b
8.w. al.
= = =
cell length in cm. sample weight size of aliquot in ml. DISCUSSION
Salicylanilide was added to tung oil varnishes of various oil lengths, extracted, and determined. Some results of these determinations are shown
in Table I. As the entire sample was used in most cases, duplicate results are not Dresented. T i e entire procedure is simple to conduct, and the relatively high absorptivity of this fungicide is responsible for the high accuracy (error less than 1%). Other-types of varnishes, based on alkyd resins and rosin esters, have been analyzed with equal accuracy.
LITERATURE CITED (1) Specification pVIIL-V-173, “Varnish,
Moisture- and Fungus-Resistant, for the Treatment of -Communications, Electronic, and Associated Electrical Equipment,,, Jan, 23, 1952, H , A ~ A L . CHEM. 21, (2) Swarm, 804 (1949).
RECEIVED for review February 6 , 1958. Accepted May 27, 1958.
DifferentiaI Voltammetry Using the Hanging Mercury Drop Electrode KENNETH J. MARTIN and IRVING SHAlN Chemistry Department, Universify of Wisconsin, Madison, Wis. The general techniques of voltammetry with continuously varying potential have been applied to a differential method in which two electrolysis cells are used, each with a hanging mercury drop electrode. Using this method, it is possible to determine 10+M solutions with increased precision, and also to analyze solutions as dilute as 10F6Mwithout removing oxygen from the electrolysis cells. Mixtures can be analyzed easily by adding individual components of the mixture to the reference cell.
0
to the problem of increasing the sensitivity and convenience of polarographic and related analytical methods, perhaps the most important have involved attempts to compensate for the residual current (3, 5, 8). Cancellation of this residual current rvould improve the accuracy of determinations a t low concentration levels and eliminate the need for pre-electrolysis of the indifferent electrolyte (6). Furthermore. compensation of unwanted electrolysis currents would permit the rapid analysis of all constituents in a mixture. I n some cases it would be possible to perform analyses without removing dissolved oxygen from the solution. A method of obtaining compensation of this type is the differential technique (2, 8 ) . Two cells are used, one containing the unknown solution with the indifferent electrolyte, the other containing only the indifferent electrolyte. The circuit is constructed so that the difference in current between the two cells is measured, Application of this technique to conventional polarography has had only limited success because of difficulty in synchronizing two dropping mercury electrodes (1, 4). F THE MANY APPROACHES
1808
ANALYTICAL CHEMISTRY
It was found that many of the difficulties encountered when using dropping mercury electrodes can be avoided by using hanging mercury drop electrodes in a two-cell differential circuit, using the general techniques of voltammetry with continuously varying potential. EXPERIMENTAL
Apparatus. T h e apparatus was similar t o t h e schematic diagram given by Delahay ( 2 ) . The polarization unit and load resistors have been described (9). The difference in potential developed across the two load resistors was amplified by a Leeds & Northrup direct current microvolt amplifier and recorded as a function of time on a Bristol potentiometer recorder. The chart speed was 20 inches per minute. The rates of voltage scan were 0.0278 volt per second and 0.0417 volt per
second, corresponding to 36 seconds for a 1.000- and 1.500-volt sweep. Cells and Electrodes. T h e hanging mercury drop electrode assembly was similar t o t h a t described previously ( 7 ) . I n preliminary experiments, separate reference electrodes were connected to each cell by means of a salt bridge, but differences in potentials of these reference electrodes resulted in large errors in matching the cell potentials when different currents were passed. A single silver-silver chloride (saturated potassium chloride) reference electrode was constructed with two salt
i
s
I
I
I 0
I -02
I
I
-06
-04
I -08
1
1
-10
VOLTS
Figure 1 . Current-voltage curves for reduction of air-free solution of 1 .OO X 10-4M thallium and 2.00 X 10-6M cadmium A. B.
