Oxalic Acid

Oxalic acid, \[H_{2}C_{2}O_{4}\] is a colourless crystalline dicarboxylic acid. It produces a colourless solution dissolving in water. If we consider the acid strength, it is a much stronger acid than acetic acid. Oxalic acid is a reducing agent and its conjugate base is called oxalate \[(C_{2}O_{4})^{2-}\]. Oxalate acts as a chelating agent for metal cations. Typically, oxalic acid also exists in the form of dihydrate with the formula \[H_{2}C_{2}O_{4}.2H_{2}O\]. Ingestion of oxalic acid through skin contact or orally proves to be fatal. Scheele in 1776 discovered oxalic acid.

Scheele produced oxalic acid by the oxidation of sugar using nitric acid and it was called saccharic acid. Now there are multiple artificial ways to produce oxalic acids. Some of them include boiling sugar and starch with nitric acid which results in the production of oxalic acid as a penultimate product of oxidation. 

Physical Properties of Oxalic Acid

• Molecular formula: \[H_{2}C_{2}O_{4}\]

• Molecular weight: 90.03gm/mole (anhydrous)

126.07gm/mole (dihydrate)

• Appearance: Orthorhombic colourless crystals

• Odour: Odourless

• Boiling point: 1570C at 1.013hPa (anhydrous)

• Melting point: 1010C, sublimes at 1500C (dihydrate)

•  190.00C (anhydrous)

• Flash point: 1660C

• Density: \[1.90gm/cm^{3}\] (anhydrous)

\[1.653gm/cm^{3}\] (dihydrate)

• Solubility: Soluble in water, absolute alcohol and ether

History Behind Manufacture of Oxalic Acid

The salts of oxalic acid from plants were produced in 1745. The Dutch botanist and physician Herman Boerhaave isolated a salt from sorrel and thus oxalic acid was discovered. By the year 1773, oxalic acid was isolated from its salt in sorrel by François Pierre Savary of Fribourg.

Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman produced oxalic acid by reacting sugar with concentrated nitric acid; Scheele called the resultant acid as såcker-Syra meaning sugar-acid or sweet acid. By 1784, it was proved that the sugar acid and the oxalic acid obtained in nature are identical.

In 1824, the German chemist Friedrich Wöhler again manufactured oxalic acid by reacting cyanogen with ammonia in an aqueous solution. This experiment may be considered as the first synthesis of a natural product.

Manufacturing of Oxalic Acid 

1. From Sodium Formate

Solid sodium hydroxide (97 - 98%) and carbon monoxide react at a 2000C temperature and 150psi pressure in an autoclave to produce sodium formate. On completion of this reaction, the pressure in the autoclave is gradually reduced and the temperature is increased to 4000C. The ceasing of the evolution of hydrogen results in the completion of the reaction.

The reaction mixture was collected in a precipitator and calcium hydroxide was added to it with stirring. This was followed by the filtration of the calcium oxalate and then sodium hydroxide was concentrated that could be reused.

The filter cake of crude calcium oxalate is taken to an acidifier with a small amount of calcium carbonate and then treated with dilute sulfuric acid. The precipitation of calcium results in the formation of calcium sulphate dihydrate (\[CaSO_{4}.2H_{2}O\]). The mother liquor was then taken to a crystallizer and concentrated to the specific gravity of 300Be. Finally, Crystals of oxalic acid in the form of dehydrating obtained which was then washed and dried.

The yield of oxalic acid dihydrate was nearly 80% by weight depending on sodium formate. It is generally sold and used, in the form of dihydrate. But anhydrous oxalic can also be obtained by heating the dihydrate to 1000C by the loss of two molecules of water.

2. Other Processes Involved

The direct acidification of sodium oxalate with methanol in the ratio of 1:3 and in the presence of 0.8 parts of concentrated sulfuric acid also produces oxalic acid. Then the insoluble sodium sulphate can be separated from the methyloxallatemethanol-sulfuric acid solution. The sodium sulphate is then filtered and the mother alcohol is hydrolysed by the addition of 3.5 parts water. This yields oxalic acid and methanol. The methanol obtained is then distilled and recycled.

The crude oxalic acid solution is then concentrated and crystallised producing the oxalic acid. Molasses can be oxidised with nitric acid to also produce oxalic acid. Similarly, oxidation of organic compounds like glycol, alcohol, fats, oat hulls, sawdust and other cellulosic materials in the presence of nitric acid also produces oxalic acid. Some of them are discussed below in detail.

3. From Propylene

This method involves the oxidation of propylene with nitric acid to produce oxalic acid. It involves a two-step process, wherein the first step propylene gets converted into α-nitrato lactic acid and the second step allows the oxidation of α-nitrato lactic acid to oxalic acid.

