Redox Titration Using Potassium Permanganate

Redox Titration Using Potassium Permanganate

You will use your KMnOsolution to analyze a solid sample containing ferrous ions Fe (II).Fe (II) is easily oxidized to ferric state Fe (III)

FeThe reaction is run in the presence of H for acid conditions.Since salts that contain ferrous and ferric ions may impart color to aqueous solutions, a small amount of H is added to complex the Fe to a colorless form.

This will result in a cleaner end point.It must also be noted that the amount of sulfuric acid added in the second step is less compared to the first : Standardization of KMnO with NaBoth these reagents are in solution so you can determine number of equivalents from normality and volume,

(or molarity and volume).

At the end point,

#eq.Red = # eq.


= (NV) Na.Where N = normality If you are using the molarity, then you need the appropriate stoichiometry.

In this case, when you balance the reaction for the redox process, the overall equation will be

That gives a molar stoichiometry of 2:5 for MnOIf you know the molarity of Cthen the molarity of KMnO will be calculated as follows:

= 2/5 x [(M C x Vol.of CYour data provides all terms except the concentration of KMnOwhich you can calculate.Percent iron (II) in unknown sample To find percent by mass of iron (II) you need mass of Iron in the sample.

Once again, it you are working with equivalence, then at the end point #eq Red = # eq Ox

= (NV) Fe (II) If you are dealing with molar equivalence, the balanced redox equation is

+ 5 FeO +

5 FeMass of Fe = 5 x [(Molarity of KMnO x Vol.

KMnO) x atomic mass of Fe] % Fe = (mass of Fe in the sample/sample mass) x 100% Your sample mass is part of your data so you need to find the grams of iron from your titration data.You know the molarity and volume of the KMnO and you can determine the gram equivalent mass of iron and the atomic mass of iron.Thus you can calculate the grams for iron present in each of your samples and then the percent iron in the unknown.Equipment

Burette, pipet, pipet filler, funnel, Erlenmeyer flasks, beaker, hot plate, balance Chemicals

Sodium Oxalate solution, KMnO solution, sulfuric acid, phosphoric acid, distilled water, Iron (II) containing salt as unknown Safety Sulfuric acid is highly corrosive, be extremely careful, be extra careful during the titration of hot solutionsProcedure Clean the buret and rinse it with distilled water.Then rinse with two 2 mL portions of your


Fill the buret with the KMnO.Because of the dark color of the KMnOsolution, you should read the top of the meniscus.Standardization of KMnOClean and rinse with distilled water three Erlenmeyer flasks and a 10.00 mL pipet.Rinse the pipet with your Na solution then pipet a 10.00 mL portion into each of the Erlenmeyer flasks.To each flask add about 50 mL-distilled water and about 15 mL of 3 M H using a graduated cylinder for measurement.

Warm the mixture to a temperature between 50 - 60 Record the initial buret reading (should not be 0.00).

Add KMnO to the first Na heated sample with mixing until a permanent faint pink color remains in the titration mixture.Record the final buret reading.Repeat with the other two Na samples.

The heating is essential to speed up this reaction that is slow at room temperature.

The reaction proceeds rapidly at warmer temperatures.It is also needed to assure that appropriate # of electrons are used in the redox process.

Calculate the molarity of the KMnOTitration of Iron (II) sample Clean and rinse three 125 or 250 ml Erlenmeyer flasks.Using the container with your unknown and weighing by difference method, transfer between 0.3 to 0.5 g of unknown to each flask.

Do these weighings extremely carefully on the analytical balance.

Dissolve each sample in about 20 ml distilled water and add about 5 mL of 3 M Hand 1 mL concentrated H.Heat the solution carefully to a temperature slightly above 60C.

Record the initial buret reading.Slowly titrate the first iron sample with the KMnO until a permanent faint pink remains.

If you did not , the end point will be slightly difficult to detect.

Record the buret reading and add a drop and check if the pink color deepens.

Record the final buret reading.

Repeat with the other two samples.Calculate the percent iron in each sample and report the average value.

