Use of an Ion-Selective Electrode for Determination of Free Chloride Ions in Water-Based Drilling Fluids
- F. Kaland (U. Hakli) | P. Ruoff (Rogaland Regional College)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- October 1986
- Document Type
- Journal Paper
- 365 - 368
- 1986. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 1.12.3 Mud logging / Surface Measurements, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 6.5.2 Water use, produced water discharge and disposal, 6.2.4 Industrial Hygiene
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The use of an ion-selective electrode (ISE) for the determination of chloride ions in water-based drilling fluids has several advantages: the measurement is simple, fast, easy to perform, and inexpensive. An ISE can be used for continuous chloride ion logging and allows analysis of colored samples where the traditional API-recommended method is difficult to use.
The determination of the chloride ion concentration in drilling fluids is very important. For example, the effect of sodium chloride on a water-based bentonite mud can be rather dramatic, as shown in Fig. 1. The increase of viscosity in Region A is generally believed to be caused by flocculation of the bentonite particles, while the decrease of viscosity in Region B is a result of aggregation of the clay.1
To determine the amount of chloride ion in a waterbased drilling fluid, API recommends titration of the filtrate with a standardized silver nitrate solution that uses chromate ions as an indication of the equivalence point.2 This method, also called the "Mohr method"3 after its inventor, requires pH adjustment. If the pH is too low, dichromate ions are formed and do not react with silver ions, making the detection of the endpoint impossible. On the other hand, when the pH of the sample is too high, silver oxide is preferentially formed instead of silver chloride. To avoid these problems, phenolphthalein, acid, and calcium carbonate are normally used to adjust the pH to about 8.3. In addition, serious endpoint-detection problems can arise when the sample is miscolored. An alternative titration method, which avoids the problems of the chromate indicator, is the use of silver or chloride ISE's working as indicator electrodes for the determination of the endpoint.3
We were looking for a method, however, that is easy and quick to. perform, applies to miscolored samples, is relatively independent of pH, is inexpensive, and can be used as a continuous mud logging device for chloride ions. It appears to us that a chloride ISE alone fulfills all these requirements in a satisfactory way.
Method and Apparatus
The use of ISE's has become an important tool in quantitative chemical analysis in many fields of industrial and laboratory applications - e.g., air and water pollution; toxicology; industrial hygiene; analysis of soil, water, and pharmaceuticals; and other industrial purposes.4 Thus we found it natural to apply a chloride ISE for the determination of chloride ions in water-based drilling muds.
The chloride ISE consists of a membrane made out of a matrix of silver chloride and silver sulfide. When the membrane is in contact with a solution containing chloride ions, the electrode develops a potential because of the chloride ion concentration. The potential is measured against a constant reference potential with a digital-volt meter or a specific ion meter.5 The measured potential corresponds to the chloride ion level in the solution and is described by the Nernst equation4:
where Epr is a constant and m is about 59 mV per log unit at 20°C [68°F].
The chloride ion activity, Ce, is related to the chloride ion concentration of free chloride ions, Cf, by the ionic activity coefficient, ?:
Ionic activity coefficients are variable and largely dependent on total ionic strength. Ionic strength is defined as
If the background ionic strength is high and constant relative to the sensed ion concentration, the activity coefficient is constant and the chloride ion activity is directly proportional to the concentration. An ionic-strength adjuster should be added to all samples so that the background ionic strength is high and constant relative to variable concentrations of chloride. We used NaNO3 as the ionic-strength adjuster, but other solutions can be used as long as they do not contain ions that would interfere with the electrode's response to the chloride ion.6
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