via Equations 11 and 13, required for the reaction to proceed as written. A similar condition applies to IN3 addition. Since Strickland, Alack, and Childs suggested no rate-determining step for their mechanism, its analysis in terms of the present kinetic results is difficult. However, it is interesting to note the similarities between their mechanism and the one proposed here. Combining Equations 11 and 12 yields step 1 of the reaction pathway. Step 2 is similar to Equation 12. The possible existence of IN3 as appears in Equation 14 was discussed earlier. d parallel may be drawn between Equation 14 and a combination of steps 3 and 4. Finally, the recombination steps (5 and 6) are approximated by the reverse direction of the equilibrium in Equation 11. The basic difference betneen the mechanism proposed by Strickland,
Mack, and Childs and the pathway suggested in the present study is the principal rate-determining step reported here. Indications of the relatively slow cleavage of the disulfide bond were made possible by the new technique which enabled measurements approaching the initial reaction rates. A more definitive statement of the relative magnitudes of the various steps proposed must await a detailed study of the catalysis by sulfhydryls.
meister, H. L., Chem. Rev. 39, 269 (1946). ( 5 ) Kice, J. L., Venier, C. G., Tetrahedron Letters 48. 3629 (1964). (6) Lenz, G: R., Marten: A . E., Biochemistry 3, 745 (1964). (7) Lgjvtrup, S., Compt. Rend. Trav. Lab. Carlsberg 27, 72 (1949). ( 8 ) Neyerstein, D., Treinin, 8., Trans. Faraday SOC.59, 1114 (1963). (9) Pardue. H. L.., Dahl. W. E . -.T.. . . . -. ~-~~~ Electroanal. Chem. 8 , 268’( 1964). (10) Pardue, H. L., Shepherd, S. A,, ANAL.CHEM.35, 21 (1963). (11) Parker, A. J., Kharasch. N.. Chem. Rev. 59, 583 (1959). (12) Parker, A. J., Kharasch, N., J . A m . Chem. So?. 82, 3071 (1960). (13) Raschig, F., Chemiker Ztg. 32, 1203 (1908): (14) Strickland, R. D., Mack, P. A., Childs, W. A., ANAL. CHEM. 32, 430 (1960). I
~
’
I
LITERATURE CITED
(1) Benson, S. W., “The Foundations of
Chemical Kinetics,” McGraw-Hill, New York. 1960. (2) Griffith, R. O., Irving, R., Trans. Faraday SOC.45, 563 (1949).
(3) Hiskey,
R. G., Thomas, B. D., Kepler, J. A., J. Org. Chem. 29, 3671
(1964). (4) Kharasch, N., Potempa, S. J., Wehr-
,
RECEIVEDfor review May 17, 1965. Accepted July 29, 1965. Investigation supported by a David Ross XR Grant from the Purdue Research Foundation.
Continuous Chloride-Ion Cornbustion Method Applied to Determination of Organochlorine Insecticide Residues F. A.
GUNTHER, T. A. MILLER, and T. E. JENKINS
Department of Entomology, University of California, Riverside, Calif.
A new continuous chloride-ion system has been developed for use with a completely automatic combustion apparatus to determine organochlorine insecticide and other residues. This dynamic, continuous flow detector system uses a silver-silver chloride vs. slow-leak calomel electrode combination in routine analysis of samples of as much as 2 grams of plant extractives, Because the present equipment accommodates samples containing from 1000 to approximately 0.03 pg. of insecticide, containing from about 70 to about 8% organically bound chlorine, the useful range of detectability is from 500 p.p.m. to approximately 0.01 p.p.m.
