~
- - ___ -
-
__li
Continuous Detection and Measurement of l o w Concentrations of Oxygen in Gases LEONARD P. PEPKOWITZ Knolls Atomic Power Laboratory, General Electric Co.,
Schenectady,
N. Y.
At all moisture concentrations below this value, the mirror will remain clear and the fluctuations below this level will not actuate the alarm system. However, when the moisture concentration rises shove this value, frost will form immediately and the instrument will actuate the alarm circuit. Thus the sensitivity of the apparatus is maintained over an extended range of concentrations limited only by the rate of diffusion of the moisture in the gas to the mirror and the instrument response time. The instrumental response is not limiting even st very low moisture concentrations because of the sensitive photoelectric system utilized ( I ) .
The oontinuous deteotion and measurement of low concentrations of oxygen in gases is of paramount importance in liquid metal heat transfer technology. This paper describes an instrument based on the dew point principle for such oontinuous detection and measurement. A sensitivity of at least 0.0005 volume % of oxygen is attained with a mean deviation for precision of &0.0001 volume yo. The instrument is applicable to the determination of oxygen in the common gases such as nitrogen, helium, the other rare gases, hydrogen, and c a r b o n dioxide.
APPARATUS
0
WE of the main difficulties in the use of Liquid alkali metals
The continuous oxygen detector was constructed by additions to tLnd modifications of the commercially available General Electric continuous dew point recorder (Catalog No. 32C109G15). The operation of the photoelectric svstem, thyratron unit, re-
as heat transfer agents is that of preventing oxygen contsmination of the system. The resulting sodium monoxide causes increased corrosion, in addition to plugging the system. As this is a cumulative effect, exacting control of the oxygen content of the inert gas blankets is required on a continuous basis. Because of the cumulstive effect, the oxygen concentrations must be held t o very law limits, oftentimes helow 0.001 volume % ’ on Systems which must operate for long periods of time. Although a sensitive and quantitative batch method for such analyses has recently been described ( S ) , ideally, B continuous monitoring instrument is required. I n contemplating the design of such an instrument, a consideration of the use of the data. c m simplify the problem. I n almost all cases, the analytical data are used to mmre the operators of the system that the oxygen concentration of the inert gas is equal to or below a predetermined specification value. If the oxygen concentration rises above this preset value, corrective measures must be taken. Thus the data of interest are not required to be ahsolute in the usual analytical chemioal sense, hut the measurement required is the deviation from the specification value. Excess oxygen in the inert gas is usually caused by a breakdown of the purification system or a leak in the apparatus, This requires immediate remedial action by the operators. Conversely, the fluctuations in the oxygen Concentration below the design value should not actuate the instrument and be the cause of repetitive false alarms. The available instrumental methods for the continuous measurement of oxygen concentration in gas-such as magnetic susceptibility, and measurement of the heat of reaction with hydrogen or polamgraphic iechniques-are not applicable in this range (below 0.005%) because of lack of sensitivity or reproducibility. However, the application of the dew point principle is ideally suited to meet the above mentioned objectives. Other chemical methods applicable to this range are adequately reviewed by Pepkowitz and Shirley (3). The application of dew point methods to absolute measurements is limited by such considerations (#) as the temperature gradient across the gas-liquid interface, inherent slowness of the diffusional process, and the hysteresis effect, in the dew evaporation. On the other hand, the method possesses relative simplicity, great sensitivity, rand applicability to gases under flow,conditions a t atmospheric pressure or above. The major advantage of the dew point method, besides its application to continuous detection, resides in its applicability to differential measurement. The temperature of the mirror can be set a t any desired dew point corresponding to the specification concentration of oxygen in the gas when converted to water.
which enables the i&rument to fu&on normally as s h e w point recorder in position 1 (dew point) ar when turned to position 3 (oxygen) makes bhe following circuit alterations to convert the instrument to the continuous oxygen detector. Connects the mirror heater to an external Variac controlled circuit and disconnects i t f r o m t h e thyratron unit, snitches the thyratron output to the alarm socket, and disconnects the automatic hourly defrosting switch. 2. Addition of an external 110-volt Variac-controlled circuit to energize and control the mirror heater. 3. Addition of a 110-volt Variac-controlled circuit to energize and control the platinum catalyst heating furnace. 4. Instrtllxtion of an indicating pyrometer for the catalyst heating furnace. 5. Installation of the gas inlet system including valves, hydrogen introduction system, and plst,inum catalyst assembly dea:rihed below.
