Quantitative Determination of Oxygen in Gases LEONARD P. PEPKOWITZ AND EDWIN L. SHIRLEY Knolls Atomic Power Laboratory, General Electric Co., Schenectady, .V. I-. Because of the extreme requirements in regard to the oxygen content of the inert gas blankets required in connection with recent developments in technology of liquid metal coolants, it is necessarj to have an accurate and sensitive method applicable to the range below 100 p.p.m. of oxygen. The method described is based on the Winkler manganous hydroxide reaction. The method is absolute with a zero blank. The standard deviation for precision is 0.77 p.p.m. of oxygen and the sensitivitlis 0.5 p.p.m. Relatively small samples can be handled, because the reactions can take place below atmospheric pressure. This fact eliminates the need for a carrier gas, thus removing one of the major sources of error. The apparatus is constructed of readily available laboratory equipment and can be made portable. Six to eight samples can be handled during the working day.
sample is of the order of 3 hours, but the samples are overlapped so that six to eight samples can be run during a working day. Only relatively small samples are required, a$ the reactions can take place below atmospheric pressure. This part eliminates the need for a carrier gas, thus removing one of the major sources of contamination. METHOD
In trying to meet the objectives listed above, the authors tried many of the described procedures and finally concluded that the classical Winkler manganous hydroxide method (6) had the most promise ab a sensitive stoichiometric procedure The fundamental reaction- involvcd are ( 4 ) :
-
-
(2)
02 = 212
to meet the optimum requirements. These are indicated in the folloning list together with the method of control or elimination used in the present paper.
T
The method described in this paper possesses most of these requirements. It is absolute with a precision of & 1 p.p.m. of oxygen and a sensitivity of 0.5 p.p.m. It is constructed of readily availahle laboratory equipment and is portable. The time per
(1)
(3) -4number of sources of error must be eliniinated or controlled
HE necessity of measuring the oxygen concentration belon 100 p.p.m. in inert blanket gases has become of increasing importance with the development of liquid metal (sodium, sodiumpotassium alloy) coolant technology. The available instrumental methods are not satisfactory in this region because of the lack of sensitivity or reproducibility. A number of chemical methods have been described; recent methods applicable to the range below 100 p.p.m. are those of Brady (f), Winslow and Liebhafsky (7), and McArthur (3). Brady and Winslow and Liebhafsky require the preparation of an oxygen-free carrier gas, which is difficult in itself, while all three methods require large samples of the test gas either to purge the analytical system or because the method is based on passing a large volume of gas through the apparatus. The previous literature has been adequately reviewed in these papers. While this report was in preparation, a paper by Williams, Blachly, and Miller (6) was published but did not contain any data to evaluate the sensitivity, precision, or accuracy. The qualifications of an ideal method for the determination of oxygen in this region should be as fol1ow.c:: Absolute, The ideal method should be stoichiometric with a zero blank if possible. Only with this requirement can one be sure that the resulting- value is not an artifact of the technique in this low range. Precise. The reproducibility of the d u e s should be a t least f 1 p.p.m. to provide the control required in some phases of liquid-metal work. Sensitive. The method should detect a t least 1 p . p n of oxygen. Simplified. The equipment should be rugged and easily constructed of standard laboratory equipment. The use of oxygenfree carrier gases should be eliminated ae a possible source of contamination. The apparatus should be portable, so that samples can be taken directly, thus avoiding possible contamhation during an intermediate transfer step. Rapid. The required time of an analysis should be such that answers will be available in a few hours as a maximum, so as to provide adequate control over the process. Small Samples. In many situations only small samples are available for analysis, such as sampling dry box atmospheres or other small static systems depending on the maintenance of inert atmospheres.
+ + 2H10 -lMn(OH), + 6Hf +2hIn+T +I? + 6H20
4 l I i i ( O € I ) ~ 02
2lI11(0H)~ 21-
Dissolved Oxygen in Reagent Solutions. In order to eliminate the use of an “oxygen-free” gas as a purging gas to sweep out the dissolved oxygen, simple vacuum techniques are applied. It is not necessary to resort to elaborate means; a simple mechanical pump suffices for this purpose. Air Oxidation of Iodide to Iodine. This is a serious source of error in the present application, as the oxidation proceeds a t a significant rate even in basic solutions. The effect is minimized by using a 1.31 sodium hydroxide-0.1M potassium iodide solution freshly prepared immediately before use. TO VACUUM
-
GAS SAMPLE
c’r Figure 1.