0 (4,
chloride method 6) consists of producing hydrogen chloride gas by combustion, trapping this gas in a dilute solution of sodium carbonate-containing chloridefree nitric acid to suppress the ultimately interfering (4, 6) carbonate ion, and measuring the chloride ion concentration by simple direct potentiometry with a silver-silver chloride us. slow leak calomel electrode system. Amounts of original insecticides are then calculated from their percentage NE TOTAL ORGANIC
1386
ANALYTICAL CHEMISTRY
chloride compositions. This combustion chloride (1, 4 , 6 , 9 , I f ) direct potentiometric method (4, 6 ) is more advantageous (1, 4 , 6 , 9 ) than the usual wet oxidation and other combustion (8) techniques for handling large numbers of samples routinely. The only two operating disadvantages of the so-called “manual” method ( 1 , 4 , 6 , 9 ) involve the two components of the union of combustion ( I , 4, 9) with direct potentiometry (4, 6 ) : manually controlled combustions, which require up to 23 minutes each, can result in lost samples from incomplete burning or from too-rapid burning, and the transfers and other operations in “batchwise” direct potentiometry readily lead to inadvertent contamination by extraneous chloride ion in the laboratory atmosphere (inadequately cleaned glassware, etc.) in this extremely sensitive detection method. Complete automation of the combustion operation and continuous chloride-ion measurement would minimize both disadvantages. A completely automatic system has been developed which burns up to 2 grams of sample extractives in a burning cycle of 7 minutes, continuously monitors and measures any chloride ion in the combustion products, and graphically displays the resulting
chloride measurement. This system is more sensitive than the earlier (1, 4 , 6 , 9 ) versions, reduces the number of process steps, and requires less time per sample. DESCRIPTION OF APPARATUS
The automatic combustion furnace has been described by White et al. (11); slight modifications for the present purpose are discussed below. A block diagram of the new detection system is shown in Figure 1. Combustion products leave the furnace A under a slightly positive pressure. Any watersoluble gases present are dissolved in a dilute nitric acid carrier solution flowing into the system a t constant rate. After temperature equilibration in bath B , the concentration of chloride ion in this flowing carrier solution is continuously measured by the electrode system E with voltage amplification through a p H meter connected to a multirange recorder. Details are presented in later sections. Furnace. The automatic furnace (11) used for combustion was built from plans kindly supplied by the Shell Development Co., with a few minor modifications. For example, a switch was added to the 2-minute program controller allowing extension of the final 2-minute burn-out period, and the bubbler was not used because our vacuum line opened to the atmosphere.
-I
carrier
Isolution flow I
E amplilier
recorder soldsr joint 3 in.
gi silvcrsilver chloride deCtrQdE
tamp! bith
B
C
7
B
Figure 2. Special sensor electrode A a i d through-flow electrode holder B
Figure 1. Block diagram of continuous-flow chloride-'ion detection system
As previously described ( I I ) , the furnace combustion tube rests between an inlet assembly and a high-temperature furnace; it is wound with a resistance wire sample heater. A cap is unscrewed to insert a porcelain, platinum, or quartz sample boat into the combustion tube through the inlet assembly. The cap is replaced and a burning cycle is initiated by pressing the furnace start button; the furnace automatically turns on nitrogen and oxygen gases, water tower vacuum, and sample heater in a programmed 7-minute burning cycle, then shuts off the heat, vacuum, and gases placing itself in a stand-by condition for the next cycle. Burning temperature is controlled by a thermocouple and amplifier system which turns off the sample heater and cools the combustion tube with an air blast when the temperature rises above a preselected threshold value. Electrodes. The silver-silver chloride electrode A :;hewn in Figure 2 is made by sealing No. 18 chemically pure silver wire to a 10/30 standard taper inner joint with either black Apiezon or white epoxy resin ( 5 ) . The protruding silver wire is coiled on a 3.7-mm. diameter rod making 3 to 5 coils which will f i t through a 10/30 standard taper outer joint in the sensor electrode cell B. The electrode shaft is cut to 3 inches. Sensor electrodes are cleaned in concentrated ammonium hydroxide solution, next in water, and lastly in concentrated nitric acid. They are plated for about 30 minutes in a saturated potassium chloride solution with a pure silver wire cathode and a 1.5-volt dry-cell battery in series with a 1 kilo-ohm resistor. The electrodes are rinsed and stored in distilled water when the plate exhibits an even color with no spots or color variation; if any irregularities are visible the electrode is cleaned again and replated in order t o produce a steady voltage a t high sensitivity. X defective silver-silver chloride coat gives characteristically unstable reactions and is susceptible to alternating current interference from the temperature bath heaters. ,4 Beckman No. 39071 frit-junction reference electrode, used for the refer-
ence potential, incorporates a liquid junction of sintered carborundum and glass. This electrode is stored in a saturated potassium chloride solution between uses. Detector System. Combustion products leave the furnace through the 14/35 standard taper ground-glass furnace junction connection (20-mm. tubing) shown in Figure 3. The products are then forced by the furnace gas pressure up through another 14/35 joint to a water tower where any watersoluble gases present are dissolved in the carrier solution. The water tower is 20 mm. 0.d. and 18 em. long, with the furnace vacuum connection a t the
top of the tower bent up to prevent loss of carrier solution and opened to the atmosphere for pressure regulation. Any packing material in the tower would tend to collect the carrier solution and interfere with the continuous flow of dissolved sample to measuring electrodes. Straight tower tubing is adequate for collection of the sample by the carrier solution. Since reproducible measurement requires a constant carrier solution flowrate, the rate is made independent of any pressure changes. I n Figure 3 a 0.0016N nitric acid carrier solution is pumped by a Beckman solution metering pump a t 11 ml. per minute from
delivery flask
reservoir
temperature bath Figure 3.
