Table II. Extraction of Diphenylurea from Aqueous Acidified Aniline Solution with 50: 50 n-Pentanol-n-hexane
1,3-Diphenylurea taken, p g . 0.00
6 0 13.7 13.7 13.7 27.4
1,3-Diphenylurea found, pg. 0.05 5.9 13.3 13.7 13.7 27.7
aniline solution and carrying this through the procedure. The results are shown in Table 111. The initial p H of the aniline solution was 7.4. Higher pH values were obtained by adding ammonium hydroxide and lower ones by adding hydrochloric acid. The incomplete reaction ai, p H 9 and above is caused by hydroxyl ion concentration and not by a reaction between aniline, phosgene, and ammonia to form phenylurea or between phosgene and ammonia to form urea. This was shown by using other bases to obtain the same recovery at the same p H values, and by
examining the reaction products for phenylurea. N t r i c oxide, if present in large quantities, will react with aniline to form a yellow-orange compound which interferes. Free chlorine will oxidize aniline to form a brown tar. These materials, however, are seldom present in concentrations high enough to interfere in practice. Large excesses of carbon monoxide (lo8) have no effect on the reaction. Samples of vent gases from gas burners produced aniline solutions of a p H from 4 to 8. They did not contain enough nitric oxide or chlorine to cause interference. Materials which are evtracted into the hexane-pentanol solution and interfere by absorbing ultraviolet light can often be eliminated by extracting the aniline solution with n-hexane before it is acidified with sulfuric acid. Diphenylurea is not appreciably soluble in n-hexane and remains in the aqueous layer. Such interferences are seldom encountered in combustion products, however.
Table 111. Effect of pH of Aniline Solution on Recovery of Phosgene
PH Recovery, %" 0.5 0 1.97 9.3 4.04 79.4 4.95 98.6 102.0 6 07 7.03 100,4 8.06 92.0 9 01 63.4 10.42 12.4 a 15.6 pg. of phosgene; 50 ml. of aqueous aniline solution (2 pg./ml.). LITERATURE CITED
w.B., ANAL.CHEM.28, 410-12 11956). ( 2 ) Lamouroux, A., Mem. Poudres 38, 383-6 (1956). W. B. CRUMMETT (1) Crummett, \
,
J. D. M C L E A N ~ Special Services Laboratory The Dow Chemical Co. Midland, Mich.
Present address, Department of Chemistry, Michigan State University, Lansing, Mich.
Use of Highly Acid Supporting Electrolytes in Polarography Observed Changes in Polarographic Waves of Selenium(IV) upon Standing SIR: The polarographic characteristics of selenium(1V) in a number of electrolytes were recently reported ( I ) . I n acid medium two waves are found, only the second being reversible. The authors have since had occasion to employ high concentrations of sulfuric acid supporting electrolytes for polarographic determinations of selenium in biological samples. I n 3144 sulfuric acid the second selenium wave decreased in height upon standing in the poiarographic cell under nitrogen with mercury dropping through the solution; the wave eventually disappeared completely. The wave also shifted to slightly more negative potentials as it decreased in height. Sometimes a milky precipitate occurred with these changes but this was not always the case. This behavior has been investigated further and the observed changes have been shown to be due to reaction of free mercury ions with intermediate reduction products of the selenium; the free mercury ions are formed from acid dissolution of the mercury electrode. EXPERIMENTAL
Reagent grade chemicals were throughout. Stock selenium(1V) tions were prepared by dissolving trograde black selenium powder
used soluspecin a
small amount of nitric acid and diluting to volume with water. All polarographic measurements were made with a Sargent Model XV polarograph. The polarographic cell was equipped with a mercury pool anode unless otherwise stated. Solutions were deoxygenated with Seaford nitrogen (Southern Oxygen Co.) for 10 minutes prior to the running of the polarogram. During the course of the polarogram a nitrogen atmosphere was maintained above the sample solution. All polarograms were obtained on a sample thermostated a t 23' ==! 0 . 2 O C. RESULTS
A polarogram was obtained on a solution of 10 pg. of selenium(1V) per ml. in 3M sulfuric acid. The original height of the second wave (after 10 minutes in the cell) was 0.90 pa. The wave almost completely disappeared upon standing for 1 hour under nitrogen in the cell [in contact with the mercury pool anode and the dropping mercvry electrode (DME)]. This decrease in the second wave height was accompanied by a change in the first wave from a fairly smooth wave to one with several maxima and minima. On further standing (after wave 2 disappeared) wave 1 became even more distorted. The maxima and minima could be eliminated by the addition of gelatin.
