The head chemist with my municipality described bicarbonate content as "a fraction of total alkalinity"
No, as we shall see, the bicarbonate content is, numerically, about 22% greater than the alkalinity number. It is correct to say that bicarbonate is responsible for only a fraction of the total alkalinity but under normal circumstances (i.e. potable water) that is usually a large fraction. The other things adding to alkalinity are carbonate ion, hydroxyl ion and the water itself (see below).
and that fraction is derived by molecular weight.
The fraction is determined by computing the distribution of carbo (carbonic, bicarbonate, carbonate) species and considering the H+ and OH- ion content of the water.
To quote him from an email:
"HCO3 (bicarbonate) is a fractional part of CaCO3 (units of the total alkalinity), by means of the molecular weights 100.086 to 61.016 (61.016/100.086 = 0.6096).
This concerns mass units only.
This says HCO3 is 60.96% of the mass of CaCO3. The titration test for these is the same, using Sulfuric Acid.
Using a test value from the annual report of 189.3 ppm(mg/L) Total Alkalinity… 189.3 X 0.6096 = 115.40 mg/L (ppm) as Bicarbonate ion (HCO3) in City Water."
This does not jive with what Bru'N Water outputs when I put in alkalinity & pH, nor does it jive with what else is said above. What gives?
What gives is that your man is all wet. Alkalinity is, as he says, measured by titrating a sample of the water with sulfuric acid. The alkalinity is defined as the number of milliequivalents required to bring 1 liter of sample to a prescribed pH. It is common practice to multiply this number by 50 (half the molecular weight of calcium carbonate) in North America and report ;ppm as calcium carbonate'. Thus in this case the alkalinity was reported at 189.3 implying that the acid required was 189.3/50 = 3.786 mEq/L. If we assume that all the acid absorption was done by bicarbonate ion i.e. that there was insignificant levels of carbonate and/or hydroxyl ions in the sample (both of which assumptions are pretty good as long as the pH is below 9 or so which it is in most cases) then we would note that each milliquivalent of acid neutralized one milliequivalent of bicarbonate. As the equivalent weight of bicarbonate is 61 mg/mEq we would multiply the 3.786 by 61 and conclude that the bicarbonate content is 231 mg/L. Bicarbonate ion concentration (in mg/L) is a bit bigger than the alkalinity (by about 61/50 or 22%) under normal circumstances.
Things are, as you will probably have no trouble appreciating, a bit more complex than this as hinted at above. The details can be found at
http://www.wetnewf.org/pdfs/carbonate-species-distribut.html. Additional information, including details (with illustrations) of how alkalinity is measured are at
http://www.wetnewf.org/pdfs/measuring-alkalinity.html.
You'll note that your man's answer is about half the correct one. He is basing it on the fact that 100 mg CaCO3 contains 61 mg of bicarbonate. He is failing to account for the fact that the factor 50 is used so that if 100 mg/L CaCO3 is dissolved
in the natural way, i.e. by carbonic acid from carbon dioxide that both alkalinity and hardness will be about 100 ppm 'as CaCO3'. The reaction is
CaCO3 + H20 + CO2 ---> Ca + 2HCO3-
Thus each 100 mg (mmol) of dissolved calcium carbonate results in 2 mmol dissolved bicarbonate ion: 1 from the CaCO3 and one from the air (or other source of CO2).
I should also note that all this sort of goes away if there are any other weak acids (such as phosphoric) in the water. There should not be in potable water (a smidgeon of phospho perhaps) but because the industry cannot always assume that to be the case there are some procedures for estimating the various components of alkalinity which do not match the carbo only case exactly. These are commonly used and are, for example, why Ward Labs reports don't balance as well as they should. Use of these approximations is not enough to account for a factor of two which is clearly attributable to this bloke's incomplete understanding of what alkalinity is and how it is computed.