Figure 2. Current-voltage curves for reduction of air-free mixture of 2.00 X 1OF4Mthallium, 1.00 X 10-4M lead, 1 .OO X 1 OU4Mbismuth, 0.33 X 1 O-4M iodate, and 1.00 X 10-4M zinc
Nondifferentially Differentially, with thallium compensated
A. 6. C.
D. E.
Differentially, with only indifferent electrolyte in reference cell Differentially, with thallium compensated Differentially, with thallium and l e a d compensated Differentially, with thallium, lead, and birmuth compensated Differentially, with thallium, lead, bismuth, and iodate compensated
bridge arms so that the same reference electrode could be used for both the reference cell arid the analytical cell. This design eliminated many difficulties in construction and matching of reference electrodes. All potentials in this work refer to this silver-silver chloride electrode. 30 particular effort was made t o match the areas of the two hanging mercury drop electrodes, as slight differences in area could be allowed for by trimming the load resistors. A typical value for the drop radius was 0.07 em.; for the area, 0.05 sq. em. Materials. All chemicals were reagent grade and used without further purification. Solutions were prepared with triply distilled water. Linde high-purity nitrogen \\-as used t o reniove oxygen from t h e cells TThen studying air-free solutions. RESULTS AND DISCUSSION
Compensation of Residual Currents. It was previously shown (7) that the analytical method of voltammetry with continuously varying potential, when used with the hanging mercury drop electrode, was about tenfold more sensitive than conventional polarography with the dropping mercury electrode. This improved sensitivity was mainly due to the tenfold decrease in residual current observed. B y making the measurfments differentially, a further fourfold reduction in the observed residual current can be obtained. This is reflected in the improved precision of measurement of the peak currents of micromolar solutions of several ions (Table I). The indifferent electrolyte used in these solutions (0.lM sodium hydroxide, 0 . 5 X sodium tartrate) \Tat
I
L.
2 0 ~ 1
0.c
05".
not purified by pre-electrolysis, which would have been required if the nondifferential technique had been used, Mixtures. Analysis of mixtures is difficult when using voltammetry with continuously varying potential owing t o t h e decay of t h e diffusion current after each peak has passed. T h e most easily reduced ion of those present can be determined as precisely as in a single component system, b u t if another substance is reduced at a potential where t h e current from the first is decaying, it is difficult to determine the proper base line. Accurate measurement of the second peak current is particularly difficult when the more easily reduced substance is present in higher concentration (7). An example of this difficulty is shown in Figure 1. Curve A is the current voltage curve obtained on a solution containing 10-4Llf thallium and 2 x 10-6;M cadmium. The indifferent electrolyte was 0.1-11 potassium chloride. The current due to the cadmium is so small that it is entirely masked by the current from the more easily reduced thallium. Merely increasing the sensitivity of the current measuring device would not reveal the cadmium wave. Horn-ever, if the same solution is run differentially with 10-4M thallium in the reference cell, the sensitivity of the measuring device can be increased to reveal clearly the cadmium reduction curve (curve B ) . Under these conditions the cadmium peak height is reproducible m-ith a relative error of less than 2.5%. To show the effectiveness of this type of compensation, a curve was obtained with a solution containing five reducible substances (Figure 2, A ) . The peaks are due to thallium (2.0 X 10-4M), lead (1.0 X 10-4Jf), bismuth (1.0 X l O - 4 M ) , iodate (0.33 X 10-4.Tf), and zinc (1.0 x
b 1 102
1 0
I
I .Q+
-01
I -06
!
1
-00
to2
Figure 3. Current-voltage curves for reduction of air-saturated solution of 1.00 X 1OU4Mthallium and 1.00 X 1 OW4M cadmium
Figure 4. reduction
A.
A. 6. C.
6. C.
Nondifferentially Differentially, with oxygen compensated Differentially, with oxygen and thallium compensated
I 0
1
I
-02
-04
! -0s
-0e
VOLTS
VOLTS
Current-voltage curves for
of air-saturated solution of
1.00 X 10-3M thallium and 2.00 1 O-jM cadmium
x
Nondifferentially Differentially, with oxygen compensated Differentially, with oxygen and thallium compensated
Table 1.