-? I
+acuum
I1
I I
-----------I
Detector system, not to scale VOL. 37, NO. 1 1, OCTOBER 1965
1387
v1
V2
1Y6
1UI
1117
I “T“ Figure 4.
a 2-liter reservoir flask to a 250-ml. Erlenmeyer delivery flask modified as shown; overflow returns to the reservoir flask. This solution then drains into the water tower through a Kontes “Varibor” stopcock set a t 10 ml. per minute. A pressure equalizing connection above the liquid level and below the microvalve maintains a head a t constant pressure. A water towerfurnace junction connection with an inside diameter less than 8 mm. causes undesirable splashing and bubbling when the furnace gases and the carrier solution pass each other a t the junction during a burning cycle. Dissolved combustion products are carried from the water tower to the measuring electrodes through 30 inches of l/Anch stainless steel tubing which is bent into a 3.5-inch diameter coil to fit inside a temperature bath and which is connected to the glass tubing by a/16-inch i d . Tygon tubing. An adaptor with side arm fits on the 19/38 standard taper furnace junction and provides an 8-mm. tubulation for connection to the terminus of solution flow a t the drain flask, again for pressure equalization. The sensor electrode cell, shown earlier in Figure 2, is shaped to prevent any condensed gases from collecting a t the electrode surface. Gases condensed a t the inlet tubing are released by milking a short rubber tube attached t o the 6 m m . capillary tube shown. Since for pressure regulation the vacuum line is always open to the atmosphere and the drain flask stopcock is always open to drain continuously, the sensor electrode holder capillary tubing is
1388
ANALYTICAL CHEMISTRY
Wiring diagram for temperature bath heater
also always left open. The sensor electrode plated wire fits in the sensor electrode cell chamber which is blown 25 mm. below the 10/30 T ground joint and is 13 to 15 mm. in diameter with the 8-mm. inlet tubing attached a t the bottom of the chamber and the 8-mm. outlet tubing attached a t the narrow end of the ground joint. As shown in Figure 3, the sensor electrode cell is positioned inside the temperature bath while the reference cell is outside the bath. The reference cell consists of the bottom 30 mm. of a 22-mm. borosilicate test tube provided with 8-mm. tubing and is attached to the sensor cell with 3/lrinch i.d. Tygon tubing. The reference electrode is positioned in its cell through a No. 3 rubber stopper. The distance between the electrodes is 5 to 7 cm. in the available space but is not critical as long as a fluid connection is maintained between the electrodes. No temperature regulation of the reference electrode is necessary. After carrying the dissolved combustion products past the measuring electrodes, the carrier solution drains off continuously through a drain flask. The drain flask with pressure equalizing interconnection as shown maintains a constant pressure head for steady flow past the measuring electrodes. The carrier solution drains through an open length of Tygon tubing which is connected to the drain flask by a stopcock. This arrangement provides a natural pressure seal and prevents any furnace gases from escaping through the pressure equalizing connection. Because noncorrosive metal tubing
is needed for optimum heat transfer in the temperature bath, the p H meter provision for solution grounding is used. Both recorder and p H meter power plugs are grounded and the stainless steel sample coil is connected t o the pH meter solution ground terminal. Alternating current pick-up from the temperature bath is avoided by filtering the heater circuits; if this pick-up occurs it is a sign of faulty plating on the silver electrode. Temperature Bath. The electronic control unit used with the constant temperature bath is a modified version of the unit repor ed by Sutzer (IO). The direct current heater, originally designed as part of the temperature sensing bridge circuit, is connected separately as the cathode resistor in the output circuit as shown in Figure 4. Two thermistors connected in opposite arms of the sensing bridge are positioned in the mineral oil bath to detect the temperature control point. Provision is made to increase the heat supplied t o the bath by adding a relay-controlled 140-watt alternating current heater. The values of certain of Sutzer’s (IO) original components are changed as shown and an additional output tube, 6BL7, is added to increase the current through the direct current heater. The relay connected in series with the direct current heater is energized when this current reaches 50 ma.; this energized relay closes the alternating current heater circuit. The operation of the phase-sensitive temperature control unit is explained elsewhere (2) and details of operation will not be given here.
