On the topic of selecting acid-base indicators
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Today, the acidity of a waste nitric acid solution (containing small amounts of copper and nickel: about 10 g/L of copper + nickel) was tested. I saw the lab technician using a standard sodium hydroxide titrant, with phenolphthalein as an indicator. I said it was wrong to do this, and suggested using a mixed indicator or methyl orange instead. The lab technician asked why I said that phenolphthalein is not suitable to use when strong acids and weak base salts are present. The color-changing range of phenolphthalein is from pH 8.2 to 10.0; the pH has to rise after copper and nickel form precipitates for it to change color, which leads to higher detection results. The laboratory technician said, \"Of course, when there are standards, we follow them. But if there are no standards, how can I determine which indicator to use?\" Could you give me a way that’s easy to remember, so I can save it and look at it when needed? I say that it’s best to study systematically the principles for selecting acid-base indicators; various acid-base indicators have specified preparation methods and color change ranges, and the choice should be based on the substance being tested – it’s not something that can be explained in just a few sentences. ==================================================== Well, if we have to make a preliminary judgment, could we say to him the following: For testing principles that do not require high precision or for which there is no clear method, phenolphthalein is the preferred choice as it makes it easy to determine the titration endpoint; of course, other options can also be used. For substances with different strengths, the principle is \"acid added to base until it becomes slightly acidic; base added to acid until it becomes slightly basic.\" In other words, a strong acid should be added to a weak base until the mixture becomes slightly acidic, while a strong base should be added to a weak acid until the mixture becomes slightly basic. Weak acids and weak bases are generally not suitable as standard titrant solutions. It’s because this lab technician has a low level of education, that’s why he can only express things in that way. However, I’m worried that it might mislead people, and I don’t know how to explain it so that it can be understood; P
This post was last edited by Yang Qing on 2021-1-8 at 13:40. Should we add another sentence: When testing waste strong acids for salt content, it’s best to stop when the pH reaches a slightly acidic level? Is a methylene red-methylene blue mixed indicator suitable? It seems like something quite basic, but I really can’t explain it right away; please forgive me, fellow sailors :)
This post was last edited by qugd on 2021-1-11 at 10:32. It is possible to find out the lowest pH value at which the metal ions present in spent acid begin to hydrolyze, so as to ensure that the titration endpoint is determined before such hydrolysis occurs. This type of waste acid is likely to be used in metal surface treatment, acidic plating solutions, or as an electrolyte in hydrometallurgical processes; its acidity level is not too low. Therefore, when determining the titration endpoint, the pH value lies in the near-neutral or slightly acidic range, resulting in minimal measurement errors. The duty of laboratory technicians is to conduct routine analyses in accordance with standards; their knowledge of analytical chemistry is limited, so it is sufficient to explain things clearly to them. It can be verified by using the prepared simulated samples. Additionally, the sample can be appropriately diluted to reduce the concentration of transition metal ions, which also helps to better control the tendency for hydrolysis. Copper and nickel ions are not a problem; it’s the trivalent iron and trivalent aluminum ions that pose more difficulties. High-valent metal ions cause hydrolysis, resulting in lower pH levels, which can affect acidity measurements. If possible, it is recommended to use a pH meter to determine the end point at a fixed pH value, thereby reducing errors resulting from human judgment.
Our samples mainly consist of waste acids from metal surface treatment and electrolytic waste liquids, etc., and generally contain various metal elements (copper, iron, zinc, nickel, etc.). When the acidity is high, it needs to be diluted before measurement. Preparing simulation samples for verification helps improve laboratory technicians’ understanding; it allows them to get a feel for them during their free time. Thank you, Teacher Q, for your guidance.
Analysts simply follow the standards, adhere to the operating procedures and regulatory requirements, and carry out their tasks in accordance with the rules. It’s impossible to understand, and not many analysts can determine how to make a choice. Any operation must follow established rules and be based on solid evidence. Standardize it and just implement it.