for 1M anion and 1M solvating agent concentrations. The varying magnitudes of these extraction constants may be attributed to the increasmg acidity of the proton of the solvating species in the order chloroform < n-amyl alcohol < p tert-butylphenol. Effect of N,N-Dimethylcaprylamide on Solvating Agent. It was of interest to determine if the schematic model postulated in Case I was reasonable. This was done by utilizing an electron donor solvating agent. This agent would not be expected to enhance partitioning of the ion pair. An experiment was carried out using fixed concentrations of dextromethorphan, bromide ion, and p-tert-butylphenol but adding increasing amaunts of dimethylcaprylamide to the organic phase. The results shown in Figure 8 indicate that the addition of the amide actually reduced the partition coefficient. These results would suggest that the amide did not act as a solvating agent even when present in rather high concentrations. Further, the decrease in partitioning at constant ptert-butylphenol could be expected, since in previous studies in these laboratories it l-as been shown that phenols formed strong association co nplexes with disubstituted aliphatic amides ( I I ) . Therefore, the addition of the amide resulted in the complexation of the phenolic solvating agent and essen(11) E. G. Shami, unpublished Ph.D. thesis, University of Wiscon-
sin. 1964.
Table 11. Effect of Various Solvating Agents on Extraction Constant for the Ion Pair Extraction of Dextromethorphan with Several Anions and Respective Molecularities Solvating Anion KO' Molecularity species 3 . 2 X 10-3 5.0 Chloroform Bromide 1.7 3.9 1-Pentanol 5 . 1 X lo7 5.3 p-tert-Butylphenol Trichloroacetate 7.0 2.5 Chloroform 51 3.0 1-Pentanol
tially prevented it from facilitating the ion-pair extraction. The requirement of a proton donor is further supported, since even when the amide was present in excess of the p-tert-butylphenol the nucleophilic agent did not enhance partitioning. ACKNOWLEDGMENT
We thank Smith Kline and French Laboratories, Philadelphia, Pa., and Warner Lambert Research Institute, Morris Plains, N. J., for support of these studies. RECEIVED for review July 11, 1966. Accepted April 10, 1967. Presented in part before the Symposium on Functional Group Analysis, Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
Colorimetric Determination of Trace Levels of Oxygen in Gases with the Photochemically Generated Methyl Viologen Radical-Cation Philip B. Sweetser E . I . du Pont de Nemocrrs and Co., Wilmington, Del. A new colorimetric method has been developed for the determination of trace amounts of oxygen in gases by the in situ photochemical generation of reduced methyl viologen (l,l'-dimethyl-4,4'-bipyridiniumdichloride). The methyl viologen reduction is thought to take place by a photochemical electron transfer reaction initiated by the light excitation of a photoreceptor, proflavine (3,6-diaminoacridine), which induces the transfer of an electron to methyl viologen from an electron donor, EDTA.. The resulting deep-blue methyl viologen radical-catilon reacts rapidly with the oxygen to produce the original oxidized methyl viologen. The method is simple, rapid, and sensitive to less than 1 ppm oxygen. One of the outstanding features of this system is the elimination of cumbersome apparatus generally required for generating and maintaining the reducing agent.