The effects of nitrogen and mercury were studied by transferring aliquots of the same solution (10 pg, Se/ml. in 3M HBSOI) to two flasks, one containing a pool of mercury. Each solution was bubbled for 8 hours with nitrogen. A polarogram was then obtained immediately after adding each of the solutions to the polarographic cell. The solution which contained no mercury yielded a height of 0.98 pa. for wave 2 and wave 1 was not distortedi.e., no change. After 1 hour in the cell, the height of wave 2 in this solution decreased to 0.85 pa. accompanied by a slight distortion of wave 1. The second solution, which had been over mercury, gave a height of 0.78 pa. for wave 2 and wave 1 was slightly distorted. Solutions of selenium(1V) in 3M sulfuric acid in the presence of air and the absence of mercury showed no change on standing, even after several weeks. To test whether the observed phenomena were associated with the sulfate species or with the high acidity of the medium, the same studies were made in 4M perchloric acid. Essentially the same results were found. The original polarogram, after deoxygenating for 10 minutes, showed wave 1 to he more drawn out in this medium (almost straight). After 3 hours in the cell, VOL. 37, NO. 3, M A R C H 1965
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wave 2 disappeared and wave 1 became badly distorted. The rate of decrease of the wave height was more rapid when the solutions were in the polarographic cell than when in the flask containing a pool of mercury. This was probably because mercury was dropping through the solution from the DME while standing. A larger and continually fresh surface area of mercury was presented to the solution. Supporting this is the fact that when a precipitate occurred, it appeared to be forming around the mercury drops as they fell through the solution (associated with mercury dissolution). A similar precipitate has been observed to form in a supporting electrolyte of 3M sulfuric acid, causing erratic behavior of the residual current. This precipitate was analyzed and found to contain mercury. Therefore, mercury apparently dissolves from the electrode. The precipitate formed around the D M E in the presence of selenium(1V) above was difficult to collect without contamination with elemental mercury. Since it was apparent that free mercury ions were being formed, an experiment was run to show that acid dissolution of mercury occurs at room temperature. Three flasks were prepared, each containing a pool of mercury. I n one was placed 3M sulfuric acid. In the second was placed 3 M sulfuric acid containing 10-3M selenium(IV), and in the third was placed lO-3M selenium(1V) in water. The flasks were allowed to stand for one day. A white precipitate occurred in the second flask and this precipitate was found to contain mercury and selenium. This was apparently the same precipitate as formed around the D M E above. A polarogram was run on the solution in flask 3 after acidifying. The normal selenium waves were found. A polarogram on the solution in flask 1 was very ragged and appeared to give an immediate cathodic current (reduction of free mercury ions). Further tests proved the existence of free mercury ions in this solution. After adjusting the p H to ca. 2, the solution was chxtracted with dithizone in chloroform. An orange color developed in the chloroform layer which was eliminated by back extraction into 6M hydrochloric acid. This indicates the presence of mercury ions (4). The dithizone extract was also analyzed by atomic absorption spectrometry and the results showed the presence of mercury. Thus, observed changes of selenium(1V) waves are apparently caused by acid dissolution of the mercury electrode with the formation of free mercury ions. The free mercury ions can then interact with intermediate selenium reduction products in the 426
0
ANALYTICAL CHEMISTRY
diffusion layer. This was substantiated by obtaining a polarogram of 1 wg. of selenium per ml. (1.26 X 10-5M) in 312.1 sulfuric acid in a Lingane H-cell equipped with a saturated calomel reference electrode (SCE). The normal waves were recorded. The solution was then made ca. l O - 5 M in mercuric sulfate and the polarogram was immediately rerun. The second selenium wave was nearly eliminated. Doubling the concentration of mercuric sulfate completely eliminated the wave. The first wave was not distorted and remained approximately constant in height. Quantitative wave height measurements of wave 1 when wave 2 decreased with time were difficult because of the high distortion of this wave which usually occurred. When the wave became distorted, it was usually broken into three steps. It appeared generally that the first two steps remained constant in height while the last step decreased slightly with increasing distortion of wave 2. If the wave was not appreciably distorted, the overall height appeared to remain constant. DISCUSSION
It is not surprising that elemental mercury can be slowly attacked by 3.V sulfuric acid (or perchloric acid). It is known that mercury is converted to mercurous or mercuric sulfate when heated with concentrated sulfuric acid (5) ; the product depends on the ratio of mercury to acid and on the time of heating. The disappearance of wave 2 upon addition of free mercury ions, either externally or from slow acid dissolution of the electrode, can be easily explained. The primary electrode reaction for the first wave is a six-electron reduction (1).
