analysis instruments aid pollution control Bernard J. Galetti and F. Curtis Snowden Process Analytical and Environmental Instrumentation Division Leeds & Northrup Co., North Wales, Pa. I9454
1 oday, we are witnessing an intensive effort to maintain the quality of water in a state suitable for our common needs. To achieve this goal, regulation and control programs are necessary. Before they can be implemented fully, however, a great deal of work must be done. Comprehensive surveys of water quality must be made to characterize the present condition of water in a particular area. Then, the data can be used to determine operating levels for water use and waste treatment by both industry and municipalities. Obviously, the complete analysis of all water and waste is of fundamental importance to a successful water quality program. Roche (1967) and Snowden (1968) have listed the constituents and properties which can be measured. Generally, the analysis required depends on the purpose of the survey or on the anticipated characteristics of the waste water. In automatic analysis systems, pH. temperature, electrolytic conductivity, dissolved oxygen, and turbidity normally are monitored. In addition, other parameters may be of interest, depending on the source and nature of the water; among them are various specific ions which can also be monitored automatically. To name only a few, the following ions might be of importance: nitrate, sulfate, phosphate, chloride, calcium, magnesium, and sodium. Presently, such ions are measured by obtaining a grab sample which is then subjected to a series of involved laboratory tests. Fortunately, recent advances in electrode technology have resulted in de34 Environmental Science & Technology
velopment of specific ion electrodes suitable, under certain conditions, for the continuous analysis of selected ions, Basically, such electrodes use a membrane-composed of glass, plastic, rare earth elements, ion exchange materials, or inorganic salts-to selectively transport charge and develop a potential attributable to a specific ion in solution. Specific ion electrodes
Specific ion electrodes may be divided into two broad classes, cation sensitive and anion sensitive. However, this may be an oversimplification. Glass membrane and liquid-liquid membrane electrodes have, in general, specificity for cations. Glass membrane electrodes, in essence, measure pH; special glass formulations result in selectivity for specific ions. (Careful consideration of glass hydration characteristics is necessary to ensure a Nernstian response of approximately 59 mv. per decade change in ionic concentration.) With some exceptions, glass electrodes have been developed empirically. Normally, the ratio of Na,O to A120, in the glass is varied to produce electrode membranes having high specificities for the monovalent cations: H+, Na+, Li+, and K+. Liquid-liquid membrane electrodes, with strong permeable membranes of either plastic or glass, are filled with a liquid ion exchange material. Electrodes of this type have been developed to determine Ca++, Mg++, and C u t + activities. The anions NO,- and C104- also can be measured with electrodes of this same basic design. Anion selective electrodes usually have a single crystal solid-state mem-
brane, or a membrane composed of a silicone rubber matrix impregnated with a suitable precipitate. In either case, the membrane contains relatively insoluble inorganic salts whose anions are similar to those which are to be measured. Typical anions measured with these electrodes are F-, CI-, Br-, I-, and 9. Selectivity-an important term used in describing the performance of specific ion electrodes-is defined simply as the ratio of an interfering ion concentration to the concentration of the specific ion of interest necessary to achieve an equal emf signal. Or, to put it another way: If the Na+/K+ selectivity ratio is 1000 for a cation sensitive glass electrode, then the electrode will respond preferentially to Na+ when the concentrations (activities) are equal. However, if the K+ concentration increases 1000-fold, the electrode will respond equally to each ionic species. The response most desired for any electrode is one which follows the generalized form of the Nernst equation. It is important to remember that, actually, we are measuring the activities, not the concentrations, of the specific ions. (Activity and concentration are equal only in very dilute solutions.) Activities, however, have been determined for concentrations down to lO-5M for many ions, and concentrations can be calculated if the mean ionic activity coefficient for a given species is known. With mixed electrolytes (ordinarily the case in water monitoring) working calibration curves can be constructed, using as the background a solution with an ionic composition similar to the actual sample to be analyzed. Obviously, the electrodes will not respond to unionized molecules or to bonded ionic species. The practical application of these electrodes to measurement of water quality and in waste treatment systems must be undertaken with care. Information on the ionic composition of the sample must be available if one is to estimate, with any degree of certainty, the possible interferences from competing ions or variations in pH. Certainly, water treatment plants, saline water analysis, ion exchange processes, and sodium zeolite regen-
feature eration facilities require measurement of Na+, C1-, and the divalent cations indicative of water hardness, Ca++ and M g + + . Fluoride ion in niunicipal water supplies and effluent wastes can be determined continuously with a fluoride ion electrode. With proper consideration, practical uses for specific ion electrodes are limitless.