IN MANY AREAS OF CHEMICAL RESEARCH, and in the chemical industry, there are numerous instances where it is important to measure the oxygen content of gases. Methods for oxygen analysis vary from a highly sensitive luminescent-bacteria1 method ( I ) , to colorinietric methods (2, 3), photonometric (1) K. P. Meyer, Helv. Phys. Acta, 15, 3 (1942). (2) L. W. Winkler, Ber., 21,2843 (1888). 20,1033 (1948). (3) L. J. Brody, ANAL.CF:EM.,
titrations of dissolved oxygen (4), dew point (5), galvanic cell-type methods (6, 7), a proposed oxygen meter based upon the photoreduction of methylene blue (8), and numerous others. Although many of the above methods are accurate and sensitive, most have some shortcomings which limit their general use. The requirement for maintaining a completely oxygen-free reagent necessitates the use of fairly cumbersome apparatus for generating and/or maintaining the reducing agent. The present paper describes a new colorimetric method which has been developed for the determination of trace amounts of oxygen in gases by the in situ photochemical generation of the reducing agent. The photochemical reducing system is based upon the coupling of Oster's (9, IO, 11) photoreducing dye system, proflavine and EDTA,
(4) T. Kuwana, ANAL.CHEM., 35, 1398 (1963). (5) L. Pepkowitz, Ibid., 27, 245 (1955). (6) P. Hersch, Nature, 169, 792 (1952). 25, 586 (1953). (7) M. G. Jacobson, ANAL.CHEM., (8) G. Oster and N. Wotherspoon, J . Chem. Phys., 22, 157 (1954). (9) G. Oster, Photographic Eng., 4, 173 (1953). (10) G. Oster and N. Wotherspoon, J . Am. Chem. SOC.,79, 4836 (1957). (11) F. Millich and G. Oster, Ibid., 81,1375 (1959). VOL. 39, NO. 8, JULY 1967
979
TEFLON STOPPER
Figure 1. Gas reaction cell
with methyl viologen (12) (1,I '-dimethyl-4,4'-bipyridiniumdichloride) to form the deep-blue stable radical-cation of reduced methyl viologen. The methyl viologen radical-cation has one of the lowest redox potentials of any reported reversible organic redox system (Eo'= -0.44 V at pH 7.0) and reacts rapidly with oxygen to regenerate the original, colorless oxidized form of methyl viologen. EXPERIMENTAL
Apparatus. A modified gas-collecting tube (50 to 500 ml depending upon oxygen range desired) is illustrated in Figure 1. Colorimetric measurements were made on a Photovolt Corp. Lumetron colorimeter Model No. 401 with a 580mp filter. The high light intensity source employed was a Tensor Model 5900 lamp which gave 120- to 200-foot-candle output at 12 inches. Reagents. A 1.0 X 10-3M solution of proflavine dihydrochloride (Mann Research Laboratory) was made up in distilled water. The 1 X 10-*M solution of methyl viologen (Mann Research Laboratory) was made in distilled water, and a 0.25M solution of EDTA, adjusted pH 6.5, was made from the Eastman disodium salt of ethylenediaminetetraacetic acid dihydrate. The 0.2M phosphate buffer (pH 6.5 was prepared from 0.2714 Na2HP04.7H20 and 0.2MNaHaP04. The stock solution of proflavine-methyl viologen-EDTA was prepared by adding: 18 ml of 1 X 10-3M proflavine, 45 ml of 1 X 10-2M methyl viologen, 36 ml of 0.25M EDTA, 90 ml of 0.2M phosphate buffer (pH 6.5) and 261 ml of distilled water. This stock solution was stored in an amber or black bottle in a dark cabinet or refrigerator. Under these conditions, the solution was stable for several months. Standard 0.1714 ferric sulfate solution was made up in 0.36N H a S 0 4 and standardized iodometrically. A dilute ferric sulfate solution, 2.0 X 10-3M, was prepared from the above standard ferric sulfate solution, and sufficient EDTA was added to give a final EDTA concentration of 0.025M. Procedure. The gas to be tested is collected, and the subsequent photochemical and oxidation reaction is carried out in a modified 50- to 500-ml gas-collecting cell (see Figure 1). A 15-ml portion of the proflavin-methyl viologen-EDTA stock solution is added to the gas-collecting cell and allowed to drain into the test-tube portion of the collecting cell. The cell and the stock solution are deaerated by passing nitrogen or the test gas through the tube for several minutes at a flow of 300-1000 cc/minute. After deaeration, the methyl viologen radical-cation reducing agent is generated by exposing the test tube containing the stock solution to the bright light source until the deep blue color of the methyl viologen is formed. The concentration of reduced methyl viologen (MVS.) is monitored by measuring the absorbance of the solution at 580 mp with the colorimeter. The photochemical generation of MV+. is continued until an initial concentration corresponding to an absorbance reading of 0.7 to 0.9 absorbance unit is obtained. It is important to note that at ~
~~
(12) L. Michaelis and E. S. Hill, J . Gen. Physiol., 16,859 (1933).