Sample Data Sheet

Standardization Trial 1Trial 2Trial 3Volume of Na

Initial buret reading

Final buret reading

Difference (mL of KMnO
Molarity of KMnO

Average Molarity Iron (II) unknown

Unknown label/number _________________ Trial 1Trial 2Trial 3Mass of the bottle with sample

Mass of the bottle (sample)

Mass of the sample (g)

Initial buret reading

Final buret reading

Difference (mL of KMnO

Moles of Iron (II)
Mass of Iron in the sample
% Iron in the value (if available) % Error

Questions 1.

A 1.874 g solid sample containing some Fe (II) is analyzed using potassium dichromate, , as the oxidizing agent in acid solution.The titration required 35.40 mL of .The half reactions occurring are Fe2-

Write a balanced overall equation for the reaction occurring during the titration.

Determine the moles of K used during the titration.Determine the percent iron in the sample.

2.Consider the titration of oxalate solution with permanganate solution in which you determined the volume of oxalate from your pipet volume.For each of the following, state whether your volume of permanganate would be too high, too low, or unaffected and why.

You did not rinse the water out of your pipet before filling it with oxalate.

You did not dry the Erlenmeyer flask before adding the pipeted oxalate solution.

You had the oxalate solution above the volume mark of the pi
Atbas 60118057
Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05 Nov 2011 ATBAS-60118057Asian-Transactions 1

Redox titration of iron using methylene blue as indicator

and its application in ore analysis

Erik Prasetyo, Fika R.Mufakhir Indonesian Institute of Sciences Jl.Ir.Sutami km.15, Tanjung Bintang Bandar Lampung, Indonesia - Dichromatometry titration procedure to determine iron content using methylene blue as redox indicator was investigated.The effectiveness of this procedure was then compared with standard dichromatometry titration in which barium diphenylamine sulfonate was applied as indicators.The result showed that the titration procedure using methylene blue could be served as alternative procedure in terms of reproducibility and accuracy.This procedure even owned advantage compared to existing standard methods, since it did not incorporate Zimmermann Reinhardt reagent as in standard permanganometry and phosphoric acid as in standard dichromatometry titration in iron determination.Keywords - methylene blue, dichromate titration, iron ore analysis.


INTRODUCTION Iron is one of the most routinely analyzed elements in laboratory.The analysis methods chosen generally depend on sensitivity or iron content in sample, sensitivity, and accuracy required.Titrimetric analysis is usually chosen for range of iron content in percent permil
redox titration using potassium permanganate

This method measures the volume of a solution of substance with known concentration (titrant) which is required to react quantitatively with a measured volume of a solution of the substance to be determined (analyte).Analyte concentration is then determined based on volume of titrant, Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05 Nov 2011 ATBAS-60118057Asian-Transactions 2

chemical reaction occurred and molecular mass of substance involved in reaction [1].Technically the end of quantitative reaction is marked by physical change including but not limited to: color, potential, pH and precipitation.Color change in the end of reaction could be imparted by the excess of colored titrant or addition of a substance known as indicator.Indicators are usually organic substances which change color according to the condition of analyte: pH, potential, concentration and species of ions contained.So far, after more than a century the application of indicators is still considered important in analytical chemistry and has yet to be restricted by application of modern instrumental technique in detecting end of reaction [2].

Several titrimetry procedures have been proposed to determine iron content.Generally these procedures are based on redox and complex forming reaction.Redox based procedure which is widely used and considered as standard includes the oxidation of iron(II) by potassium dichromate (dichromatometry) in 1 2 M sulfuric acid solution and in the presence of phosphoric acid with diphenylamine as indicator [3], with diphenylamine-4-sulfonic acid as barium or sodium salt [4], oxidation of iron(II) by potassium permanganate (permanganometry) in presence of sulfuric acid and phosphoric acid [5] and manganese(II) [1].Iron(III) is generally determined as iron(II) after reduction with tin(II) chloride.Complexation based procedure uses EDTA as titrant to form complex with iron(III) with variamine blue as indicator [6].Permanganometry own its advantage since it does not required additive substance as indicator.The permanganate itself serves as internal indicator since in the end of titration a little excess titrant could change the color of analyte from colorless to very pale pink.

