rection for the contribution due to lead; this technique is applicable provided that the lead and tin concentrations do not differ greatly (7). The analytical procedure described in this paper for the separation of tin provides, in addition, a means of recovering bismuth and lead so that the three elements can be determined using one sample weight. Bismuth and lead are contained in the organic layer after the initial extraction with APDTC and chloroform from an ammoniacal citrate solution, and can be separated from the remaining copper by shaking with an ammoniacal citrate solution containing cyanide ions. The two elements can be determined simultaneously by polarography
The ratio of oxygen to tin at which oxide formation takes place is not known and probably depends to some extent on the metallurgical operations performed on the sample. Visual inspection of the solution may not show the presence of a residue. As an example, 100 ml of a solution prepared by dissolving 5 grams of standard SSC-1, containing 42.4 ppm of insoluble tin, showed no visible residue even on standing overnight. It is advisable, therefore, that the determination of total tin in tough-pitch copper should include filtration of the sample solution, dry ashing of the filter paper, and examination of the residue for the presence of tin.
(8).
ACKNOWLEDGMENT CONCLUSION
The outcome of this investigation shows that the values obtained for small amounts of tin in tough-pitch copper can be very misleading when obtained by wet chemical procedures. (7) R.C. Rooney, Metallurgia, 74,93 (1966). (8) C. H.McMaster, Can. J. Chem., 43,405 (1965).
The author acknowledges the assistance of Mr. E. J. Murray of the X-ray diffraction laboratory for the examination and identification of the acid-insoluble residue. RECEIVED for review February 7, 1969. Accepted June 16, 1969.
Spectrophotometric Method for Determining Oxygen in Gases Samuel Kaye and J. E. Koency General Dynamics Convair, Space Science Laboratory, San Diego, Calg. 92112
MONITORING OXYGEN concentration in gas mixtures has been necessary in this laboratory in several studies dealing with propellants and life support systems. Choice of method for oxygen determinations depends largely on physical state, concentration range, application, and accuracy desired. Generally, electrochemical ( I ) , colorimetric (Z), or polarographic (3) methods may be used with oxygen in water solutions. Orsat absorption analysis, gas chromatography (4), magnetic susceptibility (3,or mass spectrometry (6), are common methods applicable to gases. We have found it useful, convenient, and rapid to make continuous measurements of oxygen in many applications by converting some of the oxygen to ozone in a glow discharge and measuring the ozone absorption at the intense ultraviolet band at 2537 A. With this method, there are no moving parts, response time may be decreased to a few seconds, oxygen is not chemically reduced so that it becomes unavailable, and there is no dependency on gravity. EXPERIMENTAL
Apparatus. A schematic diagram of the apparatus is shown in Figure 1. A pump is furnished if it is necessary to provide transport of the gas through the system. The flow rate is measured accurately by flowmeter A and is fixed at (1) J. M.Ives, E. E. Hughes, and J. K. Taylor, ANAL.CHEM.,40, 1853 (1968). (2) L. Silverman and W. Bradshaw, Anal. Chim. Acta, 12, 526 (1955). (3) K.Linhart and J. Zagmen, Chem. Anal. (Warsaw),2,183 (1957). (4) W.M.Graven, ANAL.CHEM., 31, 1197 (1959). (5) L. Pauling, R.E. Wood, and J. H. Sturdwant, J. Amer. Chem. Soc., 68, 795 (1946). (6) G. P. Barnard, “Modern Mass Spectroscopy,” Institute of Physics, London, 1953.