u
Schematic Drawing of Apparatus
Manganic Ion in Manganous Salt. This is one of the most serious sources of error, since the manganic ion (or ferric ion) in the manganous salt will be determined as oxygen. No source of manganous salt was tried that was satisfactory in thls respect. The manganous sulfate used in the determination was prepared by reaction of acid-washed pure electrolytic man anese metal in the dissolver, A (Figure l ) , with boiled and coolefO.llL’ sulfuric acid, I n the resence of excess metal only manganous ion is formed. -4suzcient amount of manganese is dissolved within 15 minutes, which is the time required for the other preliminary manipulations. Contact between Gas and Solid. One of the drawbacks of the Winkler method is the required reaction between the gas and the precipitated manganous hydroxide. This requirement is the rate-limiting step of the entire process and can be accomplished only by shaking for a long time. The resction bulb, B, was designed to facilitate the contact between the as. apd solid as well as to provide a simple means of agitation. &hls is accomplished by slowly tumbling the flask end over end with the simple appara1718
1719
V O L U M E 25, N O . 11, N O V E M B E R 1 9 5 3 tus shown in Figure 2. As the manganous hydroxide suspension m. n~drain . ~ into ~ t,he side arm. C. each revolution ensure8 that the walls of the reaction bulb will be~freshivcoated durine: each rev~~~~~~~
~~
~~~~~
hi,droxi&. Contamination during AcidiBcation. One of the conunon S O U E ~ S of error is introducing air into the reaction bulb before the basic nmnganaus hydroxide suspension is sufficiently acid. This Z O U I C ~of error is eliminated hy the simple vacuum technique used. Determination of Small Amounts of lopine. The success.af the entire method depends on the sensitivity and precision with which the iodine released in the process can he determined. In the earlier days of the investigation, the released iodine was measured volumetrically with thiosulfate, using microweight burets and starch a8 an indicator. However, more reproducible values were obtained when the iodine was extracted with toluene and measured calorimetrically. The toluene-iodine system is not odj- st,ahle hut has a higher extraction coefficient than the usual eolrents like carbon tetrachloride or chloroform.
5 . Then transfer 10 ml. of the manganous solution to the rertction bulb by means of thme-way stopcock 5. Man anom hydroxide will be precipitated in the side arm as the aci& solution is neutralized by the sodium hydroxide. Close stopcock 2, admit the required sample of gas by manipulation of stopcock 4, and record the temperature and pressure. Close stopcocks 3 and 4 and mount the reaotion bulb on the mixer which is shown in Figure 2. Tumble the reaction bulb for 2 hours with the speed adjusted so that the side arm will completely fill and empty during each revolution.
APPARATUS
The apparatus is shown schematically in Figure 1. The graduated dissolver cylinder, A, is used for the preparation of the manganous sulfate solution during the preliminary evaeuation. The 1-liter reaction bulb, B, is fitted with the side arm, C, at.tarhed by a standard-taper joint. This side arm is used for the introduction of the freshly prepared sodium hydroxidepotassium iodide solution. Stopcocks 2 and 3 are the recision vacuum type. They should be spring-loaded or p r e f e d l y have an evacuated bulb a t the end, 80 they are held in place by atmospheric presmre. This is necessary t o prevent loosening of the stopcock barrel during the mixing step. It was found convenient to have the rest of the stopcooksin the apparatus of the m e type, hut this is not a rigid requirement. The reaction bulb and the other parts of the apparatus are fitted together u-ith semiball joints to enable the disengagement of the various parts during the determinntion. The pressme tubing, F, is inserted to supply the required flexibility for t.he 8ame reason. Trap D ie required to catch the first few milliliters of the manganous sulfate before the measured amount of the remainder IS transferred to the reaction bulb. The reason for this step is to remove the air entrapped in stopcock 5 bore. Trap Diis substit,uted for D during the acidification step. Cold trap G is a lmgevolunie trap cooled by acetonedry ice slurry and is used to prevent the cazry-over of moisture to the pump. A simple mechanical pump is sufficient, as there is no need t o pump helow the vapor pressure of water to degas the solution. The closed-end manometer, Ej is used to measure the sample pressure, as the Bystem is c a p b l e of handling samples of less than atmospheric preasure. This is a valuable attribute of the method, a8 it allows for the analysis of small ssmplea (less than 1 liter STP) with the same equipment. This aspect of the method enabled the use of very small air samples to prove the accuracy of the procedure. The use of proper stopcock grease for the components of the reaction flask is important. The grease must be able to withatand the attack of the caustic solutions during the shaking priod. Apiezon type N was the only satisfactory lubricant tned. All other types rapidly developed channels, especially a t the stopcocks. PROCEDURE
In order to attain the precision indicated, i t is imperative that the reaction bulb be scrupulously clean and the stopcock grease from the preceding determination removed. Excess stopcock grease ahould not he used, as this often traps Some of the released iodine. --,
~
trolvtio manga%se metal.
