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Tincture of iodine: properties, uses, contraindications

The presence of iodine (I 2 ) makes it a fast-acting microbial agent, a property that this element shares with other halogens such as chlorine and bromine. It has a wide spectrum of activity against bacteria, spores, viruses and fungi.

Tincture of iodine. Elrond / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

In addition to I 2, it contains iodide ions I  which allow the mixture to be stabilized by forming I 3 ions  , preventing the iodine from evaporating over time. However, its antimicrobial action can be rapidly deactivated by alkalis, since other ionic species are formed that do not attack microorganisms.

It should not be confused with Lugol’s solution or povidone iodine solutions.

Medicinal product

Name

  • Tincture of iodine
  • Alcoholic iodine solution

Pharmaceutical form

Dark brown liquid with an alcohol odor.

Composition

An example of a commercial composition is the following: iodine (I 2 ) = 2.5% (w / v), potassium iodide (KI) = 2.5% (w / v), ethanol (CH 3 CH 2 OH) = 89% (v / v) and purified water.

Chemical properties of iodine

Iodine is a halogen. It is a solid in the form of blue-black flakes, melts at 113.7 ° C and boils at 184.4 ° C. It is poorly soluble in water: 0.03 g / 100 g at 20 ° C. It is soluble in ethanol, benzene, ether, and chloroform.

Crystals of iodine. Vadim Sedov / CC BY (https://creativecommons.org/licenses/by/4.0). Source: Wikimedia Commons.

Reactions of iodine in water

In water, the disproportionation or dismutation reaction occurs rapidly, forming iodide ion and hypoiodine acid:

2 + H 2 O ⇔ H + + I  + HOI

The concentration of HOI in a saturated solution of iodine at 25 ° C is negligible, since it separates into a proton and the hypoiodite ion.

HOI ⇔ H + + IO 

The following equilibrium also occurs:

3 I 2 + 3 H 2 O ⇔ IO  + 6 H + + 5 I 

The solubility of iodine in water increases when adding sodium iodide (NaI) or potassium (KI), since the triiodide ion (I  ) is formed, which is very stable:

2 + I  → I 

Lewis structure of the triiodide ion I3-. Bioika0201 / Public domain. Source: Wikimedia Commons.

Reactions of iodine in alkaline solution

Dissolving iodine in alkaline solution easily forms the hypoiodite ion:

2 + 2 OH  → I  + IO  + H 2 O

But the hypoiodite ion in alkaline solution is very unstable and immediately disproportionate, forming the iodate ion:

3 IO  ⇔ 2 I  + IO 

Other properties of tincture of iodine

It is incompatible with ammonia, iron, copper, lead, mercury and bismuth salts, mineral acids, potassium chlorate and other oxidizing compounds.

It should be kept in a tightly closed container at temperatures below 25 ° C.

Therapeutic uses

It is a solution that is applied only on the skin.

It has antiseptic and disinfectant use for shallow wounds, cuts, abrasions, and minor bacterial infections. It serves in the prophylaxis or prevention and treatment of skin infections caused by gram-positive and gram-negative bacteria.

It is the most effective disinfectant for healthy skin and is used to sterilize it before operations or before taking blood samples for analysis.

It is applied with cotton or a small brush. It is suitable for adults, children and the elderly and is indicated for external use only.

However, there are situations where it is applied to the oral mucosa, teeth or dental devices for disinfection purposes, but it can cause irritation of the oral lining, producing an itchy sensation and sometimes leading to blistering.

Tincture of iodine is sometimes used to disinfect the umbilical cord of newborn calves. Author: Gisela Kortenbruck. Source: Pixabay.

It is also used to disinfect minor animal wounds. Also in the umbilical cord after the calves are born.

Mechanism of action

Species I 2 and HOI are the most potent biocidal forms of iodine and are 97% abundant when the pH is 9 or less.

Iodine performs a powerful microbicidal action, being effective against fungi, viruses, protozoa (for example against trichomonas), cysts and spores.

Although the mechanism of action is still being studied, it is estimated that diatomic iodine (I 2 ) attacks the cell walls or capsule of the microorganism and precipitates proteins through a halogenation reaction, causing irreversible damage to the membrane, which leads to cell or virus death.

Iodine breaks down the cell membrane of bacteria. Author: Clker-Free-Vector-Images. Source: Pixabay.

Oxidation

There are also studies that indicate that I 2 can form reactive oxygen species generating oxidative stress in the cytoplasm of bacterial cells, which leads to death.

It has been determined that about 90% of the iodine absorbed by bacteria is in the form of iodide, confirming the oxidation effect. The antimicrobial mechanism works best at acidic pH.

Precautions and contraindications

It should not be used in people who are hypersensitive to iodine or iodides, or in newborn infants, or in patients with thyroid disorders or receiving lithium therapy.

It should be used with caution in pregnant women, since iodide crosses the placenta and can harm the fetus, causing hypothyroidism and thyroid goiter.

Its use during childbirth can result in absorption by both the mother and the fetus, and the infant may suffer from transient hypothyroidism.

It should also be avoided in mothers who are breastfeeding their baby, because iodide is excreted in breast milk and can cause temporary hypothyroidism in the infant.

Adverse reactions towards tincture of iodine

Prolonged use should be avoided. The skin should not be covered after its application. Not for use on large open wounds or animal bites or severe burns. It can affect the mucous membranes.

In studies carried out in patients to whom gauze soaked with tincture of iodine was placed on healthy skin, it was found that the patches had to be removed after 24 hours due to the presence of severe skin reactions.

It can cause allergic reactions, such as hives, infections under the skin, skin bleeding, fever, joint pain, swollen glands, and an abnormally high level of white blood cells.

If ingested, poisoning can occur, the symptoms of which are perception of metallic taste, abdominal pain, diarrhea, thirst and headache. It can even lead to death.

Other similar products

Lugol’s solution

The so-called “strong iodine solution” or “Lugol’s solution” is similar to tincture of iodine, but may contain higher amounts of iodine and potassium iodide in aqueous solution.

It should not be confused with tincture of iodine, as it contains alcohol while Lugol’s solution has only water as a solvent.

Commercial Lugol’s solution. Amin / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Iodophors

They are complexes of iodine with surfactants such as those derived from polyvinylpyrrolidone. Their stability is maintained in an acid medium and large amounts of potassium iodide (KI) are added to inhibit the formation of iodate.

They are not very irritating, have low toxicity, good detergency, do not stain, but lose their action in an alkaline environment.

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