Kinds Of Redox Titrations
Kinds Of Red-ox Titrations
Red-ox titration is a kind of titration based on oxidation-reduction reactions. It is a method of volumetric analysis wherein the sought-for substance is made to react by a titration technique with a measurable volume of a standard solution of known concentration.
In Oxidation-Reduction method of titration, a reducing substance is titrated with a standard solution of an oxidizing agent, or an oxidizing substance is titrated with a standard solution of a reducing agent.
More titration methods are based on red-ox reactions than any other types of reactions because so many elements may be easily oxidized or reduced.
The following are the different kind s of red-ox titrations:
I. Permanganate Process
A large number of reducing agents may be determined directly by titration with standard potassium permanganate, and many oxidizing agents may be determined indirectly. Even substances that are neither oxidizers nor reducers may be determined, by causing them to react stoichiometrically with a substance that can be titrated with permanganate.
Some substances that may be determined by the permanganate process are:
A. Determination of Iron in Limonite
- Limonite is one of the principal ores of iron. The other iron ores are hematite), and magnetite. There are three steps in the determination of Fe in an ore: (1) extraction, (2) pre-reduction, and (3) titration. Each step may be accomplished in several ways.
- Iron may be efficiently extracted from these ores with hot concentrated hydrochloric acid, but not with concentrated sulfuric acid or nitric acid. The efficiency of HCl extraction is ascribed to the formation of ferric chloride complexes.
- Stannous chloride increases the rate of extraction by reducing Fe(III) to Fe(II), and is particularly effective for hematites and magnetites.
- Titration of the Fe(II) is most frequently performed with potassium dichromate or potassium permanganate.
- In the determination of Fe in limonite, after pre-reduction with stannous chloride, manganese sulfate solution is added to form a colorless phosphate with Fe that prevents the formation of ferric chloride which obscures the potassium permanganate end point. It is then titrated with potassium permangante solution to avoid re-oxidation of Fe(II) to Fe(III) until a pale pink color remains for 15 seconds with constant shaking.
B. Determination of Calcium in Limestone
- The sample is extracted with hot HCl, which dissolves calcium carbonate and breaks down some silicates.
- The solution is diluted and ammonium oxalate is added. This causes no precipitation of calcium oxalate.
- The solution is slowly neutralized with ammonia to pH 3.7 - 4.3, whereupon calcium oxalate precipitates quantitatively.
- The calcium oxalate precipitate together with silicon dioxide is filtered through a gooch crucible and washed sparingly, since calcium carbonate is only moderately insoluble.
- The washed calcium oxalate is metathesized with dilute sulfuric acid, and the oxalic acid) liberated is then titrated with standard potassium permanganate.
C. Determination of the Oxidizing Power of Pyrolusite (impure manganese dioxide)
- An accurately weighed sample of manganese dioxide and sodium oxalate are mixed with dilute water and 6 normal sulfuric acid is added. The sulfuric acid should be freshly prepared.
- The mixture is then warmed gently until the evolution of carbon dioxide ceases and the residue is almost white or until there are no more black particles in the solution.
- The solution should not be allowed to boil as to evaporate the water, making the sulfuric acid concentrated. Do not heat the sides of the container as this may cause the reduction of the sodium oxalate. Then the solution is diluted with water and heated to just below boiling. While still hot, it is titrated with potassium permanganate to pale pink color.
II. Dichromate Process
The principal advantages of potassium dichromate as a standard oxidizing agent are (1) solutions may be prepared from potassium dichromate as a primary standard, (2) potassium dichromate solutions are very stable and may even be boiled for short times without change in titer, (3) potassium dichromate is only a moderately strong oxidizer which, does not oxidize chloride. Hence, the chief use of standard potassium dichromate is in titrating Fe(II) in the presence of chloride. On the other hand, the low oxidizing power of potassium dichromate is a drawback, for many internal red-ox indicators are unsuitable with it.
Determination of Chromium in chromite
- One of the principal minerals of Cr is chromite. Chromium is not easily extractable from ores by aqueous or concentrated acid solutions, and a first step in the analysis is to render the Cr leachable by fusion with a flux. An oxidizing flux, such as sodium peroxide oxidizes the Cr to sodium dichromate which is water soluble.
- Once cooled, the fused melt is leached with water to extract the sodium dichromate, and the excess sodium peroxide is destroyed by boiling.
- The solution is then acidified, an excess of standard ferrous sulfate is added, and the excess is back-titrated with standard potassium dichromate.
III. Iodimetric Method
• Direct method (Iodine is the titrating agent)
• Indirect method (Sodium thiosulfate is the titrating agent and the indicator is starch)
The basis of all indirect iodimetric methods is the titration of iodine with standard sodium thiosulfate. The reaction proceeds rapidly, smoothly, and stoichiometrically at pH 5-9. The same is true of the reverse titration of sodium thiosulfate with iodine, but this is seldom done.
Elementary Quantitative Analysis by W. J. Blaedel & V. W. Meloche; 2nd Edition.
Laboratory Manual In Quantitative Analysis (Classroom Manual); 1974
Classroom notes in Quantitative Chemistry