Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
Module: 2
Lecture: 10
OXALIC ACID
INTRODUCTION
Oxalic acid, H2C2O4 is a colourless crystalline dicarboxylic acid gives
colourless solution upon dissolved in water. In terms of acid strength, it is much
stronger than acetic acid. Oxalic acid is a reducing agent and its conjugate
base, known as oxalate (C2O42-), is a chelating agent for metal cations.
Typically, oxalic acid occurs as the dihydrate with the formula H2C2O4.2H2O.
Ingestion of oxalic acid through skin contact or orally is dangerous.
The oxalic acid was discovered by Scheele in 1776. Scheele was
produced oxalic acid by the oxidation of sugar using nitric acid and called it
saccharic acid. Artificial synthesis can be affected in various ways. Sugar and
starch was boiled with nitric acid which produces oxalic acid as a
penultimate product of oxidation.
Mostly commercial oxalic acid is contaminated with sulfuric acid and
alkali where alkali cannot be removed by recrystallization method using
water, but it can be removed by recrystallization using 10 - 15% hydrochloric
acid.
MANUFACTURE
1. Oxalic acid from sodium formate
Raw materials
Basis: 1000kg oxalic acid dihydrate from sodium formate
Sodium formate
1325kg
Calcium hydroxide
650kg
Sulfuric acid
950kg
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Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
Reaction
Manufacture process
Sodium formate was obtained by the reaction of solid sodium
hydroxide (97 - 98%) and carbon monoxide at 2000C temperature and 150psi
pressure in an autoclave.
Sodium oxalate was obtained, at the completion of above reaction by
reducing the pressure in autoclave and increasing the temperature to 4000C.
Reaction was completed after the evolution of hydrogen was ceased.
Hydrogen
Calcium
hydroxide
Dilute
sulfuric acid
Sodium
formate
Crystallizer
Precipitator
Acidifier
Cold air
in
Autoclave
Hot air
in
Dryer
Filter
Filter
Filter
Oxalic acid
dihydrate
Caustic soda solution
Calcium sulfate
Make-up liquor
Figure: Manufacture of Oxalic acid from Sodium formate
Block diagram of manufacturing process
Diagram with process equipment
Animation
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Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
The reaction mixture was transferred to precipitator and calcium
hydroxide was added under stirring. The calcium oxalate was filtered and
sodium hydroxide was concentrated for reuse. Filter cake of crude calcium
oxalate run to an acidifier with small amount of calcium carbonate and
treated with dilute sulfuric acid. Calcium precipitated out as calcium
sulphate dihydrate (CaSO4.2H2O). Mother liquor was run to a crystallizer and
concentrated to specific gravity of 300Be. Crystallize oxalic acid in the form of
dehydrate was washed and dried.
The yield of oxalic acid dehydrate was about 80% by weight based on
sodium formate. It is generally sold and used, as the dihydrate, but anhydrous
oxalic acid may be formed by heating the dihydrate to 1000C where it loses
2 molecules of water.
Further process
The sodium oxalate may be acidified directly to oxalic acid by mixing 1
part sodium oxalate with 3 parts of methanol and 0.8 parts of concentrated
sulfuric acid. Separate insoluble sodium sulfate, from the methyloxallatemethanol-sulfuric acid solution. Filter sodium sulfate and hydrolyze the mother
liquor by the addition of 3.5 parts water to form oxalic acid and methanol.
Distilled the methanol and recycled to the process. Concentrate the crude
oxalic acid solution and crystallized the oxalic acid.
Oxalic acid is also manufactured by the oxidation of molasses with
nitric acid. Similarly, oxalic acid can obtained from many other organic
compounds like glycol, alcohol, fats, oat hulls saw dust and other cellulosic
materials by oxidation with nitric acid.
2. Oxalic acid from propylene
Raw materials
Basis: 1000kg oxalic acid from propylene
Propylene
460kg
Nitric acid
1375kg
Oxygen
870kg
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Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
Reaction
Manufacture process
In this process oxalic acid was obtained by oxidation of propylene with
nitric acid. It is the two-step process, in first step propylene was converted into
α-nitratolactic acid and second step leads to oxidation of α-nitratolactic acid
to oxalic acid.
