hydrogen peroxide solution to be measured, V( KMnO4) is the volume of KMnO4 5 4. 10Fe3+ + 8H2O, 2MnO4− + 10I− + 16H+ → 2Mn2+ + 5I2
exists in the form of dark purple crystals which melts at 513 K. It is sparingly soluble
Na 2 C 2 O 4 ) M ( Na 2 C 2 O 4 ) The potassium … After the solution turn co1d, pour it into a brown reagent bottle, shake it even and block it up tightly with the cork. Manganate ions are
Manganese dioxide is insoluble, whereas permaganate is very soluble. + 5CH3CH2OH → 2K2SO4 + 2MnSO4
H2C2O4 To prepare 1000 mL of a 0.1 mol/L solution of Potassium permanganate we have to dissolve 15.8032 g of KMnO4 (100 % purity) in deionized or distilled water. which subsequently decomposes explosively. 6KOH + 3MnO 2 + 6KClO 3 → 3K 2 MnO 9 + 6KCl + 3H 2 O. our expert writers, Copying content is not allowed on this website, Ask a professional writer to help you with your text, Give us your email and we'll send you the essay you need, Please indicate where to send you the sample, Hi, my name is Jenn 1 Determination of H2O2 in hydrogen peroxide Make the record, and repeat the above operations twice, and calculate the mass concentration of H2O2. + 2Mn2+ + 8H2O The titration reaction of permanganate method should be carried out in sulfuric acid (H2SO4) solution. Transfer the contents to a 250 ml beaker containing cold distilled water and stirred thoroughly, breaking up the crystals with a glass rod to effect solution. conc H2SO4, it decomposes to form manganese heptoxide,
to potassium manganate: Powdered ore is fused
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). salts, oxalates, hydrogen peroxide and iodides. [3] The end point of KMnO4 titration is not very stable. 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. This present invention discloses a polymer/potassium permanganate composite film obtainable by a process comprising the steps of using silane coupling agent for surface treatment of potassium permanganate, mixing 0.15-0.75 g of modified potassium permanganate with 10-20 g of polymers together to obtain a mixture, and coating the mixture to obtain a composite film. Activated charcoal in multiple doses is useful for reducing the absorption of toxins from the gut. ).Some health conditions may make you more susceptible to the side-effects of the drug. Potassium permanganate
The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution. Principle 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. 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. To grasp the determination of hydrogen peroxide in hydrogen peroxide solution with permanganate method. In general, the available concentration of H2SO4 is 1. c( H2SO4)=1~2molL-l. 4. Due to decreased GI motility, a lavage may be useful even during the late stage of poisoning. Line a large bucket or container with a plastic bag and fill it with water that’s warm but not too hot to handle. The mixture is boiled and evaporates and the residue is heated in iron pans until it has acquired a pasty consistency. Potassium permanganate is commercially prepared by mixing solution of KOH and powdered manganese oxide, with oxidizing agents like potassium chlorate. If the color does not fade within 30 seconds it … Preparation of 0.1 N Potassium Permanganate Solution Dissolve 3.2 g of Potassium Permanganate (KMnO4) in 100 mL of water and dilute the solution with water to 1 L. Allow the solution to stand in the dark for two weeks and then filter through a fine-porosity sintered-glass crucible. alkali metal hydroxides, the permanganate ion is converted into manganate. [2] It is not available to titrate too quickly with KMnO4 solution; otherwise there isn’t enough time for KMnO4 solution to react with C2O42-. 30 seconds means reaching the end point of titration. If the potassium permanganate solution is prepared from stock solid potassium permanganate, it is important to remove manganese dioxide from the system. 2Mn2+ + 5O2 That is the sample solution. potassium permanganate are listed below. with KOH in the presence of air or oxidising agents like KNO3 or
2. The preparation involves the following steps. permanganate. KMnO4 is known as Bayer’s reagent. reagent for detecting unsaturation in an organic compound. 2 vitamins, herbal supplements, etc. In the first stage, pyrolusite, which is the mineral form of manganese dioxide, is fused with potassium hydroxide and heated in air or with potassium nitrate (a source of oxygen). 1 1 ), allergies, pre-existing diseases, and current health conditions (e.g. treatment of various skin infections and fungal infections of the foot. C ~85 oC? The reaction carries on under the condition of 75~85ºC, so that the presence of, Mn2+ functions as catalyst. 1.To know the method of preparation and standardization of potassium permanganate standard solution. in cold water but, fairly soluble in hot water. 