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The word 'acid' is known to us all. Many will know that vinegar is an acid (largely acetic acid) and that our stomach contains acid (hydrochloric acid) but what actually is an acid? There are a lot of misconceptions about acids and their opposite chemical entities - bases1. For example the strength of an acid is an entirely different thing to the corrosiveness of an acid. While it is true that strong acids such as sulphuric acid and nitric acid can and will eat through certain metals if given the chance, a much more corrosive acid is a solution of hydrogen fluoride gas in water or hydrofluoric acid. This stuff is fairly unique in its ability to eat through glass2. and has to be stored and used in something else but in the general scheme of acids it is actually a quite weak acid. Also, our genetic code is formed from DNA or 2'-deoxyribonucleic acid but our cells do not dissolve in this stuff. In short, acid strength is not the ability to smoke in test tubes in dodgy horror films and eat through space-ship hulls; it is something much different and far more boring. It is the object of this Entry to explain it all...
The Brønsted-Lowry Theory of Acids and Bases
This is generally regarded as the classical theory3. of acids and bases and encompasses those most well known to laymen. Brønsted acids include hydrochloric acid (HCl), sulphuric acid (H2SO4) and Nitric Acid (HNO3). Brønsted base examples are sodium hydroxide (NaOH - caustic soda) and sodium hydrogencarbonate (NaHCO3 - bicarbonate of soda or baking soda).
According to this theory, an acid is a substance that is a 'proton donor' and a base is a substance which is a 'proton acceptor'. But what does this mean? Well, water is famously made up of H2O molecules and these are capable of dissolving a good many other types of substance. Now an acidic substance, according to this theory will, in water, 'ionise'4. to give hydrogen ions (H+) which are simply hydrogen nuclei (which are mostly protons - hence 'proton donor') and the counter negative ion. For example, hydrochloric acid is a solution of hydrogen chloride gas in water. Upon dissolving in water, it splits up into protons and chloride (Cl-) ions. Water in fact is capable of self ionising into H+ and OH- (hydroxide) ions which are normally in balance and so the solution is neutral. When a substance such as HCl dissolves in water the solution suddenly contains a massive excess of H+ ions and so becomes acidic. When a substance such as sodium hydroxide (NaOH) dissolves in water it becomes basic or alkaline as there is now an excess of hydroxide (OH- ions). Another good example of a Brønsted base is ammonia (NH3), when dissolved in water this will 'accept' a proton from water to give a solution of ammonium ions (NH4+) and OH- ions - this is where one can most easily visualise the idea of a 'proton acceptor'.
In essence, what most of us think of as an acid is a chemical that when dissolved in water will release protons into solution (proton donor). By the same definition a base is a substance that when dissolved in water will bind to or accept protons - making the solution alkaline.
So what happens when acids and bases are mixed? Well, the protons and hydroxide ions recombine to form water and one is left with the remaining counter ions. For example, if an equal amount of hydrochloric acid and sodium hydroxide are combined then after the H+ and OH- ions have neutralised one another one is left with Na+ and Cl- ions. Evaporate the water and you have sodium chloride or common-or-garden salt. This is in fact the definition of a salt, the product obtained from the combination of an acid and a base.
ACID + BASE = SALT + WATER
The Concept of pH
Most have heard of the measure of acidity in water known as pH. Gardeners for example know that the pH of their soil is very important for the type of plants they wish to grow. Everybody has heard of 'acid rain' and some soaps are now sold at our skin's natural pH of 5.5. But hands up those who really know what it means?
This is where it starts to get all complex and horribly scientific. pH is a logarithmic quantity and is connected to the equilibrium of protons and hydroxide ions in water. pH is defined as...
pH = -log10[H+]
Now to the average layman this is likely to be a pretty meaningless equation so it's worth taking some time to explain it.
[H+] - When chemists use square brackets around something this means that they are talking about the concentration of a substance. In this instance it is the number of hydrogen ions in a unit volume of water. This concentration is measured in moles/litre5.
log10 - This signifies a logarithm in base 10. A logarithm is a mathematical function that transforms either very large or very small numbers into a more meaningful number for us to visualise. The logarithm (base 10) of 10 is 1. The logarithm of 100 is 2 and that of 1000 is 3. The number you get is essentially the power of 10 needed to get the number you started with. So if a number gives you log10(number) = 10, then the original number is 10000000000 (count the zeros). Of course for numbers that are not simple powers of 10, the log is a less simple number and one must resort to either a log table or a modern scientific calculator to give you the value of the logarithm. If one goes into negative powers of 10 such as 0.01 (10-2) then the logarithm is similarly negative. Hence log10(0.01) = -2. The pH is measured as negative logarithm because, in water, one is dealing with very small concentrations and so the minus sign transforms the numbers we get from the logarithmic transformation into the positive numbers we all like to deal with.
