Water Chemistry, For Dummies
by Grant Gussie
This article is about a topic that I approach with some trepidation, since I think that far too much attention is paid to it as it is. However, any novice aquarium keeper without a degree in chemistry would be lost while overhearing a conversation about pH and the rest, so this article is intended to cover all that. But remember, you can easily keep beautiful plants and raise healthy fishes in Calgary's tap water without paying any attention to pH whatsoever, so please don't be intimidated by all this chemistry. Just think of it as something to help you out when the club's fanatics start speaking in tongues.
Chemistry is a pretty big topic, and to introduce it, you have to become familiar with a few of the basics; such as what are atoms, protons, electrons etc . I'll start off by going through a few of the things that are needed to understand chemistry. Hopefully it's not too dry.
A molecule is the smallest indivisible unit of a chemical compound. Break a molecule in two and you have two different chemicals. Molecules are however made up of atoms, and an atom is the smallest indivisible unit of a chemical element. But it is also possible to break up an atom because an atom is in turn made up of a number of neutrons, protons, and electrons. The neutrons and protons are located in a small central nucleus that is surrounded by a cloud of electrons. The nucleus is divisible too. You can break a nucleus if you have enough energy. If you break a nucleus, you get daughter nuclei that are different chemical elements. Knocking electrons off of the atom is however relatively simple matter and it does not change what kind of atom you have.
Protons are said to have a positive electrical charge, and the electrons are said to have a negative electrical charge. Neutrons have no charge. Opposite electrical charges attract and like electrical charges repel, so the positively charged protons in the nucleus attract the negatively charged electrons.
Now, things get a little complicated. Since protons all have a positive charge they repel each other. So logically, the nucleus should fly apart because of the electrical repulsion of the protons. This is where the neutrons come in. They provide another force called the strong nuclear force that holds the nucleus together. The strong nuclear force is much stronger than the electrical force, but it has a very short range and is only felt within a nucleus.
The electrical attraction between the positively charged nucleus and the negatively charged electrons is quite strong. However, the electrons are kept from being sucked right into the nucleus by a rather subtle effect that is difficult to explain. Electrons, as it turns out, can not get too close together. Why this is so is studied in a branch of physics called quantum mechanics, which is far too complicated to get into. But as dictated by the rules of quantum mechanics, the electrons do not fall into the nucleus but are instead in shells ("orbitals") around the nucleus.
Quantum mechanics also tells us how many electrons can go into each shell. If an atom has more electrons than can fit in one shell then another shell forms to accept the extras. As it turns out, the innermost shell of an atom can hold only two electrons (at most) and the next two shells out can hold eight each. The reason that the numbers are two and eight is too difficult to explain, so I'll ask that you trust me on that one.
How many electrons an atom has in its outermost shell is very important to how that atom behaves in chemical reactions. An atom that has an outermost shell that is full of electrons will not take part in any chemical reactions at all. This is the case of the "noble gases" like helium and neon. For example, helium has two electrons, which is all the first shell can hold. With a single full shell, helium doesn't react with anything. However an atom that is missing electrons in its outermost shell will take part in chemical reactions because it wants to share an electron with another atom to fill its outer shell. This is the case of hydrogen, which has just one electron, and so has a single shell that is only half full. Because its shell is one electron short of being full, the hydrogen atom has space in its shell to share an electron with another atom. When an atom shares an electron with another atom, the two atoms are joined to form a molecule.
Another highly reactive atom is oxygen. An oxygen atom has eight electrons (two in the inner shell, six in the outer shell), so it needs two more electrons to fill its outer shell. But because a hydrogen atom (H) has only one electron, the oxygen atom (O) needs two hydrogen atoms (H 2 ) to supply those two electrons. That is why the chemical formula of water is H 2 O.
Another way atoms can join together is with ionic bonds. Ionic bonds differ from molecular bonds (where electrons are more or less shared) in that in ionic bonds one atom literally steals an electron from another atom (no sharing). The atom that steals the electron would then have more electrons than protons, so it becomes negatively charged. And the atom that lost the electron is now positively charged because it has more protons than electrons. An atom that has unequal numbers of electrons and protons (and is therefore charged) is called an ion.
A negatively charged ion is called an "anion" and a positively charged ion is called a "cation". You can keep these two names straight in your mind if you tell yourself that the first "n" in anion stands for "negative". Ions are denoted by a superscripted "+" (for cations) or "�" (for anions) symbol next to their chemical symbol, as in the examples Na + (sodium cation) and Cl - (chloride anion). The number of �'s (or +'s) tells you how many extra electrons the ion has (or is missing). For example, the carbonate anion (with two extra electrons) has the symbol CO 3 -- .
