Preparation and purification of Oxalic and formic acid
------------------------------------------------------

Oxalic Acid [1]

Many aliphatic compounds are oxidised by concentrated nitric acid, the
carbon atoms being split off in pairs, with the formation of oxalic acid.
This disruptive oxidation is shown by many carbohydrates, e.g., cane sugar,
where the chains of secondary alcohol groups, present in the molecule break
down particularly readily to give oxalic acid.

Required: Cane sugar 30g; nitric acid, 150ml.

Owing to the copious evolution of nitrous fumes, this preparation must be
carried out in a fume-cupboard having an efficient draught. Place 30g of
coarsely powdered cane sugar (sucrose) in a 750 ml flat-bottomed flask, add
150ml of concentrated nitric acid and heat the flask on a boiling
water-bath. As the mixture becomes warm, the greater part of the sugar
dissolves and a vigorous but harmless reaction, accompanied by a tremendous
evolution of nitrous fumes, takes place. Immediately the evolution of gas
starts, remove the flask from the water-bath and place it on a wooden block
or some similar non-conducting surface. When the reaction subsides (after
about 15 minutes) pour the hot solution into an evaporating-basin, wash out
the flask with about 20 e.e. of concentrated nitric acid, and then
evaporate the acid solution on the water-bath until it has a volume of
about 25ml. Some oxidation continues in the solution during the
evaporation, which is comparatively rapid. Now add about 50 ml of water to
the solution, and again evaporate to about 25ml. Cool the solution
thoroughly in ice-water; oxalic acid rapidly crystallizes. When
crystallization is complete, filter at the pump, and then recrystallize
from a small quantity of hot water. Dry by pressing between pads of
drying-paper, or in an atmospheric desiccator, but not in an oven where
partial loss of water of crystallization may occur. Yield, 10-11g. The
hydrated acid has m.p. 101øC; the anhydrous acid decomposes on heating.


Formic Acid [1]

When a mixture of anhydrous glycerol and crystalline oxalic acid, is heated
the glycerol undergoes esterification, giving first glyceryl monoxalate,
the latter, however, decomposes as the temperature reaches about 100øC,
losing carbon dioxide and giving glyceryl monoformate. On further heating,
particularly if more oxalic acid is added, the monoformate is hydrolysed
(the necessary water being provided both by the oxalic acid and by the
first reaction), and consequently a distillate of aqueous formic acid is
obtained.

Required: Glycerol 70ml; oxalic acid 40g.

Since glycerol is a very hygroscopic substance, it is necessary first to
ensure that the sample used is anhydrous. For this purpose, place about
70ml in a porcelain evaporating-basin, and heat it carefully over a gauze
(preferably in a fume-cupboard), stirring it steadily with a thermometer
until the temperature is 175-180øC then maintain this temperature for a
further 5 minutes. Allow the glycerol to cool, but while it is still warm
(i.e., before it becomes viscous) pour 50ml (63g) into a 250 ml distilling
flask containing 40g of powdered crystalline oxalic acid. Fit a thermometer
in the flask so that the bulb is completely immersed in the glycerol
mixture, and then fit a water-condenser to the flask. Heat the mixture
carefully over a gauze so that the temperature rises to 110-120øC, and then
adjust the heating so that the temperature remains within these limits. A
vigorous effervescence of carbon dioxide occurs, and the aqueous formic
acid begins slowly to distill over. When the effervescence tends to
subside, remove the Bunsen flame and allow the temperature to fall to
70-80øC: then add a further 40g. of powdered oxalic acid, and continue the
heating as before. Ultimately 25-30ml of distillate is obtained, the total
period of heating being about 1 hour.



Formic acid from oxalic acid [2]

A good yield of formic acid cannot be obtained by merely heating oxalic
acid, as a certain portion of the oxalic acid sublimes unchanged. The
oxalic acid is therefore heated with glycerol when carbon dioxide and
glyceryl monoformate are obtained, and the latter when boiled with water
yields formic acid and glycerol.

