Hydrogen peroxide

Hydrogen peroxide

Names
IUPAC name Hydrogen peroxide
Other names Dioxidane Oxidanyl Perhydroxic acid
Properties
Chemical formula	H2O2
Molar mass	34.0147 g/mol
Appearance	Very light blue liquid (conc.) Colorless liquid (dil.)
Odor	Slightly sharp
Density	1.450 g/cm3 (20 °C, pure) 1.11 g/cm3 (20 °C, 30% aq. solution)
Melting point	−0.43 °C (31.23 °F; 272.72 K)
Boiling point	150.2 °C (302.4 °F; 423.3 K)
Solubility in water	Miscibility
Solubility	Reacts with ketones Soluble in alcohol, ether Insoluble in benzene, chloroform, petroleum ether, toluene
Vapor pressure	5 mmHg (30 °C)
Acidity (pKa)	11.75
Thermochemistry
Std enthalpy of formation (ΔfHo298)	−187.80 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich (30%)
Flash point	Non-flammable
Related compounds
Related compounds	Water
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Hydrogen peroxide (H₂O₂) is a mostly clear, blue-ish liquid with similar melting and boiling points to water. It is a powerful and versatile oxidizer, but can act as a reducing agent in some circumstances. It also acts as a very weak acid (pK_a = 11.6), forming hydrated peroxide salts (such as sodium peroxide octahydrate) with alkalis in aqueous solution.

Properties

Chemical

Hydrogen peroxide disproportionates into water and oxygen gas. This happens rapidly at high temperatures or when a catalyst, such as manganese dioxide or potassium iodide, is added and this is often used to produce oxygen gas in a home chemistry setting:

H₂O₂ → H₂O + ½ O₂

Hydrogen peroxide can be used as an oxidizer, and may enhance the oxidizing capabilities on mixing. For example, mixtures of sulfuric acid and hydrogen peroxide not only will react faster than the acid alone, but will also react with organic compounds, sometimes explosively. Depending on the ratio of peroxide and sulfuric acid, there are several types: piranha solution (min H₂SO₄:H₂O₂ ratio 3:1), peroxymonosulfuric acid (H₂SO₄:H₂O₂ molar ratio of 1:1), peroxydisulfuric acid (H₂SO₄:H₂O₂ molar ratio of 1:2).

Hydrogen peroxide is dangerous as it can cause explosions when in contact with combustible materials in high concentration. An acid mixture of hydrogen peroxide and hydrochloric acid behaves like an oxidizing acid, similarly to nitric acid, and reacts with non-reactive metals such as copper:

Cu + 2 HCl + H₂O₂ → CuCl₂ + 2 H₂O

This acid mixture alone undergoes a number of slow reactions with various products, which include oxygen and chlorine in statu nascendi and hypochlorous acid, which makes it a powerful oxidizer that can be used as a much safer alternative to concentrated nitric acid for dissolving metals. However, you should never store this "green acid" as it invariably decomposes over time.

Hydrogen peroxide will reduce potassium permanganate is reduced to Mn₂₊:

2 KMnO₄ + 3 H₂O₂ → 2 MnO₂ + 2 KOH + 2 H₂O + 3 O₂

Reaction with hypochlorites, like that with sodium hypochlorite will also produce oxygen, more specifically singlet oxygen.:

NaOCl + H₂O₂ → NaCl + H₂O + O₂

This route is a safe way or neutralizing bleach, without producing any unpleasant smells.

Reaction with borax and sodium hydroxide leads to sodium perborate:

Na₂B₄O₇ + 2 NaOH + 4 H₂O₂ → 2 Na₂B₂O₄(OH)₄ + H₂O

Reaction with ozone forms trioxidane.

It converts carboxylic acids into peroxy acids

Hydrogen peroxide reacts with acetone to form the highly dangerous acetone peroxide. It also forms stable adducts with urea (urea peroxide), sodium carbonate (sodium percarbonate) and other compounds.

Hydrogen peroxide will oxidize thioethers to sulfoxides:

R-S-R' + H₂O₂ → R-S(=O)-R' + H₂O

Hydrogen peroxide solution containing ferric ion, is called Fenton's reagent, and can be used to safely neutralize many organic compounds.