DOT
1-111trbotlomless
Figure 5.
braler,ar htalir
1011
Constant temperature bath
The temperature bath detailed in Figure 5 is made \F-ith a bottomless 1-liter beaker as the heatel. element support, centered in a 4-liter beaker as container. The bottom is cut off the smaller beaker and 5 notched wood strips are fastened symmetrically around the beaker vrith thermosetting plastic. Two 6-mm. borosilicate glass tubes, bent as shown, are cemented to the beaker for heater leads. A 2000ohm direct current heater of 14.40 ohms-per-foot resistance wire is wound on the notched wood strips and a 110volt alternating current Eagle straight “Glocoil” element is wound l/z inch above the direct current heater as shown. The heater unit element is held by three metal clamps 1 inch above a wood base and is placed in the 4-liter container with the top 2 inches of the container clear to hold the temperature bath tubing. A 1/4-inch stirring rod is positioned in the center of the bath with a 3.5-inch diameter blade bent to force the contained oil down in the center, then out the bottom, up over the heater coils, past the temperature bath tubing, and back into the center of the bath. Good potentiometric measurements are made with carrier solution temperature variations of as much as 2’ C. measured a t the sensor electrode during routine burning cycles. Routine Measurement. A 400ohm resistor i b piaced across t h e recorder input terminals for all ranges used, and either t h e 5- or the 12.5-millivolt range 7s selected initially when measuring amounts of total chloride less than 5 pg. (see Figure 7 and attendant discussion). Samples prepared in nhexane extracts are evaporated to dryness in porcelain combustion boats with a drop of chloride-free white oil as keeper. h hexane solution of analytical grade D D T or dieldrin is used for standardization and reference. I%’ith the carrier solution flow on, and the temperature bath a t normally 40.0’ =I= 0.1’ C., the electrodes are placed in their holders and given approximately 15 minutes to reach equilibrium. A sample boat is sealed in the combustion tube and a burning cycle is initiated by pressing the furnace start button. During a burning cycle the gas flow and the vacuum are automatically turned on Jvhile the vacuum line and drain flask stopcock always remain open for pressure equalization At the end of a burning cycle the gas and vacuum are automatically turnedL off, the boat is
Figure 6. Time-voltage recorder responses as both peak heights and areas from successive increments of DDT (the base line arrows indicate sample insertion points)
removed from the furnace, and a new sample boat is introduced. Sample voltages are recorded continuously during each burning cycle. Peak heights of the resulting voltage curves are measured and compared to a calibration curve to determine the amount of total chloride detected and measured; peak areas are nonlinear. The silver-silver chloride electrode is subjected to a constant stream of dissolved combustion products during routine burning and any interfering metallic ions or ions other than chloride reacting with the silver-silver chloride electrode cause a high background reaction if present in excessive amounts. Dissolved contaminants are carried out of the system, but any particulate matter washed through the system will be trapped and collected in the sensor electrode cell. If the tubing and cell are not flushed and cleaned a t the end of each day, future voltage measurements will be unstable because of topochemical interaction between the sensor electrode and the collected particulate matter. Since any grease present in the tubing system will ultimately interfere with stable sensor electrode voltage, no grease of any kind is used on any of the ground glass joints. RESULTS AND DISCUSSION
The large-scale relationship between amounts of total chloride and peak heights or areas of time-voltage curves is determined by burning from 0.01 t o 1000 pg. of analytical grade DDT or dieldrin, as illustrated in Figure 6. A plot of the resulting areas shows a curvilinear exponential dependence of the type shown by the equation: Standard Curves.