.T h c reaction will produce .mank&nie-
sium ioai& I'nto side arm C and-connect t o ;eaetion bulb B;using Apieaon-N grease. Evacuate the aystem t o remove the air from the reaction bulb and to degas the caustic-iodide solution. Fifteen minutes of pumping is sufficientfor this atep. Close stopcock 1 and run 10 ml. of the manganous sulfate solution into trap D to remove the air trapped in the base of stopcock
Figure 2.
Miring Device
After the mising period, replace t.rap D with trap D,,which is filled with boiled and cooled 2M sulfuric aci?. Reassemble the apparatus, omitting dissolver A and reversing the positions of stopcocks 2 and 3. This step is required, as the semihall joint and glass tubing on this side are still clean and dry. Start the pump and open stopcock 1. When the residual gas 18 removed from D1, cautiously open stopcock 3 (now in reverged position) to pump out the gas from the reaction bulb. Besides reduein the EBSure in the reaction hulb, this oxygen-free gas P&=F tfrougkthe sulfuric acid and removes the last traces of dlssolved oxygen. When the pressure in the bulb is low enough, close stopcock 1 and heat trap D Iwith a beaker of hot water which forces the acid over into the reaction bulb. Close stopcock 3 and remove the reaction bulb from the apparatus. Mix the solutions thoroughly to acidify the contents completely and to dissolve the precipitate. Transfer the aqueous solution to B Beparatory funnel and extract with toluene. Repeat the extraction to ensure that all the iodine is removed and determine the iodine colorimetrically against 8. previously established standard curve. For higher oxygen concentrations, titration with thiosulfate to a starch end point ia satisfactory. RESULTS AND DISCUSSION
The blank value for the procedure is essentially ~ e r o . The values were obtained by carrying out the determination with no added gas hut introducing the manganous solution directly into the evacuated hulh. I n a set of nine runs, seven resulted in no detectable iodine (aero blank), while the other two gave the equivdent of 0.9 and 0.3 microliter of oxygen. There was some reason to suspect that these two values were caused by a minute leak. However, including these values, the significance of these deviations was evaluated by using the t-test difference method (e). The calculated 1 value was 1.287, which when compared with the 5% value (2.306)a t 8 degrees of freedom indicates no significance to the observed deviations and establishes that the blank value is zero for this determination.
1720 Table I. Oxygen Taken, Pl.
;2.3 02.2 47.9 44.7 41.8 31.2 32.9 29.6 28.1 21.0 20.0 17.5 17.0 14.7 13.7 11.6 7.1 7.8
ANALYTICAL CHEMISTRY Recovery of Oxygen from Known Amounts of Air Oxygen Oxygen Found, Difference Taken, rl rl rl . 53.9 +1.6 7.8 51.5 6.3 -0.7 1.7 44.0 -3.9 1.8 46.3 f1.6 46.8 +5.0 0.7 26.4 -4.8 0.9 26.9 -6.0 1.7 32.5 f2.9 1.8 26.0 -2.1 1.3 17.8 -3.2 0 21.9 fl.9 0 +4.5 22.0 0 0 19.0 +2.0 0 5.8 -8.9 -5.5 0 8.2 -0.2 0 11.4 0 8.0 +0.9 10.3 +2.5 Sum of differences for all values Average difference G)
.
.
s(d) t
tdZ3’ 5%
Oxygen Found, r l. 10.5 8.7 2.3 2.8 1.1 2.0 1.7 4.8 1.5 3.6 2.0 0.7 0.8 1.2 0.9 0.7 5.7 =
= = = =
Difference, rl +2.7 f2.4 f0.6 t1.0 f0.4
.
