Determination of Silicate in Waste Water by Atomic Absorption Inhibition Titration R. W. Looyenga and C. 0. Huber Department of Chemistry, Unicersity of Wisconsin-Milwaukee, Milwaukee, Wis. 53201 This paper reports the results of inhibition studies using a new, semi-automatic technique termed Atomic Absorption Inhibition Titration, AAIT. Magnesium absorption in a hydrogen-air flame is monitored while using a standard magnesium solution as titrant. A new, rapid method for the determination of silicate in waste water is proposed. The method is accurate and precise and is applicable down to 1pg/ml silica. AAIT also provides an improved general means of studying flame chemistry, e.g., inhibition effects. Various factors affecting inhibition by anions were investigated and are discussed. An explanation of some of the conflicting reports in the literature is offered.
CONVENTIONAL METHODS for the determination of silicate are few in number and have remained essentially unchanged for several decades ( I ) . The importance of silicate determinations is indicated by the voluminous literature on the subject. Determinations at the trace level have been limited almost exclusively to colorimetric methods based on the formation of molybdosilicic acid (1, 2). Although the heteropoly methods have been widely adopted and modified ( 2 , 3), they leave much to be desired in the routine determination of silicate. These methods require close control of pH, reaction time, temperature, reducing agent, time of measurement, etc. Molybdosilicic acid also provides the basis for the indirect determination of silicate by atomic absorption spectrometry (4, 5). Relatively little work has been reported on the direct determination of silicate by atomic absorption. Hollow cathode lamps are available. High temperature nitrous oxide-acetylene flames are required. Determination limits are in the vicinity of 1 pg/ml Sios (6-8). With the exception of atomic absorption, conventional methods are relatively cumbersome and time consuming. The need for the development of a simple, precise, and sensitive method for the routine determination of silicate is apparent. In this report we describe a method based on the inhibition by silicate of the magnesium atomic absorption signal. The sample solution is passed over an H-form cation exchange column to remove magnesium and other potentially interfering cations. A standard magnesium solution is titrated (1) F. D. Snell and C. T. Snell, “Colorimetric Methods and Analysis,” D. Van Nostrand Co., New York, N. Y., 1959. (2) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 11. Vol. 2, Interscience Publishers, New York, N. Y . , 1962. (3) E. J. King and H. Stantial, Bioclz~m.J . , 27,990 (1933). (4) G. F. Kirkbright, A. M. Smith, and T. S. West, Analyst, 92, 411 (1967). (5) T. R. Hurford and D. F. Boltz, ANAL.CHEM.,40, 379 (1968). (6) M. D. Amos and J. B. Willis, Spectroc/iim. Acta, 22, 1325, 2128 (1966). (7) J. S. Cartwright, C. Sebens, and D. C . Manning, At. Absor.ptioiz Newslett., 5, 91 (1966). (8) J. A. Bowman and J. B. Willis, ANAL.CHEM.,39, 1210 (1967). 498
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
into the sample solution with simultaneous aspiration of the titration solution into the flame. A distinct increase in the magnesium atomic absorption signal marks the end point. Proper choice of the hydrogen-air flame conditions enhance the specificity of the method. The technique may be termed atomic absorption inhibition titration (AAIT). Refractory Silicates. The inhibition effects of silicate in flame spectrometry are well documented. It has long been recognized that silicate, as well as phosphate, sulfate, and aluminate, depresses the flame emission of the alkaline earth metals (9, 10). Similar effects have been observed in the determination of these elements by atomic absorption spectrometry (11-13). Considerable work has been reported on the removal of these interferences (14-17). In most cases the actual mechanisms of these inhibition effects remain to be elucidated. Relatively little has been reported on the stoichiometry of the refractory compounds formed (15, 18), or on the analytical utilization of these effects. Limited applications to phosphate determinations have been reported (19, 20). No corresponding methods using the substantially greater inhibiting effects of silicate have appeared. The proposed method exploits the high degree of inhibition by silicate of the very sensitive magnesium atomic absorption signal. EXPERIMENTAL
Apparatus. All absorption measurements were obtained using a Jarrell-Ash Model 82-516 atomic absorption spectrometer, equipped with a 0.5-meter Ebert grating monochromator, a “tri-flame” premix, laminar-flow burner with 0.50 x 100-mm slot, and a Beckman model 1011 10-inch recorder. Aspiration directly from the titration vessel was achieved using a suitable length of Teflon (Du Pont) tubing (0.027-inch i.d.).