Since opposite charges attract, two oppositely charged ions will stick together, forming an ionic bond. Salt (sodium chloride, or NaCl) is an example of an ionically bonded chemical. Sodium has only one electron in its outer shell, so it doesn't hold onto its outermost electron very tightly. But chlorine needs only one electron to completely fill its outer shell, and therefore it has a strong attraction to the weakly held outermost sodium electron. A chlorine atom will therefore steal a sodium atom's electron. This creates a chloride anion (Cl - ) and a sodium cation (Na + ) that then bond ionically to form a salt crystal. When dissolved in water, however, the salt crystal will break up ("dissociate"), back into Na + and Cl - . This results in a mixture of free ions among the water molecules. This process is quite remarkable really; Na + and Cl - are very strongly bound to each other in a salt crystal (so salt has one of the highest melting points known) yet a little drop of water can split the bond easily.
A funny thing is that another one of the chemicals that water can dissociate is water. At any given time, a certain number of water molecules are broken up into hydrogen cations (H + ) and hydroxide anions (OH - ). If the water is pure, the concentration of hydrogen cations and hydroxide anions will both be equal to 10 -7 moles per liter. A "mole" is a number of items in the same way that a "dozen" is a number of items. One mole of atoms (or eggs, or aardvarks, or what have you) is 6.022x10 23 atoms (or eggs, or aardvarks, or what have you). So you can see that there would be somewhat more eggs in a mole than there are in a dozen.
If you are mathematically minded, you might note that the negative value of the logarithm of the molar concentration of hydrogen cations in pure water is seven. That is what pH is. So the pH of pure water is seven.
OK, so that's really a load of bull. Actually, there really is no such thing as a hydrogen cation (H + ). All of them get stuck onto at least one other water molecule to form an H 3 O + (or more complex) cation.
If you dissolve a chemical that creates H 3 O + cations when it dissolves, you are increasing the concentration of H 3 O + in the water. Since pH is the negative value of the concentration, a higher concentration of H 3 O + makes pH numerically lower. And because the scale is logarithmic, every full number decrease in pH (from 7 to 6, for example) represents a ten-fold increase in the concentration of H 3 O + .
We call water with a pH that is numerically lower than seven "acidic", and chemicals that lower the pH of water are called "acids". A common example of an acid is hydrochloric acid (HCl) which dissociates in H 2 O to form H 3 O + and Cl - .
Chemicals that deplete water of H 3 O + cations when they dissolve make pH numerically higher. We call such chemicals "alkalis" or "bases".
Different bases lower the H 3 O + concentration (and so raise pH) in different ways. For example, a base might increase pH by releasing OH - when it dissolves; like caustic soda (NaOH) does. The released OH - anions then combine with the existing H 3 O + cations to form water molecules. This lowers the H 3 O + concentration and so it raises pH.
Other alkalis can remove H 3 O + in other ways. For example, when ammonia (NH 3 ) is put into water, some of it reacts with the H 3 O + to form ammonium (NH 4 + ). The more H 3 O + is present in the water (that is, the more acidic the water is) the more ammonia will bind itself to the H 3 O + . Believe it or not, this is actually quite important to aquarists. Molecular ammonia (NH 3 ) is toxic to fish, but the ammonium ion (NH 4 + ) is not. So because ammonia forms NH 4 + in acidic water, fish in acidic water are much less susceptible to ammonia poisoning than fish in alkaline water. Biological filtration is therefore very important in areas where water is alkaline, such as Calgary.
One might assume that adding an acid to water would always cause an immediate drop in its pH. This is not however the case. Because anions dissolved in water can act as a "sponge" to absorb the added H 3 O + cations, you might be able to add an acid to water for quite a while before seeing a change in its pH.
The ability of water to resist downward changes in pH is called its "alkalinity". Conversely, the ability of water to resist upward changes in pH when a base is added to it is called its "acidity". These terms can cause a fair amount of confusion since alkalinity is sometimes (incorrectly) used as a synonym for "measure of alkaline level" and acidity is sometimes used as a synonym for "measure of acid level". This is wrong, so don't be confused by the similarity in terms. Instead, think of alkalinity as meaning "the buffering capacity that keeps the pH alkaline"; and acidity as meaning "the buffering capacity that keeps the pH acidic". However, a high alkalinity/acidity will also keep the water's pH from becoming too extreme, since it gets buffered to a moderate pH.