about 150 ml per minute. From the flowmeter, the gases pass through the glow discharge tube B. This unit consists essentially of a stainless steel inner electrode separated from an outer point-electrode by a glass tube. The gas passes through an annular space about 0.5 mm thick between the inner electrode and the glass tubing. Voltage for the glow discharge is supplied either from a luminous tube transformer or a solid state device. The details of a solid state power supply and the ozone generator tube are described in more detail elsewhere (7). Ozone is formed in the glow discharge tube and is measured continuously during the flow through the sample cell C of a d?uble beam spectrophotometer ( D ) . The wavelength 2537 A is selected for measurement by the monochromater because it is the peak of the most intense ozone absorption band (8). Procedure. Calculations of the ozone concentration are made by using the ozone absorption coefficients obtained in reference (8) to prepare a calibration curve of ozone concentration us. transmittance. The curve can be read with scale changes on our Beckman DK-1A spectrophotometer with an accuracy of 1k0.05% or ~ t 0 . % 5 transmittance corresponding to 0.05 to 0.5 ppm ozone concentration. This in turn corresponds to about 0.1 to 0.5 % oxygen depending on concentration range and conditions. Calibration curves for oxygen-containing gas mixtures are prepared by passing mixtures of increasing oxygen concentration through the apparatus and measuring the transmittance. Transmittance us. the oxygen concentration relation is then plotted as the working curve for determining the oxygen concentration of an unknown mixture. Figure 2 shows curves for oxygen-nitrogen and oxygen-helium mixtures. A sample stream from the unknown gas mixture is then passed through the 10-cm cell of the spectrophotometer. _.
(7) S. Kaye and J. E. Koency, Rev. Sci. Instrum., 40, 505 (1969). (8) M.Griggs, J. Chem. Phys., 49,857 (1968). VOL. 41, NO. 11, SEPTEMBER 1969
1491
12 K V LUMINOUS TUBE
4
N PUMP
Figure 1. Schematic diagram of oxygen analyzer system The oxygen concentration in the sample can then be read directly by comparing the transmittance with the values on the calibration curves.
rated. The per cent transmittance may then be measured in a more sensitive range. For smaller oxygen concentrations, the amount of ozone generated is raised by decreasing the flow rate and increasing the field strength so that the transmittance may be read on the 90-100% presentation of the range span. When this is done, the reproducible reading error of k0.5 or 10.05 causes an error in oxygen determination of less than +0.5 Zor 4~0.1%. When various operators independently set up the flow conditions, the reliability of the method is less. At a single oxygen concentration in nitrogen, the standard deviation in per cent transmittances was determined to be 1.79. This corresponded to maximum error of 2.1 % of the oxygen present at high concentrations and about 0 . 2 1 z maximum in the low range span. This means, for example, that a concentration of 13.0% was accurate to 13.0 10.27%. Similar tests were run with change in voltage. For these runs the standard deviation in per cent transmittance was 0.3 for 0.062 oxygen in nitrogen. This corresponded to an error of 0.026 in per cent oxygen. That is, the low concentration was determined to 0.062 i 0.026%. Although the error
RESULTS AND DISCUSSION
The method outlined above is best adapted for determining oxygen in mixtures of known gases because the oxygen ozone conversion mechanism is quite complex. The various diluent gases behave with quite different efficiencies as third body agents for transferring energy during the synthesis process (9). Extreme changes in gas composition therefore lead to erroneous results. Figure 2 shows typical results obtained with our apparatus using nitrogen and helium mixtures. These particular curves are most applicable for measuring oxygen concentrations of 2 to 16%. For higher concentrations where the curves level off and transmittance is low, the flow rate may be increased and the ozonizer voltage reduced so that less ozone is gene-
z
(9) S. W. Benson, “The Foundations of Chemical Kinetics,” McGraw-Hill Book Co., New York, 1960, p 4 2 .
IO 0
I
1
2
,
1
4
,
1
6
1
1
8
1
10
1
1
I2
~
,
14
~
16
,
18
PER CENT OXYGEN
Figure 2. Per cent transmittance us. O2concentration for Nz-02 and He-02 mixtures 1492
ANALYTICAL CHEMISTRY
20
in accuracy is thus sizable, the measurement of oxygen relative to the total gas sample is quite acceptable for our purposes. The sensitivity of the method with the apparatus described is such that less than 0.1 % oxygen may be easily detected because the transmittance of the ozone generated is less than 99%. This is easily read on an expanded scale. This sensitivity may be increased by raising the voltage of the power supply and by increasing the path length of the cell. The accuracy depends on the concentration range as shown by the curves in Figure 2. Use of the expanded scales on a Beckman DK-1A permits measurements of low concentrations which are as accurate as those where the slope of transmittance us. concentration is steep. The spectrophotometer can measure ozone concentration as low as 0.2 ppm corresponding to oxygen concentration of less than 0.01 % in some tests. The method is selective for oxygen. However, hydrogen containing gases such as methane and hydrogen itself interfere because oxygen atoms formed in the discharge tube react to form water which then invalidates the quantitative basis of the method. Oxygen-containing compounds may also impair the analysis. Pure carbon dioxide for example will give a response when passed through the discharge. In our apparatus, 15 ppm O 3 was generated from pure COZand 4.5 ppm from pure CO. These facts may suggest that under appropriate conditions the method may be applicable to analysis of COYor even CO, but we did not consider this feasible. Response time is a function of the volume of the system. Short small lines afford response times of less than 1 second.
