BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
The reaction can proceed both ways and is said to be reversible. The reaction proceeds more
easily from the right to left than from left to right.
In one liter of pure water, there will be 10 –7 (0.0000001) moles of hydrogen ions (10 –7 grams) and
10 –7 moles of hydroxide ions (17 x 10 –7 grams, as the molecular weight is 17). The concentration
of H + ions, written as {H + }, is thus 10 –7 moles/liter and similarly {OH ⎯ } = 10 –7 moles/liter and
{H + }·{OH ⎯ } = 10 –7·10 –7 = 10 –14 moles 2 /l 2 .
The pH of a solution is defined as the negative log to the base of 10 of the hydrogen ion
concentration. Thus pH = –log 10 {H + } where {H + } is in moles/liter. For pure water as above,
with {H + } = 10 –7 moles/liter:
pH = – log 10 {10 –7 } = 7 (since log 10 {10 –7 = –7}
If an acid is added, the number of hydrogen ions in the solution increases such that the product of
{H + }·{OH ⎯ } remains at 10 –14 (as long as the temperature remains constant).
Thus if hydrochloric acid is added until {H + } = 10 –3 then pH = 3 and {OH – } = 10 –11 since 10 –3 x
10 –11 = 10 –14 . The addition of an alkali to water increases {OH – }. Thus, {H + } drops and the pH
rises above 7. If caustic soda is added to 10 pH, then {H + } = 10 –10 and {OH – } = 10 –4 .
Acids always lower pH, alkalis always raise pH. A strictly neutral solution, with no acid or alkali
present, has pH 7. Acid solutions have pH less than 7; alkaline solutions have pH greater than 7.
This is illustrated below.
Table 3-7 The pH Scale
ACIDIC NEUTRAL ALKALINE
pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
[H + ]m/l 1 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 10 -11 10 -12 10 -13 10 -14
[OH ⎯⎯ ]m/l 10 -14 10 -13 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1
The pH scale, running from 0 – 14, is adequate for the majority of situations, although some acids
and alkalis, if they are very soluble, can produce pH’s outside this range. Drilling fluids are run
at between pH 6 and pH 13.5, with the majority being moderately alkaline (pH 9 – 10.5).
It is important to realize that pH is a logarithmic function. In pure water it takes ten times as
much caustic soda to increase pH from 11 → 12, as from 10 → 11, as {OH – } must be increased
by a factor of ten.
Solutions at pH 6 – 8 are generally termed “neutral”, as they are so close to pH 7 as to make very
little difference in properties.
Strong and Weak Acids and Alkalis
Strong acids are completely ionized in water such that all the molecules split up into ions. Acids
of this type include hydrochloric and sulfuric acids.
Weak acids are not fully ionized in concentrated solutions. The acid molecules are dispersed
throughout the water, but only a small proportion of them split up into ions. Thus, at the same
concentration, weak acids, such as acetic or carbonic acids, give a higher pH than strong acids, as
there are fewer H + ions in the water.
Similarly, strong alkalis, such as sodium and potassium hydroxides (the hydroxides of strongly
electropositive metals) are fully ionized in solution and give higher pH’s than weak alkalis, such
as calcium hydroxide or ammonia solution. In the case of calcium hydroxide, molecular
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-19