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Recipe example

In most cases, a doctor may prescribe hydrogen peroxide in the form of an official solution.
An official drug is a drug that is sold in the same form in pharmacies with a given concentration, which makes it possible not to indicate the concentration in the description. In this case, in Latin, the recipe for hydrogen peroxide will look like this:

• Rp.: Solutionis Hydrogenii peroxydi dilutae 100 ml DS

The prescription indicates the amount of the drug dispensed. Below the recipe indicates how to use the substance, for example, one tablespoon per 200 ml of water for rinsing the mouth.

If you need to prepare a solution of hydrogen peroxide with a different concentration, which the doctor considers necessary for medical use, the recipe will look like this:

• Solutionis Hydrogenii peroxydi dilutae 1% – 100 ml DS

This means that it is necessary to prepare a one percent solution in a volume of one hundred milliliters.

There is no need to dilute hydrogen peroxide yourself - the pharmacist will prepare a solution with the specified content and stick a label on the jar with the solution describing the recipe according to which the drug was made.

Decomposition reaction.

Pure hydrogen peroxide is a very dangerous substance, since under certain conditions its explosive decomposition is possible: H2O2 ® H2O + 1/2 O2, releasing 98 kJ per mole of H2O2 (34 g). This is a very large energy: it is greater than that released when 1 mole of HCl is formed during the explosion of a mixture of hydrogen and chlorine; it is enough to completely evaporate 2.5 times more water than is formed in this reaction. Concentrated aqueous solutions of H2O2 are also dangerous; in their presence, many organic compounds easily ignite, and such mixtures can explode upon impact. To store concentrated solutions, use vessels made of especially pure aluminum or waxed glass vessels.

More often you encounter a less concentrated 30% solution of H2O2, which is called perhydrol, but such a solution is also dangerous: it causes burns on the skin (when exposed to it, the skin immediately turns white due to the discoloration of coloring substances), and explosive boiling may occur if impurities enter. The decomposition of H2O2 and its solutions, including explosive decomposition, is caused by many substances, for example, heavy metal ions, which play the role of a catalyst, and even dust particles.

H2O2 explosions are explained by the strong exothermic nature of the reaction, the chain nature of the process and a significant decrease in the activation energy of H2O2 decomposition in the presence of various substances, as can be judged from the following data:

The enzyme catalase is found in the blood; It is thanks to it that pharmaceutical “hydrogen peroxide” “boils” from the release of oxygen when it is used to disinfect a cut finger. The reaction of decomposition of a concentrated solution of H2O2 under the action of catalase is used not only by humans; It is this reaction that helps the bombardier beetle fight enemies by releasing a hot stream at them (see EXPLOSIVES). Another enzyme, peroxidase, acts differently: it does not decompose H2O2, but in its presence the oxidation of other substances with hydrogen peroxide occurs.

Enzymes that influence the reactions of hydrogen peroxide play an important role in the life of the cell. Energy is supplied to the body by oxidation reactions involving oxygen coming from the lungs. In these reactions, H2O2 is intermediately formed, which is harmful to the cell, as it causes irreversible damage to various biomolecules. Catalase and peroxidase work together to convert H2O2 into water and oxygen.

The decomposition reaction of H2O2 often proceeds by a radical chain mechanism (see CHAIN ​​REACTIONS), with the role of the catalyst being to initiate free radicals. Thus, in a mixture of aqueous solutions of H2O2 and Fe2+ (the so-called Fenton reagent), an electron transfer reaction occurs from the Fe2+ ion to the H2O2 molecule with the formation of the Fe3+ ion and a very unstable radical anion [H2O2] . –, which immediately breaks down into the OH– anion and the free hydroxyl radical OH . (see FREE RADICALS). HE is a radical . very active. If there are organic compounds in the system, then various reactions with hydroxyl radicals are possible. Thus, aromatic compounds and hydroxy acids are oxidized (benzene, for example, turns into phenol), unsaturated compounds can attach hydroxyl groups to the double bond: CH2=CH–CH2OH + 2OH . ® HOCH2–CH(OH)–CH2–OH, and can enter into a polymerization reaction. In the absence of suitable reagents, OH . reacts with H2O2 to form the less active radical HO2 . , which is capable of reducing Fe2+ ions, which closes the catalytic cycle:

H2O2 + Fe2+ ® Fe3+ + OH . +OH–

HE . + H2O2 ® H2O + HO2 .

HO2 . + Fe3+ ® Fe2+ + O2 + H+

H+ + OH– ® H2O.

