Potassium permanganate is an oxidizing agent. It can retain its concentration over a long period under proper storage conditions. The reactions of permanganate in solution are rapid. It also acts as self-indicator as its slight excess gives a distinct pink color to the solution. It’s not a primary standard.
The ability of potassium permanganate solution to oxidize is due to the conversion of MnO4– ion to Mn2+ in acidic solution & to MnO42- in alkaline & MnO2 in neutral solution. The MnO4– ion is reduced in accordance with the following reactions.1
In acidic medium:
Equivalent Weight = Molecular Weight / No. of electrons transferred = 158/5 = 31.6≈32
So, 32 gm of KMnO4 when dissolved in 1000 ml of water = 1N KMnO4
Therefore 3.2 gm of KMnO4 when dissolved in 1000ml of water = 0.1N KMnO4
As potassium permanganate is not a primary standard it can be standardized by using sodium oxalate or oxalic acid. The former is preferred over oxalic acid as available in a higher standard of purity (99.95%). It’s available in the anhydrous form.
Equivalent weight of Na2C2O4 = Molecular weight / 2 = 134.01/ 2 = 67.01
67.01 gm of Sodium oxalate when dissolved in 1000ml of water = 1N oxalic acid
6.701 gm of Sodium oxalate when dissolved in 1000ml of water = 0.1N oxalic acid
Hence, based on the above theory our aim is to prepare and standardize potassium permanganate solution with oxalic acid.
Beaker – 250 ml
Cleaned and calibrated volumetric flask – 1000 ml
Potassium permanganate AR grade
Oxalic acid AR grade
Conc. sulfuric acid
Preparation of Potassium permanganate solution
Weigh 3.2 g of KMnO4 accurately on a watch glass. 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. Decant the solution through a small plug of glass wool supported in a funnel into a 1000 ml volumetric flask.
Add more distilled water to the beaker and repeat the above process till all the potassium permanganate gets dissolved. Make up the volume and shake well so as to affect uniform mixing. Keep the flask with stopper for 24 h and then filter through asbestos.
The organic matter present in the distilled water is decomposed by potassium permanganate resulting in the formation of MnO2 during 24 h. Keep the filtered solution in a dark glass bottle. Standardize the solution 24 h after its preparation.
Standardization of potassium permanganate solution
Dry AR grade sodium oxalate at 105-110 ºC for 2 h. Allow it to cool in a covered vessel in a desiccator. Weigh 6.7 g of above pure sodium oxalate accurately and transfer into a 1000 ml volumetric flask containing 500 ml of water. Make up the volume with distilled water.
Draw 20 ml of this solution into a conical flask and add 5 ml of conc. H2SO4 along the side of the flask. Warm the contents swirled contents up to 70ºC. Titrate against potassium permanganate solution from the burette until a faint pink color persists for 30 sec upon shaking the flask. Repeat the process until 3 concordant readings will be obtained.
Tabulation for standardization
|Volume of Sodium Oxalate solution taken (ml)
|Burette Reading of KMnO4
|Volume of KMnO4 consumed(ml)
Where N1=Normality of (COONa)2
N2=Normality of KMnO4
V1=Volume of (COONa)2
V2=Volume of KMnO4
From the above experiment it is evident that potassium permanganate can be effectively standardized by using oxalic acid. After performing the calculations, strength of the prepared potassium permanganate solution was found to be……..N.
1. Kasture AV, Wadodkar SG, Gokhale SB. Practical Pharmaceutical Chemistry-I. Nirali Prakashan; 1993:53.
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