5.7
EDTA: 直接滴定、逆滴定、および置換滴定
金属イオン分析用の EDTA 滴定の種類には、直接滴定、逆滴定、置換滴定などがあります。
直接滴定には、金属イオン溶液を目的の pH に緩衝し、標準 EDTA で終点まで直接滴定することが含まれます。最適な pH により、金属 - EDTA の条件付き生成定数が大きくなり、溶液中の遊離指示薬の色の視…
EDTA滴定には、直接滴定、バック滴定、変位滴定、間接滴定、アルカリ滴定が含まれます。
直接滴定では、金属イオン溶液をEDTAで直接滴定します—たとえば、鉛(II)イオン溶液をpH 10で緩衝し、酒石酸塩がすでに存在して水酸化鉛(II)の沈殿を防ぎます。
この溶液は、エンドポイントに到達するまで、標準のEDTAで直接滴定されます。
ただし、一部の金属イオンは反応が遅すぎたり、インジケーターをブロックしたり、EDTAなしで沈殿するものがあります。そのため、既知の過剰のEDTA溶液を添加し、2つ目の金属イオンでバック滴定します。
例えば、アルミニウムイオン溶液を過剰なEDTAと混合した後、残りのEDTAは、視覚的なインジケーターと標準的な亜鉛イオン溶液で逆滴定され、終点に到達します。
良好な指標を持たない金属イオンには、変位滴定を使用することができます。
例えば、カルシウムイオンを過剰なマグネシウム-EDTA溶液で滴定すると、マグネシウムが置換され、安定したカルシウム-EDTA複合体が形成されます。置換されたマグネシウムイオンは、標準的なEDTAで滴定され、終点に到達します。
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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.