The differences between acidity and acid, and alkalinity and alkaline, are quite important. Water can be quite alkaline, but still have a low alkalinity. For example, you can get very alkaline water by adding caustic soda (NaOH) to it, but because even a small amount of acid added to this water would cause a drastic downward shift in its pH, this water would have a low alkalinity. The reverse is true if you add a base to a highly acidic solution of hydrochloric acid (HCl). But don't try this at home, along with a rapid change in pH you are likely to get a minor explosion.
It is important to aquarists that their water has a stable, well-buffered pH. Therefore if you want acidic water you also want a high acidity, and if you want alkaline water you also want a high alkalinity. This will insure the water is well buffered and the pH is stable. In my opinion the ability to maintain a stable pH is much more important than the actual value of the pH being maintained. Fish can adapt to a stable pH that is reasonably far outside their natural range, but fish can not adapt to a changing pH regardless of its value. Because fish create acids, the pH can drop rapidly if their water is not well buffered and proper aquarium maintenance is not followed. This will stress a fish much more than living in a stable pH that is a point or two higher than that of the Amazonian stream that its grandparents were caught in.
Fortunately, fluctuating pH is not a problem if you use Calgary tap water and follow a proper water-change schedule. Calgary's water is very well buffered to a pH of about 8.2; which is certainly a high pH for most tropical fishes (at least those from Asia and South America) but at least its stable.
The reason our water is so well buffered is that our local limestone bedrock has a lot of calcium carbonate (CaCO 3 ) and magnesium carbonate (MgCO 3 ) in it. The carbonate and bicarbonate anions that are consequently dissolved in our water are an effective buffer, keeping the water at about pH 8.2. It works like this: calcium carbonate is difficult to dissolve, but when an acidic solution (such as rain) is placed on it, we get the reaction
CaCO 3 + H 3 O + => Ca ++ + HCO 3 - + H 2 O
This creates the bicarbonate anion (HCO 3 - ), which is very soluble, and so the limestone dissolves. Since this reaction also removes a H 3 O + cation, this reaction increases pH. But when the pH increases above 8.2, we have so much more OH - than H 3 O + that the reaction
HCO 3 - + OH - + Ca ++ => CaCO 3 + H 2 O
starts to occur at a faster rate than the previous reaction. The second reaction causes calcium carbonate to precipitate out of solution and also depletes OH - anions, which in turn causes an overabundance of H 3 O + cations. The pH therefore decreases back down to 8.2, at which point the first reaction starts to dominate again. This is why your tank pH is stuck on 8.2. It is also the reason why waters with a high alkalinity do not get too alkaline, but are instead buffered to a moderate pH value.
Aquarists often measure alkalinity in units of KH. This is the German scale of carbonate hardness. The prevalence of this measurement is due to the fact that most of aquarist literature originates in Germany (who came up with the scale and are loath to stop using it) and the USA (who aren't going start using the SI system until the next ice age). We shouldn't be using KH any more despite its antiquated prevalence in hobbyist literature. Alkalinity should be quoted in milliequivalents per liter (meq/L), or milligrams per liter of calcium carbonate (mg/L CaCO 3 ).
To say a sample of water has an alkalinity of 150 mg/L CaCO 3 is to say that its alkalinity is the same as that of otherwise pure water with 150 mg/L of CaCO 3 dissolved in it. There is no suggestion that the sample water actually contains 150 mg/L CaCO 3 , just that its alkalinity is the same as it would be if it did.
To convert meq/L to mg/L CaCO 3 , multiply meq/L by 50.04. To convert KH to mg/L CaCO 3 , multiply KH by 17.8.
By the way, Calgary's tap water has an alkalinity that typically ranges from about 120 mg/L CaCO 3 to 150 mg/L CaCO 3 .
Another aspect of water chemistry is hardness, which is a measure of the concentration of multiply charged ("multivalent") cations in the water. In practice this means the concentration of Ca ++ and Mg ++ , since the others are too rare to contribute much. Note that singly charged cations (like Na + ) do not contribute to hardness, just to salinity (which is completely independent of pH, alkalinity, and hardness). Water hardness is also measured with various scales, but aquarists really should use the measurement of mg/L CaCO 3 . The old German DH scale can be converted to mg/L CaCO 3 by multiplying DH by 17.8. Scales that measure hardness in parts per million (ppm) are the same as mg/L CaCO 3.
