I live in Washington DC and I am having some trouble reading my current report when it comes to alkalinity as there are several values listed.
They are:
Alkalinity- 63 PPM
Calcium hardness- 87 PPM as CaCO3
Calcium hardness- 5.1 grains per gallon CaCO3
I'm trying to figure out how to express my alkalinity as HCO3. Any help?
There is only one alkalinity value listed here: 63 ppm as CaCO3. That number is determined by adding 0.1 N sulfuric acid to a sample of 100 mL of your water until a pH value of 4.3, 4.4, 4.5 or some similar value (depending on the protocol your lab follows) is reached. As 0.1 N acid is used in a 0.1 L sample the number of mL used is equal to the number of milliequivalents per liter. That number, 1.26 mEq/L, is the alkalinity of your water. End of discussion. Or end of discussion in most of the world but here in north America that number is multiplied by 50 to get your 63 and expressed as 63 ppm as CaCO3 and the reason for doing this is that if you put 63 mg of limestone in a liter of water and dissolve it
with carbon dioxide using enough CO2 that the pH is 8.3 then the alkalinity will be 1.26 mEq/L or 63 ppm as CaCO3. This process will also put 1.26 mEq/L calcium into the water so that the calcium hardness can be expressed as 1.26 mEq/L or 63 ppm as CaCO3. This correspondence between calcium hardness and alkalinity as CaCO3 (when pH is 8.3 and CO2 was the acid) and the amount of dissolved limestone which produced them in natural waters is convenient sometimes but so confusing to people starting out in water chemistry that I really wish we would drop them and do as most of the world does: express alkalinity in mEq/L.
There is no reason to ever express alkalinity 'as bicarbonate' but there is nothing in the math to prohibit you from doing so. You can express it as sulfide if you want and the method has been set out in other posts. Divide ppm as CaCO3 by 50 to get to mEq/L and then multiply by the equivalent weight of the ion you want to express it as. For bicarbonate that is 61, for sulfide it is 32 so your alkalinity is 1.26*61 'as bicarbonate' or 1.26*32 'as sulfide'. It is, of course, ridiculous to express it 'as sulfide' because, we hope, your water contains no sulfide. It is, seemingly, less ridiculous to express it 'as bicarbonate' because, again we hope, that the only weak acid in your water is carbonic. Unfortunately, those spreadsheets and calculators that express alkalinity 'as bicarbonate' don't say 'as bicarbonate'. The just say bicarbonate leading users to believe that the number represents the amount of bicarbonate in their water. This gets me puzzled posts from people who use these spreadsheets with RO water (no bicarbonate), add lime (no bicarbonate) and find a hefty bicarbonate value in their spreadsheet's bicarbonate field.
The temptation to use bicarbonate as a proxy for alkalinity is bolstered first by the fact that if you don't add alkali other than carbonates or bicarbonates to your water and you stay below approximately pH 7.5 the bicarbonate calculated from the alkalinity as described above is pretty close. For example, at pH 7, water with alkalinity of 100 (solely due to carbonic) will have a bicarbonate content of 119.9 whereas 61*100/50 = 122. At pH 8 the bicarbonate content is 118.4, at 8.3 it is 117 and at pH 6 122.3
If you think I am on a campaign to get the spreadsheet authors to correct their products to accurately and properly describe alkalinity you are right.
Also if it is needed my Ca and Mg levels are:
Calcium- 38 ppm
Magnesium- 7 ppm
With calcium (and magnesium) the procedure is the same. You divide the hardness in ppm as CaCO3 by 50 to get the mEq/L (this time of a chelating agent as opposed to an acid): 87/50 = 1.74 and multiply by the equivalent weight of calcium (20) to get the mg/L: 20*1.74 = 34.8. That's close to, but not equal to 38. Why not? The hardness number is traditionally, as noted, determined by titration with a chelating agent. The calcium metal content may have been determined by another method such as Atomic Absorption Spectrometry or Inductively Coupled Plasma spectroscopy. These tests respond only to the metal in question. Titrations with chelants respond to anything which the chelant chelates. Thus traces of strontium, iron, magnesium that hasn't been completely removed (in the calcium hardness test the pH is raised to precipitate Mg(OH)2 ) and other polyvalent metal cations will up the number. While calcium hardness is formally defined to include only calcium this shortcoming of the traditional test is recognized. Thus there is nothing wrong with this report beyond the accepted shortcomings of the methodologies.
Not surprising that you are a little confused. I've touched on a couple of the nuances of water chemistry here of which the average guy starting out could hardly be expected to be aware.
Edit: In looking back I see that the values are averages. Many water authorities do tests on different schedules i.e. hardness may be measured on Tuesdays and Thursdays whereas ICP may be run on Mondays and Fridays. This could also be repsonsible for the apparent discrepancy between hardness derived and directly measured calcium (and magnesium) levels.
