Coulometric Determination of Uranium

Of the several redox methods, one of the more useful is the oxidation of uranium (IV) to uranium. (VI) by ferric ion. This method, described by Shiple...
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Coulometric Determination of Uranium W. N. CARSON, JR. General Electric Co., Richland, Wash.

T

EQUIPlllENT

HE determination of uranium by titration is usually acconiplished by oxidation or reduction to the V I or IV

valence state of the element. Of the several redox methods, one of the more useful is the oxidation of uranium (IV) to uranium (VI) by ferric ion. This method, described by Shipley (6), consists of pretreating the sample to remove or nullify interferences, reducing the uranium to a mixture of the uranium (111) and uranium (IV) valence states by means of a Jones reductor. and titrating with standard ferric sulfate solution back to uranium (VI) a t an elevated temperature. 9 study was made to adapt this method to a coulometric micromethod. Many modifications were found necessary, but the basic titration scheme of preliminary reduction of the uranium followed by oxidation with ferric ion proved feasible and was retained. The method proved readily adaptable for use on an automatic titrator ( 1 ) .

The construction details of the titration cell used in manual and automatic titrations are givrn in Figure 1. The indicator system is that of Reilley, Cooke, and Furman electrodes are two platinum wires dipping in the safnple. These are polarized by a constant current of 1.1 to 1.3 microamperes, as explained h y Reilley et al. The polarizing current source is composed of several 45-volt B batteries (Burpes; Z30XX) connected in parallel mith 40-megohm resistance in series. The gene1ator system is a two-compartment cell composed of a platinum anode immersed in the sample solution contained in one compartment, and a platinum cathode immersed in 1 to 1 sulfuric acid in the other compartment: the two compartments are connected by means of a silica gel salt bridge. The salt bridge is prepared by filling a tube to the required length with sodium silicate solution (40' BB.). The bottom of the tube is then im-

(4). The

/PORT

f

LEAD

COLUMN

!1

FOR EMPTYING ( I N D I C A T O ELECTRODE K R T

I - 3/14"MA

TOP VIEW

CAW. L

TO POLARIZING CIRCUIT ANOE

Yl

lNDlCATOR ELECTRODES PORCELAIN SOCKET

TO PYROMETER

Figure 1. Titration Cell Assembly 466

467

V O L U M E 25, NO. 3, M A R C H 1 9 5 3

This investigation was made to evaluate the use of coulometric techniques in the determination of uranium on a micro scale and to evaluate the use of the automatic coulometric titrator for microtitrations. The results show that samples of 0.01- to i-nig. uranium content can be automatically titrated. With care, 2 micrograms of uranium can be titrated manuall>-. The method depends upon the m e of a lead reductor to reduce the uranium to the I\' valence state and electrolytically generated bromine in the presence of iron to oxidize the uranium to the \'I valence state. The precision and accuracy of the method range from * 0.37' for 7-mg. to = 6 7 ~for 0.03-nig. samples. The method successfully uses the automatic titrator on a micro scale and extends the use of coulonietric techniques to a new field.

mersed in 1 t o 1 sulfuric acid for a few ninutes. and 1 to 1 sulfuric acid is placed on top of the silicate solution to serve as coagulant and catholyte. The acid causes a thin film of gel t o form immediately, which seals in the silicate solution. The bridge is aged for 15 minutes. Diffusion of acid from the catholyte causes all of the silicate to gel in a fern- days to a compact, Ion- resistance plug. If made properly, the bridge permits only a very small flow of catholyt,eand remains operative under the heat and acid conditions of the tit,ration with a life of several months under continuous use. The catholyte must be replenished from time to time with 1 t.o 1 sulfuric acid. The cell is heated by a small heating element (Hotspotter heating element, Fisher Scientific Co., Catalog KO.11-502-15), controlled by a Simplytrol pyrometer (Catalog So. 4533, Assenibly Products Co., Chagrin Falls, Ohio), which uses an ironconstantan thermocouple for temperature sensing. A 100-ohm 100-Tvatt variable resist,ance is placed in series with the heater circuit t o permit slower heating and extend the heater life. The temperature of titration is 95" f 1" C. and is critical. Above this t,eniperature the solution boils and ruins the determination. Below 92' the t,itration react,ions are too slow for automatic titr;rtor use, and below 90" are too slow for manual use. The condenser of the cell ret,urns water evaporated by the svieep gas from the sample. If it is not used, the evaporative losses are very large. and the sample cannot be titrated for more than a fex minutes. Rapid stirring of the sample is necessary, but the speed is not critical. The gas blnnket used to prevent air oxidation is carbon dioxide, h u t nitrogm or other inert, gas could be used. The carbon di-