Under certain conditions, chain decomposition of H2O2 is possible, the simplified mechanism of which can be represented by the diagram

HE . + H2O2 ® H2O + HO2 . 2 . + H2O2 ® H2O + O2 + OH . etc.

H2O2 decomposition reactions occur in the presence of various metals of variable valence. When bound into complex compounds, they often significantly enhance their activity. For example, copper ions are less active than iron ions, but bound in ammonia complexes [Cu(NH3)4]2+, they cause rapid decomposition of H2O2. Mn2+ ions bound in complexes with some organic compounds have a similar effect. In the presence of these ions, it was possible to measure the length of the reaction chain. To do this, we first measured the reaction rate by the rate of release of oxygen from the solution. Then an inhibitor was introduced into the solution in a very low concentration (about 10–5 mol/l), a substance that effectively reacts with free radicals and thus breaks the chain. The release of oxygen immediately stopped, but after about 10 minutes, when all the inhibitor was used up, it resumed again at the same rate. Knowing the reaction rate and the rate of chain termination, it is not difficult to calculate the length of the chain, which turned out to be equal to 103 links. The long chain length determines the high efficiency of H2O2 decomposition in the presence of the most effective catalysts, which generate free radicals at a high rate. At this chain length, the rate of H2O2 decomposition actually increases a thousand times.

Sometimes noticeable decomposition of H2O2 is caused even by traces of impurities that are almost undetectable analytically. Thus, one of the most effective catalysts turned out to be a sol of metal osmium: its strong catalytic effect was observed even at a dilution of 1:109, i.e. 1 g Os per 1000 tons of water. Active catalysts are colloidal solutions of palladium, platinum, iridium, gold, silver, as well as solid oxides of some metals - MnO2, Co2O3, PbO2, etc., which themselves do not change. Decomposition can proceed very rapidly. So, if a small pinch of MnO2 is thrown into a test tube with a 30% H2O2 solution, a column of steam with a splash of liquid escapes from the test tube. With more concentrated solutions an explosion occurs. Decomposition occurs more quietly on the surface of platinum. In this case, the reaction rate is strongly influenced by the state of the surface. The German chemist Walter Spring conducted at the end of the 19th century. such an experience. In a thoroughly cleaned and polished platinum cup, the decomposition reaction of a 38% H2O2 solution did not occur even when heated to 60 ° C. If you make a barely noticeable scratch on the bottom of the cup with a needle, then the already cold (at 12 ° C) solution begins to release on the spot scratches contain oxygen bubbles, and when heated, decomposition along this area noticeably intensifies. If spongy platinum, which has a very large surface area, is introduced into such a solution, then explosive decomposition is possible.

The rapid decomposition of H2O2 can be used for an effective lecture experiment if a surfactant (soap, shampoo) is added to the solution before adding the catalyst. The oxygen released creates a rich white foam, which has been called “elephant toothpaste.”

Some catalysts initiate non-chain decomposition of H2O2, for example:

H2O2 + 2I– + 2H+ ® 2H2O + I2

I2 + H2O2 ® 2I– + 2H+ + O2.

A non-chain reaction also occurs in the case of the oxidation of Fe2+ ions in acidic solutions: 2FeSO4 + H2O2 + H2SO4 ® Fe2(SO4)3 + 2H2O.

Since aqueous solutions almost always contain traces of various catalysts (metal ions contained in glass can also catalyze decomposition), inhibitors and stabilizers that bind metal ions are added to H2O2 solutions, even diluted ones, during long-term storage. In this case, the solutions are slightly acidified, since the action of pure water on glass produces a slightly alkaline solution, which promotes the decomposition of H2O2.

All these features of H2O2 decomposition allow us to resolve the contradiction. To obtain pure H2O2, it is necessary to carry out distillation under reduced pressure, since the substance decomposes when heated above 70 ° C and even, although very slowly, at room temperature (as stated in the Chemical Encyclopedia, at a rate of 0.5% per year). In this case, how was the boiling point at atmospheric pressure of 150.2° C, which appears in the same encyclopedia, obtained? Typically, in such cases, a physicochemical law is used: the logarithm of the vapor pressure of a liquid linearly depends on the inverse temperature (on the Kelvin scale), so if you accurately measure the vapor pressure of H2O2 at several (low) temperatures, you can easily calculate at what temperature this pressure will reach 760 mmHg And this is the boiling point under normal conditions.

