Alcalinidad y ph
2H2O = H3O+ + OH-
or
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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