PARTS PER MILLION (PPM)
analysis involves the detection of minute amounts of a variety of substances.
The expression of results in percentage would require the use of cumbersome figures.
For this reason, the results of a water analysis are usually expressed in parts
per million (ppm) instead of percentage. One part per million equals one ten-thousandth
of one percent (0.0001%), or one part (by weight) in a million parts-for example,
1 oz in 1,000,000 oz of water, or 1 lb in 1,000,000 lb of water. It makes no difference
what units are used as long as both weights are expressed in the same units.
elements are present in minute or trace quantities, the use of parts per million
results in small decimal values. Therefore, it is more convenient to use parts
per billion (ppb) in these cases. One part per billion is equal to one-thousandth
of one part per million (0.001 ppm). For example, in studies of steam purity using
a specific ion electrode to measure sodium content, values as low as 0.001 ppm
are not uncommon. This is more conveniently reported as 1.0 ppb.
times, the convention for reporting analytical results has been shifting toward
the use of milligrams per liter (mg/L) as a replacement for parts per million
and micrograms per liter (µg/L) as a replacement for parts per billion.
procedures and calculations of results are based on the milliliter (mL) rather
than the more common cubic centimeter (cc or cm3). The distinction
between the two terms is very slight. By definition, a milliliter is the volume
occupied by 1 g of water at 4°C, whereas a cubic centimeter is the volume
enclosed within a cube 1 cm on each edge (1 mL = 1.000028 cm3).
PER LITER (mg/L)
The milligrams per liter (mg/L) convention
is closely related to parts per million (ppm). This relationship is given by:
x solution density = mg/L
Thus, if the solution density is close or equal
to 1, then ppm = mg/L. This is normally the case in dilute, aqueous solutions
of the type typically found in industrial water systems. Control testing is usually
conducted without measurement of a solution's density. For common water samples,
this poses no great inaccuracy, because the density of the sample is approximately
1. Milligrams per liter (mg/L) and parts per million (ppm) begin to diverge as
the solution density varies from 1. Examples of this are a dense sludge from a
clarifier underflow (density greater than 1) or closed cooling system water with
high concentrations of organic compounds (density less than 1). All of the analytical
methods discussed in this text contain calculations required to obtain the results
in milligrams or micrograms per liter.
PER MILLION (EPM)
In reporting water analyses on an ion basis,
results are also expressed in equivalents per million (epm). Closely allied to
the use of parts per million, this approach reduces all constituents to a common
denominator-the chemical equivalent weight.
The use of equivalents per million
is not recommended for normal plant control. Parts per million is a simpler form
of expressing results and is accepted as the common standard basis of reporting
a water analysis. However, whenever extensive calculations must be performed,
the use of equivalents per million greatly simplifies the mathematics, because
all constituents are on a chemical equivalent weight basis. The remainder of this
section provides a discussion of parts per million and equivalents per million
for those who desire a working knowledge of these methods of expression for purposes
The units of ppm and epm are commonly combined in normal
reporting of water analyses, and many different constituents are frequently reported
on a common unit weight basis. For example, calcium (equivalent weight 20.0) is
reported in terms of "calcium as CaCO3"
(equivalent weight 50.0). The test for calcium is calibrated in terms of CaCO3,
so the conversion factor 2.5 (50/20) is not needed. Hardness, magnesium, alkalinity,
and free mineral acid are often reported in terms of CaCO3;
the value reported is the weight of CaCO3 that
is chemically equivalent to the amount of material present. Among these substances,
ionic balances may be calculated. When constituents are of the same unit weight
basis, they can be added or subtracted directly. For example, ppm total hardness
as CaCO3 minus ppm calcium as CaCO3
equals ppm magnesium as CaCO3. However, ppm
magnesium as Mg2+ equals 12.2 (magnesium equivalent weight) divided
by 50.0 (CaCO3 equivalent weight) times the
ppm magnesium as CaCO3.
In every case,
it is necessary to define the unit weight basis of the results-"ppm alkalinity
as CaCO3" or "ppm sulfate as SO42-
" or "ppm silica as SiO2". Where
the unit weight basis is different, calculations must be based on the use of chemical
The following rules outline where epm can be used and where ppm
must be used. In general, either may be used where an exact chemical formula is
known. When such knowledge is lacking, ppm must be used.
- The concentration
of all dissolved salts of the individually determined ions must be in ppm.
or more ions of similar properties whose joint effect is measured by a single
determination (e.g., total hardness, acidity, or alkalinity) may be reported in
either ppm or epm.
- The concentration of undissolved or suspended solids
should be reported in ppm only.
- The concentration of organic matter should
be reported in ppm only.
- The concentration of dissolved solids (by evaporation)
should be expressed as ppm only.
- Total dissolved solids by calculation
may be expressed in either ppm or epm.
- Concentration of individual gases
dissolved in water should be reported in ppm. The total concentration of each
gas when combined in water may be calculated to its respective ionic concentration
in either ppm or epm.
OF TOTAL DISSOLVED SOLIDS BY EPM
Starting with a reasonably
complete water analysis, total dissolved solids may be calculated as epm. In a
complete water analysis, the negative ion epm should equal the positive ion epm.
Where there is an excess of negative ion epm, the remaining positive ion epm is
likely to be sodium or potassium (or both). For the sake of convenience, it is
generally assumed to be sodium. Where there is an excess of positive epm, the
remaining negative epm usually is assumed to be nitrate.
To calculate dissolved
solids, convert the various constituents from ppm to epm and total the various
cations (positively charged ions) and anions (negative ions). The cations should
equal the anions. If not, add either sodium (plus) or nitrate (minus) ions to
balance the columns. Convert each component ionic epm to ppm and total to obtain
ppm dissolved solids. For example, to
convert 150 ppm calcium as CaCO3 to epm
(Table 40-1) divide by 50 (the equivalent weight of calcium carbonate) and obtain
3.0 epm. To convert 96 ppm sulfate as SO42-
to epm, divide by 48 (the equivalent weight of sulfate) and obtain 2.0 epm. After
balancing the cations and anions by adding sodium, convert to ionic ppm by multiplying
the epm by the particular ionic equivalent of weight. For example, to convert
3.0 epm calcium to ppm calcium as Ca2+, multiply by 20 (the equivalent
weight of calcium) and obtain 60 ppm calcium as Ca2+. To obtain the
ppm dissolved solids, total the ppm of the individual ions.