A constant flow infusion pump fitted with a 30-ml syringe with Teflon needle (0.044-inch i.d.) is used for titrant delivery. A convenient alternative means of titrant delivery is a constant flow device constructed from a reservoir buret with ground glass tip fitted to a suitable length of 0.027-inch i.d. Teflon tubing. Either of these systems may be adjusted (9) M. Margoshes and B. L. Vallee, ANAL.CHEM.,28, 180 (1956). (10) J. J. Diamond, ibid., 27,913 (1955). (11) D. J. David, Analyst, 84,536 (1959). (12) S. R. Koirtyohann and E. E. Pickett, ANAL.CHEM.,38, 585 (1966). (13) P. B. Adams, and W. 0. Passmore, ibid., 34,630 (1966). (14) D. J. David, Analyst, 85,495 (1960). 32,1475 (1960). (15) J. I. Dinnin, ANAL.CHEM., (16) A. C. West and W. D. Cooke, ibid., p 1471. (17) D. J. Trent and W. Slavin, At. Absorption Newsleft., 3, 118 ( 1964). (18) W. W. Harrison and W. H. Wadlin, ANAL.CHEM.,41, 374 (1969). (19) W. A. Dippel, C. E. Bricker, and N. H. Furman, ibid., 26, 553 (1954). (20) K . C. Singhal and B. K. Banerjee, Technology 5 (3), 239 ( 1968).
Figure 1. AAIT curves for addition of magnesium (100 pg/ml) to: 1. Deionized distilled water (50 ml) 2. 10 pg/ml Si02 solution, Le., 8.32 X 10-6 mole Si02
Composition of Artificial Waste Water Concn Components ( M x 103) Concn (pgiml) 0.242 Nos-, 15 KN03 NHaCl 1.11 “a+, 20 1.50 Ca2++,60 CaCL MgSO4 1.04 Mg2+,25; Table I.
NaHC03 KHnP04 Na2Si03.9H10 HCI
4.92 0.263 0.836 0.10
so4*-,100
HCOI-, 300 POF, 25
SOz,50 Total C1-, 150
to provide a suitable constant rate of titrant delivery (1-3 ml/min). Titrant delivery and aspirating tubing are physically separated and held in place by means of a large rubber stopper fitted with two short lengths of 4-mm glass tubing. Plastic beakers or wide mouth bottles, 100-150 ml in volume, serve as suitable titration vessels. The titration solution is stirred with a magnetic stirrer. Deionized water was prepared by passing singly distilled water through a mixed bed ion exchange demineralizer cartridge. Doubly distilled water prepared from a borosilicate distillation apparatus was found to contain silicate and could not be used. Removal of interfering cations from sample solutions was achieved using sulfonated polystyrene-type cation exchange resin in the hydrogen form (Amberlite IR-120, Rohm and Haas). Reagents. All solutions were prepared using reagent grade chemicals and silica-free deionized distilled water. Solutions were prepared in acid-hardened (1 :1 H?S04-HN03, overnight) volumetric flasks and stored in polyethylene containers. Standard silicate solutions were prepared by dissolving 4.732 grams of reagent grade Na2Si03.9H20in deionized water and diluting to 1.000 liter (1000 pg SiOz/ml). To prepare standard magnesium solutions, 1.673 grams of reagent grade MgCI2.6 H 2 0 (dried 4 hr at 70 “C)was diluted in deionized water to 1.000 liter for a solution 2000 Hg/ml in Mg(I1). This concentration was confirmed by titrating with a standard EDTA solution. ARTIFICIALWASTE WATER.Artificial waste water conforming to the average composition of effluent from secondary
treatment of municipal waste water (21) was prepared according to Table I. Procedure. Remove interfering cations from sample solution (pH 3-4) by ion exchange. Pipet sample solution containing up to 1.0 mg of silica into the titration vessel and dilute to 50 ml. Place the titration vessel on the titration stand, insert the aspirating and titrant delivery tubes into the solution, and start the magnetic stirrer. After the solution has begun aspirating into the flame, simultaneously (preferably by a common switch) begin adding titrant (1-3 d i m i n ) and recording. Titrate past the end point to an absorbance of one (full scale). Titrate 50 ml of deionized water, or other appropriate blank, by the above procedpre. Instrumental parameters were wavelength, 2852 A ; slit widths (fixed), entrance 100 p and exit 150 1.1; lamp current, 4 mA; Photomultiplier tube, 1P28; potential, 600 V; fuel (H2) 30 psi, 10 CFH; Oxidant (air) 4 psi, 10 CFH; and beam position (center), 8 mm above burner head. RESULTS AND DISCUSSION