Physical

Hydrogen peroxide is tinted slightly blue in high concentrations. It has boiling and melting points similar to water, but can be concentrated by fractional crystallization. Concentrations of peroxide 30% and above are considered concentrated.

Availability

Hydrogen peroxide is available readily as a disinfectant in pharmacies and grocery stores, but may only be obtained easily in low concentrations often as 3% or 6% solutions.

Higher concentration peroxide is sold for animal disinfectants, pool/spa treatments, hair bleaching. Some may contain other chemicals, including stabilizers, so read the label first.

In the EU, the sale of hydrogen peroxide solutions with a concentration higher than 12% is restricted for private individuals.

Preparation

Hydrogen peroxide can be prepared by reacting concentrated sulfuric acid and barium peroxide.

BaO₂ + H₂SO₄ → H₂O₂ + BaSO₄

The insoluble barium sulfate is filtered from the mixture.

Due to the availability of low concentrations of peroxide worldwide, concentrated hydrogen peroxide solutions are often prepared by evaporating the water from the peroxide, making sure not to boil the solution (as this will break down the peroxide).

One of the earliest large scale production of hydrogen peroxide involves the electrolysis of a solution of sulfuric acid of 50% concentration, or ammonium bisulfate solution. The resulting hydrogen peroxide solution has a concentration of 30%.

While sulfuric acid is more readily available, the ammonium bisulfate process is cheaper and has a higher cell efficiency. The electrolysis is carried out in stoneware tanks with platinum electrodes; conversion of bisulfate to the persulfate takes place at the anode. After hydrolysis of the persulfate (with steam) in an evaporator, the resulting dilute aqueous solution of H₂O₂ is separated from the bisulfate and further distilled in a stoneware distillation column. The resulting solution is approximately 30 w/o H₂O₂. Both the cathode liquor (after purification) and the bisulfate from the evaporator (and separator) are recycled back to the cells.

Industrially, hydrogen peroxide is produced via the anthraquinone process (2-ethylanthraquinone gives the best performance, as it has better selectivity), where the anthraquinone can be hydrogenated in the presence of a palladium catalyst to 9,10-anthrahydroquinone, which, upon oxidation with oxygen, reverts back to anthraquinone releasing hydrogen peroxide. Cody's Lab made a video on making hydrogen peroxide via this route.

Projects

• Make sulfuric acid
• Make peroxides
• Cleaning silver metal
• Make piranha solution
• Make Fenton's reagent
• Make copper etching solution
• Make singlet oxygen

Handling

Safety

As it is an oxidizer, high concentrations of hydrogen peroxide can ignite or detonate combustible or explosive materials. Lower concentrations are much safer, but regardless of concentration, poisonous. Concentrated hydrogen peroxide leaves weak itching or stinging burns and a white discoloration on skin.

Storage

Hydrogen peroxide solutions are best stored in cold dark places, such as a fridge. High concentration peroxides are metastable and will slowly build-up pressure, so it's recommended to open the bottles from time to time, to release the pressure, and store the solutions with ample headroom in the bottles. Phosphoric acid and EDTA are added as stabilizers.

NEVER store hydrogen peroxide near volatile organic compounds, such as acetone, as there is a risk of forming acetone peroxide.

Concentrated solutions of hydrogen peroxide are known to slowly react with glass. This does not harm the glass significantly, but it does harm the peroxide: the reaction yields sodium hydroxide, which can provoke its decomposition. It is preferable to use polyethylene to store such solutions.

Disposal

Hydrogen peroxide can be decomposed by adding a catalyst, such as manganese dioxide or iron oxides (ordinary rust will do). This method however should not be used to neutralize concentrated peroxide (>30%) as the decomposition will generate lots of heat and can lead to explosion, and the peroxide should first be diluted. The explosion that crippled the Kursk submarine for example, occured when the peroxide that leaked from a torpedo entered in contact with some rust. It's recommended to not be poured down the drain, as it will quickly decompose in the sewage and may pose an explosion hazard. Adding a sulfide, such as lead(II) sulfide, will result in lead(II) sulfate and water.

References

Relevant Sciencemadness threads

• Hydrogen Peroxide purification
• Hydrogen peroxide concentration by freezing I need to make 12% h2o2 from 3%
• Hydrogen peroxide maximum safe concentration
• Storage of 35% H2O2