980
ANALYTICAL CHEMISTRY
PH
Figure 2. Effect of pH on rate of photoreduction of methyl viologen with EDTA as electron donor this point, all direct light shining upon the absorption cell should be turned off so that generation of reduced methyl viologen does not take place during the subsequent shaking period. However, normal indirect light conditions may be allowed in the laboratory. Before the initial absorbance reading is recorded, the reduced solution is mixed by gently tipping the solution into the main body of the gas cell and then allowing it to drain back into the test tube. Thirty to 60 seconds after the initial absorbance reading has been taken, the flow of the test gas is stopped by closing the stopcocks of the gas cell, and the cell pressure is adjusted to atmospheric. The solution is then tipped into the main section of the gas cell and shaken for approximately 20-second periods, after which an absorbance reading at 580 mp is taken. The 20second shaking periods are continued, usually for a total of 1.5-2 minutes, until the absorbance readings show no further decrease. The difference in absorbance between the initial and the final absorbance reading is recorded, and the concentration of oxygen is calculated from the calibration factor of the gas-collecting cell. In gas samples which have a low O2 content-i.e., 1-10 ppm-it has also been found advisable to make an absorbance reading after stopping the flow of the test gas so that the small amount of reagent solution on the surface of the collecting cell is equilibrated with the reducing agent solution in the test tube. Prior to this reading, the methyl viologen reagent is again gently tipped into the gas-collecting cell section and then allowed to drain back into the test-tube section of the cell. Because neither proflavine nor methyl viologen are destroyed in the photochemical reaction or the subsequent oxygen reaction, the methyl viologen-proflavine reagent may be reused for numerous determinations of oxygen until the large excess of electron donor, EDTA, is depleted. Standardization. The volume of the gas-collecting cell is determined by either weighing the cell before and after filling with water or by measuring the volume of water contained in the filled cell. The absorbance sensitivity of methyl viologen is measured using the standard ferric sulfate solution. In this case, the usual procedure for determining oxygen in nitrogen is used. After sweeping the gas-collecting cell with nitrogen, the reduced methyl viologen is generated to an absorbance of approximately 0.7. A 0.20-ml portion of dilute standard ferric solution (2 X 10-3M) is then added to the gas-collecting cell with a 1.0-ml or 0.5-ml hypodermic syringe, Care should be taken to avoid any ferric solution draining into the test-tube portion of the cell. The collecting cell is again flushed out for 1-2 minutes with a moderate nitrogen flow rate to remove any traces of oxygen in the ferric solution. Absorbance of the methyl viologen solution is recorded, and the solution is tipped to the main portion of the gas-collecting cell and allowed to mix with the ferric
solution. An absorbarice reading is taken; a second mix of the solution is made by again tipping the stock solution gently into the main portion of the gas-sampling cell and again reading the absorbance. A blank solution is also run in which 0.2 ml of wai:er is added to the cell and the same procedure followed. The difference between the initial absorbance reading and the reading after the second tip is taken as the absorbance change due to the ferric oxidation of the reduced methyl vi ologen. The absorbance difference, corrected for the blank reading, is then converted to a corresponding oxygen concentration by taking into consideration the volume of the gas-collecting cell, temperature and vapor pressure of water, and the following stoichiometric reaction of methyl viologen. 2MV+,
+ + 2H+ + 2MV+' + Hz02 (PH 6.5) MV+. + Fe+3 + MVf2 + Fe+Z 0 2
(1)
Table I. Oxygen Analysis by the Methyl Viologen Method Oxygen Galvanic cell Gas Methyl viologen or coulometric Nitrogen 0.9 1.2 10.0 20.5 34.6 48.0"
10.3 20.5 35.1 50.9
Hydrogen
0.9 4.8 10.9 20.5
0.7 4.7 10.3 19.6
Carbon monoxide
2.3 10.9 11.3 20.5 20.5
11.6 11.6 21 .o 21 .o
(2)
RESULTS
The results for the determination of oxygen in nitrogen, carbon monoxide, hydrogen, and ethylene are given in Table I. These results indicate close agreement between the methyl viologen method and corresponding values obtained by a galvanic cell oxygen instrument and from coulometrically spiked gas samples. Although the normal range of oxygen concentration covered with a 300-ml gas-colle1:ting cell is approximately 0-35 ppm, it is possible to extend this range considerably by the regeneration of reduced methyl viologen during an analysis. The regeneration step is carried out after the absorbance of the methyl viologen radical-cation has been reduced to approximately 0.1 absorbance. By this technique, one can photochemically regenerate MV+. several times during the course of a single analysis and extend the oxygen range up to 200 ppm, or more, with only a slight loss in precision. The nitrogen analysis, in Table I, indicated by footnote a is an example of this procedure. DISCUSSION
