Vohammetric Determination of Metals in Low Concentrations W. W. ULLMANN,' 6. H. PFEIL, J. D. PORTER,* and W. W. SANDERSON Division o f laboratories and Research, New York State Department o f Health, Albany, N. Y.
b Voltammetric methods for determination of metals in trade wastes and receiving streams are theoretically preferable to colorimetric methods because of the rapidity of the analysis and because the method may be applied simultaneously to a variety of metals. An important limiting factor in the application of polarography to stream pollution problems, however, i s the lack of sufficient sensitivity with most of the available techniques. By use of a recently designed quiet mercury pool cathode cell in conjunction with a standard Sargent Model XXI Polarograph, determinations of cadmium, copper, nickel, lead, and zinc, in concentrations down to 0.01 mg. per liter may be accomplished with concentration of a 100-ml. sample to 10 ml. during a short preliminary processing. Chromium and iron cannot be determined b y this procedure. Data are given to indicate the accuracy and precision of the method for the analyses cited. from metal plating operations are often a difficult problem for a stream pollution control laboratory, especially nhen their composition is not knomn even qualitatively. Aprlropriately sensitive tests must be allplied for each metal of interest on the chance that it may be present, and since concentrations as lon as 0.01 mg. per liter may be significant, standard chemical procedures must lie, used with care and may he tinie consuming and burdensome. Theoretically, polarographic techniques should be ideally suited to the meawrement of many nietallic pollutants of nater because preparation of the sample is usually simple, and both qualitative and quantitatiJ e results can be obtained siniultanc~ouslyfor several metals. In practice, hon-ever, the loner limit of detection with the dropping mercury electrode and ordinary polarographic equipment is about 10-6A11,Le., ASTCS
1 Present address, Division of Sanitary Services, Connecticut Department of Health, Hartford, Conn. 2 Present address, Department of Chemistry, Union College, Schenectady, N. Y .
N
s nteredqloss bubbler
Figure 1. Polarographic cell for dropping mercury electrode and mercury pool electrode
about 0.1 mg per liter for a typical heavy metal, which falls short by a n order of magnitude of reaching the lower end of the significant concentration range of metallic pollutants in surface waters. Voltammetric techniques other than polarography may be capable of greater sensitivity, and Porter, Ullmann, and Fanderson ( 2 ) proposed the use of the mercury pool cathode of Streuli and Cooke (6,6) in a semiquantitative screening procedure for industrial n astes. The pool electrode has a much larger area than the dropping electrode, and hence the diffusion current due to a given concentration of reducible ion is lnrger. At the same time the residual current is comparatirely small, iince the area is constant and there is no necessity of continually charging a new double layer. The large ratio of diffusion current to residual current is responsible €or the high sensitivity of the pool electrode. The same kind of qualitative and quantitative information is obtainable from the pool electrode as from the conventional dropping electrode. Reducible species in the solution produce current n a t e s , or rather, peaks, characterized by half-peak potentials independent of concentration. These are often very close to the corresponding polarographic half-wave potentials. Furthermore, the peak current under properly standardized conditions is closely proportional to the concentration. Automatic scanning is essential, since peak heights depend on the rate of
voltage increase as well as on concentration. Because of the great sensitivity of the pool electrode and the fact t h a t its surface is not continually renewed, rigorous exclusion of contaminants is mandatory. The previous paper from this laboratory ( 2 ) reported the use of the mercury pool cathode for detecting copper, lead, nickel, cadmium, anti zinc in water samples. The present paper is concerned with refinements TI hich make possible cluantitatiw results ior these f i e~metals in a single sample. APPARATUS
Electrical equipment. A Sargent Model XXI Recording Polarograph or its equivalent is used. The time required for complete bridge rotation for this instrument is 13.5 minutes. For all measurements in this study the scanning rate was approximately 0.15 volt pel minute. T'olt'ammetric cell (Figure 1). A tube conbaining at its upper end a fritt'ed-glass bubbler projects a short distance upward through the bottom of the cell. The bubbler niny be made from 140- to 200-mesh borosilicate glass powder by the method of Stone and Keiss (43. The mercury pool lies in the annular space around the bubbler tube, which is sealed in such a way that the pool area, about 0.8 sq. em., is independent, IT-ithin limits, of the volume of mercury added. -4sealed-in platinum wire makes electrical contact with the pool. A 3-may stopcock permits passing nitrogen either through the bubbler or over the surface of the solution. The reference electrode, a VOL. 34,