y =
c ( l 0 Z - 1)
I n this equation y represents the total amount of chloride, C is a constant, and z represents the resulting area of the time-voltage curve. The so-called “voltage” areas from samples con-
”-/
I
I
2
3
Dieldrin in pg.
Figure 7. Representative linear peak height-microgram standard curve for dieldrin Each circle represents a single analysis
taining more than 100 pg. of chloride are difficult to calculate because of a tailing effect in the “voltage” curve. This “response delay” memory effect is briefly mentioned by Jones and Kehoe (‘7). The logarithmic character of this measurement gives a n increase in sensitivity with a decrease in sample size, however. Temperature control, inorganic ion interference, and condition of the plate on the sensor electrode are the main factors in determining the quality of measurement in the lo\\-er region with a reproducibility of 1 0 . 0 3 pg. of chloride in samples containing less than 10 pg. of organically bound chlorine. Furnace location and available laboratory space necessitated 18foot shielded electrode leads; although this length is excessive, the signal-tonoise ratio of the measuring instrument is not a n interfering factor. Because of generally increased background contribution from very large samples, plus the greater sensitivity at low chloride concentrations, the continuous chloride VOL. 37, NO. 1 1 , OCTOBER 1965
1389
Table I.
White Oil vs. Glycerol as “Keepers” ( 1 2.5 mv.)
Peak height
White oil (ml.)
Sample Kone “Keeper” alone “KeeDer” C. 2 ...UE. dieldrin “Keeper” 10 pg. dieldrin
+
I f 1 3 f l 3 f l
0.50
13 f 2
0.50
10 f 2
0.50
48
*3
0.50
43 It 3
H e p t a c hlor I O u. g_.
0
D i e l d r i n 10 fig.
€ 40 E
.I=
i
0.00 0.05 0.50
bm.1 l f l 2*1 3 f l 4 1 1 5 f l
60
v .a
Peak height (mm.)
0.00 0.05 0.50 1.00 1.50
method is ideally suited for routine detection of chloride present in amounts less than 10 wg, per sample. A standard curve to illustrate this point is shown in Figure 7 and later in Figure 9. Influence of “Keeper”. Pharmaceutical grade white oil is used almost exclusively as “keeper” (4). A comparison of white oil with reagentgrade glycerol for possible use with more polar pesticides is shown in Table I. Increasing amounts of oil cause increasing interference, but the contribution from both the oil and glycerol is negligible because it is not practical to use more than a drop or two (0.05 to 0.10 ml.) per sample. With these, or even smaller, amounts, long standing of boat plus oil results in the oil slowly creeping out of platinum, porcelain, and quartz boats; with larger amounts, creeping is rapid and must be accommodated by minimum starting time prior to burning.
50
Glycerol bl.1
30
... ...
... ...
Table II. Various Insecticides Plus 0.05 ml. of White Oil ( 1 2.5 mv.)
Insecticide None p,p’-DDT
Chlorine, Micro% grams ... ... 50
Aldrin
58
Guthion Heptachlor
0 67
Dieldrin
56
2 5 9 2 10 10 2 10 2 5 8
Peak height (mm.). 4, 4, 4 11 22 45 12 45 4 13 50 12, 13, 14 28 35
Range 12.5 mv.
