Chemicals
1) One bottle, Sulfuric Acid (0.03N)
2) One bottle, Phenolphthalein Indicator powder
3) One bottle, Bromo-creosol Green Methyl Red Indicator powder
4) One measuring spoon (0.05 g when measuring indicator powder)
Glassware
1) One Hach mixing bottle
2) One Hach plastic measuring tube
What is Alkalinity?
Alkalinity is a measure of how much acid it takes to lower the pH below
a certain level. In natural waters, most of this ability is due to the presence
of dissolved species in the carbonate system. This system consists of an
equilibrium between three chemical species:
Low pH <----------------------------------------------> High pH
H2CO3 HCO3- CO32-
This system tends to be so dominant in its contributions to alkalinity,
that we assume all of the alkalinity in the system is due to their presence.
Alkalinity is thus measured in terms of the amount of CO3(2-) present in
the system as a component of the CaCO3 molecule. This is chosen because
Calcium Carbonate in solid form, known as limestone or the minerals calcite
or aragonite, is often the primary contributor to the amount of dissolved
carbonate in the water.
To measure the alkalinity in the water, we simply add acid to it, watching
the pH and noting how much acid was added at the point where a certain
pH is reached. Our chemistry kits make this easy by using a compound known
as an indicator which changes color at a certain pH.
The Hach Alkalinity Test
If your sample is high in alkalinity, you need to follow the instructions
for the high range test. However for the sake of brevity and clarity,
in these instructions we will discuss the test as though the process was
the same for both, since the theory is very similar.
Step 1
Fill the sample bottle to the 15 mL line. We must know how much water
we're testing, so in the first step, the Hach kit has you fill a bottle
to a line marked on the side. (15 mL).
Step 2
Using the 0.05 g scoop, add a level scoop of BromCresol Green- Methyl
Red to the sample bottle. This step adds a small amount of indicator powder
to the solution, which will appear green at pH's above 4.3, and pink below
4.3.
Step 3
Add 0.03 N sulfuric acid solution dropwise to the sample bottle, counting
drops as you put them in. Note the number of drops required to produce
a full color change from green to pink.
As the acid goes in, the alkalinity in the water will use it up, until
it can't hold it any more. During this time, the pH will drop slowly,
and when all of the acid-absorbing capacity is gone, the pH will begin
to drop much more rapidly. Eventually the pH will reach 4.3, and the indicator
will change colors.
It is important to add the acid until all of the color change has taken
place. If you are unsure that a full change has occured, add another drop
and watch for further change. When no more is noted, record the number
of drops required. The number of drops multiplied by 6.8 is the alkalinity
in mg/L as CaCO3 (Calcium Carbonate).
High End Alkalinity
When we measure a water with a high alkalinity, we simply use a modified
method to save our chemicals. To do this we use a smaller initial volume
of sample water and in the end, multiply our number of drops by a different
conversion factor to account for the difference in initial volume. In
this case, we say each drop of sulfuric acid is multiplied by 17 to get
the alkalinity in mg/L as CaCO3
