Oxalic Acid

Nature

  • Also known as Ethanediolic acid, Dicarboxylic acid, acid of sugar and Salt of sorrel.
  • Oxalic acid is Colourless, transparent, odourless, prismatic crystals resembling the crystals of magnesium sulphate and zinc sulphate.
  • It has sour and slightly bitter acidic taste.
  • It is a relatively strong acid, and forms a white, dihydrate precipitate.
  • It is present in rhubarb leaves, beets and many other vegetables., Potassium oxalate, sodium oxalate and ammonium oxalate are toxic salts of oxalic acid.
Chemical Structure of Oxalic Acid

Source

  • This acid occurs naturally in plants and vegetables such as wood sorrel , rhubarb and spinach.
  • Oxalic acid also produced from the metabolism of moulds and also from the extraction of alkali.

Uses

  • It is used in paint, stain and varnish removers, rust and ink stain removers, and ceramics.
  • It is also used in general metal and equipment cleaning, wood cleaning, process engraving, printing and dyeing, bleaching, textile finishing, leather tanning, and photography.
  • It is also used in cleaning brass and copper articles.

Fatal Dose

The fatal dose of oxalic acid is 15 to 20 gm.

Fatal Period

The fatal period of oxalic acid is 1 to 2 hours.

Mode of Action

Local- It acts as corrosive when used in concentrated form and act as irritants when used in dilute form.

Systemic- After absorption, oxalic acid combines with calcium ion and causes hypocalcemia, since it reacts with calcium in plasma, and insoluble calcium oxalate is precipitated which accumulates in the liver, kidneys, heart, lungs, and blood, and is excreted in the urine. It also causes tubular necrosis and renal failure.

Sign and Symptoms

Local

  • Whitish or yellowish Corrosion of mucosa with underlying congestion. The corroded area is referred as ‚Äúscalded‚ÄĚ in appearance.
  • Production of acid haematin.

Systematic

  • Vomiting and diarrhea
  • Hypocalcemia (tetany)
  • Muscle irritability
  • Tenderness
  • Cramps
  • Convulsions
  • Accoucher‚Äôs hand due to carpopedal spasm.
  • Chavostek‚Äôs sign positive. When tapping is done over facial nerve area, there is spasm of facial muscles.
  • Metabolic acidosis
  • Renal failure
  • Uremia
  • Calcium oxalate crystals can be deposited in the liver resulting in hepatic necrosis.

Fulminating Poisoning

  • Burning pain from the mouth to the stomach.
  • There is a burning, sour, bitter taste in the mouth.
  • Constriction around the throat.
  • Nausea
  • Vomiting
  • Death usually occurs before bowels are affected, but if life is prolonged diarrhoea will occur.

Treatment

  • Local exposure: Wash the affected skin with copious water.
  • Stomach washes with calcium gluconate or lactate solution.
  • Gastric lavage with Calcium gluconate IV (10 ml, 10% solution).
  • Dialysis or exchange transfusion for renal failure.
  • Affected skin or eye should be washed copiously with water.
  • Symptomatic.
  • Dialysis and exchange transfusion for renal failure.
  • Parathyroid extract 100 units i.m. in severe case.
  • The bowels may be evacuated or by castor oil.

Detection and Forensic Examination

  • Demonstration of urinary oxalate crystals which may occur either as monohydrates (prism or needle-like), or dihydrates (tent or envelope shaped).
  • Oxalic acid can be measured in the urine by colourimetry. The normal upper limit is 40 to 50 mg/24 hours.
  • Average serum oxalate concentration is said to be 1.4 mg/L.
  • A solution of barium nitrate gives a white precipitate of barium oxalate, which is soluble in hydrochloric acid or nitric acid.
  • Litmus test.
  • Gas chromatography- mass spectrometry.
  • HPLC
  • Titration

Postmortem Aspects

  • Scalded mucosa of gastro intestinal tract.
  • Mucous membrane of mouth, tongue, pharynx, and esophagus may be bleached and has scalded appearance.
  • Kidneys show edema.
  • Congestion with oxalate crystals in renal tubules with necrosis of proximal convoluted tubule.
  • Whitish or yellowish discolorations of corroded areas.
  • The mucous membrane of the stomach is reddened or punctuate from erosions or almost black.
  • Upper part of the duodenum may be affected.
  • There will be congestion of the lungs, liver, kidneys and brain, without any local appearances.

Medico Legal Aspect

  • Accidental causes, e.g. mistaking the substance for Epsom salt, sodium bicarbonate, etc. Accidental poisoning may also result from excessive ingestion of certain vegetables rich in oxalates (rhubarb leaves,sorrel, etc.). Even tea is said to contain significant amounts of oxalate.
  • Chronic consumption of oxalic acid can lead to renal calculi with consequent renal colic.
  • Occasional cases of suicide and homicide with oxalic acid have been reported
  • Rarely, it is used to procure abortion by vaginal injection.

References

  • Dr. K.S. Narayan Reddy. The essential of forensic medicine and toxicology.34th edition.
  • VV Pillay. Modern medical toxicology.4th edition.
  • R.K.Sharma. Concise textbook of forensic medicine and toxicology. 3rd edition.
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