10.9
氧化还原滴定法：其他氧化剂和还原剂
除了碘以外，还有其他的氧化剂或还原剂可以用作氧化还原滴定法中的滴定剂。常见的氧化滴定剂包括高锰酸钾（KMnO_4）、硫酸铈（IV）和重铬酸钾（K_2Cr_2O_7）。氧化滴定剂的选择取决于稳定性、成本、分析物强度以及分析物和滴定剂之间的反应速率等因素。高锰酸钾是一种强氧化滴定剂，能够在强酸性溶液中从…
除了碘之外，还可使用多种其他氧化剂或还原剂作为氧化还原滴定中的滴定剂。
高锰酸钾是一种强氧化性滴定剂，在强酸性溶液中，它会从七价锰还原为二价锰。
在此，紫色的高锰酸根作为自身指示剂，因为锰(II)是无色的。
还原性分析物也可使用酸性四价铈溶液进行滴定，同时利用氧化还原指示剂检测终点。在滴定过程中，分析物被氧化，而四价铈被还原为三价铈。
重铬酸钾是一种中等强度的氧化性滴定剂，在酸性溶液中可将铬(VI)还原为铬(III)。重铬酸盐的滴定终点通过氧化还原指示剂来观察。
相比之下，大多数还原性滴定剂会被空气中的氧气氧化，因此很少用于氧化性分析物的直接滴定。
例如，硫代硫酸盐作为碘量滴定中典型的还原性滴定剂被间接使用。
然而，酸化后的二价铁溶液可用于滴定某些强氧化性分析物，如四价铈、六价铬和五价钒。
View the full transcript and gain access to JoVE Core videos
Q1: Why is potassium permanganate used as a self-indicator in redox titrations?
Potassium permanganate acts as a self-indicator because it changes color during the reaction. The purple permanganate reduces to colorless manganese(II) in acidic solutions, making the endpoint visually obvious when the purple color persists, indicating all analyte has been oxidized and the titration is complete.
Q2: What is the difference between potassium permanganate and potassium dichromate as oxidizing titrants?
Potassium permanganate is a strong oxidizing titrant that reduces from manganese(VII) to manganese(II) and serves as a self-indicator due to its color change. Potassium dichromate is a moderately strong oxidizing titrant that reduces from chromium(VI) to chromium(III) and requires a redox indicator for endpoint detection since its color change is insufficient.
Q3: How does cerium(IV) function in the titration of reducing analytes?
Acidic cerium(IV) solutions oxidize reducing analytes while cerium(IV) itself reduces to cerium(III). Since this reaction lacks a distinctive color change, redox indicators are employed for endpoint detection to signal when the titration is complete and all analyte has been oxidized.
Q4: Why are most reducing titrants unsuitable for direct titration of oxidizing analytes?
Most reducing titrants are readily oxidized by atmospheric oxygen, making them unstable for direct titrations of oxidizing analytes. However, acidified iron(II) solutions can titrate strong oxidizing analytes like cerium(IV), chromium(VI), and vanadium(V) because iron(II) remains sufficiently stable under acidic conditions.
Q5: What role do redox indicators play in dichromate titrations?
Potassium dichromate titrations require redox indicators because dichromate's color change is not sufficiently distinct for reliable endpoint detection without additional chemical assistance. These indicators provide a clear visual signal when the titration reaches completion, ensuring accurate results in analytical measurements.
Q6: How is thiosulfate used as a reducing titrant in iodometric titrations?
Thiosulfate is used indirectly as a reducing titrant in iodometric titrations because it is readily oxidized by atmospheric oxygen, making direct titration impractical. This indirect approach through redox titration iodimetry and iodometry allows thiosulfate to effectively determine oxidizing analytes despite its instability.
Q7: What factors determine the choice of oxidizing titrant in a redox titration?
The choice of oxidizing titrants depends on stability, cost, analyte strength, and reaction rate between the analyte and titrant. Permanganate provides self-indication, cerium(IV) works with redox indicators, and dichromate offers moderate oxidizing strength, each suited to different analytical requirements and sample compositions.