acidity of dichloropropionic acid
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It is difficult to determine the endpoint color during the acidity analysis of dichloropropionic acid produced as a byproduct of propylene oxide. Our method is: add 100 mL of carbon dioxide-free water to a 250 mL Erlenmeyer flask, then add phenolphthalein and adjust the pH to a pale pink color using sodium hydroxide. A 10 g sample is weighed into a conical flask and titrated with standardized sodium hydroxide solution until a faint pink color appears. The ending isn’t pleasing to watch.
This post was last edited by qqgd on 2026-1-2 at 08:46. Based on your analysis steps, you are using the direct titration method, that is, standard base is used to titrate any existing acid. The presence of dichloropropane may cause difficulties or delays in determining the endpoint, especially for those without extensive experience in titration analysis. If the method permits, you can try using back-titration: add an expected excess of standard base to neutralize the acid in the sample, and then back-titrate the excess base with standard acid; this may make it easier to determine the titration endpoint. Alternatively, a pH electrode can be used to indicate the endpoint, with a fixed titration pH value determining it. It is best to conduct a control test with a standard sample simultaneously during back-titration. The acids present in the dichloropropane sample are most likely some organic acids with weak acidity; the sharp change in pH at the endpoint is not distinct, which is the main reason why it is difficult to determine the titration endpoint.
According to industry standards, the acidity should be low; it’s not necessarily appropriate to use an excess of base followed by acid back-titration. Potentiometric titration or ordinary titration can be tried. Placing a white porcelain plate under the Erlenmeyer flask and using appropriate lighting should make it possible to observe a light pink color. Contact us if you have any further questions.