+ 6 H + + 6e-
H2Se03
=
HzSe
+ 3Hz0
(1)
However, a t potentials corresponding to the first wave, the D M E is depolarized by the hydrogen selenide formed at the electrode surface. Hg
+ HzSe = HgSe
+ 2H+ + 2e-
(2)
Therefore, the first wave is an overall four-electron process.
+ Hg + 4H+ + 4e- = HgSe + 3Hz0
H2Se03
(3)
At potentials more negative than the second selenium wave, reaction 2 does not occur and the overall process is reaction 1. Thus, wave 2 corresponds to a two-electron process because of depolarization of D M E by hydrogen selenide a t potentials more positive than this wave. When free mercury ions are present at the electrode
surface as the hydrogen selenide is formed, these ions will react with the hydrogen selenide and precipitate mercuric (or mercurous) selenide. HzSe
+ Hg+2 = HgSe + 2H+
(4)
Reaction 2 is therefore decreased or eliminated which effectively decreases or eliminates wave 2. One would expect, however, that wave 1 would increase to a six-electron wave as wave 2 disappeared. This is not the case; wave 1 appears to remain constant a t a net four-electron process. This again can be explained. Free mercury ions are reduced a t potentials more positive than the first selenium wave. HOWever, when the potential resides on the plateau of the selenium wave, part or all of the mercury ions in the diffusion layer (equal to the amount of reaction 2 which is eliminated) are precipitated and removed from the solution as the selenide. Therefore, the mercury ions no longer contribute to the diffusion current a t these potentials and the selenium wave appears to remain approximately constant in height. That is, for every faraday of current increase in wave 1 due to the elimination of reaction 2, there is 1 faraday of current decrease due to precipitation of mercury ions. Lingane and Niedrach (3) have observed a similar phenomenon with ammoniacal solutions of selenium (IV) containing copper(I1). Further evidence that the elimination of wave 2 is due to precipitation of the hydrogen selenide was obtained by adding either copper(I1) or zinc ions to the solution. Copper(II), which precipitates sulfide in acid medium, eliminated the wave. Zinc ions, which do not precipitate with sulfide in acid medium, did not affect the wave. Wave 1 remains nearly constant in height upon the external addition of mercuric ions and wave 1 is not distorted as wave 2 decreases, indicating that there is very little difference between the diffusion coefficients of the mercury ions and HZSe03 in this medium; also, the mercuric selenide precipitate does not distort the wave. The fact that the last step of the distorted selenium wave decreases slightly could be due to adsorption of insoluble mercury species a t the electrode surface. Probably the precipitate formed,'which has nothing to do with the electrode processes, is an insoluble mercurous salt which may be adsorbed, resulting in distortion of the wave. This is supported by the fact that the distortion is partially eliminated by the addition of gelatin. Christian and Purdy (2) have shown that insoluble mercurous phosphates are adsorbed a t the DME. The mercury-ion concentration in the bulk of the solution when the second selenium wave has disappeared is probably close to that of the selenium
(IV). This would not have to be thd case since we are dealing with processes in the diffusion layer a t the electrode surface and sufficient (trace) amounts of mercury might dissolve a t the electrode surface to react with the hydrogen selenide formed. However, the fact that wave 2 remains eliminated when running a fresh polarogram after the solution has been removed from the cell for several days indicates that there are sufficient mercury ions in the bulk of the solution to diffuse to the electrode surface along with the selenium(1V). This problem should be considered whenever working with highly acid electrolytes. Residual currents often become ragged and inconsistent results
are obtained with samples. The authors have found that difficulties are eliminated if contact with mercury is avoided while the solution is deoxygenated with nitrogen. With an H-cell fitted with an SCE, this problem is eliminated by lowering the mercury reservoir for the D M E during deoxygenation. The polarogram is obtained immediately upon raising the mercury reservoir. LITERATURE CITED
(1) Christian, G. D., Knoblock, E. C., Purdy, W. C., ANAL. CHEM.35, 1128 (1963). (2) Christian, G. D., Purdy, W. C., J. Electroanal. Chem. 3 , 363 (1962).
(3) Lingane, J. J., Niedrach, L. W., J . A m . Chem. SOC.71, 196 (1949). ( 4 ) Morrison, G. H., Freiser, H., “Solvent
Extraction in Analytical Chemistry,”
p. 218, John Wiley & Sons, New York,
1957. ( 5 ) The IClerck Index, 6th ed., p. 617, Merck & Co., Inc., Rahway, N. J., 1952.
