In natural waters, the carbonate system refers to many factors pertaining to acids and bases, including buffers, total alkalinity, pH and certain metals (especially calcium and magnesium). All of these are interrelated when it comes to water quality. We call this the carbonate system because these acids, bases and other ions originate from carbon dioxide dissolving in water (which forms carbonic acid). This system must be taken into account when looking at the total water quality picture. For instance, when calcium is present in water, it will have a greater tendency to deposit scale on coffee brewing equipment if there is a high total alkalinity, or if pH is also present.
When rainwater falls to earth, it dissolves carbon dioxide in the atmosphere and develops a low (i.e., acidic) pH. After falling through the atmosphere, the water disperses over the earth’s surface and penetrates through the soil and around mineral formations. Acidic water tends to dissolve the minerals it comes in contact with, raising the pH of the water and dissolving minerals to keep the solution in balance. Conversely, absorption of carbon dioxide present in the soil can lower the pH. In the case of ground water located far below the surface, by the time the water arrives at an aquifer or saturates the earth, its pH is higher and it contains considerable dissolved mineral content.
Acidity and alkalinity within the context of the carbonate system: if H+ and OH- ions are out of balance in a solution, it becomes either an acid (if there is an excess of H+) or a base (if there is an excess of OH- ions). Higher concentrations or either of these ions will increase rates of reaction in different ways during brewing.
Not all acids or bases dissociate immediately in a solution. Naturally occurring solutions, like water and coffee, contain a variety of compounds, including ionically bonded substances in different strengths.
You can neutralize a base or an acid by adding its converse acid or base. For example, a solution with a high pH (meaning, a greater concentration of the OH- ion) can be neutralized by adding an acid. The H+ in the added acid and the OH- already in solution combine to form a water molecule (the TDS is also increased due to the associated negative ion from the acid).
There are usually many different acids, bases and salts present in a solution. If a weak acid (or base) – one that does not dissociate easily – is present along with a strong acid (or base), only the strong acid (or base) will dissociate. If one attempts to neutralize such a solution by adding the converse acid or base, the strong acid (or base) will be neutralized, but the weaker acid (or base) may dissociate and the pH will not change.
The pH is a measurement of the concentration of H+ and OH- ions in solution at a certain point in time. Due to the presence of other un-dissociated ionically bonded structures in the solution, one may need to add a greater amount of the converse ion to measurably lower the pH. Measuring pH in a liquid can be compared to measuring a room’s temperature. One can measure how hot or cold the room is at a certain point in time, but the temperature won’t tell you how much heat would be necessary to make the room warmer. Similarly, measuring pH will not reveal how much correction will be necessary to attain the target pH (how much acid or base should be added). The measure of the amount of correction that will be needed is referred to as total acidity or total alkalinity. The most important of these is total alkalinity when dealing with municipal waters.
Total alkalinity measures the water’s ability to neutralize acids. The associated ions consist of hydroxide (OH-), carbonate (CO3 with two negative charges) and bicarbonate (HCO3-). The pH and alkalinity act in opposition to affect the relative acidity of a solution. Using the comparison of heating a room, you could attempt to lower the pH of a liquid by adding an acid, which is similar to raising the temperature of a room by turning on the furnace. However, the added acidity would be consumed by total alkalinity just as the heat that is generated initially generated is absorbed by the volume of the cold air in the room before any warmth is felt.
The presence of these ions has a dramatic effect on coffee brewing. The alkalinity and reserve total alkalinity neutralize the acids of the coffee as well as causing expansion of the coffee particles.
In tests using different combinations of water qualities, coffees brewed with water containing carbonates were the most bitter, had the highest pH (were most alkaline) and tasted flat. Water containing higher amounts of carbonates also increased dwell time significantly.
Hardness relates to the presence of calcium and magnesium in the water. Water with a high concentration of these metals is referred to as hard because of the mineral and scale deposits that can result. In extremely hard water, other metals are also commonly present. Additionally, extremely hard water indicates the presence of positively charged ions (or captions) and an equal amount of negatively charged ions (anions) comprising of chlorides, sulfates, carbonates and bicarbonates that are dissolved in solution. The anions are responsible for total alkalinity carbonate and bicarbonate (in which case the measure is of temporary hardness), or sulfate (permanent hardness).
Due to the interactions of the carbonate system and other ions, scale can form on brew machinery, especially in areas that experience temperature changes, such as spray heads and heating elements.