Theoretically, OH radicals . can also form in the absence of initiators, as a result of the rupture of a weaker O–O bond, but this requires a fairly high temperature. Despite the relatively low energy of breaking this bond in the H2O2 molecule (it is equal to 214 kJ/mol, which is 2.3 times less than for the H–OH bond in a water molecule), the O–O bond is still strong enough for hydrogen peroxide was absolutely stable at room temperature. And even at boiling point (150°C) it should decompose very slowly. Calculations show that at this temperature, decomposition of 0.5% should also occur quite slowly, even if the chain length is 1000 links. The discrepancy between calculations and experimental data is explained by catalytic decomposition caused by the smallest impurities in the liquid and the walls of the reaction vessel. Therefore, the activation energy of H2O2 decomposition measured by many authors is always significantly less than 214 kJ/mol, even “in the absence of a catalyst.” In fact, there is always a decomposition catalyst - both in the form of insignificant impurities in the solution and in the form of the walls of the vessel, which is why heating anhydrous H2O2 to boiling at atmospheric pressure has repeatedly caused explosions.

Under some conditions, the decomposition of H2O2 occurs very unusually, for example, if you heat a solution of H2O2 acidified with sulfuric acid in the presence of potassium iodate KIO3, then at certain concentrations of the reagents an oscillatory reaction is observed, and the release of oxygen periodically stops and then resumes with a period of 40 to 800 seconds .

Dispensed without a prescription

In pharmacy chains, official solutions of hydrogen peroxide are sold without a prescription as an antiseptic and disinfectant. The concentration of hydrogen peroxide is 3%; dark glass jars also contain additional sodium benzoate and purified water. The volume of the drug is 100, 50, 40 and 25 ml. To purchase such a solution, you can take a prescription, but due to the prevalence and safety of the drug, the Latin name of hydrogen peroxide is not required for purchase.

Chemical properties of H2O2.

Hydrogen peroxide is an acid, but a very weak one. The dissociation constant of H2O2H+ + HO2– at 25°C is 2.4·10–12, which is 5 orders of magnitude less than for H2S. Medium H2O2 salts of alkali and alkaline earth metals are usually called peroxides (see PEROXIDES). When dissolved in water, they are almost completely hydrolyzed: Na2O2 + 2H2O ® 2NaOH + H2O2. Hydrolysis is promoted by acidification of solutions. As an acid, H2O2 also forms acidic salts, for example, Ba(HO2)2, NaHO2, etc. Acid salts are less susceptible to hydrolysis, but easily decompose when heated, releasing oxygen: 2NaHO2 ® 2NaOH + O2. The released alkali, as in the case of H2O2, promotes decomposition.

H2O2 solutions, especially concentrated ones, have a strong oxidizing effect. Thus, when a 65% H2O2 solution is exposed to paper, sawdust and other flammable substances, they ignite. Less concentrated solutions decolorize many organic compounds, such as indigo. The oxidation of formaldehyde occurs in an unusual way: H2O2 is reduced not to water (as usual), but to free hydrogen: 2HCHO + H2O2 ® 2HCOOH + H2. If you take a 30% solution of H2O2 and a 40% solution of HCHO, then after a little heating a violent reaction begins, the liquid boils and foams. The oxidative effect of dilute H2O2 solutions is most pronounced in an acidic environment, for example, H2O2 + H2C2O4 ® 2H2O + 2CO2, but oxidation is also possible in an alkaline environment:

Na[Sn(OH)3] + H2O2 + NaOH ® Na2[Sn(OH)6]; 2K3[Cr(OH)6] + 3H2O2 ® 2KCrO4 + 2KOH + 8H2O.

Oxidation of black lead sulfide to white sulfate PbS + 4H2O2 ® PbSO4 + 4H2O can be used to restore darkened white lead in old paintings. Under the influence of light, hydrochloric acid also undergoes oxidation:

H2O2 + 2HCl ® 2H2O + Cl2. Adding H2O2 to acids greatly increases their effect on metals. Thus, copper, silver and mercury dissolve in a mixture of H2O2 and dilute H2SO4; iodine in an acidic environment is oxidized to periodic acid HIO3, sulfur dioxide to sulfuric acid, etc.

Unusually, the oxidation of potassium sodium salt of tartaric acid (Rochelle salt) occurs in the presence of cobalt chloride as a catalyst. During the reaction KOOC(CHOH)2COONa + 5H2O2 ® KHCO3 + NaHCO3 + 6H2O + 2CO2, pink CoCl2 changes color to green due to the formation of a complex compound with tartrate, the tartaric acid anion. As the reaction proceeds and the tartrate is oxidized, the complex is destroyed and the catalyst turns pink again. If copper sulfate is used as a catalyst instead of cobalt chloride, the intermediate compound, depending on the ratio of the starting reagents, will be colored orange or green. After the reaction is completed, the blue color of the copper sulfate is restored.

