V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2 LITERATURE CITED (1) Currah, J. E., McBryde, u’.A. E., Cruikshank, A. J., and Beamish, F.E., IND.ENQ.CHEM.,ANAL.ED.,18, 120 (1946). (2) Gflchrist, R., Bur. Standards J . Research, 20, 745 (1938). (3) Gilchrist, R., Chem. Reas., 32, 322 (1943). sot., 57, 2565 (4) Gilchrist, R,,and \Tichers, E., J , A ~ them, , (1935).
(5) Hiliebrand, R-. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” pp. 289, 290, New York, John Wiley & Sons, 1929. (6) Jackson, D. S., and Beamish, F. E., -$SAL. CHEM.,22, 813
1819 (7) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 358, New York, Interscience Publishers, 1944.
(8) Schoeller, W.R., and Powell, A. R., “Analysis of Minerals and Ores of the Rarer Elements,” pp. 254, 280, London, C. Griffin & Co., 1940. (9) Scott, W.W.,“Standard Methods of Chemical Analysis,” 5th ed., Vol. I, New York, D. Van Nostrand Co., 1939. (10) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” Vol. 11,pp. 137, 145, New York, John Wiley & Sons, 1942. RECEIVED for review February 11, 19.52.
(1950).
Accepted August 32, 1952.
Determination of Trace Oxygen in Gases D. D. WILLIAMS, C. H. BLACHLY,
AND R. R. MILLER Naval Research Laboratory, Washington 25, D. C.
.4rapid and accurate method for the determination of oxygen in inert cover gases was required for experiments involving oxygen-sensitive materials. The method developed is spectrophotometric, utilizing the oxygen-pyrogallol complex. The apparatus is constructed in a manner which allows for direct calibration. Blank, or reference solution, values are determined for each analysis. Equilibrium conditions are established with the aid of a 70” C. controlled temperature bath. Beer’s law is obeyed for a range of 0 to 800 micrograms of oxygen. The method has been applied to the determination of traces of oxygen in nitrogen, argon, helium, and hydrogen.
R
E C E N T projects at this laboratory involving high purity inert gas “blankets” necessitated analysis of samples for trace oxygen on a routine basis. Various difficulties were encountered in applying the methods proposed in the literature. A variety of colorimetric methods based on oxidation of ferrous iron and subsequent determination of ferric iron color complexes is reported by Mellan (4). Most of these complexes were found t o be somewhat unstable and nonreproducible. The method of Shaw ( 5 ) showed promise, but vas rather involved for routine application, as was that of Hand ( 3 ) . The latter was, however, the most sensitive and consistent of all methods tried. One
significant lack of all methods was a means of determining a reliable reagent and procedural blank. Blanks could be determined on separate samples, but the variance between blanks was greater than the per cent of oxygen in the sample in question. Most of the methods investigated involved complicated apparatus and subsequent difficulty in applying the method routinely. IOC
0 - ixio-2gms O X Y G E N o-ix 1 0 - 3 g m s O X Y G E N -3X ~ o - ~ g m OXY s GEN --CORNING F I L T E R #50
8C
10 A S P I R A T O R W 0
z a
E-
6C
z
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a 0:
t-
TO SAMPLE SOURCE
4c
8 2c
C
LJ Figure 1.
Apparatus
50
450 550 650 WAVELENGTH ( M k ) Figure 2. Transmittancy Curves
75c
The method here described was devised with an eye to accurate routine determination of traces of oxygen in gas samples. “Blanks” are determined for each run and represent true reagent and operational values. The method is colorimetric and is based on the oxidation of an alkaline solution of pyrogallic mid. The pyrogallol-oxygen complex undergoes a rearrangement before a ronstant optical density is reached. Willstatter and Heiss ( 7 ) have postulated that the oxidation of pyrogallol progresses through several intermediates. The authors have found
ANALYTICAL CHEMISTRY
1820 that this reaction may be speeded by heating. Equilibrium may also be attained, a t room temperature, by using a shaking apparatus. The equilibrium adjustment results in a decrease in optical density, original readings being as much as two times as high as final constant values, in agreement with the results reported by Tuve (6). It was found that the high original readings did not reproduce consistently nor did they plot linearly against concentrations. Hence, use was not made of the greater sensitivity provided by such readings. The successful application of this method depends upon the production of a nearly colorless solution of pyrogallol. This may be accomplished by mixing the reagents in vacuo. The apparatus is constructed in such a manner that optical density readings may be obtained without transferring the solution. This arrangement also permits a reading to be taken prior to introduction of the oxygen-bearing gas, thus providing a true blank for each run.
