FOR DIRECT IMMERSION INTO TANKS (or EXPOSED STREAMS)
FOR ATMOSPHERIC PRESSURE
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The old gives way to the new
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ROBERT H. JONES and THOMAS J. KEHOE Application Engineering Department, Beckman Instruments, Inc., Fullerton, Calif.
A New Continuous Chloride Ion Analyzer Quality control applications Monitoring industrial wastes 0
Easy to standardize
A continuous chloride ion analyzer has been developed, similar to that NEW
for pH measurement. The sensing element is a special electrode coupled to a conventional reference electrode which, in its response, approximates the Nernst equation. This analyzer can be used in industrial processes where measurement of chloride ion in aqueous solution is important, for monitoring waste streams to prevent contamination of rivers, measuring salt in processed foods, and detecting leaks in boiler plants where salt water is used for heat exchange. The general method of measurement is not new. However, publications refer only to laboratory analysis without concern for continuous measurement
Literature Background Subject Silver-silver chloride electrodes prepared by various methods are reproducible for use as reference electrodes Potentiometric titrations for chloride use silver-silver chloride electrode as sensing element Direct potentiometric measurement determines chloride ion in chlorinated insecticide residues Structurally sound billet of silversilver chloride is formed under high pressure
Ref.
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(2)
required in industrial applications. Basically, the method relies on a new silver-silver chloride sensing electrode plus a reference electrode and thermocompensator. The potential developed at the sensing electrode depends directly on the chloride ions in the solution in which electrodes are immersed. The potential is amplified and indicated by an amplifier similar to the type used for pH measurement.
(3)
Description of Analyzer
(4)
Various types of silver-silver chloride electrodes for laboratory analysis have been described (7). Because of possible
Subject to complexion formation 4 ionic activitv
0 AMPLIFIER
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Within c h l o r i d e ion concentrations from a few to 10,000 p.p.m., Special precautionsreqd for the analyzer adequate measurement. registers one 0.1 decade for each 0 60 100 I60 200 260 300 350 59 mv. of potenMILLIVOLT POTENTIAL tia I
A silver-silver chloride electrode is well suited for continuous chloride measurement VOL. 51, NO. 6
JUNE 1959
731
difficulties in using these electrodes to measure chloride concentration continuously, it became apparent that a new type of electrode was required. T o serve all applications, such an electrode must have little, if any, memory effect, must withstand rugged treatment and abrasion, and be only slightly influenced by solubility relationships. Watanabe ( 4 ) pressed a mixture of silver and silver chloride into a hard solid pellet. This, when formed into the body of a conventional electrode, is ideally suited for use in continuous chloride ion measurement.
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50
100 150 200 230 300 350 MILLIVOLT POTENTIAL
The potential generated depends on temperature
Factors Influencing Measurement Temperature. From the Nernst equation it can be seen that the generated potential depends on temperature. The linearity of potential is affected at eievated temperatures in the low parts per million ranges because solubility product relationships vary with temperature. With the thermocompensator provided, the effects of the temperature coefficient are essentially eliminated. Extraneous Potentials. The measurement of chloride ion is essentially an oxidation-reduction potential measurement. Thus, other oxidants or reductants in the solution being measured can, if in sufficient concentration, introduce a secondary potential. Applications where chloride ion is to be measured normally do not include such adverse constituents. The close proximity of metals normally cannot be tolerated at the electrodes. Thus, metallic electrode flow chambers must be avoided. The presence of dissimilar metals separated by the measuring electrodes can produce highly erroneous results, presumably because of a galvanic cell reaction. Other Factors, The surface of the sensing electrode will exhibit a certain amount of response delay when, for example, a 10-p.p.m. chloride stream is measured after being exposed to a 10,000p,p.m. chloride standard solution. Similarly, exposure of the sensing electrode to sunlight can alter the
electrode surface enough to make possible detection of a slight millivolt potential shift. Because sunlight normally is excluded from process tanks or electrode chambers: this influence is generally of little consequence. Reasonable changes in p H of the solution being measured do not interfere with the accuracy of the chloride ion measurement. When the pH values become so low or high that the hydrogen or hydroxyl ion concentration can alter the electrode, further attention must be given to this factor. A very high or low p H could cause a chemical attack upon the sensing electrode or alter the pertinent solubility relationships. Other halides or ions which form silver salts that are less soluble than silver chloride will give erroneous or interfering measurements. Again, hoxvever, such interfering constituents are not normally present where chloride must be measured.
The efficiency and reproducibility of this method of analysis, as in pH and ORP measurements, are dependent upon the application and the nature of the solution being measured. T h e stability of the measurement can be compared to that with continuous pH. Normally, the average application involving pH requires standardization on a once-perweek basis. Where the characteristics of the stream require, or maximum accuracy dictates, it may be necessary to standardize on a once-per-shift or per24-hour-day basis. Laboratory and field evaluations based on weekly standardizations indicate that the typical application can expect a continued stability within =tlOyoof the amount present. I n the ranges below 10 p.p.m., closer attention to possible interferences is required than in the ranges between 10 and 10,000 p.p.m. Below 1 p.p.m. rigorous attention must be paid to the parameters to obtain accuracy. Operation
The analyzer can be standardized with standard solutions or by titration of a grab sample. As in pH measurement, the reference standard should be at a concentration and temperature reasonably close to those of the stream to be measured. Electrodes within the process stream can be located as in pH measurement. Literature Cited (1) Bates, R. G., “Electrometric pH Deter-
minations,” pp. 202-8,
Wiley, New
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Reproducibility
Basically, the significance of the influences described above decreases as chloride ion concentration increases. I n general, an accuracy to A570 can be obtained in most applications. Increased accuracy and reproducibility are possible when measurements are made at reasonably constant temperature.
(2i6Kolthoff, I. M., Furman, N. H., Potentiometric Titrations,” pp. 144-75, Wiley, New York, 1931. (3) Helkamp, G. K., Gunther, F. A,, Wolf, 3. P., Leonard, J. E., J . Agr. Food Chem. 2, 836-9 (1954). (4) Watanabe, H., Beckman Instruments, Inc., unpublished data. RECEIVED for review July 14, 1958 ACCEPTED February 20, 1959 Instrument Society of America Symposium, Houston, Tex.: May 1958.
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Measurements of low concentrations must b e made a t constant temperature for maximum accuracy
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Below 10 p.p.m. closer attention to possible interferences is required than between 10 and 10,000 p.p.m.
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