Figure 1
Meaningless. Ionization eflect of water makes conductivity meaningless at low solute concentrations
e H,O++
ZH,O
0H-
High purity water monitor
For many years, a standard technique for establishing water purity has been to determine the electrolytic conductivity, and, hence, the total dissolved ionizable solids in a solution. Although this is a nonspecific measurement, it is often a key indicator of changes in chemical composition and pollution levels. Today, many industries are particularly interested in producing extremely pure water for process use. Until recently, it was difficult to measure water conductivity less than 1 micromho/cm. at 250' C. Conventional conductivity instruments are not practical, since they usually ignore solvent effects. Such instruments are acceptable when solute concentrations are quite high, but when making measurements of high purity water, the ionization effect of the solvent water is appreciable, The water molecules themselves ionize to form H,O+ and OH-, resulting in a conducting medium (Figure 1 ) . Since most of the water molecules are undissociated, the conductivity effect ordinarily is quite small. However, as the concentrations of solute ions present diminish-as is the case for extremely pure water-the concentration of the solvent water ions exceeds that of the solute ions. Under such conditions, not only is it exceedingly difficult, but it is almost meaningless to consider total conductivity as an indication of solute concentration. The variation of conductivity with temperature is attributable to changes in ionic mobility. The temperatureconductivity coefficient varies widely for dilute sodium chloride solutions (Figure 2 ) . Since the conductivity ratio doubles between 1.0 and 0.2 micromho, and increases by a factor of 5 between 0.2 micromho and pure water, we conclude that the water ionization equilibrium is strongly tem-
Figure 2
15
1
to rise, reaching a ratio
P water u
r
solute e ,
v
limit
The ratio at higher Concentrations approaches constancy a t 5 3
0
-----
0.2 0.4 0.6 0.8 1.0 Solution .concentration expressed as conductivity at 25" C. (micromhos per crn.)
Sensitive. Conductivity of NuCl solution is highly temperature sensitive
Figure 3
Measured total conductivity Water ion conductivity
v
Solute ion conductivity
/' Tempera tu re
Effects. Water ionization and temperature effects complicate conductivity measurements in high-purity water
perature dependent. This conclusion is supported by the fact that automatic temperature compensators designed for application to monitoring of high purity water (below 0.5 micromho) can't cope with the temperature COefficient variations. Thus, the only way to reconcile this problem appears to be to bring the sample to a predetermined, constant temperature prior to measuring the conductivity. This can be done, of course, but it would increase the complexity of the equipment and compromise the reliability of an automated system. Even with perfect temperature compensation, the background interference effect of solvent water ionization would adversely affect the accuracy of the conductivity measurement. At 2 5 3 C., the water ionization effect is approximately 0.055 micromho; at 100° C., it is 0.78 micromho. Solvent water ionization has an undesirable effect on determination of high purity water conductivity (Figure 3 ) . The measured total conductivity is the sum of solute ion conductivity and the conductivity of the water ions. Even at a constant temperature, there is a serious error in the measured conductivity. Eckfeldt, Eynon. and Kuczynski (1964) designed an instrument specifically to surmount these obstacles. Their approach was to make a conductivity difference measurement which takes advantage of the additive conductivity effect in dilute solutions. A subtraction circuit is used to make the measurement (Figure 4). The instrument agrees well with theoretical data (Figure 5 ) . We fully realize the instrument's performance when we recognize that the curve really represents a plot of small differences between two quantities of large magnitude. Thus, this differential conductometric type instrument permits accurate measurements of impurities in high purity water. Streaming current detector
A survey of instrumentation available for continuous water and waste monitoring and control will reveal a dearth of equipment suitable for meaVolume 3, Number l , January 1969 35
surement of colloids, emulsions, and other suspensions. Colloidal particles found in natural systems usually have an electrical charge; the nature and distribution of these charges have a definite influence on the stability of the system. Their rheological behavior is a function of the particlefluid interface where the charge is located. Electrokinetic phenomena are explained most readily if we assume the existence of a double layer in the vicinity of particle surfaces. The surface of a particle is capable of acquiring charge from the solution almost instantaneously, albeit reversibly. Consequently, a difference in potential will exist between the surface of a particle and the bulk solution. To maintain neutrality, the surface charge will be balanced by an equal, but opposite, charge acquired by the solution in the immediate vicinity of the particle, since counter-ions will be attracted, arid like ions repelled, by the particle. The potential difference in millivolts between the bulk solution and a point (the shear plane) in the double-layer is called the zeta-potential. This potential is a measurable physical quantity which is often used to elucidate electrokinetic behavior. If colloidal particles are inimobilized-for example, on a filter normal to the direction of flow or on the wall of a pipe through which solution is flowing-the counter-ions may be physically moved by the fluid. This movement ol charge is called a streaming current. The adsorption and desorption of charge associated with this development of a streaming current forms the basis of a measuring instrument designed by Gerdes ( 1 966). Leeds & Northrup manufacture an instrument based on this design. The Hydroscan Streaming Current Detector (Figure 6 ) can be used to make many measurements of charge-bearing particles in colloidal solutions, emulsions, and other types of suspensions. Relationships such as those between charge density, flocculation-stabilization mechanisms, and basic separation techniques certainly can be clarified by using the detector to study adsorption phenomena and colloidal properties in a particular system. As a typical example of a colloidal system, consider a simple flocculation process in the treatment of water or waste water to remove suspended solids, The nature and distribution of charge within a given solution or suspension influence the stability of the 36 Environmental Science & Technology
Figure 4
I
Circuitry. For conductance measurements at low solute concentrations, special circuitry is necessary to overcome obstacles presented by ionization of water and the effect of temperature on ion mobility. One approach to the problem is the subtraction circuit (above) designed by Eckfeldt, Eynon, and Kuczynski. This circuit is used to make conductivity difference measurements to take cidvantage o f the additive condzictivity effect in dilute solutions. Results obtained with this instrumentation agree well with theoretical data (below).