Titration Curves. Figure 1 shows the remarkable curve obtained for the titration of silicate with magnesium when using a relatively cool (fuel rich) flame. (Although titrations are normally carried out at somewhat higher flame temperatures, many of the features of the titration curve are off-scale at such conditions.) Essentially complete inhibition of the magnesium signal is observed at high silicon-to-magnesium mole ratios. Significant variations in slope occur at the whole number ratios of 0.5 and 0.25, designated by points A and B, respectively, in Figure 1. Beyond the minimum at point C, the magnesium signal increases, but with a slope considerably less than in the absence of silicate. Although points A , B, and C in Figure 1 represent constant stoichiometries, the first two are most suitable for analytical applications. Point B , however, is somewhat susceptible to diverse ions; also, it is normally off scale (absorbance >1) at the prescribed conditions. In the proposed method, point A is located by extrapolation to the base (Figure 2), and conforms to constant mole ratios over a wide range of
(21) “Cleaning Our Environment-The Chemical Basis for Action,” American Chemical Society, Washington, D. C . , 1969, p 109. ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
~
499
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limited only by the curvature (lack of sharpness) in the vicinity of the end point. Such curvature increases with silicate concentration and limits the linear portion of the signal, thereby making extrapolation to the end point more difficult. The presence of certain diverse ions (e.g., alkali metals and N H 4 + at 0.01M) tends to enhance this curvature, thereby further limiting the upper end of the concentration range. The lower end of the concentration range is determined by the minimum volume of titrant which can be accurately and precisely added to reach the end point. Loss of analyte due to aspiration is small, approximately per min for 50 ml of solution. Errors due to this loss are compensated in the calibration plot. Titration blanks include not only the presence of any silicate in reagents and solvent, but also time lapse between the addition of titrant and when the titrant reaches the flame. This time lapse depends upon aspiration rate, which is controlled by air flow rate and length and diameter of aspirator tubing. Titrant delivery rate and concentration are selected such that end points occur within 2 minutes for titrant flow rates in the range one to three milliliters per minute. Chart speed is selected to yield convenient dimensions for accurate extrapolation to the end point. Proper selection of these variables minimizes aspiration loss, dilution effects, and endpoint reading errors. In this work 100 pg/ml magnesium titrant was used for silica concentrations below 15 ,ug/ml and 200 ,ug/ml for higher concentrations of silica. Chart speeds were 2 to 5 inches per minute. Flame Conditions. Utilization of inhibition effects requires the use of relatively cool flames. Hydrogen-air flames provide a convenient temperature range to allow inhibition of magnesium by silicate with minimal interference by sulfate and phosphate. Control of the temperature of the flame is effected by the fuel-to-oxidant ratio as well as by the total gas flow rate. The effect of decreasing the fuel-to-oxidant ratio (increasing the temperature of the reducing flame) at a fixed air flow is shown in Figure 3. At lower flame temperatures, decreasing the hydrogen-to-air ratio does not affect the stoichiometry, but it does enhance the magnesium atom population beyond the end point. Figure 3 also shows that sufficiently high flame temperatures result in an apparent change in stoichiometry (shift in end point). We postulate that at these higher temperatures a fraction of the silicate forms anhydrous silica, thereby decreasing the proportion present for combination with magnesium. In addition, data obtained on titration stoichiometry and degree of inhibition show that increasing the total gas flow rate at a fixed fuel-to-oxidant ratio produces the same effects as decreasing the hydrogen-to-air ratio (Figure 3). Presumably this indicates that under the conditions studied, calorie input increases faster than the cooling effects of solvent and salt evaporation. Maximum inhibition occurs near the base of the flame. Inhibition decreases markedly in the region above the tip of the inner cone. This decrease in inhibition may be explained by the increased flame temperature due to oxidation of the excess hydrogen by entrained air, as well as the increased residence time of particles in the flame. The above considerations accent the importance of precise adjustment of instrumental parameters. Matrix Effects. The complexity of the reactions in the evaporating droplet results in subtle matrix effects. Hence,