I25 C C DROPP NG

FUNNEL

PARAFFIN OIL CHROMOUS SULFATE SOLUTION AMALGAMATED MOSSY ZINC

oxide is freed of oxygen by use of the gas scrubbing train shown in Figure 2. The gas is passed through water, chromous sulfate, and water again. The first water scrubber is used to saturate the gas t o lessen the evaporative loss in the chromous sulfate scrubbera, the second is to remove chromous sulfate spray. The first chromous sulfate scrubber is a modification of a gas scrubber described by Shaw (6). The action is as follom: The incoming gas passes through the jet and pulls or traps a small amount of solution into the center tube. This is forced up the top of the scrubber by the gas and splashes back on the glass beads in the top of the scrubber,

I

'

1

1

CELL

Figure 3.

Block Diagram of AIanual Titration Apparatus

The large surface exposed to the gas gives very efficient scrubbing. The liquid drains back to the reservoir a t the bottom of the scrubber, u here it passes through amalgamated zinc to reduce any oxidized chromium, The solution used is 15 grams per liter of chromic sulfate in 1 to 4 sulfuric acid and is replenished by additions from the reservoir. The second scrubber is used as an indicator of failure of operation of the first, as small amounts of oxygen cause a marked change from blue to green in the color of the solution. A flow of approximately 100 cc. per minute per titration cell is necessary. The lead column is filled with test lead granules. It should be rinsed thoroughly with 3 M hydrobromic acid to remove oxide and stored, when not in use, under iron rinse solution. The first samples passed through the column tend to give low results, unless rinsing with the iron rinse solution has been thorough. S o suction on the column is used. The manual titration apparatus is shown as a block diagram in Figure 3. The constant current source can be any suitable source; the one used has been described by Carson and KO ( 2 ) . The timer is a Standard Electric Time Co. Model SR.1 60 electric stop watch, used as described previously. The potentiometer i3 a Leeds and Sorthrup S o . 7655 portable potentiometer; any good student-type potentiometer can be used. The indicator system requires 3 high-impedance volt meter; a Beckman Model G or Model H-2 pH meter is satisfactory. T h e automatic titration equipment has been described ( 1 ) . TITRATION CHEMlSTRY

Figure 2.

Gas-Scrubbing Train

Cranium is reduced rapidly and quantitatively to the IF' state by a lead column in hydrochloric solution (3,8). Because large quaritities of chloride cause poor end points, hydrobromir acid is used in the present method. The lead column tends to retain small amount3 of uranium, which are significant when micro amounts are to be determined. Uranium i3 quantitatively removed by using a 0.02 ,II or greater concentration of iron in the ringe.