Analysis of Micromolar Solutions
Peak Current",
Average" Deviation,
Ion
pa.
%
Pb++ Cdi+
0.013 0.014 0.020
0.7 1.3
Zn++ 0.4 Average and average deviation of six determinations on the same solution. Q
10-4X), The solution was 0.1M in sodium hydroxide and 0.5.U in sodium tartrate. It is apparent that by the time the second peak potential is reached, the base line is hopelessly lost. However, each component can be determined by bringing the reference cell to the proper concentration of each reducible substance in succession (Figure 2, curves B, C, D,and E ) . This can be accomplished by addition of small amounts of concentrated known solution to the reference cell. After two or three components are compensated for in this manner, it is usually necessary to make up a new reference solution accurately to prevent cumulative errors from slight mismatching of the two cells owing to dilution of the reference solution. Air-Saturated Systems. For polarographic and related methods of analysis i t is normally necessary t o remove dissolved oxygen from t h e solution. With voltammetry with continuously varying potential, t h e reduction of oxygen yields two peaks, t h e second being spread out over a large potential range d u e t o irreversibility, a n d these currents (2 t o 5 pa.) interfere seriously with analyses. The oxygen current can be compensated, however, by placing a n air-saturated solution of the indifferent electrolyte in the reference cell and obtaining the data differentially. Examples of analyses that can be made with dissolved oxygen present are shown in Table 11. The thallium or cadmium waves, which normally are masked by the oxygen current, can be measured accurately when the oxygen current is compensated for by the differential technique. The low values for the 10-3M solutions are due to I R drop in the cells. Air-Saturated Mixtures. An example of what can be done with airsaturated mixtures is s h o w in Figure 3. Curve A is t h e conventional curve obtained using voltammetry with continuously varying potential for a n air-saturated solution of lO-4M thallium, cadmium, 0.1M potassium chloride, buffered to a p H of 5.4 with acetic acid-sodium acetate buffer. The thallium cannot be determined, because of the decay of the preceding oxygen current, and the cadmium reduction current is similarly masked by VOL. 30, NO. 1 1 , NOVEMBER 1958
1809
Table 11.
Peak Currents in Air-Saturated Solutions
Peak Peak Currentb Currentb Concn., (Differenti- A v . ~Dev., (Nondifferen- .Iv.~ Dev., Ion mole/liter ally), pa. 70 tially), pa. 7% Cd 1.00 x 10-3 16 6 0.7 16.9 0 2 c 1.00 x 10-4 1.72 0.6 . . 1.00 x 10-5 0.17 6. c T1+ 1.00 x 10-3 9.85 0.4 9.84 0.1 c 1.00 x 10-4 1.01 2.0 ... a Indifferent electrolyte in each solution was 0.lM potassium chloride and acetic acidsodium acetate buffer, pH 5.4. * Average and average deviation of six determinations on same solution. c Oxygen reduction currents prevent measurement. Solutions
both the 0-xygen and thallium currents. When the air-saturated indifferent electrolyte is placed in the reference cell, however, curve B is obtained differentially. The thallium can be determined without interference from the oxygen, but the cadmium current is still masked. When the reference cell is made 10-4M in thallium as well, an accurate analysis of the cadmium may be obtained as shown in c u r x C.
Figure 4 illustrates a similar solution, but in this case the thallium is lO-3M and the cadmium is 2 X lO-5M. The cadmium wave is completely masked by the oxygen and thallium currents. Compensation of the preceding waves, as shown in curve C, permits a reasonably accurate (2.2% relative error) analysis of the cadmium, even though the cadmium is not detectable in curves A and B.