3z
V O L (ml) O F T I T R A N T
Figure 2. Signals obtained for semi-automatic titration of standard silicate solutions with 100 pg/ml magnesium standard 1. 2.5 pg/ml SiOn 2. 5.0 &ml S O z 3. 10.0 pg/ml SiOt End points designated by intercept on abscissa. Titrant delivery rate: 1.50 ml/min
Table 11. Determination of Silicate by the AAIT Method Sample concn, wdml SiOz found bg/ml Re1 error, % SOz Matrix 1.oo Distilled water 0.97 -3.0 2.50 Distilled water 2.53 +1.2 5.00 Distilled water 4.98 -0.4 10.0 Distilled water 9.98 -0.2 15.0 Distilled water 15.0 0.0 25.0 Distilled water 25.0 0.0 50.0 Distilled water 49.7 -0.6 5.0 Waste water 4.97 -0.6 5.0 Waste water 4.77 -4.6 (+25 pcg/ml
10.0 25.0 50.0
in PO4) Waste water Waste water Waste water
9.81 25.5
49.1
-1.9 +2.0 -1.8
silicate concentrations (Table 11). Figure 2 reproduces actual recorder traces showing that beyond point A , the signal is sufficiently linear so that extrapolation is both convenient and accurate. Stoichiometry. The stoichiometry of the refractory compound(s) formed is obtained from Figure 1, which indicates a mole-ratio of one silicate to two magnesiums at point A . This corresponds to formation (within the evaporating droplets) of magnesium orthosilicate (Mg2Si04), which has a melting point of 1910 "C. Mechanisms to explain the shape of the curve at points B and C and beyond are intricate and provide the basis for further study. Such investigations are being continued in this laboratory. Silicate Concentration. Typical titration curves for standard silicate solutions are shown in Figure 2. Calibration graphs of end points cs. silica concentration showed excellent linearity over the range of 1 to 50 pg/ml of silica. Table I1 shows the results of determinations based on the calibration graph technique. The upper end of concentration range is 500
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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Figure 4. Effect on absorbance of magnesium (1 pg/ml) of varying the air inlet pressure to flowmeter while maintaining an air flow reading of 10 CFH Hz = 10 CFH, 30 psig
6
V O L (mll O F T I T R A N T
Figure 3. Effect of varying the hydrogen/air ratio (H2/air ratios indicated on graph) at a fixed air flow of 7.5 CFH Magnesium: 4.12 X 10-3M 4.16 X mole Silicate:
removal of cations by ion exchange is a recommended step of the procedure. Such removal is simple and effective. Without previous removal by ion exchange, alkali metals and ammonium would interfere at concentrations greater than 0.01M. Anions which do not interfere with the procedure described at 0.001M and below include acetate, bicarbonate, chloride, citrate, hydroxide, nitrate, and sulfate. Determination of Silicate in Waste Water. The applicability of the proposed method to practical samples was tested using aqueous solutions simulating effluent of the secondary treatment of waste water (21). The matrix of these solutions is summarized in Table I. Removal of interfering alkaline earth ions was achieved by column ion exchange. It was found, however, that cation exchange columns containing magnesium and/or calcium retained a small amount of the silicate. This problem was circumvented by adjusting the pH of the sample solution to about four with hydrochloric acid and using the hydrogen form of the resin. Effluent from the sodium form of the resin is neutral, resulting in a loss of silicate from solutions of pH as low as one. Sample solutions also contained a relatively high concentration of phosphate and sulfate. Interference by these ions is minimal at the prescribed flame conditions (see Procedure). The temperature dependence of the apparent stoichiometry (see Flame Conditions) at these flame conditions may require titrating standards with each set of sample solutions. The standard solutions yield a linear end point us. silica concentration calibration plot from which concentrations of samples are obtained. Each titration requires only about three minutes so that data can be obtained rapidly.
