Portable Meter for Determining Dissolved Oxygen for Use with Oil-Field Brines

Abstract

The standard Winkler method for determining dissalved oxygen yields unreliable results with many oil field brines because of the presence of high concentrations of magnesium, calcium, and ferrous salts. A method was desired which would p e r m i t rapid and convenient determination of dissolved oxygen in the field, principally to cheek the operation of brinetreating plants. A simple portable polarographio < NSITIVE chemical method for determining dissolved A"" oxygen was given by Winkler in 1888 (5). With modifications, this method will work in most aqueous solutions, but is unreliable when appreciable amounts of iron, calcium, or magnesium salts are present, as in many ail-field brines. A directreading method w a desired which would approach the socursey of the classical method and which would operate satisfactorily in these naturally ooourring brines. The instrument as described provides this operation with an accuracy of nbont +5%, and a t the same time is oampletely portable.The most practical solution to the problem of designing a directreading instrument for determining dissolved oxygen in brine appeared to be the polarographic method. The method in this case consists of the measurement of diffusion current as dissolved oxygen is reduced at a platinum or meroury cathode. Several studies (2, S), indicated that of the commonly used cathodes, the ratsting platinum microelectrode would yield the highest ratio of change in diffusion current to change in dissolved oxygen concentration. Experimentation led to the development of an apparatus which is capable of making a dissolved oxygen determination in less than 1 minute Kith the desired accuracy.The theory of dissolved oxygen analysis with the rotating platinum microelectrode is similar to that of oonventianal dropping mercury polarography. As a small electrode of platinum is rotated through a solution, a negative potential is applied to it from an external source. The negative potential is referred to a nonpoladzing reference electrode--o.g., a saturated calomel halfcell-which is also in cont.act with the solution. Neutral oxygen molecules are reduced a t a negative potential between about -0.4 and -0.8 volt, while other reducible ions likely to be encountered in brine, such BS ferrous, ferric, odoium, and magnesium, are reduced a t more negative potentials and have been found experimentally not to interfere over the range of pH from 5.0 to 9.0.The applied potential used in the original apparatus is -0.73 volt us. saturated cdomel. The change in diffusion current with ohange in dissolved oxygen concentration was found to be approximately linear for any potential in the range of from about -0.50 to -0.75 volt. Also diffusion current is practically a linear funation of applied potential over this range. It would appear that any potential more negative than about -0.45 volt could be used, but beyond -0.8 volt there is increasing probahility of interference; hence a potential of around -0.7 volt was chosen as about the optimum. APPARATUSThe apparatus described here ...

Water Analysis

“…Other studies are reported by Skopintsev and Mikhailovskaya (121), Malchow-Mpller (76), Van Meter, Gerke, and Buswell (14, Í28), and Forges and coworkers (100).…”

“…A portable meter for determining dissolved oxygen is discussed by-March (76). Automatic equipment for recording residual chlorine in water is described by Hazey (38).…”

“…March (76) describes in detail, sufficient to permit reproduction from data given, a portable, polarographic dissolved oxygen meter using a rotating platinum electrode. The instrument was designed for natural brines and the author reports that to date attempts to use the instrument with natural waters by adding sodium chloride in sufficient amount to raise the ionic content to brine values have not given satisfactory' results.…”

Instrumentation in the Laboratory

“…These include the arsenic and selenium groups and the volatile toxic metals, cadmium, lead, mercury, thallium, and zinc. The greatest use of this technique, however, is for the determination of oxygen (19,20). It is definitely superior to the Winkler method, being unaffected by most interfering substances.…”

Reaction between Ferrous Iron and Dissolved Oxygen in Brine

“…Dissolved oxygen determinations were made with a polarographic dissolved oxygen meter (17). determination of the rate of reaction between ferrous iron and oxygen has been obscured by possible changes in rate of solution of oxygen and changes in solubility of oxygen.…”