GARYI). CHRISTIAN EDWARD C. KNOBLOCK WILLIAM C. PURDY Division of Biochemistry Walter Reed Army Institute of Research Washington, D. C. 20012 and Department of Chemistry University of Maryland College Park, Md. 20742
Modification of Ferrous Ion Reduction Method for Nitroglycerin SIR: Nitroglycerin (XG) is determined in this laboratory by the familiar method of Becker and Shaefer (1). This procedure involves the reduction of the nitrate groups with a large excess of iron(I1) in glacial acetic acid. The iron (111) formed is then measured directly by titration with titanium(II1) chloride using an internal indicator, Titanium(II1) chloride, though a powerful reductant, is not a n entirely suitable titrant. The instability of the titrant is such that precision and accuracy of the titration is not always acceptable. The titanium(II1) titrant solution is very sensitive to air oxidation and has to be stored under an inert atmosphere. Solutions need daily restandardization. When this titrant is stored in an automatic buret for any length of time, it is necessary to flush a large volume through the system prior to titration. If this is not done, unacceptable precision results. The disodium salt of (ethylenedinitri1o)tetraacetic acid (EDTA) is a more suitable titrant for iron(II1) in the presence of large amounts of iron(I1). The satisfactory titration of iron(II1) with EDTA has been reported by several workers (2, 3 ) . Solutions of properly stored EDTA are very stable and require only infrequent standardization. When EDTA is used for the determination of iron(II1) formed in the NG reduction, much better precision and accuracy result. This method is applicable to the determination of IZ’G in the absence of any materials capable of reacting with XG, iron(II), iron(III), or the EDTA used as titrant. For samples received by this laboratory, it was necessary to correct only for 2-nitrodiphenylamine (2-NDPA). Other materials present did not interfere or were quantitatively removed during the extraction step.
Determination of NG by EDTA Method NG taken, mg. NG found, mg.a
Table 1.
110.0 125.0 135.0 145.0
110.0 125.1 135.3 145.1
aEach result is the average of six determinations.
EXPERIMENTAL
Reagents. A 0.20M E D T A solution was prepared by dissolving 148.8 grams of disodium (ethylenedinitril0)tetraacetate in 1 liter of warm distilled water, cooling to room temperature, and diluting to 2 liters with distilled water. This solution was standardized against calcium after Welcher ( 3 ) . A 0.70N iron(I1) solution was prepared by dissolving 137.3 grams of ferrous ammonium sulfate in a solution of 35 ml. of concentrated sulfuric acid and 350 ml. of distilled water, and diluting to 500 ml. with distilled water. Apparatus. An N G reduction flask, 500 ml., with a gas inlet and a standard taper 29/42 ground ‘glass neck fitted with a water-cooled reflux condenser with a ground glass joint, was used for refluxing. A beaker head was prepared by drilling five holes into a number 12 rubber stopper. This “head” simultaneously held a platinum electrode, calomel electrode, glass electrode, burat, and nitrogen bubbler. Procedure. Place a n accurately weighed amount of the N G sample containing 0.10 to 0.15 gram of NG into a 500-rnl. reduction flask. Add 25 ml. of glacial acetic acid and 25 ml. of a 1:l hydrochloric acid solution. Purge with nitrogen to remove air and add 25 ml. of the ferrous ammonium sulfate solution. Reflux until the sample changes in color from
yellow-orange to reddish brown and back to a yellow-orange. (This should require 5 to 10 minutes.) After refluxing, cool the flask and disconnect from the condenser. Transfer the solution to a 250-ml. tall form beaker. Place the prepared rubber stopper with the platinum-calomel indicating electrodes, glass pH electrode, buret, and nitrogen bubbler tube into the beaker. Adjust the p H to 2.5 with 30y0 sodium hydroxide. The glass and calomel electrodes are used in combination to measure pH. Titrate the iron(II1) formed in the NG reduction potentiometrically with 0.20M EDTA. The platinum and calomel electrodes are used in combination to follow the potentiometric titration. A Beckman Zeromatic p H meter or equivalent potential recording device is suitable. Run a blank in the same manner, with the exception that no sample is added. RESULTS A N D DISCUSSION
End Point Detection. Welcher (3) recommended several internal indicators for the iron(II1)-EDTA titration. A number of these indicators proved unsatisfactory under the conditions of the KG determination. These included salicylic acid, Pyrocatechol Violet, Eriochrome Black T, Variamine Blue B, Xylenol Orange, and copperPAN 1-(2-pyridylaeo)-2-naphthol. Poor end points resulted for any of a combination of reasons-e.g., iron (11) interference, high iron(II1) concentration, and high solution ionic strength. A potentiometric procedure employing a platinum-calomel indicating couple gave a suitable end point. The slope and magnitude of the end point “break” was pH dependent. A pH of 2.5 gave the best end point. Interferences. Materials frequently present with NG included varying amounts of triacetin and VOL. 37, NO. 3, MARCH 1965
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