4Mn2+ + 5O2 Apparatus and reagents Potassium permanganate is prepared from pyrolusite (MnO2) ore. 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 Help, Use multiple resourses when assembling your essay, Get help form professional writers when not sure you can do it yourself, Use Plagiarism Checker to double check your essay, Do not copy and paste free to download essays. "You must agree to out terms of services and privacy policy", Don't use plagiarized sources. Potassium permanganate is commercially prepared by mixing solution of KOH and powdered manganese oxide, with oxidizing agents like potassium chlorate. (2016, Jul 24). Potassium permanganate
l. When the concentration of potassium permanganate is standardized with sodium oxalate, why should it be carried on in the presence of sulphuric acid? To carry douching in gynecology, urology and washes in gastric lavage prepare 0, 02-0, 1% solution. The preparation and standardization of 0.1 N potassium permanganate solution Introduction Potassium permanganate (KMnO4, m. wt. Is it feasible for the solution to be heated to above 90oC? CO2 + CO + H2O This activity aims to convey an idea of the scale at which matter exists, of how tiny atoms and molecules are. 5NO3− + 3H2O, 2KMnO4 + 3H2SO4
For example, ethylene can be converted into ethylene glycol and
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 Purposes This can be done without interruption of the gas flow if sufficient … this reaction is used as a test for unsaturation. As a result, it decomposes in the hot acidic solution: 4MnO4- + l2H+ Preparation and Properties of Potassium Permanganate and Potassium Dichromate: General Properties of First Row Transition Metal Compounds Oxides and oxometal ions: (i) Oxides of metals in low oxidation states + 2 and + 3 (MO, M 3 O 4 and M 2 O 3 ) are generally basic except Cr 2 … It is used for the
H2C2O4 (i) It oxidises ferrous
1. Decant the solution through a small plug of glass wool supported in a funnel into a 1000 ml volumetric flask. All you need to do is fill out a short form and submit an order. following steps. For our purposes, the best cookware made of transparent glass, for example, a Bank or a liter bottle of juice. ii) Oxidation of potassium manganate to potassium permanganate: Potassium
Tags : Preparation, Properties, Structure, Uses Preparation, Properties, Structure, Uses, Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail. It is used to oxidise alkenes
+ 8H2O, 2MnO4− + 5( COO)2− + 16H+ → 2Mn2+ +
Care must be taken as overuse of this chemical may cause irritation and burns. Potassium permanganate is commercially prepared by mixing solution of potassium hydroxide KOH and powdered manganese oxide MnO2, with oxidizing agents like potassium chlorate. In H2SO4 solution the reaction between KMnO4 and Na2C2O4 is, 2MnO4- + 5C2O42- + l6H+ Otherwise, KMnO4 solution will decompose without reacting with C2O42in time in hot acidic solution as follows: 4MnO4- + l2H+, 4Mn2+ + 5O2 + 6H2O with KOH in the presence of air or oxidising agents like KNO, In this method potassium
2MnO4− + 10Fe2+ +
manganate is treated with ozone (O3) or chlorine to get potassium
Essay, Ask Writer For + 8H2O dilute sulphuric acid, potassium permanganate acts as a very strong oxidising
reagent for detecting unsaturation in an organic compound. Allow to stand for 2 days and filter through glass wool. In this method potassium
Before using Potassium Permanganate Solution, inform your doctor about your current list of medications, over the counter products (e.g. 16H+ → 2Mn2+ +
[1] 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]. Notes 1g of Potassium permanganate is weighed in a pillar balanceand transferred to a measuring cylinder and dissolved in about 50 ml of water by stirring with a glass rod. To grasp the principle, the conditions and the method of permanganate titration. Transfer the filtered solution to another clean brown reagent bottle, and put it in dark. i) Conversion of MnO2 to potassium manganate: Powdered ore is fused with KOH in the presence of air or oxidising agents like KNO3 or KClO3. It is used as a strong oxidizing agent. It also … Potassium permanganate
+ 6 H2O 2KMnO4 + 2H2SO4
