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Re: total alkalinity and total hardness


> 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!