ALKALINITY
I am determined to make math and
chemistry work on a website, with the recognition that they do
not yet fully support the "industry standard" chemical
and mathematical symbols! I will CONTINUE TO TRY do my best here:
To begin our discussion, let us assume that pH is REALLY the negative
log of the hydrogen ion concentration. For example the mathematical
representation in last months notebook on pH: pH = -log10 (H+)
. Let us further assume that certain molecules such as calcium
carbonate (CaCO3), sodium carbonate (Na2 CO3), and magnesium carbonate
(MgCO3), do not contain hydrogen ions (H+) , nor hydroxyl ions
(OH- ), that would appreciably impact the balance of hydrogen
and hydroxyl ion concentration in a pure water solution by physically
adding to the concentration of those species already existing
in the solution. Finally, let us agree that certain molecules
such as calcium carbonate (CaCO3 ), sodium carbonate ( Na2 CO3
), and magnesium carbonate (MgCO3 ), will be able to react with
hydrogen ions such that as any new hydrogen ions added to a solution
these molecules will "tie up" these new hydrogen ions
and prevent them from appreciably changing the pH of the solution.
We refer to this group of molecules as "alkalinity."
We will therefore loosely define alkalinity as the "buffering
capability" of the water." As we will see later, "alkalinity"
is actually much more than that!
So what types of molecules, and specific molecules are generally included in this buffering or alkalinity measurement? Generally speaking, calcium carbonate, sodium carbonate, magnesium carbonate, bicarbonates, silicates, borates, and phosphates, if occurring in the water, all contribute to this buffering action. Alkalinity is therefore a collection of many compounds and chemicals.
How is alkalinity determined? If we agree that "alkalinity" is an "acid buffering agent" that resist attempts to change the pH of the water when the addition of hydrogen ions occurs, then we can determine its buffering strength by adding an acid!
We add measured amounts of a known acid strength to the water sample we wish to know the alkalinity of, until we reach a specified pH value where we stop adding the acid (the titration "end point").
Most treatment plants add sulfuric acid (H2 SO4) to a sample until the pH drops to an end point of 4.5 pH. The concentration, (the strength of the acid), and the volume of the acid added, (the amount titrated), are used to calculate the milligrams per liter (mg/L) of alkalinity. It usually expressed as milligrams per liter (mg/L) of calcium carbonate (CaCO3), even though there may very well be many different buffering molecules other than calcium carbonate neutralizing the addition of the hydrogen ions (H+) of the acid.
Hardness is generally an elevated level of calcium (Ca) and magnesium (mg) ions in the water. Sodium (Na) and potassium (K) are NOT contributors to water hardness. Therefore if calcium and/or magnesium atoms are attached to the carbonate ion (CO3) the alkalinity is a contributor to hard water. On the other hand if sodium (Na) and/or potassium (K) atoms are attached to the carbonate ion (CO3), the alkalinity would not be a contributor to hard water. (Remember, we've re-generate water softeners by passing a saturated rock salt (sodium chloride [NaCL] and/or potassium chloride [KCl] ), solution over the resin beads to displace the magnesium (Mg) and calcium (Ca) ions captured on the surface of the resin beads.)
In an overview, let us examine the impact of biological and chemical activities in two waters: one low in alkalinity, and one high in alkalinity.
Low alkalinity water is more of headache to water and wastewater treatment plant operators as it is susceptible to wide swings in pH due to biological action or chemical action. Biological Examples: Nitrification, (please see nitrification chapters here) the result of biological activity in converting ammonia into nitrates, creates hydrogen ions which lowers the pH (increases acidity). The pH may drop dramatically if the alkalinity is very low. When waters with insufficient alkalinity undergo nitrification the pH may drop to a level that inhibits the nitifiers from converting ammonia into nitrates. Another biological activity, that of algal and bacterial activity in ponds, lakes, and other bodies (see ponds chapter) of water create greater diurnal pH fluctuations in low alkaline waters than those waters that are higher in alkalinity. Chemical Examples: A chemical action is illustrated by the addition of liquid chlorine to the water for disinfection. The chlorine reacts with water (H2 O)to create hydrochloric acid (HCl) and hydrochlorous acid (HOCl). Subsequently, the pH drops in waters with insufficient alkalinity, and may, in extreme cases, not be able to support buffering of this acid addition. In the event the pH falls out of the desired range, we have to therefore adjust the pH, which adds to the cost of the water treatment process.
High alkalinity waters are easier to treat as the biological activity and the chemical additions that produce hydrogen ions (H+) are "buffered". That is the new hydrogen ions react with the "alkalinity," and pH stays more stable, with less fluctuations; and most always with pH swings of less magnitude.
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