PRACTICUM 03 QUANTITATIVE ANALYSIS: REDOX METHOD Brief Theory Permanganometry is a titration carried out based on the reaction with potassium permanganate (KMnO4). This reaction focuses on the oxidation and reduction reactions that occur between KMnO4 and certain raw materials. Titration with KMnO4 has been known for more than a hundred years. Most titrations are carried out directly on oxidizable agents such as Fe+, soluble acids or oxalate salts and so on. Some metal ions that are not oxidized can be titrated indirectly with permanganometry such as: 1. Ca, Ba, Sr, Pb, Zn, and Hg (I) ions which can be precipitated as oxalate. After the precipitate is filtered and washed, it is dissolved in excess H2SO4 to form quantitative oxalic acid. This oxalic acid is finally titrated and the results of the titration can calculate the number of metal ions in question. 2. Ba and Pb ions can also be precipitated as chromate salts. After filtered, washed, and dissolved with acid, excess FeSO4 standard solution was also added. Some of the Fe2+ is oxidized by the chromate and the remaining amount can be determined by titrating it with KMnO4. The principle of permanganometric titration is based on oxidation and reduction reactions. Permanganometry is a titration based on redox reactions. In this reaction, MnO4- ions act as an oxidizing agent. MnO4- ions will change to Mn2+ ions in acidic conditions. This titration technique is usually used to determine the levels of oxalate or iron in a sample. In permanganometry, the titrant used is potassium permanganate. Potassium permanganate is easy to obtain and does not require an indicator unless a very dilute solution is used and has been used widely as an oxidation reagent for over a hundred years. A drop of permanganate imparts a clear pink color to the volume of solution in a titration. This color is used to indicate excess reagent. Potassium permanganate is standardized using sodium oxalate or as arsenic (III) oxide primary standards. The reaction that occurs in the process of standardizing potassium permanganate using sodium oxalate is: 5C2O4- + 2MnO4- + 16H+→10CO2 + 2Mn2+ + 8H2O The end of the titration is marked by the appearance of a pink color caused by excess permanganate. Potassium permanganate is a strong oxidizing agent. No indicators required. The weakness is in the HCl medium. Cl- can be oxidized, and so can the solution, having limited stability. Usually used in 0.1 N acid medium: MnO4- + 8 H+ + 5e-Mn2+ + 4 H2O The oxidation reaction to H2C2O4 runs slowly at room temperature. To speed up you need to warm up. Meanwhile, the reaction with As (III) requires a catalyst. The end point of permanganate is not permanent and the color can be lost due to reactions: 2 MnO4- + 3 Mn2+ + 2 H2O purple The solution in water is unstable and the water is oxidized by: 4 MnO4- + 2 H2O 4 MnO2 + 3 O2 + 4 OHIts decomposition is catalyzed by light, heat, acid-base, Mn (II) and MnO2 ions. MnO2 is usually formed from its own decomposition and is autocatalytic. To prepare a standard KMnO4 solution, the presence of MnO2 must be avoided. KMnO4 can be standardized against Na2C2O4. 2 MnO4- + 5 H2C2O4 + 6 H+ 2 Mn2+ + 10 CO2 + 8 H2O It is used for the analysis of Fe(II), H2C2O4, Ca and many other compounds. Potassium permanganate is rarely prepared by dissolving weighed quantities of its highly purified solid substance for example proanalytically in water, it is more usual to heat a freshly prepared solution to a boil and leave it on a steam bath for one or two hours then filter the solution in a non-reducing filter such as refined glass wool or through the sieve crucible of a glass maser. Permanganate reacts rapidly with many reducing agents based on this reagent, but some reagents require heating or the use of a catalyst to speed up the reaction. Were it not for the fact that many permanganate reactions are slow, many more difficulties would be encountered in the use of this reagent. For example, permanganate is an oxidizing agent, which is strong enough to oxidize Mn(II) to MnO2 according to the equation: 3Mn2+ + 2MnO4- + 2H2O→5MnO2 + 4H+ The slight excess of permanganate present at the end point of the titration is sufficient to cause the precipitation of a certain amount of MnO2. Special precautions must be taken in the preparation of permanganate solutions. Oxidized manganese catalyzes the decomposition of permanganate solution. Traces of MnO2 were originally present in the permanganate. Or formed as a result of the reaction between permanganate and traces of production agents in the water, leading to decomposition. This action usually consists of dissolving the crystals, heating to destroy the reducible substances and filtration through asbestos or sintered glass to remove MnO2. The solution is then standardized and if stored in the dark and not acidified the concentration will not change much for several months. The determination of iron in iron ores is one of the most important applications in permanganate titrations. The best acid for dissolving iron ore is hydrochloric acid and tin(II) chloride is often added to aid the solubility process. Before titrating with permanganate any iron (III) must be reduced to iron (II). This reduction can be carried out with a Jones reductant or with tin (II) chloride. Jones reductant is more recommended if the acid available is sulfuric considering that no chloride ions enter. If the solution contains hydrochloric acid, as is often the case, reduction with tin(II) chloride will be easier. Chloride is added to the hot solution of the sample and the reduction progress is followed by observing the disappearance of the yellow color of the iron ion. Tools and materials A. Tools - Test tubes and test tube racks - Stirring rod - Beaker - Measuring cup - Erlenmeyer - Burette - Statives and Claims B. Material - Potassium Permanganate (KMnO4) - Oxalic Acid (H2C2O4) - Sulfuric Acid (H2SO4) - Phosphoric Acid (H3P.O4) - Iron (III) Chloride (FeCl3) - Iron tablets (Fe) Work procedures 1. Procedure for Preparing 0.05 N KMnO4 Solution - Weighed 0.79 grams of KMnO4 crystals and put them in a beaker glass. - Add up to 100 ml of distilled water to the beaker glass. The solution is stirred thoroughly and heated until boiling. - After boiling, the solution is cooled. 2. Standardization Procedure for 0.05 N KMnO4 Solution - A total of 10 ml of H solution2C2O40.05 N into an Erlenmeyer and mixed with 10 ml H2SO41 N. - The solution was stirred thoroughly and heated to a temperature of 70-80 oC. - Titrate with KMnO4 solution until a stable rose red color is obtained. - Note the volume of KMnO4 used. - The above experiment was repeated three times. - Calculated KMnO4 concentration. 3. Procedure for Determining Iron (Fe) Content - A total of 0.001 g of Fe tablet sample dissolved in concentrated HCl was diluted to 100 mL of distilled water - Pipette 10 sample solutions and put them in an Erlenmeyer. - Added 10 ml H2SO41 N and 2 ml H3PO4 85%. - Titrate slowly with 0.05 N KMnO4 solution until a solution with a stable rose red color is obtained. - Note the volume of KMnO4 used. - The above experiment was repeated three times. - Calculate the Fe content in the sample.