The difficulties inherent in the use of amalgam electrodes for the estimation of alkali and alkaline-earth cations have greatly restricted our knowledge of the electrochemistry of many negatively charged colloidal systems. Using the recently described membrane electrode (8), which consists of a film of electrodialyzed bentonite dried at 490°C., we are now able to make measurements over a range of potassium-ion activities from /10 to about ¿v/ , with reasonable accuracy. The application of this technique to agar and to four clays is here described.The exact significance of activity measurements in colloidal systems has frequently been discussed. Pauli and Matula (11) introduced the concept that potentiometrically determined activities represent the mean activity of the ions in the electrical double layer surrounding the particles and in the intermicellar liquids. Wiegner (13), in discussing the "suspension effect", suggested that it was even possible for ions in the inner part of the double layer to affect an electrode. Rabinovitch and Kargin (12) maintained that both of these views were erroneous on the grounds that for any system in equilibrium the activity of all its parts must be equal; hence the intermicellar liquid should have the same activity as the interior of the electrical double layer. However, thermodynamically, it is not possible to envisage a separation of the intermicellar liquid without work being done on or by the system, nor can one expect thermodynamic arguments to throw light on the constitution of the electrical double layer.Rabinovitch and Kargin also drew attention to three practical difficulties which might lead to error: namely, the adsorption of foreign electrolytes, electrode poisoning, and the possibility of insoluble complexes being formed between the ion under investigation and the colloidal surface. It is believed that the determinations reported below are not appreciably affected by these sources of error. II. experimentalThe apparatus (7) and technique (8) used in obtaining potassium-ion activities have been fully described. In the present instance a 0.1059 molal potassium
The experiments described in papers I and II of this series (1, 2) have demonstrated how potassium-ion activities, below 0.1 molal, may be determined both in true solutions and in suitable colloidal systems. In view of the close similarity 1 Paper No. 768, Journal Series, Department of Soils, Missouri Agricultural Experiment Station.2 The authors are indebted to the Research Council of the University of Missouri for a grant-in-aid which has made this work possible. 3 In the papers of this series such terms as "potassium-ion activity" and "ammonium-ion activity" are frequently employed. This does not imply that we are dealing with an independent method of determining the activities of cations alone. This cannot be done without making non-thermodynamic assumptions. The methods used have the same validity as pH measurements and the cationic activities referred to have the same meaning as hydrogen-ion activities calculated from such measurements.4 Paper IV begins on page 327.
Many of the aqueous drilling muds used in the field are prepared with bentonite and treated with sodium hydroxide and quebracho. In view of limitations of published literature on such systems, extensive tests were conducted to show the effects, singly and in combination with each other, of sodium hydroxide and quebracho in a 4.8% SUBpension of bentonite. In addition, this study summarizes the eKect of sodium chloride, which is frequently encountered as a contaminant, and of sodium carboxymethylcellulose, which is used to achieve low water-loss values and to facilitate control of the physical properties.QUEOUS drilling fluids for field operations are generally pre-A pared by the addition of clays to water. Of the clays used in preparing muds, bentonite is the most common. This clay is also introduced into the mud during penetration of certain shales. Because of its highly colloidal nature, it is of particular value in producing systems having a low water-loss value. To achieve water-loss values even lower than those that may be obtained with clay suspensions, various additives, usually organic materials of high molecular weight (gums, starches, cellulose, and their modifications), have been employed.In addition to treatment for control of water loss, a treatment is necessary for control of the rheological properties-viscosity and gel strengths. To control these properties, several alternatives are available. One, dilution with water, is applied particularly when the solids content of the mud is high. This procedure is not universally applicable, because the dilution reduces the density of the mud and may involve expenditure of an excessive amount of weighting agent. A second alternative is to treat the mud with the so-called "viscosity reducers." The chemicals most frequently added for this purpose include sodium silicate, phosphates, quebracho (tannates), and products related to the tannates.In the treatment of aqueous drilling fluids the effect of contamination due to electrolytes which may be introduced during drilling must be known. Of these sodium chloride, calcium sulfate, and cement are the most common: Despite the numerous problems pertaining to these contaminants, a survey of the literature revealed that much of the information is limited or general rather than specific in nature. This may in part be attributed to the fact that each operation involves a mud that is slightly different from that involved in another operation; and the problems with each system are different. However, as a result of various studies in this laboratory, it appeared highly desirable to obtain extensive information on the most common treatments which are used to control the physical propertieswater loss, viscosity, and gel strengths-and to study the effect of electrolytes on these treatments. In order to utilize the results of this study most fully, the experiments were conducted with a simple system-a 4.8% suspension of bentonite-because this clay is usually present in field muds. This report covers that part of the study relati...
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