Galvanic Cell for Determining Oxygen in Gases Containing Carbon Dioxide Karl Koyama,' Hanford Laboratories Operation, General Electric Co.,Richland, Wash.
o
FORESTALL
the
oxidation
of
Tgraphite in nuclear reactors, the oxygen concentration in the reactor gas must be kept to a minimum. A continuous analyzer that provides immediate knowledge of the oxygen content is tlcsirable for this purpose. The galvanic cell described by Hersch (4) and Bomyer and H u t t ( 2 ) offers advantages from the standpoint of cost and simplicity. However. the potassium hydroxide electrolyte is unsuitable for reactor gas containing carbon dioxide because of the formation of potassium carbonate. The substitution of saturated potassium bicarbonate solution as the electrolyte provctl satisfactory for recording low concentrations of oxygen in carbon dioxide, carbon monoxide, nitrogen, hydrogen, lielium, and mixtures of these. The theory and operation of the galvanic cell liave been summarized b y Hersch (4). GALVANIC CELL OXYGEN ANALYZER
The oxygen cell (Figure 1) is similar to Bomyer and Hutt's design, except that the oxygen generator is separated from the cell to obtain better mixing of the generated oxygen with the gas stream before arriving at the cell. The cell is 6 inches in over-all length with a cathode compartment 15 inches in outside diameter equipped n i t h a stopper Added to this are a n anode compartment, gas inlet and outlet tubes, and a sealed platinum nire in contact with the silver cathode. The cathode is made by bending a 7,'s X 41/2 inch sheet of 20-mil silver in a n inverted U-shaped trough and cutting a notch in one end for the platinum nire. The silver is nashed to remove grease and sulfide5 and finally rinsed in ammonia, The anode, about 5 X 30 X 2 mm. thick, consists of porous cadmium metal electroplated onto a nickel screen from cadmium hydroxide in a dilute solution of potassium hydroxide. The spongy metal is gently pressed into shape and wrapped with the platinum wire of the electrode holder as shown in Figure 1. The electrolyte of saturated potassium bicarbonate solution is added to the cell to cover all the anode and about half the cathode. The various components of the oyygen analyzer in contact with the gas stream are: flowmeter, humidifier containing saturated potassium bicarbonate solution, calibration unit consisting of a chromous sulfate scrub solution and a n electrolytic oxygen generator, and the galvanic oxygen cell. Gas flow is Present address, General Atomic Division, General Dynamice Corp., San Diego 12, Calif.
normally through the flowmeter, humidifier, and then directly through the oxygen cell. During calibration, however, a three-way solenoid valve diverts the gas stream through the solution of chromous sulfate to remove the oxygen. The gas then passes through the oxygen generator, where known quantities of electrolytically generated oxygen and hydrogen are added before the gas stream enters the galvanic cell. The generator consists of two platinized-platinum electrodes in a n electrolyte of dilute sulfuric acid. Current to the oxygen generator is obtained from Carson's constant current supply ( 3 ) . The chromous sulfate solution is prepared by placing 0.2M chromic sulfate in 131 sulfuric acid over a layer of amalgamated zinc and allowing i t to reduce t o the chromous state. A thin layer of mineral oil on top of the solution protects it against air oxidation. A 100-ohm load resistance is connected t o the cell, and the potential
R.TLET
Figure 1. 150
r
/
Galvanic oxygen cell
l
l
I
I
I
l
l
l
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I
drop across this resistance, obtained with a recording potentiometer, is a measure of the current. -1 voltage from a zero control circuit similar to the one used by Baker et al. ( I ) suppresses the small blank current of the cell. The analyzer is placed in service by passing oxygen-free gas through the cell. The desired response is usually obtained in less than 1 hour, but this can be reduced by short-circuiting the cell during flushing. This treatment consumes the e w e s oxygen present in the vicinity of the cathode, a t the same time establishing equilibrium I\ ith respect t o the anode. RESULTS
The typical calibration curve of Figure 2 was obtained hy sweeping known amounts of gcwrated oxygen through the cell with a const'ant flow of carbon dioxide. By changing the gas stream to carbon monoxide and to a gas mixture, other points which coincided with the original curve were obtained. The gas mixture was mostly carbon dioxide, n i t h the remainder nitrogen, carbon monoxide, and helium. Though not given. a test showed the calihration of t'he cell to 1~ equally good for nit'rogen, hydrogen, or helium gas. Each point on the ciirvc was obtained ox-er a p e r i d which varied from 30 niinut'rs to 4 hours. 1 long-term precision to better than - 2 7 , n-as obtained from week-end operation on tank carbon dioxide. Tlic analyzer respondcd immediately to changes in oxygen conccntrat'ion. Equilibrium was reached in less than a minute, except lylien attaining the zero rratliiig after operating a t high oxygen levels; this required from 5 to 10 minutes. Sensitivity of the cell n-as ncarly constant for flow rat'es between 100 aiid 300 cc. per minute. From this) a flow rate of 150 cc. per niinutt. m ?chosen for the gas stream. DISCUSSION
0
002
004 006 008 010 OXYGEN CONCENTRATION, PERCENT
Figure 2. Calibration curve for oxygen in various gases
The current output of the c ~ l lwas sensitive t o sudden tenipwature changes. Placing the cell in a. small cabinet protected it against air movements aiid rapid temperature fluctuations. For long cell life, control of the humidity of the gas stream t o the analyzer was required to prevent either evaporation or dilution of the electrolyte. Hubbling the gas through a solution of potassium bicarbonate was adequate, VOL. 32, NO. 8, JULY 1960
1053
Calibration of the analyzer with electrolytically generated oxygen was convenient, but the output current from the oxygen cell occasionally fluctuated, because of irregular release of oxygen bubbles of nonuniform size from the surface of the platinum electrode. -4 steadier cell current was obtained after the two platinized electrodes of the oxygen generator had been placed close together, to allow the evolved hydrogen to furnish some stirring action
of the electrolyte to aid in the release of oxygen bubbles. The galvanic cell can be designed for use over a wide range of oxygen concentrations, It requires a minimum of electronic equipment and where a record is unnecessary, an ammeter in the circuit is sufficient to indicate the oxygen concentration. LITERATURE CITED
(1) Baker,
W. J., Combs, J. F., Zinn,
T. L., \Totring, A. LT., ITa11, R. F., Ind. Eng. Chem. 51, i 2 i (1959). (2) Bomyer, P., Hutt, E. C., Cnited Kingdom Atomic !yeapons Research Establishment, Rept. AWRE-0-14/54 (February 1954, unclassified). ( 3 )1565 (19501. "* lj.2 J r . ~-''AL. 22, (4) Hers&, p., ~~~~t~~~~~~ practzce11, 81i, 937 (1957).
