5.7
EDTA: 직접, 역, 변위 적정
금속 이온 분석을 위한 EDTA 적정 유형에는 직접 적정, 역적정, 대체 적정이 포함됩니다.
직접 적정에는 금속 이온 용액을 원하는 pH로 완충하고 종말점까지 표준 EDTA로 직접 적정하는 작업이 포함됩니다. 최적의 pH는 금속-EDTA의 큰 조건부 형성 상수와 용액…
EDTA 적정에는 직접적정, 역적정, 변위적정, 간접적정, 알칼리 적정이 포함됩니다.
직접 적정에서 금속 이온 용액은 EDTA로 직접 적정됩니다—예를 들어, 납(II) 이온 용액은 pH 10에서 완충되며, 주석산염은 수산화납(II) 침전을 방지하기 위해 이미 존재합니다.
이 솔루션은 종말점에 도달할 때까지 표준 EDTA로 직접 적정됩니다.
그러나 일부 금속 이온은 너무 느리게 반응하거나 지표를 차단하고 일부는 EDTA 없이 침전됩니다. 따라서, 알려진 과량의 EDTA 용액이 첨가되고 두 번째 금속 이온으로 역 적정됩니다.
예를 들어, 알루미늄 이온 용액과 과량의 EDTA를 혼합한 후 나머지 EDTA는 시각적 지시약과 표준 아연 이온 용액으로 역적정되어 종말점에 도달합니다.
좋은 지표가 없는 금속 이온의 경우 변위 적정을 사용할 수 있습니다.
예를 들어, 마그네슘-EDTA 용액이 과도하게 함유된 칼슘 이온을 적정하면 마그네슘이 대체되어 안정적인 칼슘-EDTA 복합체를 형성합니다. 변위된 마그네슘 이온은 종말점에 도달하기 위해 표준 EDTA로 적정됩니다.
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Q1: When should you use direct titration with EDTA instead of back-titration?
Direct titration works best when metal ions react quickly with EDTA and don't interfere with indicator visibility. The metal ion solution is buffered to the desired pH and directly titrated with standard EDTA until the endpoint is reached. However, if metal ions react too slowly, block indicators, or precipitate without EDTA, back-titration becomes necessary instead.
Q2: What is the purpose of auxiliary complexing reagents in EDTA titrations?
Auxiliary complexing reagents prevent metal hydroxide precipitation and maintain the concentration of free metal ions in solution during titration. For example, tartrate is added to a lead(II) ion solution buffered at pH 10 to prevent lead(II) hydroxide from precipitating, ensuring accurate titration results with standard EDTA.
Q3: How does back-titration work when metal ions block EDTA indicators?
Back-titration adds a known excess of EDTA solution to the metal ion, then buffers the solution to the desired pH. The remaining EDTA is then back-titrated with a standard solution of a second metal ion, such as zinc, using a visual indicator to reach the endpoint. This method works when direct titration fails due to slow reactions or indicator interference.
Q4: Why is displacement titration used for calcium ions instead of direct titration?
Displacement titration is used when metal ions do not react adequately with the indicator. Since solochrome black is a poor indicator for directly titrating calcium ions, an excess magnesium–EDTA solution is used instead. Calcium ions displace magnesium to form a more stable complex, and the released magnesium is then titrated with standard EDTA to determine the endpoint.
Q5: What role does pH play in optimizing EDTA titrations?
The optimum pH ensures a large conditional formation constant of the metal-EDTA complex and maintains visibility of the free indicator color in solution. Buffering the metal ion solution to the desired pH before titration with standard EDTA is essential for accurate results and reliable endpoint detection across all titration methods.
Q6: How do you determine which EDTA titration method to use for a specific metal ion?
Choose direct titration if the metal ion reacts quickly and doesn't block indicators. Use back-titration if the metal ion reacts slowly, precipitates, or interferes with indicator visibility. Use displacement titration if the metal ion lacks a suitable indicator. Each method requires buffering to the appropriate pH and using standard EDTA or a second metal ion solution to reach the endpoint.
Q7: What happens to magnesium ions during displacement titration of calcium?
During displacement titration, calcium ions are titrated with excess magnesium–EDTA solution. The calcium ions displace all magnesium ions from the complex to form a more stable calcium–EDTA complex. The free magnesium ions released into solution are equivalent to the original calcium ion concentration and are then titrated with standard EDTA to obtain the endpoint.