Figure 1.
245
Gas inlet and eatal3st assembl,
246
ANALYTICAL CHEMISTRY
The gas inlet system is comprised of a by-pass valve assembly, a one-way hydrogen metering and injection system, and a heated plat'inum catalyst section. The details are shown in Figure 1. The two-valve arrangement on the copper inlet line allows the test gas to pass direct,ly into the instrument when operating as a dew point recorder (top valve open, bottom valve closed) or passes the gas through the hydrogen inlet section and the catalyst section (top valve closed, bottom valve open). The glass stopcock a t the bottom of the mercury valve assembly is for init,ial purging to remove oxygen from the hydrogen metering system. The simple one-way mercury valve system was used to introduce hydrogen from a No. 4 cylinder of extra-dry grade hydrogen as the gas source. At the very slow rate of addition after the initial purging, a No. 4 cylinder will last for many months. The one-way mercury valve system also helps to control the effect of the back pressure of the test gas and acts as a simple flowmeter. This is accomplished by regulating the gas flow to approximately 10 bubbles of hydrogen per minute. In actual practice this very simple arrangcment proved to be reliable for long periods of time after the initial adjustment and the maintenance of a steady pressure of the test gas. At the very low concentrations of oxygen and hydrogen involved, the conversion to water will proceed only in the presence of a catalyst. Accordingly, the heated platinum catalyst section \vas incorporated into the gas inlet system as shown in Figure 1. This has proved effective even in the range of oxygen concentrations below 0.001%. The connection between the glass tubing and the copper system a t the exit end of the heated section is made with silicone rubber tubing which will withstand the elevated (150" C.) temperature. All other glass-to-metal connections are butt joints made with short 1engt)hsof Tygon tubing. Although in actus1 practice the alarm outlet is connected to an external alarm system during the development work, the change in oxygen concentration \ v u followed by noting the light intensity of a 60-watt bulb plugged into the alarm outlet. This proved to be very convenien.t, a s the rate of dew deposition was indicated by the change in light intensity of the bulb. For actual records, a recording ammeter with a suitable series resistance was used. Figure 3 shows a front view 6f the instrument with the dex point cell and phototube assembly visible. Figure 4 is a view of the left side of the instrument showing the auxiliary heating circuits, pyrometer, selection switch, and alarm outlet.
0.021% ' oxygen). Sitrogen from the general laboratory supply was passed into the instrument through a three-way stopcock. This gas contained -0.0301, of oxygen as the sum of the oxygen and moisture in the gas The response of the instrument was followed by noting the response time and the brilliance of the 60-watt light bulb plugged into the alarm socket. The response was practically instantaneous and the brilliance was equal to the normal output 3n the usual 110-volt circuit. When the gas supply was switched back to the cylinder of helium, the light emission decreased as the frost on the mirror evaporated back into the dry gas. After some 5 minutes, depending on the amount of frost that accumulated on the mirror, the output signal was back to zero and the mirror was clear. As the last traces of frost were evaporated, there was a continuous decrease in the light emission of the bulb. The sequence could be repeated a t will by switching from one gas supply to the other To determine the sensitivity and response characteristics of the instrument, various gases contained in standard cylinders Tvere tested by first determining the straight dew point of' the unknown gas and then adjusting the mirror temperature ti, 1' to 2" above this value with the selection switch turned to oxygen. The recording microammeter was connected to the alarm outlet and the pen was adjusted to zero. When a steady qtate was attained, hydrogen was introduced a t the preset rate. \\ ithin a few seconds, the milliammeter needle indicated the presence of oxygen. IT-hen the pen approached the top of its excursion (usually off scale) the hydrogen flow was stopped and after a variable period of time depending on the oxygen concentration in the gas, the pen returned to or approached the initial setting. h number of such tracings arc' shown in Figure 2. The hydrogen !vas introduced a t point -4 and turned off a t point B. The rate of' rice of the milliammeter tracing indicates the response timca; howver, in some cases the instrument wa3 switched off