I n general, the consequence of increasing amount of white oil is a rise in background as sensed by the electrode system, but in the presence of various insecticides and various substrate extractives this “reaction” between excessive amounts of oil and electrodes can nullify any meaningful chloride measurement. Illustrative data are shown in Table I1 and plotted in Figure 8 as micrograms of organically bound chlorine us. peak in mm. The linear relationship at constant amount of white oil is clear; if the amount of white oil is varied, however, this relationship becomes curvilinear. Increasing amounts of white oil broaden all peaks, especially in the 5-mv. range,
”
1
2
3 4 5 6 Amount of chloride in p g .
7
Figure 9. Effect of recorder range upon slopes of calibration curves Each point i s a single analysis
with the appearance of an occasional second small peak and often severe tailing. Day-to-Day Reproducibility a n d Replicability. Day-to-day variations in standard curves occur as illustrated in Table 111, depending in part upon the age of the silver-silver chloride electrode. Freshly prepared electrodes are unstable and should be stored in distilled water several days before use, and always in distilled water when not in use.
c
-*
20 Day-to-Day Reproducibility and Replicability with DDT Plus 0.05 rnl. of White Oil per Sample ( 1 2.5 mv.) Peak height (mm.). n5 Date pgyC11 pg. C12 pg. c13 pg. c15 pg. c18/17 ... 23, 24, 24,26 35,36,40 62,63 8/18 7,9 11, i i , ‘ i 2 20,29 56,56 8/19 ... 11, 13, 14, 14 20,20,20 28, 3 i , ’ i i ,31 ... 12,14 25 34,37 ... 8/24 8/25 6,‘7,’6 12, 14, 15 35,37 ... 8/26 ... 9,9, 11, 14 23,25’ ’ 37,41 ... 11 15, 16, 16 ... 42,42 ... 8/27 Bath temperature a Either black Apiezon wax or white epoxy resin electrode stems. 42.0” f 0.1” C., solution flow rate 11 f 2 ml. per minute. No distinction made as to electrode stem or to freshness of electrode. Table 111.
a IO ‘Guthion a n d B l a n k I
C
1
I
I
I
I
I
2 3 4 5 6 Amount o f c h l o r i d e in pg.
7
Figure 8. Various amounts of various insecticides at constant amount of “keeper” Guthion contains no organically bound chlorine
1390
ANALYTICAL CHEMISTRY
Accuracy. Chloride-free corn meal samples are fortified with various organochlorine compounds a t t h e 0.4-p.p.ni. chloride level. The compounds are rhlorotetracycline, chloranilihenicol, p,p’-DDT, griseofulvin, caldariomycin, and. Drosophilin A ; except for D D T , these are all naturally occurring chlorine-containing compounds. The compounds are added t o the corn meal both in solution and in the dry form. After b l h g blended, each sample is air dried, then extracted with n-hexane in the ueual manner. The hexane extracts are aliquoted, concentrated, and analyzed in quadruplicate by the present technique as well as by neutron activation ( 3 ) . Results are shown in Table IV. Range Affects. T h e slope of a calibration curve i:j dependent upon t h e range used for t h e recorder. I n Figure 9, DDT standard curves are plotted for 4 range:, from 5 to 50 mv. t o illustrate this paint. Other Factors. As discussed by Helmkamp et al. (6) and as mentioned earlier, certain inorganic ions interfere in this method. .ilso, precision temperature control a t the sensor electrode is essential by virtue of the Sernst equation.
