It is important to make a distinction between sterilization and disinfection. The first refers to the elimination of all viruses and life forms in a medium or surface, whereas disinfection does not require the complete destruction of contaminating elements, only a fraction, according to the level of asepsis that is desired . Whenever possible, work should be done with techniques that are able to inhibit the spread of contaminating agents after disinfection and, especially, sterilization.

Disinfection/sterilization agents can be classified by chemical or physical action, depending on the mechanism for inactivation of life forms. Among physical agents we have moist heat (steam) or dry heat, UV radiation and ionizing radiation. Chemical agents include, for example, ethylene oxide, glutaraldehyde and the well-known ethanol. ¹

The recent public health crisis caused by Covid-19 has brought to the surface the need to contain and isolate the virus, which spreads through direct contact or with proximity to infected people, as viral particles can be expelled by the vector and be carried by small droplets of water, mucus or solids. That demonstrates the importance of physical barriers, such as masks, which despite having channels and holes hundreds or thousands of times larger than the viral particles, yet they are capable of capturing them because the materials are formed by tangles of fibers, where eventually the contaminated particles collide and lose their kinetic energy, being trapped on the fibrous surface by mechanical or electrostatic forces.

As for surface disinfection, the United States Center for Infectious Disease Control and Prevention (CDC) reports that Covid-19 transmission occurs mainly through air, in droplets and aerosols, but it is known that the virus can remain viable on a variety of materials for hours or even days. ²

Surface cleaning can be done with a 70% (mass/mass) alcohol solution or sodium hypochlorite solution with 0.1% (1000 ppm) active chlorine (³, ⁴) (this percentage is in mass units and refers to the amount of active chlorine in a certain amount of solution). Always be careful with sodium hypochlorite as it is an oxidizing, corrosive and bleaching agent and can damage certain materials such as fabrics, metals and even certain polymers, such as polycarbonate.

The preparation of 70% alcohol solution can be carried out very simply with a scale and any commercial alcohol with a concentration higher than 70%. Simply dilute the alcohol in water until the desired concentration is obtained.

Let us take as an example the preparation of a 70% alcoholic solution from a 94% alcohol. We know that the mass fraction of ethanol in the 94% alcohol solution ($ m_1 $) is 0.94, so if we start from 1 Kg of solution:

$$ m_ {ethanol} = 0.94 * 1 Kg = 0.94 Kg $$

As in the 70% alcohol solution ($ m_2 $), the ethanol fraction should be 0.70:

$$ 0.70 = \frac {m_ {ethanol}} {m_2} $$

$$ 0.70 = \frac {0.94 Kg} {m_2} $$

$$ m_2 = \frac {0.94 Kg} {0.70} = 1.34 Kg $$

Therefore, the difference between the mass of the 94% solution and the 70% solution is the amount of water that we must add to reach the 70% concentration:

$$ m_ {water-to-add} = m_2 - m_1 = 1.34 Kg - 1 Kg = 340 g $$

In this way, adding 340 g of water to 1 Kg of 94% ethanol solution gives 1.34 Kg of 70% alcohol. The reason that ethanol-water solutions (70% is the optimum concentration) are more effective than anhydrous ethanol against pathological agents is that the main mechanism of microbial killing in ethanol is protein denaturing, and proteins are more quickly denatured in the presence of water ³, which also decreases the volatility of the solution and prevents the rapid evaporation of ethanol, increasing the contact time. The addition of surfactants to alcoholic solutions is also carried out in some cases, as many pathogens have lipid layers that are destroyed by the action of detergents, the reduction of surface tension also contributes to the mechanical removal of contaminated particles.

Note 1: When working with large volumes perform the addition slowly, the ethanol-water dilution is an exothermic process, it releases heat due to the energy released when hydrogen bonds are formed. For this reason, too, we use mass instead of volumes to take our measurements, the water-ethanol solution deviates a lot from ideality and presents a decrease in volume when mixed, due in large part to the aforementioned hydrogen bonds.

Note 2: ethanol is flammable, handle with care, away from sources of ignition, in a well-ventilated place and with Note 1 in mind.

Obtaining 70% alcohol from less concentrated solutions is only possible by distillation processes (it can be simple) or extraction.

Solutions with 0.1% active chlorine can be prepared from bleach, commercial ones usually have 2% to 2.5% active chlorine. Starting from the equation below:

$$ C_1 V_1 = C_2 V_2 $$

With C = concentration and V = volume.

We want to produce 5 L of 0.1% solution from 2% solution, so:

$$ 2 \% * V_1 = 0.1 \% * 5 L $$

And we found $ V_1 = 0.25 L = 250 mL $. Therefore, to prepare 5 L of hypochlorite disinfectant solution, we need 250 mL of 2% solution (commercial bleach), which must be topped up with water until it reaches 5 L. That is, 1 part bleach for 19 parts of water ⁴.

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