ANALYTICAL CHEMISTRY

468 Fortunately, iron is required for the titration media as a catalyst for the oxidation of uranium, so its use in the rinse solution does not introduce complications. The effect of iron on the indicator system is such that the higher the iron concentration, the poorer the end point. Hence, the iron concentration in the rinse should be kept at 0.02 di, the minimum concentration required for rinsing. The presence of sulfate, phosphate, and other ions that form fnsoluble lead salts is not harmful as long as the solution passed through the reductor is at least 3 M in hydrobromic acid, Hot acid causes gas formation Tvith no harm to the column, but the rate of flow through the colun~nis l o ~ e r e d . PROCEDURE

Reagents. Test lead, C.P. granules, screened to remove the fines that pass a 100-mesh sieve. Hydrobromic acid, concentrated c.P., 48%. Iron Bromide, 1 M. Dissolve 55 grams of electrolytic iron (Standard Sample Go., Ames, Iowa) in 500 ml. of concentrated hydrobromic acid and 30 ml. of bromine. Dilute to 1 liter after dissolution. Iron Rinse Solution, 0.02 M iron and 3 M hydrobromic acid. Add 336 ml. of concentrated hydrobromic acid to 20 ml. of 1 M iron bromide and dilute to 1 liter. Boil gently for 5 minutes just before use under a blanket of oxygen-free inert gas, if less than 0.5-mg. samples of uranium are used. Sample Size. The cell dimensions given in Figure 1 are suitable for samples containing 0.01 to 7 mg. of uranium, that can be reduced to approximately 2 ml. in the pretreatment steps. Other samples can be accommodated by changing the cell size to handle the required volume of rinse solution. As a guide, the automatic titration requires a uranium concentration in the final mg. per/ml. or greater; the o p titration medium of 5 X timum concentration is 0.1 mg. per ml. The manual titration is limited mainly by the blank: concentrations of 10-4 mg. per ml. have been titrated successfully. Pretreatment. The sample is prepared for reduction by removing interferences and adjusting to 3 M hydrobromic acid content. For samples containing nitrate, dichromate, permanganate, and other easily reduced substances, the sample is boiled with concentrated hydrobromic acid until the bromine formed is expelled. For samples containing reducing interferences, the solution is heated with hydrobromic acid and bromine. Most of the excess bromine should be removed by boiling for a few minutes prior to reduction. For other interferences which are not removed by these simple treatments, see below. Manual Titration. Pretreat the required sample to remove interferences. Outgas the titration cell thoroughly with a rapid stream of inert gas. Transfer the sample quantitatively to the top of the lead reductor, and pass it through the reductor a t a rate of approximately 5 ml. per minute. Rinse the sample vessel three times nith 2- to 3-ml. portions of iron rinse solution, and pass rinses through the reductor, allowing the reductor to drain to the top of the lead between rinses. Turn on the heater and stirrer after the first rinse has been added. Rinse the column once with a 3-ml. portion of iron rinse solution. The final volume in the titration cell should be 12 to 15 ml. The sample and rinse solution should be approximately 25” C. or warmer. Allow the sample to heat to 95” C. and start the electrolysis by clqsing the “titrate” switch on the constant current source. Continue the electrolysis until the voltage across the indicator electrodes is 200 mv. From this point, titrate in 5-second or 0.1-minute increments. The indicator system should be allowed to reach equilibrium between increments of electrolysis. The above procedure needs a slight modification with samples of low uranium content. With samples containing less than 0.5 mg. of uranium, the iron rinse solution must be boiled for a few minutes before use to expel oxygen. Automatic Titration Procedure. This procedure refers to the use of the automatic titrator (1). Set the controls of the titrator as follows: Constant c u r r e n t at factor current desired p H meter to 0 to 800 mv., and standardiza Trigger at 175 mv. Time of end-point d u r a t i o n a t 30 seconds Pyrometer c o n t a c t at 95’ C.

