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sIdebar 5.1 water analyses
Most reported chemical analyses of water are reported in mg/kg of solution, which equals parts per million (ppm) by
weight, or in mg/l of solution. However, most chemical calculations are carried out using moles/kg of solvent (which
can be either steam or liquid water). Moles/kg of solvent is called molality, and the unit is molal, abbreviated m.
Using a variety of methods, it has been established that one mole of a substance contains 6.0221e23 atoms (or
molecules), which is often called Avogadro’s number. The gram formula weight of a substance, also known as the
molecular weight, is the mass, in grams, of Avogadro’s number of atoms of that substance. These masses have been
tabulated for known elements and compounds and can be accessed either in reference materials or online. An
excellent reference for molecular weights of compounds is the National Institute of Standards and Technology (NIST)
(http://www.nist.gov and http://webbook.nist.gov/chemistry/).
Converting from mg/kg of solute species i to the molal concentration of i requires calculating the number of
moles of i in the solution and multiplying that quantity by the amount of solvent in 1 kg of the solution:
molal conc. of i = [(mg/kg i )/(1000 × mol. wt. of i)] × (1000/(1000-∑(z)/1000), (5S.1)
where ∑(z) is the sum of the concentration of all dissolved species.
Conversions such as this, where the number of moles of a species is determined, is an important first step in
establishing the quality of an analysis. A poor quality analysis can lead to miscalculations of reservoir temperature or
other important resource estimates, which can become costly errors. An important test of the quality of an analysis
is to determine the charge balance. Since all solutions are electrically neutral, the total negative charge attributable
to the number of moles of anions must exactly match the total positive charge attributable to the number of moles
of cations. For example, in a chemical analysis of a water that has only dissolved salt (i.e., halite = NaCl), the only
solutes in the water will be sodium (Na ) and chlorine (Cl ) ions and a negligible amount of NaCl (aq). If the analysis
+
–
+
is of high quality, the number of moles of Na multiplied by the charge on the sodium atom (i.e., 1.0) computed
from the reported concentrations should be within a few percentage of the computed number of moles of Cl mul-
–
tiplied by the charge on the chlorine atom (i.e., –1.0). If the difference is greater than 10%, the analysis should be
considered inadequate and not used. For more complex solutions, the total positive and negative charges (computed