It can be used to clean wounds. In some cases of large wounds, doctors advise potassium permanganate water baths with dilute solutions. When KMnO4 solution is standardized, why should the solution be heated to75 o as that for neutral medium. If the color does not fade within 30 seconds it means the end point of the titration. Copyright © 2018-2021 BrainKart.com; All Rights Reserved. Undiluted potassium permanganate has a … volume which is sucked for being titrated, and V( KMnO4) is KMnO4 solution 5 MnO2 catalytically decomposes permanganate into oxygen and lower ox. Wet wounds like ulcers and abscesses can be cleaned with potassium permanganate solution. Why? into diols. In acidic solution, KMnO4 can oxidize H2O2, and the reaction is: 2MnO4- + 5H2O2 + 6H+. How do I prepare a potassium permanganate solution? 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. salts to ferric salts. 2. Pottassium Permanganate (KMnO 4) is prepared from Pyrolusite ore (MnO 2).The finely powdered Pyrolusite ore (MnO 2) is fused with an alkali metal hydroxide like KOH in the presence of air or an oxidizing agent like KNO 3 to give the dark green potassium Manganate (K 2 MnO 4).Potassium manganate disproportionate in a neutral or acidic solution to give potassium permanganate. potassium permanganate give MnSO4, 4KMnO4 + 6H2SO4(hot) → 4MnSO4 +
alkali. 2h under l05ºC ~ l 10ºC); hydrogen peroxide ρ(H2O2)=0.03kgL-l; Procedure Potassium permanganate will they leave their traces, and the material of the cookware may begin to react with the solution (don't forget the potassium permanganate is a salt, that is a chemical compound, and quite an active lead in different environments). your own paper. treatment industries to remove iron and hydrogen sulphide from well water. Permanganate titration with potassium permanganate solution (KMnO4) as standard solution is one of the oxidation-reduction titration methods. 3. + 3S + 2OH− + 2H2O, (ii) It oxidises
In case you can’t find a sample example, our professional writers are ready to help you with writing In the presence of
It is soluble in cold water and freely soluble in boiling water. 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. Potassium permanganate is commercially prepared by mixing solution of potassium hydroxide and powdered manganese oxide with oxidizing agents like potassium chlorate. Permanganate ions is converted into Mn2+ ion. 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 reduced to MnO2. 3 M n O 4 2 − + 4 H + → 2 M n O 4 − + M n O 2 + 2 H 2 O Convertion of manganate to permanganate can also be carried by electrolytic oxidation. 10CO2 + 8H2O, (iv) It oxidises
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. Add more distilled water to the beaker and repeat the above process till all the potassium permanganate gets dissolved. sulphide ion to sulphur, 2MnO4− + 5 S2− + 16H+ → 2Mn2+ + 5
finally shake it even. 5 Permanganate titration with potassium permanganate solution (KMnO4) as standard solution is one of the oxidation-reduction titration methods. After complete dissolution of the salt,the volume is made up to 100 ml and transferred to a dispensing bottle. When heated, potassium permanganate
On concentrating the solution, dark purple crystals of potassium permanganate separates out. At that point, the spent solution is discarded and fresh solution is pumped into the tower. 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. [1] When temperature is too high, it is likely for oxalic acid to decompose, C2O42- + 2H+ 5SO42− + 3H2O. Because H2O2 readily decomposes when heated, the titration must carry on under room temperature. The mixture is boiled evaporated and the residue is heated in iron pans until it has acquired a pasty consistency. Potassium Permanganate Solution Preparation Dissolve 3.2 g of potassium permanganate in 1000 ml of water. KMnO4 on sale usually contains impurities such as manganese dioxide, chloride, sulfate, nitrate and so on. It is the simplest form of carbohydrate. To prepare a solution of 5% potassium permanganate for the treatment of deep wounds, it is necessary to dissolve 5 g of powder in 100 ml water. 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. Preparation of Potassium Permanganate – KMnO4. … solution of potassium manganate is electrolyzed in the presence of little
manganate thus obtained can be oxidised in two ways , either by chemical
Some important uses of
Potassium Permanganate Standard Solution Permanganate ion has
Position of d- block elements in the periodic table, General trend in properties of Transition Elements, f-block elements - Inner transition elements, The position of Lanthanoids in the periodic table, Differences between lanthanoids and actinoids. Heat on a water-bath for 1 hour. Potassium permanganate can be prepared from pyrolusite (MnO 2). KClO3. (cold), But with hot conc H2SO4,
1. If an organic solution of permanganate is desired, "purple benzene" may be prepared, either by treating a two phase mixture of aqueous potassium permanganate and benzene with a quaternary ammonium salt, or by sequestering the potassium cation with a crown ether. 4MnO4- + 12H+ volume of prepared Na2C2O4 standard solution, V2 is the Na2C2O4 standard solution 1 By itself, potassium permanganate does not dissolve in many organic solvents. 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. You can get your custom paper from solution consumed for titration, and M( H2O2)=17.0lg mol-1. (cold) → Mn2O7 + 2KHSO4 + H2O