A waste water sample was selected for this work in order to show freedom from relatively high concentrations of a number of common anions. The presence of phosphate in these samples shows that other anions which inhibit magnesium absorption in flames do not necessarily interfere in the AAIT method. If silicate forms in preference (by an equilibrium or a kinetic mechanism) to the salt of another ion, then the end point of the titration will be evident and its position unchanged on the titrant axis. When the matrix allows use of a cooler flame, the stoichiometry is not temperature dependent (Figure 2) and can be applied from day to day. A cooler flame also results in increased sensitivity of silicate measurements. Determinations at 0.1 pg SiOz/ml are possible. Results of determinations of silicate in artificial waste water solutions are summarized in Table 11. Sample solutions ranged from 5 to 50 pg/ml in silica. Freedom from phosphate interference is demonstrated by the successful determination of 5 pg/ml of silica in the presence of 50 pg/ml orthophosphate. Results of five determinations of waste water solutions containing 5 pg/ml of silica indicated a relative standard deviation of 0.45z. The detection limit (3 X estimated std dev or blank) is 0.1 pg SiO,/ml. General Discussion. Because of the critical nature of the flame temperature, which is dependent upon fuel-to-oxidant ratio, total gas flow rate, and location within the flame, it is not surprising that conflicting reports concerning inhibition by anions appear in the literature. An additional source of confusion results from the fact that at a constant reading of a falling ball flowmeter, the true flow rate varies significantly with inlet pressure. Figure 4 shows the effect of varying the inlet pressure while maintaining SL constant flowmeter reading. Both the actual flow rate and aspiration rate increase with increasing inlet pressure. It must be asserted that when using a falling ball flowmeter to specify flame conditions, both flowmeter inlet pressure and flow rate should be stated. It is unfortunate that some manufacturers ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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do not stipulate the inlet pressure at which their flowmeters were calibrated. The special nature of the inhibition titration curve (Figure 1) elucidates another important source of confusion concerning the extent of inhibition by anions. A given absorption value may correspond to as many as three different silicate concentrations. This precludes the determinations of silicate cia its direct inhibiting effect on standard magnesium solutions. To date, most inhibition methods are based on this technique. The reported inhibition titration curve, with its reversals in slope, is not unique to the magnesium-silicate system. Similarly sloped curves were obtained for the titration of silicate with calcium and phosphate with magnesium or calcium. Huber and Crawford (22) observed a similar titration curve for phosphate inhibition of magnesium and attributed these effects to refractory compound stoichiometry.
