I wouldn't take that page too seriously. If this is representative of the quality of work we get for our tax dollars then, - well never mind.
"Alkalinity is usually measured using sulfuric acid with a digital titrator. Sulfuric acid is added to the water sample in measured amounts until the three main forms of alkalinity (bicarbonate, carbonate, and hydroxide) are converted to carbonic acid."
No amount of sulfuric acid will convert hydroxyl ion to carbonic acid.
"At pH 10, hydroxide (if present) reacts to form water."
And it will do the same at pH 9 or 4.3 if it is present (and it is but not in very great quantitiy).
"At pH 8.3, carbonate is converted to bicarbonate."
Only 98% of it is.
"At pH 4.5, it is certain that all carbonate and bicarbonate are converted to carbonic acid."
Assuming that one started at a rather high pH (such that there is unapprecibale carbonic acid in the system) and that the total of carbonate and bicarbonate were 1 mmol/L at pH 4.5 there would be 0.013 (1.3% of the total) mmol/L of bicarbonate remaining.
"Below this pH, the water is unable to neutralize the sulfuric acid and there is a linear relationship between the amount of sulfuric acid added to the sample and the change in the pH of the sample."
Not so. The remaining 1.3% bicarbonate is able to neutralize more acid.
"So, additional sulfuric acid is added to the sample to reduce the pH of 4.5 by exactly 0.3 pH units (which corresponds to an exact doubling of the pH) to a pH of 4.2."
Reducing 4.5 to 4.2 is doubling? Sounds like this guy is one of O's economic advisers. What doubles is the hydrogen ion concentration.
"However, the exact pH at which the conversion of these bases might have happened, or total alkalinity, is still unknown."
Some conversion takes place at pH 9, some at 8, some at 4.3 and some at any other pH you care to choose.
"This procedure uses an equation derived from the slope of the line described above to extrapolate back to the amount of sulfuric acid that was added to actually convert all the bases to carbonic acid."
All the bases are never converted. Even at pH 4.2 a fraction (0.65%) of the original carbo remains.
The multiplier (0.1) then converts this to total alkalinity as mg/L CaCO3. "
Where does this come from? The answer is that they are clearly describing Hach's digital titrator without saying so and using the 0.1600 N cartridges (obviously, if you don't know about Hach's system, as would be the case for most readers, this would mean nothing to you) instead of the 1.600 N cartridge which gives 1 ppm as CaCO3 per digit for a 100 mL sample. Note that the piece doesn't mention sample size until the example.
There is no 'correct' end point pH for alkalinity. Alkalinity is a measure of the amount of acid required to move between two pH's whatever they may be. Standard methods, which the EPA piece references but seems to ignore, clearly states that the end point is arbitrary but must be stated in the report. This is because anyone who understands the chemistry can calculate the amount of bicarbonate in the original sample given the sample pH, the end point and the amount of acid it took to go between the two pH values. Popular end points are pH 4.3 (Methyl Orange end point), pH 4.4 (used by Ward Labs) pH 4.5 (ASTM) and the equivalence end point at which the remaining bicarbonate is equivalent to the hydrogen ion concentration.
I haven't figured out just what they are trying to do here by subtracting off the extra acid required to go another 0.3 pH. Why do this from pH 4.5 - why not 4.4 or 4.1? The difference is just another alkalinity: the alkalinity between 4.5 and 4.2 and is, by itself, as is the alkalinity between any other pH pair, sufficient to determine the amount of carbo in the system. Once you have that the bicarbonate can be calculated trivially from the pH.