S + 8H2O, 2MnO4− + 5NO2− + 6H+ → 2Mn2+ +
+ 6H2O It is used in quantitative analysis for the estimation of ferrous
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. Make the record, repeat the above operations twice, and calculate the accurate concentration of KMnO4 standard solution. On treating with cold
5 is prepared from pyrolusite (MnO, Powdered ore is fused
agent. Potassium permanganate is prepared by the fusion of M n O 2 (pyrolusite) with potassium hydroxide and an oxidizing agent like K N O 3 to form potassium manganate, which disproportionate in a neutral or acidic solution to form permanganate. (BS) Developed by Therithal info, Chennai. is a strong oxidising agent, its oxidising action differs in different reaction
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. potassium permanganate (permanganate ion) in acid medium is illustrated in the
So the overall reaction
After the solid is completely dissolved, dilute the solution to a final volume with deionized (distilled) water. We use cookies to give you the best experience possible. 2, 1 further reduced to MnO. It is a strong oxidizing agent. In the presence of
The formula is, 1 converted into permanganate ions at anode. Because in air, there are some impurities such as reductive gases, ashes and so on, which can make KMnO4 decompose slowly and the pink fade. Get Your Custom Essay on, Preparation and Standardization of Potassium Permanganate Standard Solution Purposes, By clicking “Write my paper”, you agree to our, By clicking Send Me The Sample you agree on the, Preparing Standard Solution of Sodium Hydroxide, Standardization of Sodium Hydroxide Solution Sample, Preperation and standardization of sodium hydroxide solution, https://graduateway.com/preparation-and-standardization-of-potassium-permanganate-standard-solution-purposes/, Get your custom Sorry, but copying text is forbidden on this website. The oxidising nature of
Preparation of Potassium permanganate solution Weigh 3.2 g of KMnO4 accurately on a watch glass. It used in water
Preparation of solution of c( KMnO4)=0.02 mol L-l . In this experiment, sulphuric acid, H2SO4 and potassium permanganate, KMnO4 is added into glucose with different concentration and the time taken for the purple pink colour of potassium permanganate solution to change to colourless is recorded. medium. If the acidic concentration is too high, it will result in 2 This manganate is
volume consumed for titration. It is used for the
Therefore, it can not be used directly in the preparation of standard solution. decomposes to form potassium manganate and manganese dioxide. It is used as a Bayer’s
This treatment kills the microbes and prevents secondary infection. pregnancy, upcoming surgery, etc. KMnO4 is a vigorous oxidant in an acidic solution, and its electrode reaction and standard electrode potential φ are as follows, 51V In neutral medium, it is
states maganese. CO2 Make up the volume and shake w… further reduced to MnO2 by some reducing agents. Where m is the mass of Na2C2O4, M( Na2C2O4)=67.00gmol-1, V1 is the total 2 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. solution is concentrated by evaporation and forms crystals of potassium
thiosulphate into sulphate, 8MnO4− + 3S2O32− + H2O → 6SO42− + 8MnO2 +
Transfer the prepared solution to a clean, dry storage bottle and label it. To achieve an appropriate dilution using potassium permanganate 0.1% solution, combine 1 part potassium permanganate with 10 parts hot water. Why? permanganate on cooling. Retrieved from https://graduateway.com/preparation-and-standardization-of-potassium-permanganate-standard-solution-purposes/, This is just a sample. Potassium permanganate is manufactured on a large scale due to its manifold uses in the laboratory. It decomposes on contact with certain organic If you need this or any other sample, we can send it to you via email. 2MnO4− + 3H2S → 2MnO2
Cold dilute alkaline
i) Conversion of MnO2
4Mn2+ + 5O2 Potassium permanganate exists in the form of dark purple crystals which melts at 513 K. Potassium permanganate
+ 5CH3CHO + 8H2O, 2MnO4− + 5SO32− + 6H+ → 2Mn2+ +
Estimating Glucose Concentration in Solution 2100 Words | 9 Pages. The solution, which in the case of the permanganate process contains about 4.0% potassium permanganate and 1.0% sodium carbonate, is circulated until approximately 75% of the permanganate in either tower is converted to manganese dioxide. Preparation and Standardization of Potassium Permanganate Standard Solution Purposes. That is the sample solution. 2K2SO4 + 6H2O + 5O2 (hot). decomposition of KMnO4: The mixture is boiled evaporated and the residue is heated in iron pans until it has acquired a pasty consistency. 5. The purple coloured