30g of glycerol is weighed in an evaporating dish and dehydrated by
heating on a sand bath until a thermometer immersed in the liquid indicates
a temperature of 176-180øC. The mass is then introduced into a 250ml
distilling flask and 30g of oxalic acid dihydrate is added. The side tube
of the flask is attached to a condenser, and a thermometer introduced
through the neck of the flask dips into the liquid. The mixture is heated
gradually on a sand bath, and the temperature maintained at about 110ø
until the evolution of carbon dioxide gas slackens. When this occurs,
another 30g of oxalic acid is added. The heating is renewed, the reaction
recommences, and an aqueous solution of formic acid distills over into the
receiver. The reaction is a continuous one, and an abundant supply of the
distillate may be obtained by the addition of similar quantities of oxalic
acid at intervals. The water of crystallization of the oxalic acid serves
to hydrolyze the glyceryl monoformate. The residue in the flask after each
distillation contains this ester. The product in this experiment is an
aqueous solution of formic acid.



Formic acid purification [3]

An aqueous solution of formic acid is obtained by distilling the sodium
salt with dilute sulphuric acid:

2 HCO2Na + H2SO4 = 2 HCO2H + Na2SO4

Anhydrous formic acid is obtained from the aqueous solution (70-77 %) by
the addition of butyl formate followed by distillation. The first fraction
is an azeotrope of ester and water, and then the excess of ester is removed
from the formic acid by fractionation.



Formic acid data and purification [4]

Formic acid [64-18-6] mw 46.0 (anhydr.), Fp. 83°, b 25°/40mm, 100.7°/760mm,
n 1.37140, n(25) 1.36938, d 1.22. Anhydrous formic acid can be obtained by
direct fractional distillation under reduced pressure, the receiver being
cooled in ice-water. The use of P2O5 or CaCl2 as dehydrating agents is
unsatisfactory. Reagent grade 88% formic acid can be satisfactorily dried
by refluxing with phthalic anhydride for 6 hr and then distilling.
Alternatively, if it is left in contact with freshly prepared anhydrous
CuSO4 for several days about one half of the water is removed from 88%
formic acid: distn. removes the remainder. Boric anhydride (prepared by
melting boric acid in an oven at a high temperature, cooling in a
desiccator, and powdering) is a suitable dehydrating agent for 98% formic
acid; after prolonged stirring with the anhydride the formic acid is distd.
under vac. Formic acid can be further purified by fractional crystn. using
partial freezing.



References:

[1] Mann & Saunders, Practical Organic Chemistry, 3rd Ed.
[2] Sudborough & Campbell, Practical Organic Chemistry
[3] Finar, Organic Chemistry, vol. 1, 6th ed.
[4] Perrin & Armarego, Purification of Laboratory Chemicals, 3rd ed.


--------------------------------------------------------------------------

Preparation of formic acid via Ethanol/Hypochlorite - by RatHead

Reflux Ethanol with sodium hypochlorite either as bleaching powder or as 5%
bleach solution. Mol ratios 1:1 Ethanol will trichlorinate and react with
the sodium hydroxide produced thus breaking the e thanol at the C-C bond
and condensing out as CCl3 and formic acid. The formic will react with the
sodium hydroxide to make the formate salt. Distill off the chlorform or
chloro/water if using 5% hypochlorite. Sodium formate can be mixed with
conc. sulpur ic to reconstitute the acid by mixing conc. H2SO4 with sodium
formate in water to make 85-90% formic. Anhydrous formic can be prepared by
dissolving excess sodium formate in 85-90% formic then adding conc.
sulphuric acid. The reason for the two step pro cess is while some of the
formate will convert if we attempt an intial anhydrous conversion of dry
formate and 18 molar H2SO4 a great deal of product will convert to CO2
instead by allowing the reaction to be moderated intially by water the
unwanted produ ction of CO2 is minimized. Hey mate, does this add to your
Hardware/grocery store value. BTW save the chloroform it's an excellent
solvent and if ya need sodium acetate use acetone instead of ethanol. The
acetate can be used as is or converted to the a cid as before.

RatHead

--------------------------------------------------------------------------

RATHEAD!!! Stop telling such nonsense!!! Don't add conc. H2SO4 to aq. HCOOH
or Na formate solution! This will produce CO, not CO2. CO is a deadly poison
gas. Think a little, we don't want dead underground chemists, do YOU?