Hydrogen peroxide reacts completely differently in the presence of strong oxidizing agents, as well as substances that easily release oxygen. In such cases, H2O2 can also act as a reducing agent with the simultaneous release of oxygen (the so-called reductive decomposition of H2O2), for example:

2KMnO4 + 5H2O2 + 3H2SO4 ® K2SO4 + 2MnSO4 + 5O2 + 8H2O;

Ag2O + H2O2 ® 2Ag + H2O + O2;

O3 + H2O2 ® H2O + 2O2;

NaOCl + H2O2 ® NaCl + H2O + O2.

The last reaction is interesting because it produces excited oxygen molecules that emit orange fluorescence (see ACTIVE CHLORINE). Similarly, metallic gold is released from solutions of gold salts, metallic mercury is obtained from mercury oxide, etc. This unusual property of H2O2 allows, for example, the oxidation of potassium hexacyanoferrate(II) and then, by changing the conditions, reducing the reaction product to the original compound using the same reagent. The first reaction occurs in an acidic environment, the second in an alkaline environment:

2K4[Fe(CN)6] + H2O2 + H2SO4 ® 2K3[Fe(CN)6] + K2SO4 + 2H2O;

2K3[Fe(CN)6] + H2O2 + 2KOH ® 2K4[Fe(CN)6] + 2H2O + O2.

(“The dual character” of H2O2 allowed one chemistry teacher to compare hydrogen peroxide with the hero of the famous English writer Stevenson’s story The Strange Case of Dr. Jekyll and Mister Hyde; under the influence of the composition he invented, he could dramatically change his character, turning from a respectable gentleman into a bloodthirsty maniac.)

Effect of hydrogen peroxide

The drug is an antiseptic with oxidizing properties. It is also able to stop minor bleeding, such as capillary bleeding. When the solution comes into contact with the outer layer of the skin or mucous membrane, oxygen radicals are released. Thanks to this, organic substances are deactivated: blood, suppuration, proteins.

The antiseptic effect is due to the influence of active oxygen on the membranes of bacterial cells and pathogenic microflora. As a result of oxidation of cell membranes, bacteria die. But hydrogen peroxide is not a sterilizer - after time, microflora multiplies again on the treated surface.

Physical properties.

Also on topic:

PEROXIDES

Pure hydrogen peroxide is very different from the familiar 3% H2O2 solution that is found in your home medicine cabinet. First of all, it is almost one and a half times heavier than water (density at 20°C is 1.45 g/cm3). H2O2 freezes at a temperature slightly lower than the freezing point of water - at minus 0.41 ° C, but if you quickly cool a pure liquid, it usually does not freeze, but is supercooled, turning into a transparent glassy mass. Solutions of H2O2 freeze at a much lower temperature: a 30% solution at minus 30° C, and a 60% solution at minus 53° C. H2O2 boils at a temperature higher than ordinary water - at 150.2° C. H2O2 wets glass worse than water, and this leads to an interesting phenomenon during the slow distillation of aqueous solutions: while water is distilled from the solution, it, as usual, comes from the refrigerator to the receiver in the form of drops; when H2O2 begins to distill, the liquid comes out of the refrigerator in the form of a continuous thin stream. On the skin, pure hydrogen peroxide and its concentrated solutions leave white spots and cause a burning sensation due to a severe chemical burn.

In an article devoted to the production of hydrogen peroxide, Tenard did not very successfully compare this substance with syrup; perhaps he meant that pure H2O2, like sugar syrup, strongly refracts light. Indeed, the refractive index of anhydrous H2O2 (1.41) is much greater than that of water (1.33). However, either as a result of misinterpretation, or because of poor translation from French, almost all textbooks still write that pure hydrogen peroxide is a “thick, syrupy liquid,” and even explain this theoretically by the formation of hydrogen bonds. But water also forms hydrogen bonds. In fact, the viscosity of H2O2 is the same as that of slightly cooled (to about 13 ° C) water, but it cannot be said that cool water is thick like syrup.

Why is the drug prescribed?

According to the designation of the European Pharmacopoeia, hydrogen peroxide is called hydrogen peroxide. In Latin, hydrogen peroxide sounds like Hydrogenii peroxydi dilutae - it is by this name that the doctor writes a prescription.

Please note that you cannot purchase hydrogen peroxide in European pharmacy chains without a doctor’s prescription, despite the high safety of the medicine. This practice applies to all medications - prescriptions for them must be issued by a doctor.

The drug is prescribed for rinsing and washing for diseases of the mouth and throat, and gynecological pathologies. The antiseptic is effective for treating shallow wounds and against nosebleeds.