Interferences. No studies of interferences have been made; however, strongly oxidizing gases-Le., chlorine, fluorine, bromine-would be expected to interfere and should be removed prior to analysis. One of the most serious interferences in any O2analysis is from the introduction of atmospheric 0 2 into the test apparatus through diffusion. Because all rubber and plastic tubing diffuse oxygen through their walls, it is important to keep this source of error to a minimum. This may be done by carrying out the following steps: All rubber tubing connections should be kept as short as possible. The tubing used should be of a material which allows a minimum rate of O2 diffusion; butyl rubber is one of the best in this respect. Gum rubber and plastic tubing have very high O2diffusion rates; thus, they should not be used. Because the amount of O2 diffusion into a gas stream is dependent upon the flow rate, it is important to keep gas flows at least 200-1500 cc./minute, if possible. By taking these precautions, the 0 2 contamination from the atmosphere can be reduced to less than 1 ppm, even with several feet of butyl tubing connected to the gas-collecting cell. Photochemistry of the Methyl Viologen-Proflavine System. The mechanism for formation of the methyl viologen radicalcation is thought to takl? place by a photochemical electron
Ethylene
...
1.4
1.3 4.5 8.4 12.5
5.3 10.5 13.5 (I
Methyl viologen generated during analysis.
transfer reaction in which proflavine acts as the photoreceptor. In its excited state, proflavine apparently abstracts an electron from EDTA and transfers it to methyl viologen, thereby resulting in the formation of a stable methyl viologen radical-cation. EDTA
+ PF + M V 2
A h 4
light
-
EDTA. . .PF*. . .MV+2
(EDTA+)
+ PF + MV+'
(3)
The exact fate of the EDTA molecule during the photochemical reaction is not known. The important feature of this electron donor is that the oxidized form of EDTA does not interfere with the subsequent oxidation-reduction reactions of methyl viologen and oxygen. Studies designed to determine the effect of pH on the photoreduction of methyl viologen were carried out using a solution which was4 X lO-SMproflavine, O.OlMEDTA, 2.4 X lO-4M methyl viologen, and 0.04M in the appropriate buffer. The variation in rate of photoreduction of methyl viologen with pH, at a constant light intensity, is given in Figure 2. It can be seen from these data that the maximum rate of MV+. formation takes place between pH 5.0 and 7.0. Quantum yield studies for the photoreduction of methyl viologen at 450 mp were carried out using potassium ferrioxalate for calibration of the light source (13). A quantum yield of 0.50 was obtained from a solution at pH 6.0 containing 2 X lO-5M proflavine, 4 x lO-4M methyl viologen, and 4 X 10-2M EDTA. By reducing the concentration of proflavine to 1 x 10-6M, the quantum yield was increased to 0.60. It is interesting to note that Oster reports a quantum yield of only 0.01 for the photoreduction of proflavine when allylthiourea was used as electron donor (11). Thus, by using EDTA as electron donor and by coupling the reducing dye system to a good electron acceptor, such as methyl viologen, it is possible to increase the quantum yield at least 60-fold. (13) C. G. Hatchard and C. A. Parker, Proc. Roy. SOC.(London), A235, 518 (1956). VOL 39, NO. 8, JULY 1967
981
Stoichiometry of Methyl Viologen Reactions. Studies with coulometrically generated oxygen and ferric sulfate standardized methyl viologen indicate that although methyl viologen radical-cation is a very potent reducing agent, the MV+. oxygen reaction proceeds stoichiometrically to the H202stage in solution buffered at pH 6.5. If the pH of the methyl viologen-proflavine solution is increased to 10.0, oxygen is reduced completely to H20. Although the sensitivity for oxygen at this pH is nearly two times that at pH 6.5,
the photochemical formation of reduced methyl viologen is considerably slower at pH 10-i.e., 2-3 minutes us. 20-30 seconds ; therefore all of the measured results were carried out at pH 6.5. However, if maximum sensitivity is desired, the pH 10 conditions could be employed with only a moderate increase in the analysis time. RECEIVED for review March 13, 1967. Accepted April 20, 1967. Division of Analytical Chemistry, 152nd Meeting ACS, New York, September 1966.