OSMIUM

--------------------------------------------------------------------------

OOPS sorry forgot the 2 but sodium formate does react to make formic acid in
aqueous. That is the standard lab prep method for it. It is only when we go
for the anhydrous conditions that the monoxide production becomes a problem
then that can be reduced by working up to 85-90% formic to buffer further
additions. Still if ya don't want the formic acid the hypochlorite cleavage
of ethanol or acetone is a neat way to transform solvents. Sometimes
chloroform is needed.

RatHead

--------------------------------------------------------------------------
Posted by Cherrie baby on January 13, 1998:

Scanned Info. On Formic acid:

From: Perrin & Armarego, "Purification of Laboratory Chemicals", 3rd ed.
Formic acid [64-18-6] M 46.0 (anhydr.), Fp. 83ƒ, b 25ƒ/40mm, 100.7ƒ/760mm, 
n 1.37140, n(25) 1.36938, d 1.22. Anhydrous formic acid can be obtained by 
direct fractional distillation under reduced pressure, the receiver being 
cooled in ice-water. The use of P2O5 or CaCl2 as dehydrating agents is 
unsatisfactory. Reagent grade 88% formic acid can be satisfactorily dried 
by refluxing with phthalic anhydride for 6 hr and then distilling. 
Alternatively, if it is left in contact with freshly prepared anhydrous 
CuSO4 for several days about one half of the water is removed from 88% 
formic acid: distn. removes the remainder. Boric anhydride (prepared by 
melting boric acid in an oven at a high temperature, cooling in a
desiccator, and powdering) is a suitable dehydrating agent for 98% formic 
acid; after prolonged stirring with the anhydride the formic acid is distd. 
under vac. Formic acid can be further purified by fractional crystn. 
using partial freezing. 

From: Finar "Organic Chemistry, vol. 1", 6th ed.
An aqueous solution of formic acid is obtained by distilling the sodium 
salt with dilute sulphuric acid:

2 HCO2Na + H2SO4 = 2 HCO2H + Na2SO4

Anhydrous formic acid is obtained from the aqueous solution (70-77 %) by 
the addition of butyl formate followed by distillation. The first fraction 
is an azeotrope of ester and water, and then the excess of ester is 
removed from the formic acid by fractionation. 

Anhydrous Formic acid can be obtained from Lead formate by reaction with 
hydrogen sulphide [this is given for information purposes only]. Note 
that several clandestine chemists have died as a result of inhaling 
hydrogen sulphide fumes. H2S is nasty.

Mann & Saunders also has a prep. of formic acid!

From: Mann & Saunders, "Practical Organic Chemistry", 3rd ed. 

Continued from prep (See above)

To obtain lead formate, add about 100 mL of water to the distillate 
[from the prep. of formic acid] and stir powdered lead carbonate into the 
gently heated solution until no further effervescence occurs. The boil 
the mixture vigorously and filter at the pump. Evaporate the clear 
filtrate by direct boiling until crystals appear on the surface, and then 
allow to cool, finally chilling in ice-water. The lead formate separates 
as colourless crystals; filter off, wash with a small quantity of cold 
water, and dry. Yield, about 6 gms.

Lead formate is only slightly soluble in cold water, and insoluble in hot 
absolute alcohol: it can therefore be readily distinguished from lead 
acetate or "sugar of lead" because, quite apart from chemical tests, the 
acetate is readily soluble in cold water and moderately soluble in alcohol.

Lead formate separates from aqueous solution without water of crystallisation. 
It is therefore frequently used for the preparation of anhydrous formic acid. 
For this purpose, the powdered lead formate is placed in the inner tube of 
an ordinary jacketed condenser, and there held loosely in position by plugs 
of glass-wool. The condenser is then clamped in an oblique position and the 
lower end fitted into a receiver closed with a calcium chloride tube. 
A current of dry hydrogen sulphide is passed down the inner tube of the 
condenser, whilst steam is passed through the jacket. (ie. the reaction 
takes place at 100ƒ) The formic acid which is liberated 
[ Pb(HCOO)2 + H2S = 2 HCOOH + PbS ] collects in the receiver, and is then 
purified from dissolved hydrogen sulphide by redistillation over a further 
quantity of lead formate.

--------------------------------------------------------------------------


