BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Table 3-3
Formula and Valency of Common Ions formed From Groups of Atoms
Name of Ion Formula Valency
Ammonium
+
NH 4 1
Nitrate
+
NO 3 1
Hydroxide OH¯ 1
Hydrogen Carbonate
HCO 3¯
1
Hydrogen Sulfate HSO 4¯ 1
Hydrogen Sulfite HSO 3¯ 1
Dihydrogen Phosphate H 2 PO 4¯ 1
Sulfate SO 42¯ 2
Carbonate CO 32¯ 2
Sulfite SO 32¯ 2
Hydrogen Phosphate HPO 42¯ 2
Phosphate PO 43¯ 3
Some elements, particularly nitrogen, sulfur, and iron, can show more than one valency, as there
is more than one way for them to obtain a stable electron configuration. The valency is written in
brackets when describing the compound. Thus iron (II) chloride is FeCl 2 , iron (III) chloride is
FeCl 3 .
The concept of valency also applies to groups of atoms which carry a charge, such as the sulfate
group SO 2⎯ 4 , which has valency 2. The valency in this case equals the number of electrons which
have already been transferred, and thus equals the magnitude of the charge on the ion. Table 3-3
gives the commonly occurring ions of this type.
Determination of Bond Type
Ionic and covalent bonding can both be present in the same compound. Figure 3-4 shows the
structure of sodium carbonate. The two sodium ions and the carbonate ion are held together by
ionic bonding, but the carbonate ion itself contains three covalent carbon-oxygen bonds.
Figure 3-5 Sodium Carbonate Structure (schematic)
In this example both types of bonding are present and are easily distinguishable. However, this is
not always the case. Many ionic bonds have a certain amount of covalent character. This can be
thought of as an incomplete transfer of the electrons involved so that the atom that has lost them
still retains a small share. An example of this type of bonding would be zinc bromide, ZnBr 2 ,
which is used to prepare high density completion fluids. This compound can nominally be
written as Zn 2+ Br⎯ 2 , but in fact the bond has considerable covalent character in that the zinc
retains partial control over the two electrons which it has lost.
Conversely, many covalent bonds show varying degrees of ionic character. Some atoms are
greedy for electrons and when they enter into a covalent bond they try to take more than their fair
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-9