[H+][OH-] = 10-14 - This equation, although slightly simplified6, represents the equilibrium between protons and hydroxide ions in water. It is also known as the self ionisation constant of water. Other solvents have similar self ionisation constants and this will be discussed later. No matter what you do or add to water, this product remains the same. The concentration of protons, multiplied by the concentration of hydroxide ions is always 10 to the power of -14 or 0.00000000000001. From this, one can see why logarithms are used. -log10(0.00000000000001) = 14, a much more easily visualised number (and easier to type!). It is from this product that pH is calculated. At neutral pH the number of protons is exactly equal to the number of hydroxide ions and this is equal to 10-7 moles/litre (-log1010-7 = 7), so the pH of a neutral solution is 7.
What happens when one adds an acid or base? Well if one adds an acidic chemical to water then the concentration of H+ ions increases. If one adds hydrochloric acid to water then the concentration of protons in the water will rise to around 0.1 moles/litre - giving the solution a pH of 1 (log100.1 = -1). In this instance the concentration of hydroxide ions in solution has of course dropped to 10-13 moles/litre. In the opposite scenario, one can dissolve a basic chemical such as sodium hydroxide (caustic soda) to water and this will increase the concentration of hydroxide ions in water, for this substance [OH-] = 100 moles/litre and so [H+] = 10-14 moles/litre and so pH = 14.
This is the scale of pH in water. An acid solution will have a pH of 0-7. An alkaline solution will have a pH of 7-147. Your skin has a natural acidity of pH 5.5 as already mentioned, it is therefore slightly acidic. Acid rain sounds nasty (and lets face it, it isn't too good) but one should note that generally this means that it varies from around pH 5.5-6 so it is only really mildly acidic - if it was pH 1 then it really would be trouble8. There are quite a few ways of measuring the pH of a solution. The most famous method is probably that of using Litmus paper. This contains a dye that has an acidic proton. At basic pH, this is removed from the dye molecule and the anionic version of this dye is coloured blue. Under acidic conditions the protonated version is coloured red and so this is a very useful indicator of whether a solution is acidic or alkaline. This of course is qualitative not quantitative. In fact, there are quite a few dyes that have this colour changing property and they vary in the pH at which the colour change occurs9. If one therefore mixes a few of these dyes together, one will get a gradual colour change according to pH and this gives a much more accurate measurement of the pH of the solution. This is known as universal indicator. Of course the modern chemist has a far more accurate means of measuring pH than this. The modern pH meter relies on a pH electrode which is an electrode filled with potassium chloride solution (KCl in water) and the voltage generated by this electrode is dependent upon the proton concentration in the solution one is measuring. Measurement of this voltage against that generated by standard solutions of known pH gives a highly accurate pH value for the solution in question.
Strength of Acidity and Basicity.
Now that pH has been defined, it is possible to discuss the relative strengths of various acids and bases.
A strong acid is one that when dissolved in water will completely ionise and give a high concentration of protons and so a low pH. HCl, H2SO4 and HNO3 all do this and you only need to add a small amount of these to water to give solutions of pH 1. These are strong acids, the proton concentration is the same as the acid concentration. If one dissolves acetic acid (CH3COOH), the main ingredient in vinegar to water it does not completely ionise (into CH3COO- and H+) and so the same small quantity added to water gives a much higher (though still acidic) pH, acetic acid is a weak acid - a typical solution will be around pH 5, to get this to pH 1, much more than 0.1 moles/litre of pure acetic acid must be added. The example of HF or hydrofluoric acid given at the beginning of this entry is also in this category. Here it should be remembered that the pH scale is logarithmic and that consequently hydrochloric acid (HCl) is not five times stronger an acid than acetic acid, it is 105 times or 100000 times stronger. For bases the situation is similar. Sodium hydroxide or caustic soda is a very strong base. In water it completely ionises and so gives a high concentration of hydroxide ions - a solution of NaOH in water is at pH 14. Weaker bases (ammonia is a good example) will not completely ionise to give high concentrations of OH- ions and so will give a lower (although still alkaline) pH. DNA is also mentioned above. It has a structure where its backbone is made up of a string of phosphate (PO43-) groups linked together. The parent acid of these is called orthophosphoric acid H3PO4 and is a strong acid. However, there is complicated chemistry going on inside your cells and so although DNA when taken on its own in water is strongly acidic, this does not mean that it is a corrosive acid and in fact there are many basic chemicals attached or linked to it inside the cell which neutralise this acidity anyhow.