pyrogallic acid solution is allowed t o drain into A by carefully opening stopcock E. The reaction with the alkali is vigorous but not violent. The resulting solution should be nearly water white. The apparatus is removed by loosening joints I and F. The apparatus is supported in a water bath controlled at 70" C. in such a manner that the liquid level in the flask is below the level of the hot water. Optical density readings are taken a t 15-minute intervals until consecutive readings are constant. Two readings normally suffice for the blank and three or four for final sample readings. 500
REAGENTS
Distilled water. Pyrogallic acid (4% solution prepared fresh for each determination). Potassium hydroxide (10% solution prepared in the apparatus). Most of the work reported here was done with potassium hydroxide. Some work with sodium hydroxide, however, indicated a somewhat more sensitive reaction. The sodium salt might also be preferable because of its lower water content. SPECIAL EQUIPMENT
-4 photoelectric colorimeter or spectrophotometer equipped with a tube-type sample holder. A Klett-Summerson colorimeter was used by the authors, following the determination of transmittancy curves and filter selection on a Beckman spectrophotometer. High vacuum system. A controlled temperature bath (70" C.) or a shaking mechanism. APPARATUS
The apparatus shown (Figure 1) consists of a 1-liter calibrated volume, A , equipped with a tubulation, B , of such dimensions as to fit snugly in the sample holder of the colorimeter. The stopcock, ball joint, and T assembly (H, I , and M ) provide access to the sample source and high vacuum manifold. The joint, stopcock, and ball joint assembly ( D , E, and F ) provide for reagent introduction and postdetermination cleaning. Flask C is a degassing chamber for the pyrogallol solution. The enclosed volume assembly ( J ,K , and L ) is used for calibration purposes only. I t is sealed to the apparatus a t X during calibration, following which it may be removed. PRECALlBRATION OPERATIONS
Two samples representing approximately 300 and 900 micrograms of oxygen in a 4% pyrogallol-10% potassium hydroxide solution were prepared in sealed ampoules. An additional sample of such concentration as to read "infinity" on the Klett-Summerson colorimeter was also prepared, The per cent transmittance of these samples was determined on a Beckman spectrophotometer, using a 10% potassium hydroxide solution as a 100% base. The values obtained are plotted in Figure 2. The transmittance curves are not ideal, but sufficient absorbancy for operation in the 400 to 500 mp range is indicated. Superimposed over these data is the transmittance curve of a Corning No. 5030 filter. The K-S No. 42 filter, also peaked a t 420 mp, was later found to be more sensitive and was used by the authors for final calibration. Corning S o . 5030 is satisfactory, however, and allows for a somewhat extended range of concentration and more precision. PROCEDURE
Solid potassium or sodium hydroxide(2.5 grams)is placed in flask A through the joint, D , which is then capped with assembly D, E , F. The apparatus is connected to the vacuum manifold a t Z and evacuated, with gentle flaming to remove adsorbed gases. Flask C is attached and 25 ml. of distilled water containing 1gram of pyrogallic acid is added. T joint G is connected to a water aspirator and the solution is degassed. When this operation is completed, stopcock H i s closed and all but 0.5 ml. of the
200
400
600
000
MICROGRAMS OF O X Y G E N
Figure 3.
Calibration Curves
When the blank is constant, the apparatus is attached through the T assembly, M , to both the vacuum system and the gas sample source. After the volume above stopcock H has been purged, A is filled to some predetermined pressure dependent upon the anticipated oxygen content of the sample. (In lieu of working at partial pressures in a set volume apparatus, flasks of various capacities may be constructed and each operated a t atmospheric pressure according to the oxygen content of the gas being analyzed.) Stopcock H is closed and the apparatus is placed in the hot bath. Readings are again taken at regular intervals until consecutive readings are constant. The difference between the optical densities of the sample and the blank is interpreted as grams of oxygen through the medium of a calibration graph or equation. Suitable calculations based on the volume of the apparatus, ambient temperature, the pressure to v hich the apparatus was filled, and the density of gas in question provide an ansq-er in weight or volume per cent, as desired. CALIBRATION
This method and apparatus lend themselves to an ease of calibration not inherent to most methods of this type.
A known volume, L, is enclosed b e h e e n two stopcocks, J and K , Figure 1. This assembly is sealed to the upper periphery of flask A. Calibration is then affected by following the procedure outlined above through the point of blank determination. At this point, stopcock K is opened to fill the small volume, L , with air a t room conditions of temperature and pressure. (Oxygen may be used if desired, but air is satisfactory and is more convenient.) ilfter K is closed, J is opened to alloty the trapped air to enter A . (This operation leaves the calibrator essentially evacuated and ready for further additions.) This knoan quantity of oxygen is then carried through the remaining steps in the procedure. After the final equilibration and reading, further "shots" of air or oxygen are admitted and successive points determined until an optical density beyond the sensitive range of the measuring instrument is reached. Figure 3 contains typical calibration curves determined by the method described. The curves are essentially linear for the range
1821
V O L U M E 2 4 , N O . 11, N O V E M B E R 1 9 5 2 500 0 - 10% I ( O H - 4 % P Y R O G A L L O L 0 - 20%KOH-4%
400
b-
20%KOH-8%
PYROGALLOL PYROGALLOL
0 - 10% K O H - 8 % P Y R O G A L L O L
“0
200
400
600
M I C R O G R A M S OF OXYGEN
Figure 4.