The spectrophotometer sample cell may also be shortened but a larger 10-cm cell provides greater sensitivity without great loss in response time when the flow is 150 ml/minute. The sample gas must be dried and the outer electrode epoxy coated to prevent moisture from affecting the electrical field or the kinetics of the 0 2 $ 0 3 conversion. The kinetics is perturbed also if the temperature is not maintained constant. Less ozone is generated at higher temperatures. In general, higher voltage produces more ozone, except ozone destruction becomes noticeable at extremely high voltages. However, the discharge usually punctures the glass tube first. In general, lower flow rates increase residence time in the electrical field so that more ozone is generated per unit of oxygen. In fact, 100% conversion of Oz to O3 is possible in a static system at cryogenic temperatures (IO). In spite of the number of variables which affect the method, these are easily fixed for any given gas mixture. The same factors which cause variable ozone output when uncontrolled, also afford very great flexibility in the range of oxygen concentration for which the method is effective. The method then provides a continuous rapid presentation of the oxygen concentration in a mixture and is a simple way to monitor gas streams. RECEIVED for review December 9, 1968. Accepted June 20, 1969. (10) M. Griggs and S. Kaye, Reo. Sci. Instrum., 39, 1685 (1968).
CORRESPONDENCE Correlation of Linear Sweep Voltammetric and Chronoam perometric Data for n-Value Determinations SIR: One of the most important parameters characterizing an electrode process is its n-value, the number of electrons transferred. If the diffusion coefficient of the electroactive species is known, the determination of n is usually straightforward. However, diffusion coefficients are obtained, in most instances, by electrochemical methods in which an n-value is assumed. Controlled potential coulometry is often the most unequivocal method of determining n, but one must be cognizant of the possibility of interferences from homogeneous chemical reactions in such electrolyses which may lead to spurious nvalues. Thin-layer electrolysis is an ideal choice for n-value determinations since a knowledge of diffusion coefficients is not necessary ( I ) . The extensive use of thin-layer electrolysis has been hampered by the problems of fabrication of thin-layer cells, We report a method for determination of electrochemical n-values which is based on quantitative comparisons of linear sweep voltametric and chronoamperometric data. The method does not require previous knowledge of a diffusion coefficient or electrode area. Mueller and Adams used a similar approach but were interested in an experimental evaluation of the Randles-Sevcik constant ( 2 , 3 ) . The peak current, ip,for a linear sweep voltammogram of a reversible redox system is given by ( 4 ) : i, = 2.68 X lo6 n 3 I 2A C O X ’ D O X ~ / ~ ~ ~(1) /~
where L‘ is the voltage sweep rate in volts/sec, and all other terms have their usual significance. In the case of chronoamperometry in the diffusion-controlled region of the currentvoltage curve, irrespective of the reversibility of the electrode reaction, the usual Cottrell relationship holds (5): =
nF,LfCox”ox’i2f-’/2
(2)
If we divide Equation 1 by Equation 2 and rearrange, the foliowing relationship results : (3)
This relationship between i P / u 1 i 2and hereafter termed R, offers an attractively simple way to obtain an n-value for an electrode process.
(1) C. R. Christensen and F. C. Anson, ANAL.CHEM.,35, 205
(1963).
(2) R. N. Adams, “Electrochemistry at Solid Electrodes,” Marcel Dekker, Inc., New York, 1969, p 128. (3) T. R. Mueller and R. N. Adams, A n d . Chim. Acta, 25, 482
(1961).
(4) R. S. Nicholson and I. Shain, ANAL.CHEM., 36, 706 (1964). ( 5 ) F. G. Cottrell, 2.Physik Clnem.,42, 385 (1902).
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