This up to now is all rosy, lovely and easy to cope with. Unfortunately it is not the complete story. While it is enough for your gardener or soap manufacturer, it is not sufficient for chemists and other scientist who need to use chemicals that do not dissolve in water and which sometimes do not have protons to donate and/or accept. In addition, this definition is limited to acids and bases in aqueous (water) solution. Using water, one cannot tell the difference in acid strength between say HCl and HNO3 as both give the same pH when dissolved. It is, however, possible to distinguish between them using other solvents.
Ammonia when in its liquid form (at atmospheric pressure it is liquid below -33°C) has a much lower ionisation constant than water and so measurements can be made in liquid ammonia solution to distinguish between very strong acids. If an acid chemical reacts with it other than in and acid/base reaction, then of course it is not a suitable medium and other liquids may be used.
Of course, the term pH is no longer relevant when not using water solution or referring to acids which will give the same value of acidity in water and so chemists use a more accurate and all encompassing term for acid strength. Acidity is measured by chemists using the term pKa.
Ka - The Acid-Base Equilibrium Constant
If you've got this far, you've probably noticed by now that when an acid loses its proton, the negative ion formed must, in order to reform the acid, accept it back again and is therefore by the Brønsted-Lowry theory a base. It is called the conjugate base of the acid. Similarly, any base, when accepting a proton, becomes an acid, capable of donating that proton back again to reform the original base - it is the so-called conjugate acid of that base. This backwards and forwards reaction happens with extreme rapidity and forms a dynamic equilibrium so that it appears to be static at one position. The way to visualise this is to think of turning a tap on hard into a sink without the plug hole covered. The water will keep running through the plug hole and water will keep pouring into the sink but after a while the level of water in the sink will appear to remain unchanged - the system is now in dynamic equilibrium. The level of water in the sink will depend on how hard the tap is flowing - this is equivalent to the measure of acidity in the acid-base equilibrium. The measure of acidity is the so-called position of equilibrium which is measured by the equilibrium constant which chemists call Ka.
A general acid-base reaction is the sum of two half equations, by measurement of the position of equilibrium in many complete equations it has become possible to work out where the equilibrium lies in half-equations which define the acidity of specific acids. A general half-equation is shown below. AH is the acid, H is the released proton and A is the conjugate base of the starting acid - charges on ions have been left out for simplicity.
AH ------------------> A + H Ka = [A][H]/[AH]
Hence the equilibrium constant Ka is the concentration of conjugate base A multiplied by the concentrations of protons H then divided by the concentration of the starting acid AH. The value of this number is an indication of where the position of equilibrium lies. A value of 1 for Ka indicates that the position of equilibrium lies half way, a very small number indicates that the position of equilibrium is very much to the left hand side - that is that very little of the acid exists in its ionised form and that it is has a low propensity to donate protons - ie, it is a weak acid. Conversely, a strong acid's equilibrium will lie far to the right and so the value of Ka will be very high. Again, it is convenient to use a logarithmic form for the value of this equilibrium and so the term pKa is used and yes, you guessed it, the little p yet again stands for using negative base 10 logarithm.
pKa = -log10Ka
As with pH, a low value indicates strong acidity and this time the numbers can get very low indeed (a negative pKa indicates a very strong acid indeed - too strong to be measured accurately in water). The values of pKa are difficult to compare if measured in different solvents as the position of this equilibrium will vary from solvent to solvent. pKa values are therefore conventionally quoted relative to water and so those calculated in other solvents should not be taken as exact but more of an estimate (albeit a pretty darn good estimate).
A Selection of pKa Values for Some Interesting Chemicals
Those values in bold represent exactly measured values, those in ordinary text are approximate.
A few things should be noted from the above table. The first entry does not show a value; this is because this complex acid is so strong that it has defied measurement. It has been possible in this solution to dissolve and protonate methane from CH4 to CH5+. This may not seem amazing to a layman, but to a chemist this is the equivalent of water into wine or ice caps appearing on the Sun. For this reason solutions such as these are such amazingly powerful acids that they are termed superacids. Water gets two mentions in this table, one for its protonation to H3O+ and one for its deprotonation to give hydroxide ions. This classes water into a special group of chemicals that have the ability to react both as a base and and as an acid - such compounds are termed amphoteric. It was earlier discussed that in aqueous solution it is impossible distinguish strong acids, the above table shows clearly that hydrochloric acid HCl has a lower pKa than nitric acid HNO3 and hence is a stronger acid. Now it is clear from the last three entries in this table that these chemicals are much happier in their protonated or 'acid' form. In fact one would struggle under ordinary circumstances to describe them as acids at all. This of course is the case, in fact their conjugate bases are extraordinarily strong bases. Organic chemists will use the salts of NH2- and CH3CH2CH2CH2- (eg, NaNH2 and CH3CH2CH2CH2Li10) in research to specifically deprotonate complex chemicals and use the resultant anion to make a new chemical. Using such techniques new and wonderful organic chemicals can be made as potential new drugs for previously untreatable diseases. Attention should be played to both acid and conjugate base columns in this table. Species appearing towards the top left are very strong acids and those to the bottom right are very strong bases.
