Chemistry Lab Experiment 2

Reaction of Potassium Manganate with Oxalic acid

Mashiat and Kausar13B

Purpose of experiment• To carry out oxidation of hydrated

Oxalic acid (Ethanedioic acid) (H2C2O4.xH2O) by Potassium manganate (VII) (KMnO4) in the presence of acid by titration.

• To determine the value of x in H2C2O4.xH2O

2MnO4- (aq) + 5H2C2O4 (aq) + 6H+ (aq) 2Mn2+ (aq) + 10CO2(g) + 8H2O(l)

PROCEDURE

Reagents

FA1: 5.00 dm-3 of H2C2O4.xH2OFA2: 2.37 dm-3 of KMnO41.00 moldm-3 H2SO440 cm-3 distilled water

Apparatus used

BuretteBeaker PipetteBunsen burnerThermometer Conical flaskMeasuring cylinder

Method 1. Burette is filled with purple

KMnO4 solution.2. 25.0 cm3 of H2C2O4.xH2O

pipetted into conical flask.3. 25 cm3 of 1.00 moldm-3 of

H2S04 poured using measuring cylinder.

4. 40 cm3 distilled water added using measuring cylinder.

5. The flask is heated using Bunsen burner and temperature observed on thermometer till a little over 65◦C.

6. Flask placed under burette and titration carried out.

7. Flask is swirled continuously until end-point reached.

Results & Observation

1) A strong pungent smell was released when the acidified solution was heated using Bunsen burner. 2) On reaching end-point, the color of the solution turned from colorless to a permanent pale pink completing the oxidation of Oxalic acid. Hence, no indicator is required as KMnO4 acts as an indicator itself.

Conclusion• Volume of FA2 used with 25.0 cm3 of

FA1: 27.00 cm3 • Moles of KMnO4 used: 0.0004 mol• Moles of Oxalic acid reacted: 0.001

mol• Value of x in C2H2O4.xH2O: 1.95

mol

What I learnt• Potassium permanganate is a strong

oxidizing agent.• Excess dilute H2SO4 is needed, otherwise

it forms brown ppt. of hydrated manganese dioxide, interfering with obtaining the correct end-point.

• The oxidation reaction is slow in room temperature, so heating the acidified solution is required to speed up the rate. Heating also releases carbon dioxide that helps the reaction to be completed.

Safety Precautions

• The upper meniscus should be used for burette readings of colored solutions (KMnO4) as the lower meniscus isn’t clearly visible.

• Handle flask carefully while heating. Hold the neck of flask using folded paper towel if it’s too hot to hold safely.

• Wear gloves while heating and prevent skin contact with Oxalic acid as it’s corrosive.

• Potassium permanganate decomposes in bright light, and so cannot be exposed in these conditions for long.

Real-life Applications of Oxalate

• Reducing agent for development of photographic films.

• Bleaching agent for wood and stone. Wood turns grey when exposed to weather elements. When Oxalic acid is added to it, the natural color of wood is retained.

• Removing stains and rust. Oxalic acid is a gentle stain remover that effectively gets rid of stains without wearing the base away.

• Wastewater treatment as Oxalic acid can effectively remove Calcium from wastewater.

Critical-thinking question

Why is dilute H2SO4 added in KMnO4 titrations instead of HCl or HBr and HNO3?

Ans: Potassium manganate(VII) can't be used in titrations in the presence of ions like chloride or bromide as they consume some KMnO4 and liberate Cl2 and Br2 , while HNO3 is a stronger oxidizing agent than KMnO4 .