Table IV. Comparative Analytical Results of Standard CI- Fortified (0.4 p.p.m.) Corn Meal Samples b y the Present Method vs. Neutron-Activation Analysis
C1- found, p.p.m. Seutron-activation Chlorinated compound added Present method analysis” None 0.0,o. l , o .0 0.08 f 0.02 Chlorotetracycline 0.5 f 0.1 0.36 f 0.08 0.5 f 0.2 0.50 f 0.10 Chloramphenicol 0.5 f 0.1 0.49 f 0.03 P,P‘-DDT Griseofulvin 0 . 5 It 0 . 1 0.53 f 0.10 0.41 f 0.08 Caldariomy cin 0.4 f0.1 Drosophilin A 1.3 f 0.3 1.83 f 0.03 0.02 f 0 . 0 1 Solvent alone 0.015 f 0.003 a After 1OX concentration of the extractive solution. ACKNOWLEDGMENT
The authors are grateful to John E. Leonard, of Beckman Instruments, Fullerton, Calif., for invaluable assistance and suggestions during the entire course of this work. LITERATURE CITED
(1) Agazzi, E. J., Peters, E. D., Brooks,
F. R., ANAL.CHEM.2 5 , 237 (1953). ( 2 ) Aronson, 11. H., Kezer, C. F., “100 Electronic Circuits,” Vol. 1, p . 155,
Pittsburgh Instruments Publishing Co., Pittsburgh, Pa., 1957. (3) Guinn, I-.P., Schmitt, R. A,, Residue Rev. 5 , 148 (1964). (4) Gunther, F. A., Blinn, R. C., “Analysis of Insectirides and Acaricides,”
PP. 357-70, Interscience-Wiley, Sew Yorl Proc. I.R.E. 4 3 , 701 I
_
^
_
(19.55). \ _ _ _ .
(11) White, T. T., Penther, C. J., Tait,
P. C., Brooks, F. R., ANAL. CHEM. 25, 1664 (1953). RECEIVEDfor review April 19, 196s. Accepted J ~ l y22, 1965. Supported in part by U.S.P.H.S. grant No. EF-00029.
Spectrophotometric Determination of Antimony R. M. MATULIS and J.
C. GUYON
Department of Chemistry, Universify of Missouri, Columbia, M o .
b A sensitive, spectrophotometric technique for antimony has been developed. The method i s based upon the enhancement b y antimiony o f a blue hue due to the reductiori of the molybdate aGgregate near p t i 1.4. The addition of a large excess of Na2S04 minimized the blank and aided the reproducibility of the system. Spectral data indicate an interaction of Sb(lll) with the molybdate aggregate under conditions of the analysis. The method i s sensitive for antim’ony in the range of 0.02 to 1.6 p.p.m. The molar absorptivity can b e calculated to b e 1.1 X The method i s subject to several interferences but the antimony can be cleanly separated as the volatile chloride in a Scherrer apparatus. Good results were obtained on NBS 5 4 d Sn-base bearing metal.
lo5.
A
NEW, SENSITIVE spectrophotometric technique for determining micro amounts of antimony has been developed. Many of the current methods in use employ organic chromogenic
reagents which form antimony complexes extractable into nonpolar phases on which absorbance measurements are made. The most common of these techniques have employed Rhodamine B ( 7 , 13, 17, 26, 28), methyl violet (16, 23), Brilliant Green (6, 11, 24), phenylfluorone, (8, 19, 21), and crystal violet (12, 25). These methods are satisfactorily sensitive, but suffer from a number of interferences, and require extraction steps. The method proposed here has even greater sensitivity than the above techniques and has no extraction step. The interferences are readily separable. Other current methods rely upon complex formation in the aqueous phase to develop color proportional to the antimony concentration. Iodide ion in acid solution (4, 5 , 14, 18, 20, 27), or with iodide and pyridine (2) have been used, but not with the sensitivity of the above methods. The yellow complex formed with thiourea has been applied with some success (1). KO heteropoly acid technique has been developed in which antimony be-
comes the central element coordinated with molybdate or other hetero-atoms. Advantage has been taken of the sensitivity of heteropoly acids to weak reducing agents in acidic solutions. Sb (111) has been used to reduce molybdophosphoric acid (9), molybdovanadosilicic acid (10) and molybdotungstophosphoric acid (15) to the soluble “heteropoly blue,” the intensity of the blue hue being proportional to the Sb (111) content. This paper reports the results of a study designed to develop a spectrophotometric method for antimony and to continue an investigation into heteropoly acid formation. EXPERIMENTAL
Absorbance measurements were made either on a Cary Model 12 recording spectrophotometer, or on a Beckman D U spectrophotometer each with matched 1-em. quartz cells against double distilled water . Emission spectra were taken on a Jarrell-Ash 480-cm. emission spectrograph with a Wadsworth mounting. Apparatus.
VOL. 37, NO. 11, OCTOBER 1965
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