NO. 2,
FEBRUARY 1962
213
small helix of No. 24 silver wire chloridized by brief anodic treatment in 10% hydrochloric acid, is suspended in a U-shaped sidearm. The inside of the cell is coated with a silicone preparation (Beckman Instruments, Inc. Desicote) to prevent penetration of the sample between the glass and the mercury. The cell serves also for conventional polarography if the dropping electrode is inserted into the upper portion of the sample compartment. Nitrogen purification train. Tank nitrogen is led through a Vycor tube filled with copper turnings and heated to 450" C. in a n electric furnace (E. H. Sargent B: Co.), then successively through a trap, a bubbler to indicate flow rate, and finally the cell. The trap prevents backflow of liquid into the furnace. Mercury purifier. A Sargent mercury oxidizer and a Sargent gold adhesion apparatus are used. Deaeration apparatus. A test tube (15-mm. inside diameter) is graduated a t 10 ml. and provided with a sidearm about 15 mm. above the graduation. REAGENTS
Mercury, redistilled, National Formulary grade. Used mercury is purified. Ammonia solution. Tank ammonia gas is passed through a glass wool trap into chilled double distilled water until the concentration reaches about 7M. Alternatively (S), 900 ml. of concentrated ammonium hydroxide in a 1500ml. distilling flask is distilled into a 1liter polyethylene bottle containing 250 ml. of chilled double distilled water until the volume of liquid in the bottle has increased to 900 ml. The tip of the condenser is kept below the surface of the liquid. Saturated ammonium sulfate solution. 751 ammonia solution is neutralized with concentrated sulfuric acid slowly and with extreme caution. Heavy metals are removed by extraction with dithizone in carbon tetrachloride and the solution is concentrated to saturation by evaporation of excess water. PROCEDURE
Preparation of Sample. Rinse all glassware, including the cell, successively with concentrated nitric acid, distilled water, and twice with double distilled mater. Evaporate 100 ml. of sample with 0.1 ml. of concentrated sulfuric acid in a n Erlenmeyer flask and bring to fumes. Add concentrated nitric acid in increments of several drops with heating between additions until the solution clears and color disappears. Wash down the sides of the flask with double distilled water. Add a few crystals of sodium sulfite to reduce hexavalent chromium and again bring to fumes. Cool the flask, neutralize with ammonia to precipitate iron and boil off the excess ammonia. Filter through a Hirsch funnel directly into the deaeration apparatus and bring up to the 10-ml. mark with double distilled water. Pass nitrogen through 214
ANALYTICAL CHEMISTRY
Electrolyle 0 18M(NHq)2 SO4
w
a
2 20
$0.20
-____
0
-0.40
- 0.80
VOLTS Figure 2. Current voltage curve of sample 10 in (NH4)&04
the solution using a tube with a small orifice for a t least 10 minutes before transfer to the voltammetric cell. Preparation of Cell. Add saturated ammonium sulfate solution to the sidearm of the cell to fill about half its height. Add carefully sufficient saturated potassium chloride solution to the outer end of the sidearm t o force the ammonium sulfate just t o the junction of the sidearm with the main body of the cell. The less dense potassium chloride solution forms a layer above the ammonium sulfate solution. Insert the chloridized silver wire reference electrode into the potassium chloride solution. The first voltage scan with a freshly prepared mercury pool cathode usually gives an irregular curve with a high residual current. Basic salts, oxides of mercury, and traces of organic matter are probably responsible. Preconditioning of the mercury by a blank run eliminates most of these irregularities. Add double distilled water to the main section of the cell to a level just below the upper nitrogen inlet tube, and add to it 10 drops of ammonium sulfate solution and 2 drops of 95% ethyl alcohol. The latter lowers surface tension thus permitting more rapid removal of oxygen in the succeeding step ( I j . Bubble nitrogen through the solution for 5 to 7 minutes, add 0.3 to 0.4 ml. of mercury from a small buret, making sure that the platinum wire is covered and that the mercury forms a complete ring. Bubble nitrogen through the solution for an additional 3 t o 5 minutes, and then turn the stopcock to bypass it over the solution. Scan the voltage rapidly up and down several times manually, and then make one or two automatic scans a t the instrument scanning rate of 0.15 volt per minute for a 2-volt scan. This process serves to smooth out irregularities beliered to be due to minor contaminants of the mercury or possibly to organic substances which occur in initial runs. It does not affect peak heights in the sample runs which follow. Sample Runs. Remove the blank from the cell by aspiration, taking care not to disturb the mercury surface although all of the aqueous liquid must be removed. Transfer the sample
