The pH of an acid or base is not an indication of its strength: for example, weak acids can still show low value pH values. It is therefore important to consider the strength and concentration of acids and bases as well as the pH value.
The terms strength and concentration are frequently confused when used in the context of acids/ bases or alkalis that dissociate into ions to a limited extent when dissolved in water. The confusion is caused by the fact that in common usage strength and concentration are interchangeable terms.
However, when referring to acids or bases (also called alkalis), the terms strong or weak refer to the relative amount of substance present in ionic form compared to the amount of the same substance present in (non-ionic) molecular form. An acid or base is strong if it is entirely or almost entirely present in solution in ionic form cations (+) and anions (-). Hydrochloric acid (HCl) is a strong acid because in water, it dissociates almost entirely as H+ and Cl- ions. Acetic acid (vinegar) on the other hand, is only slightly dissociated into ions when dissolved in water. Most of the acetic acid dissolved in water is present in the non-ionic molecular form.
This distinction matters. Stronger acids or bases are more reactive and corrosive and therefore pose more of a hazard to the responders, requiring measures to be taken to reduce the risks posed.
Examples of strong acids
Examples of weak acids
Hydrochloric acid (HCl)
Hydrobromic acid (HBr)
Nitric acid (HNO3)
Sulphuric or sulfuric acid (H2SO4)
Perchloric acid (HClO4)
Organic acids such as carboxylic acids (R-COOH) of which acetic acid or vinegar (CH3COOH) is a very common example.
Hydrofluoric acid (HF) – note that although HF is a weak acid, it still poses very serious hazards.
Hydrocyanic acid (HCN).
Carbonic acid (H2CO3).
Sulphurous or sulfurous acid (H2SO3).Nitrous acid (HNO2).
Examples of strong bases
Examples of weak bases
Potassium hydroxide (KOH)
Barium hydroxide (Ba(OH)2)
Sodium hydroxide (NaOH)
Calcium hydroxide (Ca(OH)2)
Lithium hydroxide (LiOH)
Magnesium hydroxide (Mg(OH)2)
Sodium carbonate NaCO3.
|10||Milk of magnesia|
Toxic products of combustion
When a material burns (combusts) it undergoes a chemical reaction that usually involves atmospheric oxygen. This results in products of combustion, some of which may pose particular hazards to health. Hydrocarbons and many common materials such as paper, wood and plastic contain carbon. When burned they produce oxides of carbon: carbon dioxide and carbon monoxide. If smoke is produced in a fire, it will contain particulates, usually of carbon. As carbon monoxide is a toxic gas it can be assumed that all smoke is toxic and products of combustion will either be toxic or asphyxiant.
Particular concern needs to taken when the products of combustion are corrosive or acutely toxic. These tend to be produced when the chemical products undergoing combustion contain certain elements such as halogens (flourine, chlorine, bromine, iodine), metals and particularly alkali metals (lithium, potassium, sodium) or certain non-metallic elements (specifically nitrogen, sulphur, phosphorus, arsenic) in their compound structure. Compounds containing these elements are commonly found in products such as detergents, fertilisers and pesticides. If in any doubt as to whether fumes are likely to contain these compounds, consult the safety data sheet (SDS) or seek further advice from the manufacturer or from a scientific adviser.
Chlorinated hydrocarbons, for example, produce hydrochloric acid (HCl) fumes when they decompose at high temperatures (when exposed to a fire or hot surfaces). If HCl fumes are inhaled by responders, this will lead to acid forming in the lungs, affect their working could lead to more serious health effects such as pulmonary oedema (which can occur up to 48 hours after exposure and can result in death). Anyone who is suspected of having breathed in toxic or corrosive fumes should seek immediate medical attention.
Toxicity is the intrinsic capacity of a chemical to adversely affect an organism.
The level of toxicity is distinguished in legislation according to the United Nations Globally Harmonised System for classification and labelling (GHS) system and can be highly-toxic, toxic, harmful, etc., based on the lethal dose. The most common ratings concern acute toxicity (a short-term but potentially high exposure). Substances can also have chronically toxic effects (from repeat and cumulative exposure over time). The differences between acute and chronic toxicity are reflected in the criteria for classifying products for transport and supply. For example, cancer is much more likely to develop from repeated occupational exposure to a carcinogen than a one-off acute exposure.
The dose-effect relationship is the relationship between dose and effect on the individual level. An increase in dose may increase the intensity of an effect, or a more severe effect may result. Some toxic effects, such as death or cancer, are not graded but are ‘all or none’ effects.
A dose is often expressed as the amount of a substance entering an organism (such as a person) and is expressed in units such as mg/kg body weight. A dose threshold is a dose level below which no observable effect occurs. Thresholds are thought to exist for certain effects, like acute toxic effects; but not for others, like carcinogenic effects or exposure to radiation. The lethal dose (LD50 or LC50) is the dose causing 50 percent death in an animal population. LD50 is given as a measure of the acute toxicity of the chemical substance. The lower the LD50 the higher the acute toxicity. There is no necessary correlation between acute and chronic toxicity.
Further information on eco-toxicity and how to interpret the various values such as biochemical oxygen demand (BOD), toxicity, bioaccumulation and persistence can be found in the Environmental Protection Handbook.
Dispersal of chemicals in water
|Boiling point||Vapour pressure||Specific gravity||Solubility||Expected behaviour in water|
|Below ambient||Very high||Any||Insoluble||All liquid will rapidly boil from surface of water.|
|Below ambient||Very high||Below that of water||Low or partial||Most liquid will rapidly boil off but some will dissolve. Some of the dissolved liquid will evaporate.|
|Below ambient||Very high||Any||High||At least 50 per cent will rapidly boil off; the rest will dissolve. Some of the dissolved liquid will evaporate later.|
|Above ambient||Any||Below that of water||Insoluble||Liquid will float, forming a slick. Those with significant vapour pressure will evaporate over time.|
|Above ambient||Any||Below that of water||Insoluble||Liquid will float but will dissolve over time. Those with significant vapour pressure may simultaneously evaporate.|
|Above ambient||Any||Below that of water||Low or partial||Liquids will rapidly dissolve in water up to the limit (if any) of their solubility. Some evaporation may take place over time if vapour pressure is significant.|
|Above ambient||Any||Below that of water||High||Liquids will rapidly dissolve in water up to the limit (if any) of their solubility. Some evaporation may take place over time if vapour pressure is significant.|
|Above ambient||Any||Near that of water||Insoluble||Difficult to assess. May float on or beneath surface or disperse through the water column. Some evaporation may occur from surface over time if vapour pressure is significant.|
|Above ambient||Any||Near that of water||Low or partial||Will behave as ‘insoluble’ chemicals above at first and eventually dissolve. Some evaporation may take place over time.|
|Above ambient||Any||Any||High||Will rapidly dissolve up to the limit (if any) of their solubility. Some evaporation may take place over time.|
|Above ambient||Any||Above that of water||Insoluble||Will sink to the bottom and stay there. May collect in deep water pockets.|
|Above ambient||Any||Above that of water||Low or partial||Will sink to the bottom and then dissolve over time.|
|Above ambient||Any||Above that of water||High||Will rapidly dissolve up to the limit (if any) of their solubility. Some evaporation may take place from the surface over time if vapour pressure is significant.|