Figure 5
0.30
I
1
I
1
of water sample
g 0.25
L
0
.E 0.15
/
Predicted f r o m 25" C conductivity value I
0 . 0 5 1Solute / ion indication
I
given by high purity
20
30
40
50
Temperature,"C.
-1 60
~
70
dispersed solids. If one can increase the probability of collision between adjacent particles, and simultaneously promote their adhesion after collision has occurred, the particles gradually increase in size; eventually, they form large, dense agglomerates which are readily separated from the liquid by ordinary mechanical means. The probability of collision can be enhanced by agitation; adhesion on the other hand, can be promoted by the addition of p.p.m. concentrations of flocculants, which alter the physical and electrical stability of the suspended particles. The practical use of the detector in this process might well be to determine the effective charge condition of the influent, and then to correlate the signal level with empirical data to establish the optimum flocculant dosage. In this manner, flocculant consumption is related to the actual concentration of colloidal material which must be treated, as well as the volumetric flow rate of the influent. Closer control of flocculant minimizes possibility of underdosing or overdosing, both of which can result in less than optimum operation. Furthermore, substantial reductions in chemical costs can result from not overdosing. The measurement of a streaming current with the detector might imply the instrument reading corresponds with zeta-potential. Such a correspondence has not been established, primarily because the instrument also responds to variations in charge density contributed by other sourcesfor example, ionic concentrations including pH. Thus, the instrument detects a current which results from adsorption of solute and solvent ions, as well as from the adsorption of colloidal particles on the surfaces of the annulus formed by the boot and piston. Under these circumstances, the magnitude of the signal from the instrument does not represent the true streaming current. Instead, it is an empirical quantity (nevertheless reproducible under like conditions) which can be correlated with descriptive methods and established experimental procedures to detect and control the concentraltion of charged materials down to p.p.b. levels. This is a technique which can be of paramount importance to the analyst. These approaches can resolve but a small portion of the demonstrated needs for proper water treatment and
Figure 6
Synchronous motor
I
I Cam 1 I
I
Attenuator
m Meter
amplifier
rectifiei
Sample solution
Suspensions. Operation o f streaming current detector depends on the physical movement of counter-ions when charged colloidal particles in suspension are rendered immobile, f o r example, on a filter normal to the direction of flow or on the wall of a pipe through which the solution is flowing. Block diagram above is a description of Leeds & Northrup instrnment that can be used to make measurements of charge-bearing particles in colloidal solutions, emulsions, and other types of suspensions.
I
Electrodes
effective waste control processes. Many other instruments also are required to monitor and maintain the quality of our natural waters, but those described represent the results of some recently applied, new techniques. ADDITIONAL READING
Eckfeldt, E. L., Eynon, J. U., and Kuczynski, E. R., “High Purity Water Monitor,” Proceedings of the American Power Conference, Vol. XXVI (1964). Gerdes, W . F., Instrument Society of America Journal, 13, 38 ( 1 9 6 6 ) . Roche, W. M., “Instrumentation’s Role in Maintaining Water Quality,” Instr. Tech. 14, No. 9, 55-8 (1967). Snowden, F. C., “Instrumentation for Pollution Monitoring and Control,’’ Proceedings of the Texas A&M Twenty-Third Annual Symposiuni on Instrumentation for the Process Industries, 1968. (In press).
Bernard J. Galetti is senior engineer in the process analytical and environmental instrumentation division, Leeds & Northrup Co., a position he has held since 1963. His work at Leeds & Northrup has involved the design and application of analysis instrumentation. Previously (1957-63), he was research chemist, corporate research, at United Stares Steel Corp., specializing in imtrumental analysis. Galetti received his B.S. f r o m the University o f Pittsburgh in 1961. H e is author of technical papers in the field, and has lectured on electrochemistry and analysis instrumentation.
F. C. SnowdenI is general manager of ,” r” ‘F n m r r v ~-.._ nnnLeeds & Nod..I l lI.r lytical and environmental instrumentation division. H e has been with Leeds & N o r t h r u p w h e r e his work involves design and application of analvsis instrumentation to indusfrial and environmental m e aYurements end control -for 17 years. :Ynowden received his .-~..”-7r.-B.S. f r o m Lafayat. ~ v ~ r r x~,l””li rI ~ L and completed 2 years of graduate work at Lehigh University. The author of technical papers in analytical chemistry, electronic circuit design, and environmental sciences, Snowden is a member of ACS, Instrument Society of America, Air Pollution Control Association, Water Pollution Control Federation, and Air & Water Quality Committee-Greater Philadelphia Chamber of Comnzerce. _l_ I
Volume 3, Number 1, January 1969 37
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