Page 171 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.7

The results of the analysis can be expressed in many ways. A common method is to report
the concentration of ions in solution by the weight of an element or compound per liter of water,
expressed as milligrams per liter of water (mg/L). Another method is expressed in parts per mil-
lion (ppm), which can be expressed either by the weight of an impurity compared to the weight
of water, abbreviated w/w (weight to weight), or by the volume of the impurity compared to the
volume of water, abbreviated v/v. Other units are also used, such as grains per gallon (gpg) and
equivalents per million (epm). mg/L differs from ppm in expressing a proportion in weight per
volume. This finds specific use in analysis of saline waters. Where the specific gravity of a liquid
is around 1, mg/L and ppm are equal. Grains per gallon (gpg) is a term used in the discussion of
ion-exchange equipment capabilities, where 1 gpg = 17.1 ppm.
As previously explained, compounds break down into ions when dissolved. Although
chemists can measure the amounts of each ion present in a sample, it is not practical to find
the total amount of each compound that actually went into solution. In practice, the actual
method of analysis measures ions only. Using the ionic form in measurement when report-
ing impurities makes it easier to interpret the results.
To further simplify reporting, it is desirable to reduce all ions present in solution to a
common denominator. The common denominator is calcium carbonate. This is accom-
plished by comparing the equivalent weight of all ions present and expressing them as the
ppm anion and cation equivalent of calcium carbonate. The main reason for this is that the
molecular weight of calcium carbonate is 100 and its equivalent weight is 40. This method
of expression is a widely accepted, but not universal, standard for reporting water analysis.
Table 4.2 presents the conversion factors used for major impurities. Figure 4.1 illustrates a
typical water analysis report indicating impurities in ppm, equivalents useful in calculating
reacting chemicals, and a comparison of positive and negative ions.

pH
When alkalines (bases) are mixed in water, hydroxyl ions result. In a mixture of acid and
water, hydrogen ions result. pH is a measurement of the hydrogen-ion concentration of a
solution. Since the balance of hydroxyl (cation) and hydrogen ions (anion) must be con-
stant, changes in one ion concentration produces a corresponding change in the other. The
pH value is calculated from the logarithmic reciprocal of the hydrogen-ion concentration
in water. The pH scale ranges from 0 to 14, with 0 being acid and 14 being alkaline. 7.0 is
neutral. A change of one unit represents a tenfold increase (or decrease) in strength. pH is
not a measure of alkalinity.

TABLE 4.2 Converting ppm of Impurities to ppm of Calcium Carbonate

Cations Ionic ppm multiplier Anions Ionic ppm multiplier
Hydrogen 50.00 Hydroxide 2.94
Ammonium ` 2.78 Chloride 1.41
Sodium 2.18 Bicarbonate 0.82
Potassium 1.28 Nitrate 0.81
Magnesium 4.10 Bisulfate 0.52
Calcium 2.50 Carbonate 1.67
Ferrous 1.79 Sulfate 1.04
Ferric 2.69
Cupric 1.57 Other
Zinc 1.53
Aluminum 5.55 Carbon dioxide 2.27
Chromic 2.89 Silica 1.67

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