To know the method of preparation and standardization of potassium permanganate standard solution. To grasp the principle, the conditions and the method of permanganate titration. To grasp the determination of hydrogen peroxide in hydrogen peroxide solution with permanganate method.
Principle
Permanganate titration with potassium permanganate solution (KMnO4) as standard solution is one of the oxidation-reduction titration methods. KMnO4 is a vigorous oxidant in an acidic solution, and its electrode reaction and standard electrode potential φ are as follows, MnO4- + 8H+ Mn2+ + 4H2O φ = + l.51V
The contents of many reducible substances can be determined directly or indirectly by the permanganate method.
The titration reaction of permanganate method should be carried out in sulfuric acid (H2SO4) solution. Because nitric acid (HNO3) is of oxidization, and because hydrochloric acid (HCl) is of reduction, they are unavailable to adjust the acidic concentration of solution. In general, the available concentration of H2SO4 is 1 c( H2SO4)=1~2molL-l. If the acidic concentration is too high, it will result in 2 decomposition of KMnO4: 4MnO4- + 12H+ 4Mn2+ + 5O2 + 6H2O
If it is too low, the reduction product of KMnO4 is MnO2 which is brown deposit, and this kind of reaction can not be used in titration reaction, either. KMnO4 on sale usually contains impurities such as manganese dioxide, chloride, sulfate, nitrate and so on. Therefore, it can not be used directly in the preparation of standard solution. Moreover, because the oxidization ability of KMnO4 is strong, and it readily reacts with reductive substances such as organic impurities in water, ashes in air and so on, it easily decomposes when exposed to light. When it is prepared, its solution must be boiled or be dissolved with cold distilled water and then kept in brown reagent bottle in dark.
The primary standard substances used in standardizing KMnO4 solution include sodium oxalate(Na2C2O4), oxalic acid (H2C2O4 2H2O), arsenic trioxide (As2O3) etc because it has no crystalline water or water absorption, Na2C2O4 is often used. In H2SO4 solution the reaction between KMnO4 and Na2C2O4 is 2MnO4- + 5C2O42- + l6H+ CO2 + 2Mn2+ + 8H2O
The reaction carries on under the condition of 75~85ºC, so that the presence of Mn2+ functions as catalyst. At the beginning of titration, the KMnO4 solution must be titrated drop by drop for the speed of reaction is very slow; during the course of titration, Mn2+ is gradually produced in the solution, and it increases the reaction speed, so titration speed may be increased slightly, but don’t drop KMnO4 solution with a flow. Otherwise, KMnO4 solution will decompose without reacting with C2O42in time in hot acidic solution as follows: 4MnO4- + l2H+ 4Mn2+ + 5O2 + 6H2O
Because KMnO4 solution itself has color, and when the reaction reaches its stoichiometric point in the course of titration, slightly excessive amount of KMnO4 solution is added to make the solution pink, and no additional indicator is needed. In acidic solution, KMnO4 can oxidize H2O2, and the reaction is: 2MnO4- + 5H2O2 + 6H+ 2Mn2+ + 5O2 + 8H2O
Because H2O2 readily decomposes when heated, the titration must carry on under room temperature.
H2O2 with mass concentration ρ(H2O2) ==0.025~0.035kg L-l is a common medical sterilizing reagent. The mass concentration of H2O2 on sale is ρ(H2O2)=0.03kg L-1or ρ(H2O2))=0.3kg L-1, which is extremely instable, and before titration it is necessarily diluted with distilled water to a certain mass concentration to reduce sampling error.
Apparatus Analytical balance; wash bottle; acid buret of 50mL; pipet of 25mL; beaker of 50mL;, beaker of 500mL；measuring pipet of 5mL; volumetric flask of 250mL; Erlenmeyer flask of 250mL; measuring cylinder of l0mL; 1 Reagents c( H2SO4)=6molL-1 ; solid KMnO4 ; solid Na2C2O4 (dried in oven for 2
2h under l05ºC ~ l 10ºC); hydrogen peroxide ρ(H2O2)=0.03kgL-l;
Preparation of solution of c( KMnO4)=0.02 mol L-l .
Scale about 0.2g solid KMnO4 on balance, put it in a 500mL of beaker, add 250mL of distilled water into it, heat and stir it until the solid is dissolved completely. After the solution turn co1d, pour it into a brown reagent bottle, shake it even and block it up tightly with the cork. After standing still in dark for 7~l0 days, the solution is filtered with a funnel (or draw the upper solution out of the bottle with a sucker). Transfer the filtered solution to another clean brown reagent bottle, and put it in dark. Preparation of Na2C2O4 standard solution.
Precisely scale primary standard substance of Na2 C2O4 0.35~0.40g on an analytical balance, put it in a little beaker, dissolve it with about 20mL of distilled water, then transfer the solution to a 250mL volumetric flask, wash the little beaker 2~3 times with distilled water, transfer all the solution to the volumetric flask, add distilled water to the scale, and finally shake it even.
Standardization of KMnO4 solution. Draw 25.00mL Na2C2O2 standard solution with a pipet into an Erlenmeyer flask of 1 250mL, and add l0mL of c( H2SO4) =6molL-l solution in it, and heat it until the 2 vapor rises (75 oC ~80oC), but don’t make it boil. During continuous shaking , titrate it with KMnO4 solution drop after drop only when the pink of the first drop fades, increasing the dropping speed gradually, but never drop KMnO4 solution in a flow[2]. When the stoichiometric point of titration reaction is reached, the pink fades slowly, so the dropping speed should be decreased again, and the flask should be shaken continuously until the solution shows a pink color. If the color does not fade within 30 seconds, it means the end point of the titration . Make the record, repeat the above operations twice, and calculate the accurate concentration of KMnO4 standard solution.
Determination of H2O2 in hydrogen peroxide. Transfer 2.50mL hydrogen peroxide with measuring pipet into a 250 mL volumetric flask, dilute it to scale with distilled water, and mix it completely. That is the sample solution. Transfer 25.00mL sample solution with pipet into a 250 mL Erlenmeyer 1 flask, add 5mL solution of c( H2SO4)=6mol L-‘, mix it even, and then titrate it to the 2 pink with KMnO4 standard solution. If the color does not fade within 30 seconds it means the end point of the titration. Make the record, and repeat the above operations twice, and calculate the mass concentration of H2O2. The formula is as follows, 1.
Where V (sample) is the sucked volume of the sample solution , V1 is prepared total volume of hydrogen peroxide solution to be measured, V2 is transferred volume of 1 hydrogen peroxide solution to be measured, V( KMnO4) is the volume of KMnO4 5 1 solution consumed for titration, and M( H2O2)=17.0lg mol-1. 2
Questions
l. When the concentration of potassium permanganate is standardized with sodium oxalate, why should it be carried on in the presence of sulphuric acid? How about hydrochloric acid or nitric acid? Why?
2. When KMnO4 solution is standardized, why should the solution be heated to75 o C ~85 oC? Is it feasible for the solution to be heated to above 90oC? Why?
