Acid/Base Concept

by Fred Scaffidi, Chemist 

The corrosivity of a material, that is, the ability for a chemical to cause visible destruction to skin and other tissues is an important parameter in emergency response. The acids are well known for their ability to corrode. A splash of acid on the skin may cause a severe burn and scarring. Though highly dangerous, these materials are used in many industrial processes on a huge scale, and are found in just about every household (for example, cleaners). What makes a material acidic (or basic)? What are the common approaches to dealing with these chemicals in a spill scenario?

Recall from your early chemistry: Acidic materials dissolve in water to produce a net surplus of hydrogen ions [H+]; ions being electrically charged chemicals in water. Basic materials also dissolve in water and produce hydroxyl ions [OH-]. It is the concentration of these ions which determines the strength of an acid or a base. Strong acids produce higher concentrations of H+ than weak acids (similarly for bases). We can generalize and say that: most common acids have a high solubility in water i.e. you can put them in water, dissolve them and generate ions in solution; a few highly concentrated acids can be flammable and others may be sufficiently oxidizing to ignite combustible materials; acids react with metals, sometimes slowly, to produce flammable and explosive hydrogen gas; acids neutralize bases, in other words, hydrogen ions react with hydroxyl ions to produce water and resultant heat:

ACID + BASE <--> a salt + water + heat.

Caustic Soda (a base, chemical formula NaOH) will neutralize a spill of hydrochloric acid (acid, chemical formula HCl). The overall reaction is:

Na+ + OH- + H+ + Cl- <--> H2O + NaCl (table salt).

(the above reaction would produce considerable heat and fumes)

The presence of water is critical to the acid/base concept. One might consider adding water to acids and bases in order to dilute them and hence make them less concentrated, less hazardous. One rule of thumb in the chemical laboratory is that acid is always added to water; never water to acid. Addition of water to concentrated acid, the only way to combine the two in a spill situation, can cause splashing and bubbling of acid. Dilution may also cause the solution to heat up (heat of dilution) which will reduce the solubility of the acid in water and generate acid fumes. Most of you are familiar with the term pH used to indicate how acidic or basic a solution is (scale is below).

Solutions with pH between 0 and 6 are acid, pH 7 is neutral and pH 8-14 solutions are basic. pH readings may be taken using litmus paper and observing the colour change. The pH scale as shown is a logarithmic scale, that is factors of 10 separate each value. For example, a 1 litre spill of a strong acid (pH=1) would require 10,000 litres of water to be diluted to pH=5. Dilution to neutral pH would require 1,000,000 litres of water (although this wouldn't be necessary as pH=5 would represent a low hazard for skin contact). Addition of water has the added drawback of spreading the spill around.

Abrupt addition of neutralizing agents to concentrated acids in a spill situation will cause fuming and boiling. Neutralisation is normally done in a laboratory under highly controlled conditions. Consider that some neutralising agents can be just as hazardous as the original acid. One would have to add just enough neutraliser to do the job, and no more than that as one could pass the endpoint and turn the solution into a strong base. A concentrated sulphuric acid spill can be neutralised using sodium bicarbonate (baking soda) fairly safely while neutralising with caustic soda could produce a lot of heat and fumes. Sodium bicarbonate has a lower heat of reaction, however it will cause fierce bubbling due to the production of carbon dioxide gas. Sodium bicarbonate is not a particularly hazardous material and an excess may be added without concern. Crushed limestone is also an excellent choice, however the heat of reaction will be higher and hence greater fuming. Neutralisation of a large quantity of acid at the site would require the presence of personnel very familiar with this procedure. It is recommended that recovery of as much spilled material as possible should be done prior to neutralisation.

Clearly there are several chemical properties attributable to acids in general. The release of acid fumes due to neutralization or dilution seems to be the most grave concern. In the case of hydrocyanic acid for example, release of hydrogen cyanide could be fatal. The question of proper protective clothing will be discussed in a future article.

REFERENCES

• Environment Canada TIPS Manuals, Environmental Protection Service, February 1984
• Chemistry: A Conceptual Approach, 4th Edition, Mortimer copyright 1979
• Handbook of Chemistry and Physics, 71st Edition, CRC Press, 1990
• Initial Emergency Response Guide, CANUTEC, 1992 Edition, TP7341E

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Publication: TDG Dangerous Goods Newsletter, Vol. 15, No. 1, Spring 1995.

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