The inhibition curve is also not unique to the absorption process. Similarly shaped curves were obtained using flame emission to monitor titrations of silicate with magnesium and calcium. Hence, most of the effects and data herein reported apply equally well to flame emission, and the method developed should also be applicable to flame emission. The proposed technique, AAIT, provides a new, unique, and convenient means of studying not only inhibition effects, but other high temperature chemistry as well. The ease of varying stoichiometry and the rapidity with which data may be obtained should result in the revelation of many new aspects of flame chemistry. A particularly attractive aspect of the proposed method is the ease with which it could be automated for the routine determination of large numbers of samples. Work is continuing on the determination of phosphate, sulfate, and other anions by the same general procedure.
(22) C. 0. Huber and W. C. Crawford, Abstracts 160th National
RECEIVED for review September 10, 1970. Accepted December 15, 1970. This work supported by Grant No. 16020-DHD from the Federal Water Pollution Control Administration, Department of Interior.
Meeting of the American Chemical Society, Chicago, Ill., September 1970, No. A71.
Titration Errors and Curve Shapes in Potentiometric Titrations Employing Ion-Selective Indicator Electrodes Franklin A. Schultz Department of Chemistry, Florida Atlantic University, Boca Raton, Fla. 33432 Calculated titration curves and errors are presented, illustrating the effect of interfering ions on potentiometric titrations which employ ion-selective indicator electrodes. The presence of interfering ions in the sample solution or titrant distorts the titration curve and causes the inflection point to precede the equivalence point. Using an isovalent precipitation titration as a model, the titration error increases as the sample ion concentration decreases and as the interfering ion concentration, solubility product constant, and dilution factor increase. The calculated titration error can be as large as several per cent.
ION-SELECTIVE ELECTRODES have been developed rapidly in recent years and have widespread application in direct potentiometric measurements and as indicator electrodes in potentiometric titrations. In direct potentiometry the need to correct for the presence of interfering ions has long been recognized and is a serious factor limiting the accuracy of the method. For example, with full Nernstian response at 25 "C of 59.2 mV per decade change in concentration, a 4z error in concentration results from an error of only 1.0 mV in the potentiometric measurement. Potentiometric titrations, however, are generally regarded as being more accurate than direct potentiometric measurements and little consideration has been given to the effect of interfering ions on such titrations. An intuitive appraisal of the subject suggests that the presence of interfering ions will distort a titration curve and cause the inflection point to occur at a point other than the equivalence point. If these points are assumed to coincide, an error will result in the titration. Several authors have encountered distorted titration curves 502
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
when performing potentiometric titrations with ion-selective indicator electrodes in the presence of interfering ions (1-5). However, no quantitative treatment of the error involved has been presented to date. This paper describes the effect of interfering ions on potentiometric titration curves obtained with ion-selective indicator electrodes. In earlier work Meites and coworkers (6-8) derived important fundamental relationships regarding the location of the inflection point in acid-base, precipitation, and chelometric titrations. When the effects of dilution were rigorously included, it was a general conclusion that the inflection point and equivalence point of the titration curve do not coincide. These derivations assumed an ideal indicator electrode, however, and the resulting titration errors were small and not likely to be experimentally detectable. Whitfield et al. (9, 10) have calculated titration curves specifically for chelometric titrations of calcium and magnesium with calcium
(1) R. J. Baczuk and R. J. DuBois, ANAL.CHEM., 40, 685 (1968). (2) A. K. Mukherji, Anal. Chim. Acfa,40, 354 (1968). 41, 111 (1969). (3) G. A. Rechnitz and T. M. Hseu, ANAL.CHEM., (4) M. Whitfield and J. V. Leyendekkers, Anal. Chim. Acta, 45, 383 (1969). ( 5 ) J. S. DiGregorio and M. D. Morris, ANAL. CHEM.,42, 94 (1970). (6) L. Meites and J. A. Goldman, Anal. Chim. Acta, 29,472 (1963). (7) Ibid., 30, 18 (1964). (8) L. Meites and T. Meites, ibid., 37,1 (1967). (9) . , M. Whitfield. J. V. Levendekkers, and J. D. Kerr, ibid., 45, 399 (1969). (10) M. Whitfield and J. V. Leyendekkers, ibid.,46,63 (1969).