WORK performed under Contract S o . AT(45-1)-1350 betn-een the U. S. Atomic Energy Commission and General Electric Co.
Rapid Method for Moisture and Fat Determination in Biological Muterials Hugo E. Wistreich, John E. Thompson, and Endel Karmas, Reliable Packing Co., Research Laboratory, Chicago OISTCRE
and fat are routinely de-
in many laboratories, b y n/r teImined . ' '
the use of an air oven, vacuum oven, azeotropic distillation, and Soxhlet extraction. This paper describes a method for the simultaneous extraction of moisture and fat from biological materials. APPARATUS
The apparatus is composed of a reflux condenser, a Bidwell-Sterling-type receiver arm, and a specially designed flask (patent applied for). The flask (Figure 1) has a volume of 780 nil. and is divided into tn-o compartments b>- a curved separation. Compartment A is directly under the mouth of the flask and is approximately one fourth or less the size of compartment B. The separation between the compartments is water-tight, so that the solvent can flow from one compartment into the other only by overflowing the separation. A new design, where the small compartment is located in the center of the flask, is now being tested. PROCEDURE
Three hundred milliliters of toluene, tetrachloroethylene, or any other azeotrope-forming fat solvent, immiscible with water, are added t o the flask. The solvent level in compartment B must be a t least 2 em. below the level in -4. Approximately 10 grams of the material to be analyzed are placed in a tared 9-cm. Soxhlet extraction thimble. The thimble is plugged with a tared glass no01 plug. The thimble is lowered into compartment A of the flask and the apparatus assembled. The flask is heated (hot plate or heating mantel) and distillaTable 1.
Product Bacon Pork skin Pork muscle Ground meat Sausage 1054
e
1 $ 29/26
p rm
Figure 1. Special compartments
flask,
showing
tion is continued for 2 hours. The 901vent and water azeotrope separates ir. the condenser. n'ater collects in the receiver, while the solvent returns to the flask. All the returning solvent falls into compartment A and then overflons into compartment B. As the solvent evaporates a t a n equal rate from both Compartments, 5 times as much solvent returns to compartment A as evaporates from it. This flow of solvent carries the fat extracted from the sample into compartment B. The fat eatraction i j very rapid because the solvent is boiling. After distilling for 2 hours (or when 110 more water is visible in the condensate .
Comparison of Results Obtained in Meat Product Analysis
(Values listed are individual) Moisture, % Fat, Kew method Air oven Kew method 22.8 66.9 22.4 66.6 22.3 23.4 60.0 28.0 28.5 10.8 52.0 56.0 5.9 73.2 73.5 31.6 52.5 53.5 27.5 51.2 51.0
ANALYTICAL CHEMISTRY
9% Soxhlet 66.4 65.9 60.1 9.0 6.1
31.9 27.2
9, 111
heating is discontinued and the apparatus is let cool. The apparatus is disconnected, and the thimble is removed. allowed to drain, and placed for 1 hour in a n air oven to evaporate residual solvent. The water receiver is set aside. At the end of 1hour the volume of water in the receiver is read. According t o Schley [IND. ENG.CHEX, ANAL. ED. 16, 720 (1944)] the water droplets adhering to the walls of the receiver may be set free by rinsing with a surface active agent containing some solvent. Tv-een 20 and sodium lauryl sulfate were recommended. The dried thimble is weighed and weight of the residue is determined. F a t may be determined by evaporation of the solvent, after transferring quantitatively into a dry evaporator flask, or as it is more frequently done, by difference. Moisture reading (ml.) X 100 - yGmoisture sample weight (grams) Residue weight (grams) X 100 = '%residue sample weight (grams) 100 - ( % moisture %residue) = %fat
+
RESULTS AND DISCUSSION
Xunierous meat products were analyzed and the results compared to those obtained from air-oven moisture determinations a t 110" C., folloned b y a Soxhlet fat extraction (Table I). The moisture percentages determined by the neIv method are slightly higher than the air-oven determinations. This is probably because there is no case-hardening n-hen the new method is used. The fat determination values are comparable to those obtained by Soxhlet extraction. The data in Table I were selected at random from a total of 60 comparative determinations. The results nere obtained in 3 hours as compared to 24 hours needed in the other methods. ACKNOWLEDGMENT
The authors express their appreciation to the Delmar Scientific Laboratories, Chicago, Ill., for help in building the special flask.