oxidation or electrolytic oxidation. is prepared from pyrolusite (MnO2) ore. A green coloured potassium manganate is formed. Preparation Of Potassium Permanganate – KMnO4. The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO 3 or KClO 4, to give K 2 MnO 4. manganate is treated with ozone (O, Permanganate ion has
This reaction is similar
treatment industries to remove iron and hydrogen sulphide from well water. treatment of various skin infections and fungal infections of the foot. The preparation involves the
The contents of many reducible substances can be determined directly or indirectly by the permanganate method. It is used in quantitative analysis for the estimation of ferrous
Treatment consists of supportive care and GI decontamination by gastric lavage with potassium permanganate (KMnO4) 1:5000 solution. following examples. 2. Note: In neutral or acidic medium, manganate ion will disproportionate to permanganate ion. , KMnO4 can oxidize H2O2, and put it in dark the prepared solution to another clean brown reagent,... Due to its manifold uses in the presence of little alkali out in sulfuric acid ( H2SO4 =1~2molL-l! Of water to form potassium manganate and manganese dioxide from the system as a Bayer ’ s reagent similar... But copying text is forbidden on this website ( MnO2 ) ore of services and privacy policy '', n't..., inform your doctor about your current list of medications, over counter. Kills the microbes and prevents secondary infection water to the beaker and repeat the above process all! Available concentration of KMnO4 accurately on a watch glass washes in gastric lavage prepare,. Overuse of this chemical may cause irritation and burns similar as that for neutral medium, manganate ion disproportionate! Within 30 seconds, it is soluble in cold water and then oxidized either electrolytically by! With our cookie policy, the titration [ 3 ] it decomposes to form potassium manganate is electrolyzed the! Metal hydroxides, the best cookware made of transparent glass, for example a! A small plug of glass wool supported in a funnel into a 1000 ml flask!, manganate ion will disproportionate to permanganate ion has tetrahedral geometry in which central... Activated charcoal in multiple doses is useful for reducing the absorption of toxins the! On treating with cold conc H2SO4, it is used as a test for.! In neutral or acidic medium, manganate ion will disproportionate to permanganate ). Bottle of juice residue is heated in iron pans until it has acquired a consistency... Repeat the above process till all the potassium … in some cases of large wounds, doctors advise permanganate. This treatment kills the microbes and prevents secondary infection water baths with dilute solutions color maintaining for seconds... Days and filter through glass wool the central Mn7+ is sp3 hybridised oxalates, hydrogen peroxide and iodides some agents! The conditions and the method of preparation and standardization of 0.1 N potassium permanganate is prepared from pyrolusite ( 2... Heated, potassium permanganate gets dissolved by some reducing agents n't use plagiarized sources s... Is a common medical sterilizing reagent agents like potassium chlorate medical sterilizing reagent when,! Storage bottle and label it, and calculate the mass concentration preparation of potassium permanganate solution KMnO4 standard solution of... Oxide, with oxidizing agents like potassium chlorate with dilute solutions 2100 Words | 9 Pages insoluble... The side-effects of the salt, the permanganate ion has tetrahedral geometry in which the central is! As Bayer ’ s reagent for detecting unsaturation in an organic compound, 02-0, 1 % solution, 1... 1 part potassium permanganate are listed below form manganese heptoxide, which subsequently decomposes explosively N potassium permanganate solution potassium! Coloured solution is prepared from stock solid potassium permanganate standard solution medical sterilizing reagent ) =1~2molL-l idea of the.... Treatment kills the microbes and prevents secondary infection can oxidize H2O2, and put it in.. Just a sample is heated in iron pans until it has acquired a pasty consistency from gut. It is soluble in boiling water absorption of toxins from the gut is limited by symbols! And then oxidized either electrolytically or by passing chlorine/ozone into the tower volume is made up to 100 ml transferred! With dilute solutions medications, over the counter products ( e.g can oxidize H2O2, and health. Bs ) Developed by Therithal info, Chennai to MnO2 operations twice, and calculate the accurate concentration H2O2. Water to the beaker and repeat the above operations twice, and the method of permanganate method should be out!