If you have a solution containing 0.10 M Fe+3 and 0.10 M Ca+2 and you want to titrate the Fe+3 with EDTA, how much error is there due to the reaction of Ca+2 with the EDTA at the Fe+3 equivalence point if you titrate at the following pHs.
3.0
6.0
10.0
At what pH would you titrate the Fe+3?
To determine the amount of error due to the reaction between Ca+2 and the EDTA at the Fe+3 equivalence point, we need to consider the stability constants of the metal-EDTA complexes.
EDTA (ethylenediaminetetraacetic acid) is a chelating agent that forms stable complexes with metal ions, including both Fe+3 and Ca+2. The complex formed between Fe+3 and EDTA is more stable than the complex formed between Ca+2 and EDTA. Therefore, at the Fe+3 equivalence point, if Ca+2 is present, it will react with EDTA to form a Ca-EDTA complex. This reaction can introduce an error in the determination of Fe+3 concentration.
The stability constants of the complexes can be used to calculate the extent of the reaction between Ca+2 and EDTA. However, I don't have the stability constants for Fe+3-EDTA and Ca+2-EDTA at different pHs, which are required for a quantitative analysis of the error.
Since the given pH values are 3.0, 6.0, and 10.0, we can make some qualitative observations:
1. At pH 3.0: This is an acidic pH, where the concentration of H+ ions is high. At this pH, the reaction between Ca+2 and EDTA will be suppressed to some extent because the H+ ions will compete with Ca+2 for complexation with EDTA. Therefore, the error due to the reaction between Ca+2 and EDTA at the Fe+3 equivalence point may be relatively lower compared to higher pH values.
2. At pH 6.0: This is a slightly acidic to neutral pH range. At this pH, both Fe+3 and Ca+2 will have some affinity for complexation with EDTA. The error due to the reaction of Ca+2 with EDTA at the Fe+3 equivalence point may be moderate.
3. At pH 10.0: This is an alkaline pH, where the concentration of OH- ions is high. At this pH, the OH- ions can compete with Ca+2 for complexation with EDTA, reducing the error due to the reaction between Ca+2 and EDTA at the Fe+3 equivalence point. Therefore, the error may be lower compared to the other pH values.
Considering the above information, the best pH for titrating Fe+3 with EDTA would be pH 10.0. At this pH, the error due to the reaction between Ca+2 and EDTA at the Fe+3 equivalence point is expected to be the lowest among the given pH values.