+A.l +S.O f0.2
+3.6 f2.0 +0.7 f0.8 f1.2 fO.9 f0.7
f5.7
-1 . 5 0.0677 3.592 0.0835 2.042
In order to evaluate the accuracy of the present method, a large number of recoveries was made using air as the standard. An accurately calibrated 100-ml. gasometer was placed between the reaction bulb and a McLeod gage. The gasometer -was fitted with a three-way stopcock, one arm of which was connected to a McLeod gage and the other arm to the reaction bulb. The standards were prepared by pumping down the gasometer to predetermined pressure as measured by the McLeod gage. Then the stopcock was turned to connect the calibrated volume with the reaction bulb and the gas was pushed into the reaction bulb by displacement with mercury. No helium was added. The rest of the determination was carried out as described above. Thirty-five such determinations were made, varying from an oxygen equivalent of 52 to 0 microliters in order to cover the critical range of application. These data are presented in Table I. In evaluating the recovery data which are shown in Table I, it is essential to point out that the inherent precision of these data is poorer than the true precision of the method because of the added manipulations and associated errors in measuring and transferring the known air samples. The actual precision of the present method is discussed below. As there is no blank involved in the method which might indicate a constant analytical bias or cast doubt on the stoichiometry
of the method, the proof of absolute accuracy depends on demonstrating that the differences in Table I do not deviate significantly from zero. This is most conveniently done by computing the t value for the average algebraic difference, using the method of differences (8). The standard deviation, s(d), for the mean difference is 3.592 and the equivalent t value is 0.0835. Since the significant (5%) t value for 30 degrees of freedom is 2.042,it can be stated that the deviation of the differences from zero is completely negligible and therefore the present method is stoichiometric and accurate within the experimental errors involved in the method of preparing the standard samples. The precision of the method was ascertained by a series of repetitive runs on a tank of relatively pure helium. The oxygen concentration was very low, which affords a test of precision under the most adverse circumstances. The cylinder used was approximately half emptied in the course of the earlier experiments and therefore provided a sample without the layering (nonmixing) effect almost always encountered in high-pressure gas cylinders. If this fact is not recognized, erroneous and discouraging results will be obtained. In a series of seven determinations (0.8 liter STP) an average value of 5.0 p.p.m. was obtained with extreme deviations of f1.0 and -0.8 p.p.m. The standard deviation for precision was 0.77 p,p.m., thus fulfilling the objective of a 1 p.p.m. of oxygen. precision which is at least =I= ACKNOWLEDGMENT
The authors are indebted to Florence Blinn of this laboratory for many of the determinations included in this report and to W. S. Horton, also of the laboratory, for many helpful discussions during the course of this project. LITERATURE CITED
Brady, L. J., ~ ~ X A CHEY., L . 20, 1033 (1948). (2) Davies, 0. L., “Statistical Methods in Research and Production,” London, Oliver and Boyd, 1949. (3) McArthur, I. A., J . A p p l . Chem., 2, 91 (1952). (4) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” Vol. 11, New York, John Wiley & Sons, 1942. ( 5 ) Williams, D. D., Blachly, C. H., and llliller, R. R., ANAL.CHEM., (1)
24, 1819 (1952).
(6) Winkler, L. W., Ber., 21, 2843 (1888). (7) Winslow, E. H., and Liebhafsky, H. A , , ANAL.CHEM.,18, 665 (1946). RECEIVED for review February 6, 1953. Accepted August 24, 1953. T h e Knolls Atomic Power Laboratory is operated by the General Electric Co. for t h e Atomic Energy Commission. T h e work prepared here was carried out under contract No. W-31-109 Eng-52.
Infrared Spectra of Metal lsopropoxides JEROME V. BELL, JULIUS HEISLER, HARVEY TANNENBAUM, AND JEROME GOLDENSON Chemical and Radiological Laboratories, Army Chemical Center, M d .
T
H E data outlined in this paper originate from a limited investigation into the properties of metal alkoxides. Special interest was devoted to finding absorption bands characteristic of the isopropoxy grouping as it occurs in a variety of molecules. In addition, characteristic absorptions were derived for butoxy, ethoxy, and methoxy groups from spectra available in the literature. The four spectra determined in this work (Figures 1 to 4) do not appear to be available in the literature and constitute an addition to the small library of spectra of distinctly metallic atoms linked by covalent forces to organic groups. The present work indicates that absorption bands in the 8.5- to 9.4-micron region consistently appear in the spectra of molecules
containing the isopropoxy group, which enables a distinction to be made between this group and most other alkoxy groups. EXPERIMENTAL WORK
The infrared spectra s h o m were recorded in the rock-salt region with a Perkin-Elmer Model 21 spectrometer. The sample cell used was approximately 0.12 mm. thick and the compensating cell was approximately 0.11 mm. thick. Carbon tetrachloride and carbon disulfide were employed as solvents. The absorption maxima of the metal isopropoxides are listed in Table I. MATERIALS
Triisopropyl phosphite (phosphorus triisopropoxide), P(0iC3H7)3,was prepared by Thomas Dawson, Chemical and Radio-