NITROGEN 0 0 0 0 8 % 02 OEWPOINT - 60'F. MIRROR TEMP 59-C
OPERATION OF INSTRUMENT
-
Start the refrigerator units by following instructions ( I ) , and energize the heater circuit for the platinum catalyst furnace. While the refrigerator is cooling the mirror, adjust the temperature of the furnace t,o 150' C. Close the top valve and open the bottom valve in the gas inlet system. With the selector switch on dew point follow the operating instructions (1). Set the sample gas flow a t approxiniat,ely 0.1 cu. foot per minute (0.05 cu. foot per minute for helium). Introduce hydrogen a t approximately 10 blibbles per minute through the mercury seal on top of the fritted glass disk. Record the dew point until the lines are purged and a reproducible dew-point record is attained. When a steady state is realized, turn the selector switch to oxygen. By successive adjustment of the mirror heating control 011 the left side of the cabinet, adjust the mirror temperature to the reference dew-point temperature. This is based on the allowable oxygen concentrntion in the test gas and can be ascertained from the dew-point table ( I ) , remembering that each mole of oxygen will be equivalent to 2 moles of water, so that the values in the table must be divided by 2. When a steady state has been attained, the mirror should remain clear, unless the gas is higher in total oxygen than allowed. To use the instrument as a straight dew-point recorder, turn selector switch to dew point, open the top valve, close the bottom valve of the gas inlet system, and follow instruction in ( 1 ) . To determine the approximate total oxygen content of the gas including moisture, place selector switch a t dew point, and measure dew point of gap plus added hydrogen passed over the heated catalyst. The difference between the straight dew-point value and this value is equivalent to the oxygen concentration in the gas.
A9GON 0 0025 7' 02 DEWPONNT- 5 9 * F . MIRROR TEMP-57'F.
-
NITROGEN DEWPClhlT 0 55'F 045%
02
VlRR07 T E M P - 5 4 ' F
RESULTS AND DISCUSSION
The first tests, afterthe flow conditions for the gases and the temperature of the cntalyst had been ascertained, were to pass helium containing 0.001% of oxygen through the apparitus with the mirror temperature set a t -50" F. (dew point, equivalent to
MlLLlAMPS
Figure 2.
_L
Response characteristics and sensitivity of continuous oxygen detector
V O L U M E 2 7 , N O , 2, F E B R U A R Y 1 9 5 5
247
the presenoe of rtpproximatdy 0.01% moisture in the sample gases. This emphasises the inherent sensitivity of the instrument and dew-paint methods in general. I n the range of interest (less than 0.01% oxygen) the average equivalent of the l a F. differential, which can easily he attained, is approximately 0.0001% oxygen, and the oxygen equivalent decreases as the dew-point value of the test, gas is lowered. Thus a t 0.001% (-85' F.), the 1" F. differential is equivalent to 0.00003~0of oxygen. It is one of the attributes of the dew-point method t h a t the sensitivity 8s a function of temperature increases with a decrease in the dew-point temperature. Thus a t the lower end of the scale a 5" F. differential between -95' and -100" F. is equivalent to a change in the moisture content of O.OOOZ%, while in the 0.01% range a '5 F. change is equivalent to 0.0027% of moisture.
near the top of the excursion to save the pen from injury when hitting thestop. The response time of the equipment was very rapid. Although the test gas had to pass through some 5 feet of tubing, the instrument resoonded within 5 seconds after the introduction of the hydr6gen. The recovery time after s t o.. m i n e the hvdroeen flow was a " _ function of the oxygen concentration in the gas and flow rate. I n the lower concentration the recovery was relatively very rapid, hut a t the higher concentrations of oxygen it tailed off more slowly, as shown in Figure 2. I n the high oxygen concentrations much more water was frozen aut on the mirror and the hysteresis effect is noticeable. A t the lower concentrations the thin frost filmevaporate rapidly, minimizing the hysteresis effect and increasing th? rccovery rate.
Figure 4.
Table 1. Oxygen Concentration in T e s t Gases Cylinder
No.
K-989247 11-669 807768 261242 544
Cas
NB N*
A A He
Der point of Gas
-- 60 55
- 59 69 - 54 ~
newpoint
+oas H1 -56 - 9
-55 -49
-60
A
H~O.
