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**To**:**Aquatic-Plants at actwin_com****Subject**:**Re: total alkalinity and total hardness****From**:**"Roger S. Miller" <rgrmill at rt66_com>**- Date: Tue, 14 Oct 1997 14:53:57 -0500 (EST)
- In-Reply-To: <199710141846.OAA04828 at acme_actwin.com>

> Date: Tue, 14 Oct 1997 00:56:24 -0700 > From: Stephen Pushak <teban at powersonic_bc.ca> > Subject: total alkalinity and total hardness > > I think I understand that KH measures carbonate ion concentration and GH > measures total dissolved solids. Knowing the total alkalinity and total > hardness, and the concentrations of Cl-, NO3-, SO4--, Ca++ and Mg++ can > we deduce CO3--, Na+ and K+ concentrations? (assuming typical ground > water from a well) > KH most often measures bicarbonate ion concentration, not carbonate, and yes, you can calculate carbonate given KH and pH. You can estimate by difference the total concentration of Na+K, but not the concentrations of the separate ions. This was once very commonly done. The resulting value is expressed as Na. I've completed the estimates and tabled them below: mg/l mg/meq meq/l Chloride 45 -35.45 -1.27 Nitrate 22.2 -62.00 -0.36 Sulfate 24 -48.03 -0.50 Barium <0.02 Calcium 92 20.04 4.59 Magnesium 12.2 12.15 1.00 Ferrous iron <0.02 Manganese <0.02 Copper <0.02 Zinc 0.36 Chromium <0.02 Phosphate 0.041 pH 7.1 Total Hardness 280 Total Alkalinity 330 50.04 -6.59 Na+K 72 22.99 3.13 HCO3 402 61.02 CO2 51 CO3 0 For Na+K, the procedure is to calculate an electrical balance for all major ions. Here I assumed that the nitrate was expressed as nitrate - if it is expressed as N then this is heavily polluted water. The column headed "mg/meq" shows the weight of each major constituent needed to make one millimole of charge (a milliequivalent), and the column headed "meq/l" shows the milliequivants for each major constituent. The sign reflects the polarity of the charged ion. The sum of the meq/l column (exclusive of the Na+K entry) is -3.13 meq/l. The water must be electrically neutral, so the concentration of sodium + potassium must provide enough positive charge to counterbalance that total: there must be +3.13 meq/l of sodium+potassium. Standard practice is to represent that total as all sodium, since sodium is normally much more common than potassium and without some other source of information you have no way to know how the total might be broken down. 72 mg/l of sodium is needed to get 3.13 meq/l. I also used pH and alkalinity to get the dissolved inorganic species. Bicarbonate is calculated from alkalinity: divide 330 mg/l alkalinity by 50.04 to get meq/l of alkalinity, then multiply meq/l by 61.02 to get mg/l of bicarbonate. This assumes all alkalinity is in the form of bicarbonate. CO2 was calculated from bicarb and pH with CO2 = HCO3*1.6*10^(6-pH) and CO3 was calculated from bicarb and pH with CO3 = HCO3*0.553*10^(pH-10) based on pK=10.25. The calculation actually produced a value of 0.28 mg/l CO3, which is functionally 0 because of the precision in the alkalinity value. If CO3 were not negligible, then the assumption that the alkalinity was entirely bicarb would be incorrect and a more complicated calculation would be needed. Roger Miller In Albuquerque, where the pumpkin crop is in!

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