atoms consist of three types of particles. Protons
and Neutrons form the Nucleus of the atom. Protons have an effective positive charge of 1. Thus, for example,
if there were 3 protons in the atom, the overall charge of the nucleus would be +3. Electrons orbit the nucleus in layers or "Shells" and have an effective negative charge of 1. For every naturally
occurring element, the number of electrons in the atom equals the number of protons and the effective charges of each particle
are canceled out. The elements can be tabulated in order of increasing number of protons, or Atomic Number. This is known as the Periodic Table. As the number of electrons also progressively increase,
different shells of electrons are filled, the ones closest to the nucleus being filled first. These shells can be thought
of as the orbits of planets around the sun, being of increasing radius from the center.
are formed when electrons are lost or gained from the atom when an energy change is applied. For example, a sodium atom has
only one electron in its outer shell, 2 other inner shells being completely full. This electron is easily lost to reveal the
more chemically stable full shell. Since a negative charge has been lost from the atom there is now an overall charge of +1,
which indicates for formation of an ion, I.E., Na+. Similarly, the chlorine atom has an outer shell of 7 electrons
and would require 1 more to complete this shell. Therefore, an additional electron is accepted which provides a full outer
shell of electrons, and an overall charge of -1, i.e., Cl-. Generally, up to a total of three electrons can be
transferred in this way forming Monovalent,
Divalent and Trivalent ions. Positively charged ions are
known as Cations and negatively charged
ions are known as Anions. Therefore,
Al3+ is a trivalent cation and S2- is a divalent ion. Three monovalent anions
would be required with one trivalent cation to produce a compound where the charges are balanced.
to the obvious attractions of cations and anions many Ionic
Compounds exist in which the overall charge of the Molecule is zero. When these compounds are dissolved in water, however, the individual ions tend to
drift apart (disassociate), producing a conducive solution of Electrolyte.
examples of disassociation:
Hydrochloric Acid: HCl → H+ + Cl-
Sodium Hydroxide: NaOH → Na+ + OH-
Ammonium Chloride: NH4Cl → NH4+ + Cl-
Sodium Carbonate: Na2CO3 → Na+ + Na+ + CO32-
such as OH-, NH4+ and CO32- will not dissociate further in aqueous solutions. So,
Na2CO3 consists of two monovalent sodium ions and one divalent carbonate ion.
units to express weights and concentrations can be used in chemistry. Indeed, different units exist which can refer to the
same measurement. The examples given below are applicable to solutions only:
1 part per million = 1
ppm = 1 mg/l = 1 mgl-1 = 1mg/kg = 1 mgkg-1
1 part per billion = 1
ppb = 1 µg/l = 1µgl-1
= 1 µg/kg = 1 µgkg-1
Furthermore, in water chemistry,
parts per million can be expressed as:
ppm = amount dissolved
in 1 liter in grams x 1000
To illustrate the tiny
quantities involved, 1 part per million can be thought of as one second occurring in every 11 ½ days and one part per billion
as 1 second in every 31 years and 8 months!
For higher concentrations,
the units can be expressed in parts per thousands, or percentages by weight or volume:
0.1% = 1 ppth = 1 g/l =
Mole is a convenient unit which is
used to compare different chemical elements and compounds. It is based on the Relative Atomic Mass of each atom which is different for each element. The mass of any atom is made up essentially
from Protons and Neutrons with each of these particles having an effective mass of 1. The weight of an electron is negligible.
1 mole of any substance, therefore, is simply its atomic or molecular mass in grams, i.e., atomic or molecular weight.
A. 1 mole of Sodium would be 23 g (Na
has 11 protons and 12 neutrons)
B. 1 mole of Potassium Chloride would
be 74 g (the atomic weight of K is 39 and relative weight of Cl is 35)
C. 46g of Sodium would be equivalent
to 2 moles of Na
Molarity of solutions is determined
by the weight of solid equivalent in moles dissolved in 1 liter of solution.
A. Molar Potassium Chloride solution would be 74g of
solid KCl dissolved in 1 liter of solution.
B. 0.1M KCl would be 7.4 g dissolved in 1 liter of solution.
analyzers may be scaled in measurement units directly by the probes or system, e.g. measuring Ammonia (NH3). They
may also be scaled in terms of an associate element or compound, e.g. Ammonia may be expressed in terms of Nitrogen (N), or
Ammonium (NH4+). The choice of unit depends on the application or industry.
has a total molecular mass of 17. Nitrogen has an atomic mass of 14. Therefore, every 17g of NH3 contains 14 g
of N or a ratio of 1.21:1 NH3 to N. Therefore, for an Ammonia (NH3) instrument scaled 1-100 ppm as N,
the probe would actually be detecting 1.2 – 120 ppm NH3.Similarly, Phosphate (PO43-)
analyzers can also be scaled as phosphorous, (P). The ratio of PO43- to P is 95/31 or 3.06:1.
a nitrate (NO3-) the ratio of NO3- to N is 4.42:1.
concentrations of ions in solution can be arranged in ascending order or scale, the most widely known scale being pH which
is used to express the hydrogen ion concentrations in terms of acidity and alkalinity. In this case the ‘p” in
pH comes from the German word ‘Potenz’ meaning power and, ‘H’ from the concentration of Hydrogen ions
in solution. Similarly, other p scales exist for other ions. For example, pOH, pNa, pCl, etc.
relationship between a p-scale and the concentration of a specific ion is the negative logarithm of the concentration of the
pION = -log10[ION] where [ION] is the concentration
of that ion.
A. 2 pH is equivalent to 1 x 10-2 grams of hydrogen ions
in a liter of solution.
B. 4 pNa is equivalent to 1 x 10-4 grams of sodium in
a liter of solution.