Lewis Acids and Bases - The 'Modern' Theory
Brønsted and Lowry's theory of acids and bases works very well in water and other protic solvents (those capable of 'donating' protons), but it is not sufficient to explain all acid-base type reactions. For example if one dissolves the salt magnesium chloride (MgCl2) into water one will get a slightly acidic solution. This is down to magnesium coordinating water molecules around itself in solution which results in the ionisation of some water molecules to release protons. Clearly though the Mg2+ ion cannot donate protons of its own accord as it has none to give yet by the Brønsted-Lowry theory it cannot be considered an acid. There are many other examples such as this in chemistry and chemists regard the Brønsted-Lowry theory as being largely inadequate.
This is where an American chemist called Gilbert Newton Lewis (1875-1946) came up with a new all-singing, all-dancing theory that includes all of the acids and bases already defined by the Brønsted-Lowry theory but also allows other non-conforming chemicals to be considered acids and bases too11.
A Lewis acid is an electron pair acceptor.
A Lewis base is an electron pair donor12.
By this theory all the acids originally discussed can still be considered acids and bases just as they were before. An H+ ion is still simply a proton with no electrons at all around it. If a molecule such as ammonia comes up to it then the ammonia acts as a Lewis base, donates its lovely spare pair of electrons to the proton and you get the ammonium NH4+ species just as you did before. The difference really is in the emphasis you put on the reaction. Instead of the proton getting donated to the ammonia the reaction is now considered a donation of a pair of electrons to the proton. Simple and very effective. If one reconsiders the Mg2+ ion dissolving in water. The metal ion is highly charged in a positive type fashion and it forces a coordination sphere of water molecules to aggregate around it in such a fashion as to minimise this positive charge. The water molecules behave like little bar magnets dangled around a powerful electromagnet and the more electron rich oxygen ends of the H2O molecules point towards the positively charged metal ion and the less electron rich hydrogen ends point away. For the water molecules closest to the metal ion this interaction is so strong that it is advantageous for the water molecule to simply donate an electron pair into a coordination bond between itself and the magnesium ion and lose a proton in the process - thereby rendering the solution acidic. The Mg2+ ion is therefore considered by the modern chemist to be a Lewis acid13.
The Lewis acid-base theory allows some weird and wonderful molecules and reactions to be considered acid-base malarchy. For example, the chemical entity boron trifluoride (BF3) is not a happy little molecule. There are six electrons in the outer electron shell of the boron atom and this is not good - usually eight are required around such atoms for a molecule to be stable in our environment (see the entry on Electron Shells and Orbitals). It therefore will very readily react with anything that can donate a pair of electrons to it to complete this eight electron arrangement. If you therefore react BF3 with NH314, then the two combine to give the Lewis-salt BF3NH3 - a nice, stable compound15. Boron trifluoride is much used in research chemistry as a strong Lewis acid. The beauty of using Lewis acids such as MgCl2 and BF3 is that they have nothing whatsoever to do with water. Water is all well and good to life as we know it, but to a chemist desperately trying to make that breakthrough molecule it can spell disaster. Many chemicals used in research are water-sensitive and so it is often necessary to exclude water from reactions used to created new molecules. If an acid-base reaction is needed to do what is required in the absence of water then Lewis acids and bases are the meat and drink of the synthetic chemist.
And There's More
Acid-base reactions are pretty much the simplest of all reactions involved in chemistry. They concern only the movement of a pair of electrons from one species to another and don't generally involve the rearrangement, elimination or substitution of the various atoms involved in the species you are dealing with. That is why acid strength is not the same as corrosiveness. The corrosiveness of acids such as nitric acid can be down to the ability of these chemicals to perform oxidation-reduction reactions. These are chemical reactions of an entirely different nature (although your regular pentry physicist might disagree) and are the subject of another entry.
There has also been no mention of temperature effects in this entry. Suffice it to say that acid-base equilibria like anything else are variant depending on what temperature the system is at. All quantities and relative acidities mentioned above are at standard laboratory 'room temperature' (25°C). If the temperature varies from this then calculated values above are no longer applicable but in our normal range of temperatures on this planet the results would be qualitatively the same. Beyond our atmospheric temperatures and pressures on Earth, you really don't want to know, it's all terribly, terribly complicated.
Some humans are said to possess an acid tongue but no chemist has ever detected these beings moving pairs of electrons around. This is the subject of ongoing research...