You may also come across something called carbonate hardness or temporary hardness. This is that part of the hardness that came from dissolving bicarbonate and carbonate compounds of calcium and magnesium. Temporary hardness is so-named because the bicarbonate and carbonate precipitate out of water in the form of calcium carbonate so easily, as for example when you boil the water. It is the temporary hardness that is responsible for the limestone scale that builds up in kettles over time.
The term KH is sometimes used as a synonym for carbonate hardness. This practice causes a lot of confusion because KH is also used as a synonym for alkalinity. Remember that alkalinity and carbonate hardness are quite different things. However, in practice, almost all of a fresh water's alkalinity would come from its dissolved carbonates, so the water's alkalinity would be almost equal (numerically) to its carbonate hardness. You would be well advised however to avoid using the two terms interchangeably. To set the record straight, those pet-shop test kits that give their results in units of KH actually measure alkalinity, not carbonate hardness, despite what their booklets may say. Yes, I know its confusing. Let's hope that KH soon gets dropped along with all those other archaic units like gallons and furlongs per fortnight.
There is also something called "permanent hardness", which is the hardness caused by dissolving calcium or magnesium compounds of chloride, sulphate, phosphate, and nitrate. Because these compounds do not precipitate out of water during boiling, these compounds are said to give the water "permanent" hardness.
You may also see something called total hardness. This is just the numerical total of temporary hardness and permanent hardness. To avoid confusion, you shouldn't use the term total hardness at all. Just call it hardness, defined as the concentration of multivalent cations (principally Ca ++ and Mg ++ ) expressed as an equivalent value in mg/L CaCO 3 .
Calgary's tap water has a hardness that ranges from about 120 mg/L CaCO 3 to 250 mg/L CaCO 3 , which qualifies it as being only moderately hard. Hardness is lowest during the Rocky Mountain snowmelt of late spring through summer, and highest in winter.
A lot of confusion exists on the relationship between alkalinity, hardness, and pH. In fact these terms are sometimes used interchangeably. I get a chuckle whenever I see that commercial with that couple who say "we have hard water" while holding up a piece of dark litmus paper. Gopher nuggets. Litmus paper tests pH, not hardness.
Hardness, alkalinity, and pH are however interrelated. Natural fresh waters ultimately come from rain, which has almost no alkalinity or hardness, and just a slightly acidic pH. But if the water then acquires an alkaline pH, it must have been modified so that it also got a high alkalinity too. If it did not, the acidifying effects of atmospheric carbon dioxide, air pollution, and organic acids would soon lower the pH back into the acidic range. But it is the dissolution of calcitic rocks (and the subsequent release of carbonates) that provides natural water with alkalinity. So water with a high alkalinity is therefore almost guaranteed to have passed over calcitic rocks. And since such water would have also gained Ca ++ (and thus hardness) at the same time as it gained its carbonates, the water would be hard as well. So natural water sources that are high in pH also tend to have a high alkalinity and hardness. But, on the other hand, an unpolluted natural water source with a low pH has assuredly not contacted calcitic rocks and therefore retains the low alkalinity and hardness it had as rain.
Surprisingly, pure water can gain a small amount of alkalinity by simple exposing it to air; or rather, the carbon dioxide in the air. Carbon dioxide (CO 2 ) raises alkalinity (and lowers pH) because of the reaction:
CO 2 + H 2 O + H 2 O <=> HCO 3 - + H 3 O +
In chemically stable water this reaction occurs in both directions at equal rates at any given time. This reaction will equalize so that the pH of pure water exposed to air is about 6.8 or so at a temperature of 20C. However, if you suddenly lower pH by adding an acid, raise the HCO 3 - concentration by adding baking soda, or lower the CO 2 concentration by heating, the reverse reaction will temporarily dominate over the forward reaction. This will create free CO 2 gas and raise the pH until a new balance is achieved.
This process is the cause of the limestone scale in your kettle. As the temperature of the kettle's water rises, CO 2 is driven off. Bicarbonate then reacts with H 3 O + to rebalance the equation, but this raises the pH of the water. Recalling the second reaction in this article,
HCO 3 - + OH - + Ca ++ => CaCO 3 + H 2 O
we see that when there is an overabundance of OH - (as we have when we have a high pH) the bicarbonate anion (HCO 3 - ) will react with OH - and calcium to form calcium carbonate (CaCO 3 ). Calcium carbonate is insoluble and therefore precipitates out. The result is scale deposits in your kettle.
So that's your introduction to water chemistry. Although admittedly a little long, hopefully it wasn't too daunting. Next month I'll talk about the practical matter of how to manipulate pH, alkalinity, and hardness