through the sidearm of the deaeration apparatus to the cell. Maintain a good current of nitrogen while the blank is being removed and the sample added. Add 2 drops of ethyl alcohol, and bubble nitrogen through the sample in the cell for 5 to 7 minutes to remove any oxygen introduced during the transfer. Meanwhile make a few rapid manual scans to further precondition the mercury. Bypass the nitrogen over the solution, and make a t least two automatic scans from f0.2 to -1.4 volts. By making more than one scan the risk of mistaking an accidental and transitory peak for a significant one is reduced. Bfter the peaks for the various metals in the sample have been located, rescan the sample automatically as before, but hold the voltage stationary just beyond each peak until the current decays, and then allow automatic scanning to proceed. I n this manner metals whose half-peak potentials lie close together may usually be evaluated without interference. \Then satisfactory curves have been obtained in the ammonium sulfate medium, make the solution about 0.411 with respect t o ammonium hydroxide. Bubble nitrogen for a few minutes t o ensure good mixing and to remove any oxygen which may have been introduced with the ammonium hydroxide. Make a t least two automatic scans from 0.0 to -1.4 volts a t the rate of 0.15 volt per minute. After the solution has been made ammoniacal, it is essential not to allow the pool electrode to become positive, since troublesome mercury-ammonia complexes may form. Finally, rescan the sample, pausing just beyond each peak, as described above. RESULTS
Determination of half-peak potentials (Ellz)and peak heights (h) is ' somewhat less certain when the pool cathode is used than in conventional polarography. The most satisfactory way to proceed is to draw t\vo parallel lines tangent to the curve a t the turning points, marking the lower and upper limits of the excursion associated with the particular metal. The half-peak potential is then given by a point on the curve lying half way between these parallel lines, and the peak height is the vertical distance between the lines. These definitions are illustrated in Figures 2 and 3. Figure 3 shows also mutual interference of peaks and the effectiveness of the prescribed technique in overcoming it. In an alkaline medium the half-peak potentials of copper (-0.32 volt) and lead (-0.38 volt) are so close that their peaks merge and appear as one. Similar interference between nickel and zinc, also shown in Figure 3, was eliminated by allowing the current due to reduction of nickel t o decay before advancing the scan further to obtain the zinc peak. I n Table I are presented the half-peak potentials and peak heights found for
zinc, and cadmium but nickel must be determined in ammoniacal solution since it gives no wave unless a n excess of ammonia is present. Chromic, chromate, and ferric ions give no useful peaks, but the latter two ions interfere with the determination of other metals. These interferences are, hon-ever, easily eliminated as described in the procedure. Table I1 summarizes data obtained in
Reproducible potentials and heights were obtained for copper, lead, cadmium, nickel, and zinc. Copper gives two distinct peaks in ammoniacal solution and one in ammonium sulfate solution. Lead, cadmium, and zinc also give peaks in both ammoniacal and ammonium sulfate solutions. I n general, however, the ammonium sulfate supporting electrolyte serves best for copper, lead,
Electrolyte 0.4M NH40HtO.I8M(NH4)2504
tn
nickel current decoys
L
Table I.
Behavior of Various Metals with Mercury Pool Voltammetry
-0 80
Supporting Electrolyte
VOLTS
iNH,).SOa . , ~ Relative peak Relative peak Half peak height pa./pg. Half peak height Ma./pg. potential (v.) of element potential (v.) of element +O 05- 0.00 0.161f 0.008 -0.07--0.0!3 0.041i 0.005 -0.32--0.33 0.060f 0.003 -0.33--0.38 0.0363z 0.006 -0.38 0.022i 0.005 -0.50--0.54 0.092 0.006 -0.65--0.67 0.094f 0.006 S o peak -0.88--0.93 0.097i 0.003 -0.96--1.02 0.180~0.010-1.17--1.1!3 0.180150.028 0.10 Wave is difficult E1 '2 indeterniin- Wave is difficult to measure ant peak a t t o measure 4-0.10, -0.03 If peaks occur they are extremely low 1%-ithEl.* = -0.86--0.88 May cause interference with zinc determination
-0.20-040
0.4.11 ",OH
0.18M ISHd)2SOa -,~
Figure sample
3. Current voltage curve of 10 in ("&SO4 and N H 4 0 H
stymate solutions of copper, lead, cadmium, nickel, zinc, chromium(VI), and chromium(II1) salts in ammonium sulfate solution 1% ith and without free ammonia. Potentials are given with reference to the silver-silver chloride electrode in saturated potassium chloride solution. T o convert to the basis of the saturated calomel electrode, add -0.03 \Tolt.