Effect of Reagent Concentration
reported. Specific conditions prevailing at each calibration are shown on the figure. DETERMINATION OF OPTIMUM CONCENTRATION OF REAGENTS
Before final calibration, a series of runs was made to determine desirable reagent concentrations. The effects of various concentrations of both the alkali hydroxide and the pyrogallic acid are shown in Figure 4. Excess pyrogallol results in nonlinearity a t higher concentrations of oxygen. Runs were also made with no hydroxide and with sodium carbonate; sensitivity was low,
however. The 4% pyrogallol in 10% potassium hydroxide has proved to be reliable and easily handled. Various stopcock greases were used with no apparent affect upon results. Care must be taken to prevent grease being carried into the reagents, however, as the cloudy fiolutions thus obtained will give spuriously high results. -4 cellulose-base grease was found t o be most satisfactory because of the ease of cleaning the apparatus following a determination. The usual procedure was to place the apparatus, after washing with water, in a muffle furnace and heat to 550’ C. for a short time. Various solvents also effect good cleansing with this type of grease. The bulk of the work a t this laboratory was performed on nitrogen, but oxygen in helium, argon, and hydrogen has also been determined by this method. While the oxygen content of tank carbon dioxide \vas not determined, runs involving the pure gas demonstrated that no spurious colors viere introduced by the presence of this gas. Because of their reaction xith hydroxide, a revision of sampling technique n-ould be required when large percentages of carbon dioxide or any acid gas were present in the sample. I t seems reasonable that a sybtem of “stripping” such as that proposed by Deinum and Dam ( 2 ) and used more recently by Brooks et al. ( 1 ) could be applied to this procedure, making it useful for the determination of dissolved as well as gaseous oxygen. I t is further felt that use of the liquid in question as the solvent for the reagents, or the use of water as a “stripping” agent, might also give satisfactory results, though such an application would certainly require further investigation. LITERATURE CITED (1) Brooks, F. R., Dimbat, M.,Treseder, R. S.,and Lykken, L., A l n a ~ .CHEM., 24, 520 (1952). (2) Deinum, H. W., and Dam, J. W., Anal. Chim. Acta, 3 , 3 5 3 (1949). (3) Hand, P. G. T., J . Chem. Soc.. 123, 2573 (1923). (4) hfellan, I., “Organic Reagents in Inorganic Analysis,” Philadelphia, Pa., Blakiston Co., 1941. (5) Shaw, J. A , IXD.ENG.CHEX.,ANAL.ED., 14, 891 (1942) (6) Tuve, R. L., U. S. Patent 2,440,315 (1948). (7) IVillst%tter, R., and Heiss, H., Ann., 433, 17 (1923). RECEIVED for reriew June 2 , 1952.
.4ccepted -4ugust 16, 1952.
Determination of Arsenic in Fruits and Vegetables J. C. BARTLET, MARGARET WOOD, AND R. A. CHAPMAN Food and Drug Laboratories, Department of National Health and Welfare, Ottawa, Canada
M
ANY procedures have been developed for the determination of small quantities of arsenic. The Gutzeit method ( 1 ) has
been in use for many years but its accuracy is limited and reproducibility of stains depends on a number of factors ( 2 , s ) . For microgram quantities of arsenic, Cassil and Wichman (4)have employed a microtitration procedure after distillation of arsine. Arsenic has also been isolated by distillation as arsine (11, 1 4 ) , the trichloride ( 5 , Is), and the pentavalent bromide (9, IO), and by carbon tetrachloride extraction as arsenic xanthate (8). Following isolation arsenic was estimated colorimetrically as the molybdenum blue complex, Hydrazine sulfate has been recommended by a number of investigators (9, 12, 1 4 ) as the reducing agent in the molybdenum blue procedure, although stannous chloride ( 1 6 ) and reduced molybdate ( 1 7 ) have also been used. Preliminary experiments with the bromide distillation procedure (9) gave promising results. However, in the course of the study several points of technique were found to be critical, and these factors were investigated more intensively. The following procedure, which is essentially that of Magnuson and Watson ( 9 ) , was finally adopted.
APPARATUS
Either a Cenco 250-natt variable heatcr or a Precision Scientific 750-watt Ful-Kontrol heater equipped with pipe clay rings or an asbestos ring was employed. The area of the bottom of the flask exposed to the heat was approximately 4.5 cm. in diameter. The distillation apparatus as described by Magnuson and Watson (9) was used. In all, three traps were employed; trap 1 was exactly as described by Magnuson and Watson ( 9 ) , while traps 2 and 3 had no baffle in the lower chamber. All the traps were purchased from the Scientific Glass Apparatus Co. REAGENTS
Potassium Bromide Solution, 30% u~eight/volume. Dissolve 30 grams of reagent grade potassium bromide in water without heating. Filter as soon as it is dissolved through a coarse filter paper and dilute to 100 ml. Molybdenum Color Reagent. A4dd 10 ml. of concentrated sulfuric acid to 40 ml. of water, cool, dissolve in this solution 1.0 gram of ammonium molybdate, and dilute to 100 ml. Hydrazine Sulfate Solution, 0.05% weight/volume in water. Standard Pentavalent Arsenic Solution. Dissolve 1.5 grams of arsenic pentoxide in 100 ml. of -V sodium hydroxide, add 600 ml. of distilled water, neutralize with 100 ml. of 1 N hydrochloric