Nitric acid
Second
reactor
First
reactor
Nitric acid
recovery
Propylene
Oxygen
Oxygen
Intermediate
reservoir
Nitric acid
Crystallizer
Catalyst
Filter
Autoclave
Mother
liquor
Cold
air out
Dryer
Hot
air
Oxalic acid
Figure: Manufacture of Oxalic acid from Propylene
Block diagram of manufacturing process
Diagram with process equipment
Animation
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Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
In the first reactor, propylene was introduced where it was reacted with
nitric acid. Propylene to nitric acid molar ratio is kept at 0.01 - 0.5. The mixture
from first reactor was run into second reactor where it was reacted with
oxygen. In this reactor vapour of nitric acid was continuously evolved from
the top of the vessel. The stream from vessel contains intermediate and trace
amount of nitric acid was run into reservoir tank where acid was evolved.
α-nitratolactic acid from reservoir was then run into autoclave where it was
oxidized at 45 - 1000C in the presence of mixed acid as catalyst. In this step
α-nitratolactic acid is converted into oxalic acid dihydrate. The crude
product was then transfer into a crystallizer where oxalic acid crystals
obtained. The slurry from crystallizer is filtered and sent for drying operation
where dried oxalic acid is obtained.
3. Oxalic acid dimethyl oxalate process
Raw materials
Basis: 1000kg oxalic acid from methyl nitrite and carbon monoxide
Carbon monoxide
957kg
Methyl nitrite
1330kg
Water
395kg
Reaction
Manufacture process
In this process, circulating gas containing CO and regenerated methyl
nitrite from regeneration column was pressurized and fed to the reactor. In
this vessel dimethyl oxalate was produced while methyl nitrite is mostly
consumed. The dimethyl oxalate and unconverted mixture was run into the
condenser where methanol was added. Uncondensed vapour contains
methyl nitrite, water and methanol was drawn to regeneration column where
NOx and oxygen are added to regenerate the methyl nitrite which is to be
recycled.
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Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
Methyl
nitrite
Reactor
Regeneration
column
NOx
Carbon
monoxide
Oxygen
Methanol
H2O and CH3OH
Distillation
column
Steam
Crystallizer
Water
Hydrolyzer
Water vapour
Condenser
Filter
Mother
liquor
Cold
air out
Dryer
Hot
air
Oxalic acid
Figure: Manufacture of Oxalic acid by Dimethyl oxalate process
Block diagram of manufacturing process
Diagram with process equipment
Animation
Condensed dimethyl oxalate from condenser was run into the
distillation column where water vapours were removed while dimethyl
oxalate was sent for hydrolysis where water is added. After the hydrolysis of
oxalate, slurry was sent to the crystallizer where a crystal of oxalic acid is
obtained. The slurry is then filtered and passed from dryer and dried oxalic
acid stored.
Engineering aspects
In propylene process, sulfuric acid is used which results into corrosion
problem at the oxidation step. Oxidation reaction can be made speedy by
using large reactor. Possibility of unstable by-product formation along with
α-nitratolactic acid leads to explosion or decomposition.
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Module:2
Lecture:10 Oxalic acid
Dr. N. K. Patel
PROPERTIES
 Molecular formula
 Molecular weight




Appearance
Odour
Boiling point
Melting point
 Flash point
 Density
 Solubility
: C2H2O4
: 90.03gm/mole (anhydrous)
126.07gm/mole (dihydrate)
: Orthorhombic colourless crystals
: Odourless
: 1570C at 1.013hPa (anhydrous)
: 1010C, sublimes at 1500C (dihydrate)
190.00C (anhydrous)
: 1660C
: 1.90gm/cm3 (anhydrous)
1.653gm/cm3 (dihydrate)
: Soluble in water, absolute alcohol and
ether
USES
 Oxalic acid is used as a mordant in dyeing processes
 For the removal of rust oxalic acid is used for cleaning or bleaching
application
 3.2% solution of oxalic acid or its vaporized form is used by beekeepers
as a miticide against the parasitic varroa mite
 As a bleaching agent for wood, baking powder, pulpwood and stone
 To remove mustard, ink, different food stains as well as other types of
stains
 As a reducing agent to develop the photographic film
 To remove calcium from wastewater
 As a grinding agent for polishing the marble
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