Table
II.
Metal Copper Copper Lead Cadmium Nickel Zinc Chromium( T I ) Chromium( 111) Iron( 111)
Precision and Accuracy for Determination of Various Metals b y Mercury Pool Voltammetry
Supporting Electrolyte
~-
0.18X(PiHc)2S04 A4v.a recovered Range (mg./1.) (mg./l.) 0.016 0.0144.019 0,051 0.039-0.062 0,100 0.092-0.106
-4dded Metal (mg./l. ) Copper 0.01 Copper 0.05 Copper 0.10 Copper ... Lead 0.01 Lead 0.05 Lead 0.10 Cadmium 0.01 Cadmium 0.05 Cadmium 0.10 Xickel 0.01 Sickel 0.05 Sickel 0.10 Zinc 0.01 Zinc 0.05 Zinc 0.10 a Average of 10 replicate Table 111.
+ 0.1831
~
...
0:ooi
0.047 0.100 0.007 0.054 0,100
Added (mg./l.)
0,001 0,007 0,005
...
0.048-0.063 0.096-0.109
0 : io2
...
...
0.10 0.01 0.05 0
...
0.049 0,100 portions.
0.10
0.10 0.10
0,006
0.007-0.010 0.046-0.054 0.092-0.109
...
... 0 :ioo 0,100 ...
...
0:ooo 0,003 0.017 0.001 0.006
0.005-0.008
...
Std. dev. (mg./l.)
0:Obl 0.001 0.001
0.4144",OH -4v.a recovered (mg./l.)
... 0.10
+ 0.1834(KH,)?SOc Std. dev. (mg./l.)
Range (mg./l.)
...
... 0,090~0,124 0.093-0.107
o:oi2 0.005
o .ogiio.155
0:0-3
O.OS&O.lOQ
0:006 0,001
...
...
...
0 :io2 0.009
0.100
0.007-0.011 0.048-0.061 0.097-0.107
o:ioo
O.O8$-b. 126
0.055
...
0,003
0.003
...
...
...
...
o:oo1
Determination of Various Metals in Twelve Synthetic Trade Wastes b y Mercury Pool Voltammetry
llilligrams per liter of sample Cadmium SamReple Added covered 1 0.01 0.02 0.10 0.10 0.00 0.00 0.00 0.01 1 .oo 0.82 0.00 0.00 8
9 10 11 12
0.05 0.05 0.05 0.05 0.05
0.10
0.05 0.05
0.04 0.05 0.05
0.11
Copper Added
Recovered
0.05
0.05
1 .oo 0.10
0.00
0.01 0.05
0.05 0.05 0.05 0.05 0.05 0.10
0.92 0.10 0.01 0.01 0.06 0.07 0.06
Lead Added 1 .oo 0.10
0.05 0.00 0.00 0.01 0.05
0.04
0.05 0.05
0.06 0.06
0.05 0.05 0.10
0.15
Recovered 1.20 0.11 0.07 0.01 0.01 0.01 0.07 0.07 0.04 0.06 0.08 0.10
Kickel Added 0.10 0.05 1 .oo 0.01 0.05 0.00 0.05
0.05
Recovered 0.13 0.04 1.04 0.02 0.06 0.00 0.07
0.05 0.04 0.07
0.05 0.05 0.05
0.05
0.10
0.11
Zinc Added
Recovered
0.05
0.04
0.01
0.03
0.05 1.00
0.05
0.10
0.09 0.04 0.10 0.05 0.10 0.16 0.07 0.10
0.05 0.05 0.05 0.05 0.05 0.05
0.10
0.95
Interfering Substances Added ChroOrganic mium Iron matter
...
... ... ... 100 ... ...
100 100 100 100
... ...
...
... ...
100
... 100 100 100 100
VOL. 34, NO. 2, FEBRUARY 1962
100 ... 100
... ... ...
100
...
