10.9
Titulação Redox: Outros Agentes Oxidantes e Redutores
Além do iodo, outros agentes oxidantes ou redutores podem servir como titulantes em titulações redox. Titulantes oxidantes comuns incluem KMnO_4, céri…
Além do iodo, vários outros agentes oxidantes ou redutores podem ser empregados como titulantes em titulações redox.
O permanganato de potássio é um titulante oxidante forte que se reduz de manganês (VII) para manganês (II) em uma solução fortemente ácida.
Aqui, o permanganato roxo atua como um autoindicador porque o manganês (II) é incolor.
Os analitos redutores também podem ser titulados usando soluções ácidas de cério (IV) enquanto usam indicadores redox para detecção de ponto final. Na titulação, o analito é oxidado enquanto o cério (IV) se reduz a cério (III).
O dicromato de potássio é um titulante oxidante moderadamente forte que se reduz de cromo (VI) para cromo (III) em soluções ácidas. O ponto final do dicromato é observado usando um indicador redox.
Em contraste, a maioria dos titulantes redutores é oxidada pelo oxigênio atmosférico, por isso raramente são usados para a titulação direta de analitos oxidantes.
Por exemplo, o tiossulfato é usado indiretamente como um titulante redutor típico para titulações iodométricas.
No entanto, uma solução acidificada de ferro (II) pode ser usada para titular alguns analitos oxidantes fortes, como cério (IV), cromo (VI) e vanádio (V).
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.