However this procedure has disadvantages in terms of potassium permanganate instability and its characteristic as very strong oxidizing agent.Permanganate instability (easily decomposed and difficult to obtain in pure form) does not meet the criteria as primary standard, so it must be frequently standardized with oxalic acid before use.As strong oxidizing agent permanganate tends to react with chloride ion present in analyte.This chloride generally comes from the aqua regia used in sample destruction.So that to lower Asian Transactions on Basic & Applied Sciences (ATBAS ISSN: 2221-4291) Volume 01 Issue 05 Nov 2011 ATBAS-60118057Asian-Transactions 3

the tendency of permanganate reacts with chloride and increase its tendency to react with iron(II), manganese(II) sulfate and phosphoric acid are added (Zimmerman Reinhardt reagent).

Potassium dichromate solves the problem posed by permanganometry in terms of stability and its characteristics as a weaker oxidizing agent compared to permanganate.So that in titration dichromate does not react with chloride to some extent (max.2 M).

However, the lack of detectable of color change in the end of titration makes this procedure require diphenylamine-4-sulfonate (DAS) as indicator.In fact the procedure requires phosphoric acid addition to change redox potential system of iron(II) dichromate from +0.56 V to +0.78 V, in order to get near to the transition potential of DAS (+0.76 V) so that the color change (end of titration) occurs in the end of reaction [4].The use of phosphoric acid could be avoided if an appropriate indicator is used.An indicator which might be proposed is methylene blue (MB).MB (Figure 1) is a substance easily obtained in the market due to its broad application which is not limited only to chemical applications but also to biology as staining agents, medicine, pharmaceutical and 1.Methylene blue structures.

MB as a redox indicator has a potential transition which depends on the pH of the medium [2]

(+0.532 V at pH 0).This value is close to potential redox system Fe (II) - dikromate (+0.56 V), so it is assumed that methylene blue could be used to detect the end point in titration.

This paper will review the possibility of methylene blue as indica
To facilitate the reduction of Mnreaction must be carried out in an acidic medium through the addition of .

Without the H, the reaction may stall at MnIn today's exercise you will use the relationship between sodium oxalate and potassium permanganate to determine the molar concentration of a permanganate solution.

You will then use the standardized permanganate solution to determine the molar concentration of an unknown oxalate solution.Weigh approximately 0.5 grams of

Nasample to a 50 ml volumetric flask.

Dissolve the Nain about 30 ml

Make sure the Na is completely dissolved.

Once all the Nais dissolved, dilute the solution to the mark with

Transfer the 50 ml of Nasolution to an Erlenmeyer flask and label.Standardization of KMn0Obtain in a clean dry beaker approximately 75 ml of KMnOwith two 1 ml portions of the KMnOsolution.

Fill the buret with the solution making sure no air remains in the tip (the tip must also be filled with KMnOsolution).

Record the Pipet 10.00 ml of the standard Nasolution into a 250 ml flask.

Add approximately 60 ml of distilled water and 12 ml of 6.0 M H4.

The reaction is slow and solution to boil.Titrate slowly the KMnOsolution into the

Nasolution until a faint pink color remains for at least 30 seconds.

Record Perform a second titration.

Determination of Unknown Naml of unknown into a 250 mL Erlenmeywater, 12 ml of 6 M Hbefore.

Perform a second titration.REDOX is an acronym for what type of reaction?

Define the term oxidizing agent; reducing agent.

today's exercise? the reducing
the reaction? What is the name of the laboratory technique used in today's exercise?

Why is it not necessary to use an indicator in this ach to report sheet.4
__________moles Volume of Solution
__________mL Molarity of

NaStandardization buret > Volume KMnOused
Volume titrated
> > 4
Average Molarity of KMnODetermination of Unknown > 4

_________ mL
Initial buret mL

Volume ( ALL calculations on separate paper and attach to report sheet.Consider the balanced ionic equation for the reaction in today's exercise: 2-

, the oxalate salt is Na all react to form three different sulfate salts.

Based on these facts, write the balanced
molecular eWhy is it not necessary to know accurately the volume of Hnecessary to accurately know the volumes of KMnO solutions? If the equation for a redox reaction is balanced, will free electrons appear in the equation as either reactants or products?

Explain your answer.

reduced how many moles of CO are produced?

What would have happened if you forgot to put in the sulfuric acid when you ran the reaction? llowing reactions.

Label them as OX or RED.().
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