Prepare the sample and introduce it into the titration cell in the same manner as for the manual titration. Turn the “titrate” switch to “on.” The titrator will now heat the sample to the titration temperature, titrate it to the end point, and turn itself off. The titration is complete when the “timed out” pilot turns on. Remove the titrated sample and rinse the cell thoroughly. Record the time and turn the titrate switch to “reset.” The titrator is now ready for the next sample. Computation. The number of milliequivalents of uranium is computed by multiplying the current in milliamperes by the time in seconds and dividing by 96,500. This divided by 2 gives the millimoles of uranium titrated. For samples with less than 1 mg. of uranium, a blank correction is usually required. This is determined by manual titration a t low currents with the same amount of electrolyte used in the analysis and represents (unless impurities are present) the time necessary to generate sufficient ferric ion to give an end point. It is inversely proportional to the current and can be calculated for any current once it is known for a particular current. The use of factor currents is recommended for routine use to simplify the computation of results. Interferences. A number of interferences exist for both the reduction and the titration. These are discussed in detail by Sill and Peterson (8) and Shipley ( 6 ) . A limited study of the interferences with the method was made, and the results are summarized in Table I. The tests were made as follows: For each element a large excess (50 to 100 times the molar equivalent of uranium) was added as a solution of the compound listed in the table to 2-mg. samples of uranium. Concentrated hydrobromic acid was added, and the sample solutions were boiled down to expel any volatile reaction products. The solution was diluted with dilute hydrobromic acid, reduced in the usual manner, and titrated. For elements listed as “KOInterference,” 3 to 4 samples showed no change in titer from the standard sample. For elements marked “ S o Interference-Precipitates,” the element precipitates out in the lead column completely enough not to interfere with the titration. This eventually ruins the lead column for reduction. Small amounts (0.1 mg.) of these elements may be removed by internal electrolysis with a lead-platinum couple in the presence of 3 M hydrobromic acid, but larger amounts require other separations or frequent replace ment of the lead column.

Table I.

Resume of Interference Studies on Titration of Uranium

Element Aluminum Antimony Arsenic Bismuth Boron C a1ciu m Chlorine Chromium Cobalt Copper Fluorine Gold Iodine Magnesium Manganese Mercur Molyb&num h’ickel Nitrogen Palladium Phosphorus Platinum Silicon Sulfur Tin Titanium Tungsten Vanadium Zinc Zirconium Formate

Added AB AlCla NaiSbO4 Nat.4804 BiClr HBOa CaCln HCI Ka*Crr07 COCI, cuso4 NaF AuCIa KI RIgCI, KhInO4 HgClr KanMOO4 XiClr HNOi PdClz NaHnPOd H,PtCh h’a,SiOr NarSO4 SnC14 TiCla NaaWO4 NaVOa ZnCla ZrOCh HCOOH

Oxalate

h’atC104

Sulfide

Kats

Sulfite

XatSOa

NHa Sulfamate

NHiCl HSOiNHt

Remarks Does not interfere Nonstoichiometric interference Nonstoichiometric interference Does not interfere, precipitates Does not interfere, precipitates Does not interfere, precipitates Does not interfere,preci itates Nonstoichiometric inte2erence Does not interfere Does not interfere, precipitates Nonstoichiometric interference Does not interfere, precipitates Interferes Does not interfere Does not interfere Does not interfere Nonstoichiometric interference Does not interfere Does not interfere Does not interfere, precipitates Does not interfere u p to 30: 1 mole ratio P0,:U Nonstoichiometric interference Does not interfere Does not interfere Nonstoichiometric interference Nonstoichiometric interference Interferes Nonstoichiometric interference Does not interfere Do& not interfere Interferes. removed b v oxidation with bromina prior to reduction Interferes. removed by oxidation with bromine prior to reduction Interferes, removed by oxidation with bromine prior to reduction Interferes, removed by oxidation with bromine prior to reduction Does not interfere Does not interfere

V O L U M E 25, NO. 3, M A R C H 1 9 5 3

469 one unusual instance, the hydrobromic acid was found to contain 0.5 mg. per liter of uranium, n-hich is significant in view Current, Ma of the large amounts of the acid required 10.00 per determination. 10.00 7.00 Of the elements not tested, the alkalies, 6.00 alkaline earths, t h o r i u m , h a f n i u m , 2.00 1.00 scandium, tantalum indium, cadmium, 0.533 0.206 and rare earths (except possibly euro0.206 pium, ( 7 ) ] would not be expected to in0.200 terfere. Any element or compound which prccipitates upon the addition of acid likely to occlude uranium. Tellurium and some of the metals not tested may interfere.

