Acid is highly corrosive; the skull and crossbones warning on an acid label is there for a reason; acids will etch or erode anything in its path, they are also very effective in dissolving metals and etching glass. Acid will keep reacting with any surface it’s applied to if it isn’t neutralized by an alkaline product. It’s pH decreases when the solution is diluted with water (H20) (this results in an acid becoming more reactive (stronger) whereas the effect of water on an alkaline is to make it become weaker or changes when there are other substances that are dissolved in the solution.
Hydrochloric Acid (HCI): or Muriatic Acid, its historical but still occasionally used name, this is a highly corrosive acid (pH of -1) and is often used to clean calcium carbonate build up from the inside of kettles or from around water faucets and from shower heads;
Sulphuric Acid (H2 SO4): this is a common acid in both the laboratory and industry. It is both highly corrosive and economical to manufacture, which makes it the reagent of choice for many applications;
Phosphoric Acid (H3PO4): this acid is used to remove rust and rust stains from metal tools and from car bodies undergoing repairs;
Nitric Acid (HNO): this is another common laboratory acid used as a reagent in many chemical tests and experiments due to the fact that almost all of its products (salts) are soluble in water;
Hydrofluoric Acid (HF): This acid is extremely corrosive and has the unique property of being able to etch (eat away) glass. Consequently it is used in industry to write signs on glass windows in stores and office buidings or on glass products.
Strong acids include the heavier hydrohalic acids: Hydrochloric Acid (HCl), Hydrobromic Acid (HBr), and Hydroiodic Acid (HI). However, Hydrofluoric Acid (HF) is relatively weak. Acids are acids by virtue of the presence of an excess of hydrogen ions in the solution, Their salts are created when the positive hydrogen ions are replaced with positive metal ions, for example when Hydrochloric Acid (HCL) reacts with Sodium (Na) to produce NaCl with the release of H2 gas.
Before using any chemicals or solvents, carefully read all of the cautions and medical information on the products container and / or MSDS. There is a very fine line between effective cleaning and dangerous to use products; always follow all of the manufacturer’s recommendations. Ensure that you wear eye protection and acid-proof gloves; this is mandatory. No matter how safe an acidic product is advertised, never run the risk of it splashing in your eye or it permeating your skin.
Use a product that has a low acid concentration level, 10%, or less, with as high a pH level (> 2.0) as you can find (i.e. Sulphuric, Phosphoric and Nitric acid) this constitute a less aggressive product. A strong base (alkali) is just as corrosive as a strong reactive (acid)
What is pH and what does it mean? Simply put the potential of hydrogen (pH) scale is a set of numbers between 0 and 14 where 0 is the most acidic and 14 is the most alkaline (caustic) with 7 in the middle, being neutral. Conceptually, since most people in our industry are interested in the acid side of the system, the smaller the number, and the more acidic the system.
Unfortunately the scale is logarithmic, for every integer that the scale decreases the material is 10 times more acidic. Those of us in earthquake country know all too well the consequences of a change of from 6 to 7 on the logarithmic, Richter scale. The difference in the pH scale is just as dramatic and therefore just as misleading.
A substance that is neither acidic nor basic is neutral; pure water has a neutral pH of 7.0 Each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10 times 10) more acidic than a pH of 6. The same holds true for pH values above 7, each of which is ten times more alkaline than the next lower whole value. For example, a pH of 10 is ten times more alkaline than a pH of 9
A few examples of maximum acid strength might be helpful:
Acid rain has a pH of 3.5-4.0. Bird excrement contain highly acidic concentrates of uric acid (pH 3.0-4.5) Sea water has pH of 7.75 to 8.25 Tap water has a pH of between 5.5 and 6.5
Organic acids Citric acid, found in lemons, grapefruit has a pH of 2.0 Oxalic acid, found in spinach has a pH of 1.8 Vinegar has a pH of 2.5
‘Weak’ mineral acids Hydrofluoric acid has a pH of 3.14 Phosphoric acid has pH of 2.2
‘Strong’ mineral acids Hydrochloric acid, aka Muriatic acid has a pH of -1.0 Sulphuric acid, battery acid, has a pH of 0
A couple of things to note from this list are; a) How very strong ‘strong acids’ are b) How relatively weak yet dangerous Hydrofluoric acid (HL) is c) And how small and misleading the difference is between organic acids and weak mineral acids. As an example, hydrofluoric acid (HF), one of the most highly regulated, most dangerous acids and the only acid that will dissolve glass is a weak acid. d) pH decreases when the solution is diluted ( this results in an acid becoming more reactive (stronger) were as H20 effect on an alkaline is to make it become weaker or changes when there are other substances that are dissolved in the solution. e) An acids reaction with water- the hydration reaction of sulphuric acid is highly exothermic. If water is added to concentrate sulphuric acid, it can boil and spit dangerously. One should always add the acid to the water rather than the water to the acid.
Just to complicate things a bit more, mixing some strong mineral acids together with other relatively weak acids can create a solution of “super acid”. This can result in acid strengths of up to 12 times higher than either acid by itself and are not measured accurately by the normal pH scale. In theory, this is the result obtained by adding HF to other mineral acids in wheel cleaners. Or mixing several weak acids together can have a synergistic effect where together these acids do more than just strong solutions of each one separately.
In practical applications, a formulator can get identical pH values by using various quantities of different acids. For example very small amounts of very strong acids or larger amounts of weaker acids can result in the same pH. The addition of a molecule of water to a chemical compound, without forming any other products is known as hydration (i.e. dilution causes the pH to decrease) But since pH is not a very good indicator of the strength of the acid in every system, this approach won’t always produce the desired results. Each acid has properties that make it most useful for certain jobs. For example Citric acid is quite good at picking up Calcium ions in solution so acts as a good water softener where the same pH of a Sulphuric acid solution would be worthless for that application.
My point here is that, it is not possible to judge how well a product will do the job it is designed for just by measuring the pH and stronger is not always better. So, acid strength is always relative to the system you’re measuring and what materials are in danger of being dissolved or attacked by the acid pH is a measure of the relative strength of an acid but the key word is relative. (See also Alkalinity, Potential of Hydrogen – pH)