Preparation and Evaluation of Standards for the Determination of Hydrogen in Steel F. R. Coe, Norman Jenkins, and D. H. Parker British Welding Research Association, Abington Hall, Cambridge, England The preparation of steel specimens with precisely known hydrogen contents is described. Inasmuch as the hydrogen concentration in these specimens remains constant indefinitely at room temperature and the level is predetermined independently of analytical techniques, the specimens can be used as primary standards in gas analysis. Conditions for the successful batch production of hydrogen-in-steel standards have been established. The precision and reliability of standards produced in this way have been examined and it is shown that hydrogen concentrations may be quoted with confidence to within 2%.
THEHIGH SOLUBILITY and diffusivity of hydrogen in steel at elevated temperatures together with the very low solubility but still appreciable diffusivity at room temperature, produce a problem unique in the field of metallurgical analysis. In few other cases is the act of initial sampling fraught with so many difficulties and uncertainties. In metallurgical analysis generally, a clear distinction can be drawn between errors due to sampling and errors arising from the method of analysis itself. Usually the latter can be investigated successfully by the use of a carefully prepared synthetic sample and the degree of intermethod and interlaboratory agreement established. Once this information has been obtained the performance of methods and laboratories using practical materials of certified homogeneity can be studied. In the case of determinations of hydrogen in steel no such scheme has so far proved entirely satisfactory. Replicate analyses by one operator in one laboratory using one apparatus rarely, if ever, show the precision that normally would be considered essential in quantitative chemical analysis. Taken at the interlaboratory level even greater, and sometimes embarrassing, discrepancies are revealed. There is reason to suppose that hydrogen itself can be analytically estimated with a high degree of precision because of the inherent sensitivity of the detection methods used. Some indication of analytical precision can be obtained by calibrating the apparatus against carefully metered volumes of pure hydrogen gas. While such procedures may demonstrate analytical precision, it is generally recognized that the presence of a steel specimen is desirable to compensate for matrix effects and possible side reactions. Only by using a steel specimen out of which a known 982
ANALYTICAL CHEMISTRY
amount of hydrogen is extracted can one begin to simulate routine practice and investigate the possible accuracy of the determination. The use of steel specimens artificially hydrogenated to a predetermined level goes some way toward fulfilling this requirement, and has formed the basis of several recent cooperative investigations. The preparation of such specimens is not an easy task, whether the hydrogen is introduced by thermal, pressure, or electrolytic techniques, and storage and handling prior to analysis present the same difficulties that are encountered in the handling of routine hydrogen samples. Those who have had experience with this type of investigation will probably agree that analytical variables tend to become obscured by the uncertainties regarding the hydrogenated samples themselves. Determination of hydrogen still requires a certain degree of skill and experience, and few operators are quick to admit that their own apparatus could be giving erroneous results. Interlaboratory differences are therefore too conveniently attributable to “sampling errors,” and while there is ample justification for regarding the sampling stage with considerable suspicion, it will not be possible to effect an objective assessment of sampling efficiency until the capabilities and limitations of the various types of analysis equipment are known with confidence. The need to investigate possible methods of preparing standards for hydrogen determination is therefore clear and this report describes work which has culminated in the development of a synthetic specimen showing all the features of a good primary standard. GENERAL CONSIDERATIONS
A certified reference material must fulfill certain requirements which have been defined by Hoffman (1) as follows: homogeneity, permanence, integrity of certified value, and form convenient for intended use. In addition, the material must be readily available to the potential user at a reasonable price. To be classified as a secondary standard the certified value can be provided by a single reputable source without independent check. As a primary standard the composition must be certified by a (1) J. I. Hoffman, ANAL.CHEM.,31, 1934(1959).
