ACID RAIN 5
negative charge (due to SO 42 ) when the concentrations are
expressed in eq/L (or m eq/L).
For most precipitation samples, the major ions are those
listed in Eq. (1):HCa Mg NH aSO22
442 ()()( )()()()()(ΝΚΝΟ 3 ))( )( C1OH )(HCO 3 )(1)with each ion concentration expressed in m eq/L. In prac-
tice, if the actual measurements are inserted into Eq. (1),
then agreement within about 15% for the two sides of the
equation is probably acceptable for any one sample. Greater
deviations indicate that one or more ions were measured
inaccurately or that an important ion has not been measured.
For example, in some samples Al^3 ^ contributes a signifi-
cant amount and therefore needs to be included in Eq. (1).
It should be noted that assumptions concerning the parent
compounds of the ions are not necessary. However, if one
did know, for example, that all Na^ ^ and all Cl^ ^ resulted from
the dissolution of a single compound such as NaCl, then
these two ions would not be necessary in Eq. (1) since they
cancel out on the two sides of the equation.
There are actually two useful checks as to whether or not
all the major ions have been measured. First, one compares
to see that the sum of the negative charges is approximately
equal to the sum of the positive charges. If all the sodium
and chloride ions come entirely from the compound NaCl,
then this first check would produce an equality, even if these
major ions were not measured. The second check is whether
the calculated conductivity is equal to the measured conduc-
tivity. The calculated conductivity is the sum of all the ions
(in Eq. (1)) multiplied by the factors listed in Table 1. Forlow pH samples of rain or melted snow (i.e., pH 4.5),
H^ ^ is the major contributor to the calculated conductivity
because of the relatively large value of its factor in Table 1.
For precipitation samples, bicarbonate concentration is
usually not measured. Thus both (HCO 3 ) and (OH^ ^ ) must
be calculated from the measured pH. To calculate (OH^ ^ ) and
(HCO 3) the following relationships for the dissociation of
water and for the solubility and first and second dissocia-
tions of carbon dioxide in water are used:Chemical ReactionHO^2 OH H
(2a)Pco22 2H O CO· (2b)H O CO^22 · H HCO^3
(2c)HCO^33 H CO 2
(2d)Equilibrium RelationshipK W (OH^ ^ )(H^ ^ ) (3)KHO CO
H Pco22
2()·
(4)KHHCO(^1) HO CO
3
22
()()
()·
(5)
K
HCO
HCO
2
3
2
3
()( )
()
(6)
For 25°C, K W 10 ^2 ( m eq L^ ^1 )^2 , K H 0.34 10 ^6 m eq
L^ ^1 , K 1 4.5 10 ^1 m eq L^ ^1 , and K 2 9.4 10 ^5 m eq L^ ^1.
HCO
CO
H
K
3
3
2
2
()
()
()
(7a)
For T 25°C and pH 8, (H^ ^ ) 0.01 m eq/L and thus:
ΗCO
CO^10
106
3
3
2 5
()
()
001
94
.
.
(7b)
TABLE 1
Conductance Factors at 25C a
Ion mS/cm per meq/L
H 0.3500
HCO 3 0.0436
Ca^2 0.0520
Cl 0.0759
Mg^2 0.0466
NO 3 0.0710
K 0.0720
Na 0.0489
SO 42 0.0739
NH 4 0.0745
a From Standard Methods for
the Examination of Water and
Wastewater, American Public
Health Association, Inc., Wash.,
D.C., 13th Edition.
C001_001_r03.indd 5C001_001_r03.indd 5 11/18/2005 10:07:26 AM11/18/2005 10:07:26 AM