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Acid solutions, especially those of some strong acids such as sulfuric and nitric acids, are among the most widely used solutions in the laboratory for different purposes. For this reason, it is important to know how to prepare them properly, especially considering how dangerous these substances can be when they are highly concentrated.
A solution is a homogeneous mixture between two or more components, one of which acts as a solvent. Although its preparation may seem simple, it is not a simple matter of mixing two components in the right proportions and that’s it. In fact, the order in which the components are mixed and the intermediate steps that must be carried out when mixing them matters, even if the end result is exactly the same.
This leads us to ask ourselves when preparing a solution of a strong acid, what should we add first, the acid (solute) or the water (solvent)? In other words, should we add the water to the acid, or should we do it the other way around?
The answer is that when preparing an acid solution, we should always start by adding pure water and then slowly add the acid to the water . In the following sections, we will learn why it is dangerous to prepare strong acid solutions, what are the associated risks, what safety measures we should always take when working with this class of substances, and what are the steps necessary to prepare acid solutions safely. safe.
The risks of preparing strong acid solutions
The way in which a solution is prepared is especially relevant in the case of the preparation of solutions from concentrated acids. Doing it the wrong way or in the wrong order can be very dangerous, leading to possible strong acid spills or dangerous splashes that can cause severe burns to the skin, eyes, or mucous membranes.
The reason behind this is that the dissolution and ionization process of a strong acid is very exothermic (ie, it releases a lot of heat). If proper precautions are not taken, all this heat can be concentrated in a small volume of water, quickly heating it to its boiling point and causing a violent boil of the solution that sprays concentrated acid in all directions. These splashes can be very dangerous and if we are not careful, we could go blind or, in the best of cases, marked by a burn on the skin.
How to avoid splashing when diluting concentrated strong acids?
The key to preparing the solution safely is understanding what causes spatter so you can avoid it. As we have just seen, the problem is the large amount of heat that is released by dissolving and ionizing the acid in the water.
It is not possible to prevent this heat from being released, however we can either cause it to be released in a controlled manner, or we can take steps to dissipate all the heat quickly, thus preventing the solution from getting hot enough to boil. The following are three different ways to achieve these goals.
1 Mix slowly
The first can be achieved by mixing the two components, water and acid, very slowly. This will limit the amount of heat being released, giving the heat time to flow from the solution into the vial, flask, or beaker where we are preparing the solution.
2 Add acid to water and not the other way around
The second, we can easily achieve by first adding a good amount of pure water as a cushion and then adding the acid to the water under constant agitation. Since the acid is being dissolved in a very large volume of water in this case, the heat is distributed over a much larger mass and does not cause the dramatic rise in temperature that leads to splashing.
3 Use an ice bath
Another additional measure that can be taken is to prepare the solution in a container submerged in an ice water bath. The low temperature requires more heat to heat to a boil. In addition, the large temperature difference between the point where the two components are mixed and the ice bath accelerates the flow of heat from the solution to the ice bath, causing the solution to cool more quickly and thus dissipating all the heat released. for the mix.
4 All of the above
There is no reason to choose only one of the above measures, so if we want to be more sure, we can combine them all. That is, we first add a cushion of water and then add the acid slowly and with constant stirring while keeping everything cool in an ice bath.
Additional safety precautions when preparing strong acid solutions from concentrated acid
Preparing an acid solution following the above recommendations minimizes the chances of accidents. However, it does not nullify them completely. It is always possible for something to go wrong, which is why it is necessary to take other additional safety measures that are common in any science laboratory.
These measures are:
- Wear laboratory gloves: latex or neoprene gloves are resistant to many aggressive chemicals such as strong acids. Using gloves allows us to safely handle the instruments that come into contact with these substances, thus protecting our hands from burns.
- Always wear a lab coat: the lab coat helps protect most of our skin from contact with any chemical substance that may splash on us. Also, in case of splashes, it is easy to remove without coming into contact with the affected area.
- Wear safety glasses: The eyes are very delicate organs and a strong acid burn is likely to result in partial or complete loss of vision, which can be avoided by wearing glasses.
- Make sure the emergency eyewash works: every laboratory should have an eyewash that ejects two jets of water to quickly remove any splashes without having to use our hands. These safety equipment should be checked periodically to make sure they are working properly.
- Keep a neutralizing solution on hand: Another important safety measure is to always have a 5% or 10% aqueous solution of sodium bicarbonate on hand . This solution is used to quickly neutralize any acid spill, avoiding possible additional accidents.
- Working in a fume hood: many concentrated acids are fuming. This means that, inside the bottle, the liquid phase is in equilibrium with the acid or anhydride that generates it in the gas phase. Uncover the bottle, the vapors can be very irritating to the eyes and the respiratory system, so it is only recommended to uncover these concentrated reagents inside a fume hood.
References
Fertiberia. (2005, January 4). Safety data sheet for nitric acid (20% to 70% HNO3) . https://www.ecosmep.com/cabecera/upload/fichas/7819.pdf
Hurum, D. (nd). Laboratory Safety . Northwestern University. https://faculty.washington.edu/korshin/Class-486/AEESP-safety-notes.pdf
Traful. (2014, August 4). Preparation of a solution from a commercial acid . Youtube. https://www.youtube.com/watch?v=hgZMMh6056s
Autonomous University of Madrid. (nd). PREPARATION OF SOLUTIONS AND USE OF THE STOICHIOMETRY OF A CHEMICAL REACTION . http://www.qfa.uam.es/qb/practicas/P1-guion.pdf
National Autonomous University of Mexico. (nd). SAFETY SHEET III HYDROCHLORIC ACID . https://quimica.unam.mx/wp-content/uploads/2016/12/3hshcl.pdf
neighbor of picasso (2011, October 6). Preparation of solutions in the laboratory . https://vecinadelpicasso.wordpress.com/2011/10/06/preparacion-de-dissoluciones-en-el-laboratorio/