Vol. %
0,". Vol. %
0.0016 0.0911 0.0017
0.0008 0.0456 0,0009 o 0025
o.on5o 0.0022
o.oni1
Left-hand view of oxveen
o+,
Val. % 0.0008
0,0523 0.0010
o.ooii o.001i
b Method oi Pephowit. and Sliirlry ( 8 ) .
The effect of Raw rate is indient,ed in the response curve for helium (Figure 2). Because of the higher heat capacity of helium the flow rate is reduced to 0.05 cu. foot per minute to avoid heating the mirror. The slower recovery time as shown in Figure 2 reflects this effect, as the rate of frost removal from the mirror is decreased because of the smaller volume of gas passing over it. The sensitivity of the instrument can be set very conservatively a t 0.0005 volume % of oxygen. Although the lowest eoncontration indicated in Figure 2 is 0.0008% by volume, a full scale deflection was obtained. As no lower concentration of oxygen wzs available to ascertain directly tho sensitivity of the instrument and assuming that there may be a threshold value required for the reaction of oxygen with hydrogen under the described flaw and temperature conditions, the 0.0005% sensitivity is claimed. The oxygen values detected and subsequently measured were in
As a ULU ab L/UUTIC~LLUU~. wt: ( ~ ~ L ~ C L I V B W S SVI lint macrument for quantitative measurement and incidentally to confirm that the observed responses were due only to oxygen and moisture, an estimate was made of the oxygen concentrations in several test gases. This was done by determining the straight dew point of the gas and the dew point of the gas when hydrogen is added. The oxygen value is the difference between these values expressed as volume per cent. of moisture divided by two. The gases were then analyzed by the method of Pepkawitz and Shirley (8)to determine the oxygen values. The data. obtaiued are given in Table I and indicate exceptionally good agreement between the two methods of analysis. Thc data given in column 6 are individual values and are coinpared with the averages of two or three determinations listed in column 7. These data can thus he taken as an indicntion of the accuracy of the method. The instrument. is capable of exceptionally goad precision. The data in Table I1 present two series of replicate measurements on nitrogen and holium a t the lowest available oxygen concentrations. The data were'taken in groups of three, spaced a week apart t o avoid the reflection of any ooustant instrument error which m y be present a t any givcn time. The mean dcviation is 'LtO.0001volume %.
ANALYTICAL CHEMISTRY
248 Table 11.
Replicate Data Obtained with Nitrogen and Helium
Kitrogen Containing 0.00083 Vol. '70 0 2 " Dew Dew point Ozr iioint, H:, vel. 0 F. F. 7%
-
+
- 60 -GO -GO
- 5G - 59
-$;
0 0008
0.0011
-aa
-56 -52
0.0008 0.0010 0.0009 0.0008
__
-0a
-57 AI'. 0.0009 .\lean dev. &0.0001 Clieniical analysis ( 3 ) .
-61
0
Helium Containing 0 00113 Vol. % Oaa Dew Dew point Oa. point, Hz, vol.
F.
- 54 - 54 - 54 -51 - 02 - 50
+
' F.
%
-50 -49
0.0011 0.0014
0.0011 0.0013 0.0010 0.0011 0.0012
-50 -47 -49 -47
4v. Mean dev. =kO.OOOI
I n high prescure gas cylinders, there is almost always stratification, which, if not recognized, will produce erroneous and discouraging resultq. This is almost always true of the first samples of gas removed. These replicate runs were made on gases fram cylinders which were halt emptied in the course of the preliminary experiments. In addition a number of cylinders contained free water and the moisture conteiit of the gas increased as the cylinder was depleted; this should be taken into account if the moisture content as well w the oxygen content must be controlled or avoided. The contribution in terms of total oxygen (H20 0 2 ) of the added hydrogen, is negligible. The hydrogen flow is 5 cc. per minute while the test gas flows a t -2800 cc. per minute. The total osygen (H,O 02) in the electrolytic hydrogen used was 0.04 volume %. Thus the positive error from this source is only 0.00007%. Because of this negligible effect, there is no need to use any special grade of hydrogen nor to resort to elaborate
+
+
means of cleanup, such as diffusion through palladium. Stoichiometrically 5 cc. per minute of hydrogen are in excess of the usual oxygen concentrations in gases. This rate is 0.22 millimole per minute, which would be equivalent to 0.0892 volume yo of oxygen in the test gas a t a flow rate of 2.8 liters per minute. There are no interfering compounds in the common gases such as nitrogen, argon, hydrogen, helium, or carbon dioxide. The only care required is to exclude particulate matter and oil vapors which may deposit on the mirror and necessitate cleaning. ,4 simple glass wool filter in the gas supply will usually eliminate such interference. As the freezing point of carbon dioxide is - 110" F., and i t can be liquefied only under pressure, there is no interference from carbon dioxide. ACKNOWLEDGMENT
The author is particularly indebted to William N o a k of this laboratory for his valuable assistance in fabricating the prototype instrument and obtaining most of the data, and Florence Blinn of this laboratory for performing the chemical analyses for oxygen in the test gases. LITERATURE CITED
Y. "General Electric Dewpoint Recorder Instructions," GIE-40444. ( 2 ) Ilixson. A. W., and White, C. E., IND. ENG.CHEM...LS.AL. ED., ( I ) General Electric Co., Schenectady, S .