LITERATURE CITED
(1) Airey, L., Smales, A. A,, Analyst 75,
287 (1950). (2) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 372, Interscience, New York, 1954. (3) Ferrett, D. J., Phillips, C. S. G., Trans. Faraday SOC.51, 980 (1955). (4) Heyrovsky, J., Anal. Chim. Acta 2, 533 (1948). (5) Lingane, J. J., Kerlinger, H., ANAL. CHEM.12,750 (1940). (6) Jleites, L., Ibid., 27, 416 (1955). (7) Ross, J. W.,DeXars, R. D., Shain, I., Ibzd., 28, 1768 (1956). (8) Semerano, G., Riccoboni, L., Gazz. chzm. ital. 72, 297 (1942). (9) Shain, I., Crittenden, A. L., ANAL. CHEM. 26,281 (1954). RECEIVEDfor revien- March 20, 1958. Accepted June 4, 1958. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1958. Work supported in part by the U. S. Atomic Energy Commission and in part by the Research Committee, Graduate School, Gniversity of Wisconsin, with funds made available by the Wisconsin Alumni Research Foundation.
Colorimetric Method for Continuous Recording Analysis of Atmospheric Sulfur Dioxide HAROLD
L. HELWIG and CHESTER L.
GORDON
Air Sanitation laboratory, California State Department of Public Health, Berkeley, Calif. )Sulfur dioxide in the atmosphere can be determined continuously with automatic sampling and recording apparatus. The reaction of sulfurous acid with pararosaniline hydrochloride and formaldehyde produces a red-violet color which is measured colorimetrically. At p H 1.5 this reagent is stable and exhibits good resistance to interferences at concentrations found in the atmosphere. Concentration limits for the instrument were established up to 5 p.p.m. but these can b e considerably extended or reduced.
S
is an intermediate in the manufacture of sulfuric acid. It is also used in the manufacture of sodium sulfite, and in refrigeration, bleaching, fumigating, and preserving. Released, too, as a by-product in the burning of sulfur-containing coals and oils and in the smelting of many ores, it is undoubtedly one of the most important of the air pollutants. As available conductometric methods for measuring sulfur dioxide concentrations on a continuous basis were not considered specific, work was initiated to ‘C‘LFUR DIOXIDE
1 8 10
ANALYTICAL CHEMISTRY
determine the feasibility of using colorimetric methods. The basic fuchsin-formaldehyde reagent developed by Steigmann (8, 9) has been dealt with in detail in a number of papers on the determination of atmospheric sulfur dioxide (6, 7 , 10). Atkins (1) found that the color response obtained by combining sulfur dioxide with fuchsin-formaldehyde reagent in a n acid medium follows Beer’s law up to a concentration of 1 mg. of sulfur dioxide in 500 ml. of solution at 25” t o 30” C. West and Gaeke (11) found that their colorimetric system obeys Beer’s law up to approximately 25 y of sulfur dioxide per 10 ml. of reagent. Moore, Cole, and Kata ( 6 ) did concurrent studies of sulfur dioxide and nitrogen dioxide in Kindsor, Ontario. using the conductometric and fuchsinaldehyde method for sulfur dioxide and the Saltzman method for nitrogen dioxide, and sampling with automatic impingers. They found that conductometric measurements of sulfur dioxide gave higher values than the chromogenic reactions of fuchsin-aldehyde and sulfite, and attributed this to nitrogen dioxide interference a t threshold levels.
They applied a factor based on nitrogen dioxide levels to the colorimetric values and found that these agreed with conductometric values. The present authors believe that correction for the effect of nitrogen dioxide at levels found in the atmosphere would not be significant for the colorimetric reagent used in the studies described. As a point of departure, the method proposed by K e s t and Gaeke (11) utilizing sodium tetrachloromercurate with pararosaniline hydrochloride and formaldehyde was chosen for primary investigation. It is acidic in nature and the oxidizability of sulfite solutions in the alkaline ranges, it was believed, might be avoided by working in the acid range. illso, the methods described previoudy (1, 6-10) with modifications were felt not to be as easily adaptable to continuous recording instruments a6 the K e s t method. The criteria felt necessary for colorimetric measurements in continuous recording machines were determined to be the follon.ing: Specificity of reagent for substance Time for color response