In the first reactor, propylene was introduced where it reacts with nitric acid. The molar ratio of propylene and nitric acid is kept at 0.01 - 0.5. The mixture from the first reactor was taken into the second reactor where it reacts with oxygen. In this step, the vapour of nitric acid continuously evolves from the top of the vessel. The stream from the vessel has an intermediate and a trace amount of nitric acid. It was then taken into the reservoir tank where acid was evolved. α-nitratolactic acid from the reservoir was then collected into an autoclave where its oxidation occurs at 45 - 1000C in the presence of mixed acid as a catalyst. In this step, α-nitratolactic acid gets converted into oxalic acid dihydrate. The crude product was then taken into a crystallizer where oxalic acid crystals are obtained. The slurry obtained from the crystallizer is filtered and sent for drying operation to generate dried oxalic acid.

Dimethyl Oxalate Process

In this process, methyl nitrite is regenerated from circulating gas containing CO and from the regeneration column it is pressurised and then fed to the reactor. In this vessel, dimethyl oxalate was produced but the consumption of methyl nitrite takes place. Freshly brewed dimethyl oxalate and unconverted mixture were then taken into the condenser where methanol was added. Uncondensed vapour containing methyl nitrite, water and methanol were drawn to the regeneration column accompanied by the addition of NOx and oxygen to regenerate the methyl nitrite which is to be recycled.

Condensed dimethyl oxalate from the condenser, when taken into the distillation column, removes water vapours while dimethyl oxalate was hydrolysed. After the completion of hydrolysis of oxalate, the slurry was sent to the crystallizer to generate a crystal of oxalic acid. The slurry is then filtered and passed from the dryer to store dried oxalic acid.

In the propylene process, the usage of sulfuric acid results in corrosion problems at the oxidation step. The oxidation reaction can be enhanced by a large reactor. There is a possibility of unstable by-product formation along with α-nitrato lactic acid and it may even lead to explosion or decomposition.

Oxalic Acid and Divalent Ions

The titration of 0·01m oxalic acid solutions with various levels of \[Zn^{2+}\] against standard KMnO4 solutions results in a linear relationship between the amount of precipitated oxalate and the metal ion concentrations. Same holds true for \[Ca^{2+}\], but here higher proportions of the oxalate ions were precipitated at an equivalent metal ion concentration. The inhibitory action became more noteworthy when the metal ions present in the solution were in a 1:1:1 molar ratio. In all cases, more the proportion of \[Mg^{2+}\] results in less precipitation of the oxalates of the other two metals.

Uses of Oxalic Acid

• Oxalic acid can be used as a mordant in dyeing processes

• Oxalic acid can remove rust and thus can be used for cleaning or bleaching

• Beekeepers use a 3.2% solution of oxalic acid or its vaporised form

• Helps in the removal of mustard, ink, different food stains as well as other types of

• stains

• It is the reducing agent used for developing the photographic film

• Calcium can be removed from wastewater using oxalic acid

Impact of Oxalic Acid on Health

Oxalic acid corrodes tissue. When ingested, oxalic acid results in the removal of calcium from the blood. Kidney damage can be the outcome as the calcium gets removed from the blood in the form of calcium oxalate, which obstructs the kidney tubules.

1. Inhalation is Harmful as it may result in severe irritation and burns of the nose, throat, and respiratory tract.

2. Ingestion of it results in renal damage causing bloody urine.

3. If it comes in contact with skin, severe itching and burning sensations prevail, dermatitis, cyanosis of the fingers and possible ulceration may be the outcome of prolonged skin contact.

4. Eye Contact may result in corrosive effects.

5. Chronic exposure may lead to inflammation of the upper respiratory tract.

Disposal Considerations

Since the harm caused on the human body by oxalic acid is irreversible to a certain extent, there should be proper disposal of wastes generated from and due to oxalic acid. Though it is not counted as RCRA hazardous waste, a few characteristics of the hazardous wastes may be prevalent and thus it demands appropriate analysis for the determination of specific disposal requirements

Physical Properties of Oxalic Acid

The molecular formula of oxalic acid is \[C_{2}H_{2}O_{4}\] with a molecular weight of 90.03gm/mole in anhydrous form while in the dihydrate state, its molecular weight is 126.07gm/mole. It has colourless crystals in orthorhombic structures and is odourless in nature. Its boiling point in anhydrous form is 1570°C at 1.013 hPa and its melting point is at 1010°C. Its anhydrous form sublimes at 190°C and its dihydrate state has a sublimation point of 1500°C. It has a density of \[1.90 gm/cm^{3}\] in the anhydrous state and \[1.653 gm/cm^{3}\] in the dihydrate state. Oxalic acid is soluble in water and in absolute alcohol as well as in ether.

Conclusion

Most commercially used oxalic acid has the contamination of sulfuric acid and alkali and here the alkali cannot be removed by recrystallization method using water, but the removal can be done by the method of recrystallization using 10 - 15% hydrochloric acid. This article serves you with almost all the facts you need on Oxalic Acids. Peruse away for better understanding, my friend!