pH AND ALKALINITY CONTROL Introduction The importance of obtaining correct pH measurements has greater emphasis today than at any other time. Its measurement is necessary both in the field and the laboratory as pH directly affects the functioning of drilling fluid additives. pH pH is the measurement of the relative degree of acidity or alkalinity of an aqueous solution. The numerical reading ofthe pH scale ranges from 1 (acid) to 14 (base), with 7 being the neutral point. It is known that the value of pH in the ranges stated are directly related to the effective or active acid concentration of a solution by the negative logarithm of the hydrogen ion concentration. The equation may be written as follows: pH = - log [H+] The [H+] in the equation represents the hydrogen ions in thesolution, and can be referred to as the strength of the hydrogen ion in solution. If we have a strong acid of 0.01 molar, the pH is equal to 2 because the hydrogen ion concentration is as follows: H+ is 10-2 molar pH = - log [10 -2] pH = 2 The pH scale is based on the dissociation constant of water. In distilled water, there are a + few molecules that react with one another to form hydronium ions (H3O ).This accounts for the acidic properties. The base properties are given by hydroxyl (OH-) ions of the solution. The following reaction is how they focus:
2H2O = H3O+ + OH-
H2O = H+ + OH-
Distilled water, at 25° C, will dissociate until the acid (H+) and the base (OH-) concentrations are equal. So the neutral pH value is as follows: 1 x 10–7 molar The product of both concentrations isthe equilibrium constant for water. It is referred to as the dissociation constant Kw: KW = (H+) (OH-) Kw = [1 x 10–7] [1 x10-7] –14 Kw = [1 x 10 ] In this equation, the hydrogen (H+) ion concentration equals 1 x 10-7, thus the pH is 7. This is referred to as the neutral point. In the continuous phase of drilling fluids, water is the primary aqueous solution. Drilling fluids allow us theflexibility of an increase in either the acid or base concentration, and
an increase in either concentration always results in a decrease in the other concentration. To demonstrate how a strong base material will effect the hydroxyl ion concentration, a solution of strong caustic soda (0.01 NaOH) will be used in the example below:
Kw 10 -14 = = 10 -12 -2 [OH] 10 –12 pH = -Log [10 ] pH = 12 H+ = ThepH chart in Figure No. 1 clearly shows the relationship of pH value to the activity of base/acid solutions. It must also be remembered that: All concentrations are expressed in moles/liter (H+) x (OH-) = Kw = 1 x 10-14 moles/liter Kw = Equilibrium constant for water As (H+) decreases, (OH-) must increase if the product is to remain constant (1 x 10-14 moles/liter) = Kw (H+) moles/liter 1 x 10-0 1x 10-1 1 x 10-2 1 x 10-3 1 x 10-4 1 x 10-5 1 x 10-6 1 x 10-7 1 x 10-8 1 x 10-9 1 x 10-10 1 x 10-11 1 x 10-12 1 x 10-13 1 x 10-14 Figure 1 API recommends two methods for measuring the pH of drilling fluids. They are: • Colorimetric method - paper strips impregnated with indicator dye. • accurate to only 0.5 pH unit • improper field storage, temperature and humidity affects dye • high saltconcentrations affect readings (10,000 mg/L or more) • dark filtrates affect recognition pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 (OH-) moles/liter 1 x 10-14 1 x 10-13 1 x 10-12 1 x 10-11 1 x 10-10 1 x 10-9 1 x 10-8 1 x 10-7 1 x 10-6 1 x 10-5 1 x 10-4 1 x 10-3 1 x 10-2 1 x 10-1 1 x 10-0
Very High concentrati on ACID
Very Low Concentrati on ACID
Very Low Concentrati on BASIC
Very High Concentration BASIC
Electrometric method - pH meter with a glass electrode • accurate to within ± 0.02 pH unit • can be standardized by calibrating with buffer solutions • temperature compensated • pH predicted by low voltage, which is very accurate • poor accuracy with saturated NaCl and KCl solutions
E = Eo 2.303 (RT) pH F
where: E = measured voltage Eo = total constant voltage in the...
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