100 100 100
215
the analysis of 10 different solutions of each of five metals, copper, lead, cadmium, nickel, and zinc, a t each of the three concentrations. 0.01,0.05, and 0.10 mg. per liter. The accuracy and reproducibility were for the most part mithin the limits ordinarily associated with quantitative determinations, and the sensitivity was at least equivalent to 1 pg. of the metal in the IO-ml. volume of the cell. Table 111 shows the results of applying the procedure to 12 samples prepared to simulate trade wastes. Each 100-ml. sample contained two or more metals in amounts between 1 and 100 fig. Some samples contained also iron, chromium, and organic matter (tartaric acid). Satisfactory recovery was obtained. The high values for zinc in samples 7 , 9, and 10 were due to interference by
iron, which was apparently not completely removed by filtration through Whatman No. 40 paper. A change to a medium porosity glass filter eliminated the interference in samples 11 and 12. The spurious recoveries in samples 4,5, and 6 were probably due to metallic contaminants in the reagents, the mercury, or the glassware. SUMMARY
B y use of a mercury pool cathode 11ith a silver-silver chloride anode, voltammetric measurement of copper, zinc, cadmium, lead, and nickel is made with satisfactory accuracy in concentrations down to 1 pg. per 10 ml. of cell solution. By conccntrating a 100-ml. sample to 10 nil., an original concentration of 0.01 mg. per liter may be determined.
LITERATURE CITED
(1) Kolthoff, I. l f . , Lingane, J J., “Polarography.” 2nd ed., Interscience,
Kew York, 1952. (2) Porter, J. D., Ullmann, W. W.,
Sanderson, K. W., “Polarographic Scanning of Industrial Waite Samples,” Proc. Ind. Waste Conf.. 1 4 t h Conf.;
E n g z n e e r z n g Bull , P u r d u e r m v e r s z t y , 44, 587-606 (1960). (3) “Standard Methods for the Examination of Water and \Tastewater,” 11th ed., p. 365, =Imerican Public Health Association, Inc., S e w York, 1960. (4) Stone, H. IV,, Keiss, L. C . Im. ENQ. CHEM., h 4 L . E D . 11, 220 (1939). ( 5 ) Streuli, C. .I.,Cooke, TT. D., AXAL. CHEM2 5 , 1691-6 (1953) ( 6 ) Ibzd., 2 6 , 963-70 (1954).
RECEIVED for reviev September 20, 1961. Accepted November 30, 1961. Division of Water and Waste Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961.
Polarographic Reduction of 1,2,4-Benzothiadiazine-l 1-dioxides and Related Compounds
,
ALLEN 1. COHEN, B. T. KEELER, N. H. COY, and H. L. YALE Squibb Institute for Medical Research, New Brunswick, N. 1.
The reduction of trifluoromethylaryl compounds in aqueous (protonated) and N,N-dimethylformamide (nonprotonated) solutions involves all three fluorine atoms; in water the three fluorine atoms are removed simultaneously, while in N,N-dimethylformamide reduction occurs in one, two, or three steps, depending upon the number and type of substituents on the aryl ring. A number of sulfamyl-, carboxyl-, and aminotrifluoromethylbenzenes and 6-(trifluoromethyl)-l,2,4benzothiadiazine-1,I-dioxides have been studied; the correlation between structure and polarographic behavior is discussed.
R
several halogenated derivatives of 1,2,4 - benzothiadiazine - 1,l - dioside have been found to be useful as diuretics (9, IS). One such derivative is bendroflumethiaaide, 3 -benzyl - 3,4 - dihydro - 6(trifluoromethyl) - 7 - sulfamyl - 2H1,3,4 benzothiadiazine - 1,l - dioxide; the Squibb trade-mark is Naturetin. Previously, ultraviolet (2, 9, 10) and infrared (12, IS) spectroscopy has been employed to coArm chemical structures. This study was conducted to show the relationship of reduction potential and structure of these compounds and their precursors. ECENTLY,
-
216
ANALYTICAL CHEMISTRY
Lund (8) reported the reduction of 6 - trifluoromethyl - 7 - sulfamyl - 3,4dihydro - 1,2,4 - benzothiadiazine - Illdioxide in aqueous media. Large scale electrochemical reduction showed that the three fluorines were removed and replaced by hydrogen. Under the same conditions, however, 3-trifluoromethyl - 4 - sulfamylaniline and 3 - trifluoromethylaniline were not reducible. Chlorobenzene, previously reported unreducible, was found to be rcducible in N,N-dimethylformamide (DMF) (7). I n the present study, all trifluoromethylaryl compounds are polarographically reducible in anhydrous D N F and the half-wave potential@) influenced by substitution in the aromatic ring. STRUCTURAL CONSIDERATIONS
The ability of a substituent to withdraw or donate electrons is manifested by induction (-I, +I) and resonance (-R, f R ) (6). Negative inductive (-1) and negative resonance (-R) substitution make the aromatic ring electron-deficient and enhance the ease of reduction of the compound or reducible substituent. Conversely, an inductive-positive, resonance-positive group makes the ring electron-rich and hinders reduction of the compound or reducible group. A substituent may simultaneously donate electrons through
resonance (+I
