(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 NG 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 NG 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 pH 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 pH 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
427
about 17, 2-KDP.1. Introducing varying amounts of triacetin caused no interference in the N G determination. A nonstoichiometric positive error was caused by 2-NDPA. This error, however, was small but constant and was corrected for in the titration. One milligram of 2X D P A required the equivalent of 0.0034 meq. of E D T A in the titration. Precision and Accuracy. T o determine the accuracy and precision of the K G determination using E D T A to titrate the iron(III), four standard samples mere prepared. Six determinations were made on each sample using the E D T A titration. (The results are shown in Table I.) The method showed a positive bias of
0.09%. The 957, confidence limit for a single NG determination using EDTA was =k0.24% a t the 70 to 90% level. Becker and Shaefer reported a 95% confidence limit of =k0.69% at the 100% level for the titanium(II1) method using a potentiometric end point. For further comparison of the methods, this laboratory has also obtained data for the titanium(II1) titration with a potentiometric end point. A 95YG confidence limit of *0.33% a t the 100% level was established. Previous work here has shown the potentiometric end point gave better precision and accuracy than the visual end point methods, so no precision data was obtained using the visual end point.
LITERATURE CITED
(1) Becker, W. W., Shaefer, W. E., “Determination of Nitro, Nitroso and Nitrate Groups,” Organic Analysis Vol. 11, g . 101, Interscience, New York, 1954. (2) Flaschka, H., Barnard, A. J., Jr., Borad, W. C., Chemist Analyst 47, 52 (1958). (3) Welcher, F. J., “The Analytical Uses of Ethylenediaminetetraacetic Acid,” . 222-9, Van Nostrand, Princeton, J., 1958.
i?
R. S. LAMBERT R. J. DUBOIS
Hercules Powder Co. Bacchus Works Magna, Utah
Fluorescence of the Aluminum-[l-(2-Pyridylazo)-2-Naphthol] Complex SIR: b recent development in our laboratory has been a fluorescent indicator for the qualitative determination of the aluminum ion (3). .4n aqueous fluorometric determination of aluminum using Pontachrome Blue Black R has been reported by Weissler and White (6). The present paper reports the use of PAX for the determination of aluminum in ethanol water solutions. The method consists in preparing an aluminum-PAN complex in 957, ethanol and measuring the aluminum content fluorometrically. PAN [1-(2-pyridylazo)-2-naphthol], was first reported by Liu ( 5 ) to form complexes with the heavy metals. Cheng and Bray (1) have also noted further properties of PAN; and Flaschka (2) has investigated the use of PAN as an indicator in conjunction with EDTA. I t has been noted that a number of aromatic fluorescent complexes of aluminum all possess a
Table I.
phenolic hydroxyl group either ortho or para to the complexing site ( 4 ) ; PA?; belongs in this category since it has its phenolic hydroxyl group ortho to an azo group. The aluminum-PA?; mixture exhibits several properties not shared by other metallo-PAP; chelates. No reaction is detectable between the aluminum ion and PAX in aqueous solution; however, an orange or yellow solution results when the two substances are mixed in variety of organic solvents. The fluorescence color of the complex as well as the color of
I
Fluorescence Color
PAN
Solvent plex EtOHo Et,O C6H6 Cc1, CHCL Cr B Br Y Y Y Fe Br Br Y Y Y M n P P Y Y Y Ni R P U P R Y Z n R R R R R C o L L Y G G A l O Y Y Y Y B-blue, Br-brown, G-green, L-lavender, 0-orange, P-pink, Pu-purple, R-red, Yyellow. a Identical colors were also obtained in RleOH, PrOH, BuOH, AmOH, HexOH and acetone. com-
428
ANALYTICAL CHEMISTRY
other transition metal complexes is given in Table I. The other property which was observed for the AI-PAN complex is that it fluoresces with an orange-yellow color in alcohol and acetone. A typical calibration curve can be plotted from the fluorescence observed in a 95y0 ethanol media. The aluminum-PAN ratio need not be constant since only the complex fluoresces and not the excess PAN. A linear plot was obtained over the range of 4 to 12 X lo-* mg. of Alf3 per 50 ml. of 95% ethanol-water solution using 2.5 x gram per ml. of PAN. These alcoholic solutions were prepared by volumetrically diluting 1 ml. of 5 x 10-3~1PAX and an appropriate volume ion from hl(N03)3.9H20 of 10-3h‘ to 50 ml. with 95% ethanol. The solutions used to obtain a calibration curve are given in Table 11. The fluorescence was measured with a Coleman Photo-fluorometer (Model 12-B) using the primary filter No. 12-222 (B-2) which passes the 436 mp line and the secondary filter S o . 14-212 (PC-2) which stops light below 530 mp. Two excitation maxima, 350 mu and 545 mpu, permit the use of primary filters 320 to 345 mp or 500 to 550 mp,
Table
Figure 1 . Variation of fluorescence with per cent ethanol
II.
Aluminum-PAN Solutions
111. of M g of A1 +s 10-3M Al+3 per 50 ml. 1 33 0 036 2 00 0 054 2 67 0 072 3 33 0 090 4 00 0 108
Al: PAK
ratio 029 043 058 072 082
0 0 0 0 0
