Problems with complexometric titration
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When analyzing the aluminum ion content, an excess of EDTA standard solution is typically added, the pH value is adjusted to around 3, and heating is used to promote the complexation between aluminum ions and EDTA. Adjust the pH to 5–6 again, add xylene orange indicator, and titrate with zinc standard solution. However, it is said that aluminum ions have a blocking effect on xylene orange. If there is indeed a sealing effect, then cannot the aforementioned methods be used for aluminum ion analysis?
It exists within a specific pH range! ! ! ! ! ! ! !
This is a method for determining aluminum using back-titration, which avoids the problem of closure. During direct titration, the indicator cannot function properly due to masking effects.
As far as I know, the blocking effect occurs because the stability of the complex formed between a certain ion and the indicator is greater than that of the complex formed between it and the reactant (EDTA). Therefore, its stability remains unchanged whether it is titrated directly or by back-titration. Therefore, I am not very convinced by the claim that back-titration avoids the problem of indicator interference. Friend on the 2nd floor, could you explain in detail at what pH value the indicator gets blocked?
This post was last edited by xwtsq on 2010-6-27 at 20:46. Aluminum ions cannot be directly titrated with EDTA because the reaction rate between aluminum ions and EDTA is too slow, failing to meet the conditions for direct titration. Saying it has a blocking effect on indicators means that in the presence of trace amounts of aluminum ions, xylene orange is blocked if lead, zinc, chromium, mercury, and other such ions are titrated directly. One can look up the condition stability constants for the reaction between aluminum ions and EDTA as well as xylene orange.
Reply to 5# xwtsq: Can it be assumed that in a normal titration, at the endpoint the EDTA displaces the indicator that is complexed with the metal ions, thereby causing a color change? When the indicator is blocked, since the stability of the complex formed between this metal ion and the indicator exceeds that of the complex formed with EDTA, EDTA is unable to displace the indicator, preventing proper color development If this view is correct, then the indicator blocking phenomenon is inevitable, regardless of whether aluminum ions are present in large quantities. Unless under different pH conditions, the complexation stability of aluminum with EDTA exceeds that of aluminum with the indicator.