10, 235 (1938). (3) Pepkowita, L. P., and Shirley, E. L., .IN.AL. CHEM.,25, 1718 (1953). RECEIVED f o r review July 31, 1954. Accepted October 18, 1954. T h e Knolls Atomic Power Laboratory is operated by t h e General Electric Co. for the Atomic Energy CommisSion. Work carried ont rinder Contract So. W-31109 Eng-52.
Determination of Benzo[~]pyrene in Complex Mixtures Use of Catalytic Iodination on Activated Alumina RUSSEL TYE, MARY JANE GRAF, and A. WESLEY HORTON Kettering Laboratory, University of Cincinnati, Cincinnati, O h i o
This paper descrihes the isolation and identification of the polyc>clic h?drocarhon, benzo[a]pqrene, in a residual product of the cata1)tic cracking of petroleum, together with the anal) tical method developed for the estimation of the concentration of this compound in products of refining operations. 4 selected fraction of a given sample is obtained by the use of a standardized chromatograph). Two equal portions of the fraction are taken, and one of them is subjected to a catalj tic iodination on a column of actitated alumina. Spectrophotometric measurement of the difference in absorbance hetween the iodinated portion and its mate is then used to determine the concentration of benzo[a]p?rene present.
A
HIGHLIGHT of the many years of effort by numerous investigators to identifl- the carcinogenic constituents in certain tars produced from coal was the isolation and synthetic proof of structure of benzojalpyrene by Hieger (1'0)and Cook and Hewett (6). .\]though there has been no direct proof of the relationship of the pure h j drocarbon to human cancer, such effects on certain experimental animals have been well documented (9). I n the course of the current investigation of the possible carcinogenic properties of high boiling products from petroleum
refining operations, i t was observed that the ultraviolet absorption spectrum of a distilled fraction of a catalyticallj cracked residuum showed maxima indicative of the presenre of benzopyrene in significant concentration. The compound u as isolated by the follom-ing procedure in a sufficient state of puiity for positive identification As a matter of conservation of time, no attempt was made a t quantitative separation. Fourteen hundred grams of the oil R ere subjected to a simple vacuum distillation. A fraction, boiling a t 205" to 275" C. a t 0.5 mm. of mercury and weighing 175 grams, was subjected to chromatography on alumina, and fractions enriched in benzopyrene \$ere selected spectrophotometrically. The chromatographic procedure was repeated five times, i~esultingin 6.5 grams of a red semisolid, the ultraviolet absorption spectrum of a-hich indicated a content of benzopyrene between 0.8 and 2.5 grams (curve I, Figure 1). The formation of iodine complexes (4,1 4 ) folloa-ed by filtration was used to remove pcrylene and other unideptified compounds, reducing the weight of the concentrate to 5 grams. This was dissolved in benzene and extracted with cold, concentrated sulfuric acid. The acid was diluted with ice, and the resulting precipitate was dissolved in benzene (for spectrum, see curve 11, Figure 1). Further fractionation by chromatography and fractional crystallization from n-heptane produced 0.116 gram of (#rudebenzopyrene, having substantially the same ultraviolet absorption as that of a n authentic sample. Further recrystallizations from n-heptane and from eth? I alcohol produced 12 mg. of crystals, melting point 175-176.5' C. (corrected). The melting point of a mixture with synthetic benzo-