Table 11. Automatic Titration Results on Uranium Standards Standard,

Sample Size,

G./L. 278.9 278.9 27.89 27.89 27.89 2.789 2.789 2.789 0.2789 0.2789

PI.

25 2o

loo 50

20 100 50 20

50

Found, Av., Alg. 6.98 6.99 5.58 5.60 2.79 2.80 1.40 1.41 0.558 0.555 0.279 0.278 0,137 0.140 0.0558 0.0536 0.0279 0.0290 0.0140 0.0157 Taken.

Precision,

76

hIg.

0.29 0.31 0.38 0.46 0.50 f 0.92 f 2.3 f 3.0 f 5.6 f14.8 i f f f f

Av. Titration Time, Min. 9.444 7.569 5.412 3.192 3.798 3.861 3.674 4.014 2.402 1.568

Table 111. Analyses of Standard Uranium Samples No. of

Detns.

6 6 6

Sample

Accepted Value Uranium, per cent 84.62 Gaol Uranium. mg. per gram 21.56 UOzClz(So1ution I) UO~Cb(Bo1ution 11) 2.538

Found

Precision

84.46

50.28

21.56 2.559

f0.18 10.026

Oxidizing agents, such as nitrates, are not fully reduced in the column but oxidize iron in the hot hydrobromic acid medium, causing low results. This type of interference is removed by boiling down the sample with concentrated hydrobromic acid prior to reduction Reducing agents, such as formate and oxalate, which react with bromine, interfere but can be removed by treating the sample with bromine and acid prior to reduction. The admission of air to the lead column can cause errors by the formation of peroxide. Cooke (3)reported no error ip caused by admission of air; Sill and Peterson ( 8 ) , however, found that large quantities of peroxide are formed under certain conditions. In their method, peroxide caused high results; in this method, peroxide will give low results, as i t readily oxidizes the ferrous iron present in hot titration media Use of hydrobromic acid and iron in the rinse apparently permits only traces of peroxide to form with the present procedure, but these traces are sufficiently large to require elimination of oxygen from the rinse solution when 500-microgram samples of uranium or less are titrated, although the error appears negligible for larger quantities of uranium. 'Good technique would require reasonable precautions against introducing air to the column during reduction, since gross errors will be obtained if air reaches the hot titration media, aside from the possible peroxide formation in the column. The elenientswhich are listed in Table I as giving a nonstoichicF metric interference give incomplete reduction or incomplete retention of the reduced material on the column. The amount of excess oxidant, required varied without relation to the amount of interference added. Tungsten interfered in this study by reduction to an as yet undetermined compound, which required an amount of excess oxidant that v a s proportional to the quantity of tungsten added. This relation held up to the solubility limit of KOa in the acid rinse used. No attempt beyond this preliminary test was made to eliminate or elucidate the interference. Iodine interfered by obscuring the end point for both the derivative polarographic and the potentiometric indicator systems. The cause of this is not known. An unsuccessful attempt t o remove iodine was made by treating with hydrobromic acid and bromine. In a number of cases, the interferences can be nullified without separation. Typical of these are the chromium and fluoride interferences. Chromium interference is due to the formation of chromous ion in the column and is removed by pretitration. Fluoride interference is due to complexing ferric ion and is removed by complexing with zirconium prior to reduction. As is often the case with microdeterminations, the reagents used in the titration frequently contain significant amounts of interfering impurities. The use of high purity or primary standard iron and acid rather than the commercially available salt to make the iron bromide is recommended for low level work. In

Blank, hlin. 0 0 0 0.020 0.050 0.100

0.188 0.500 0.500 0.500

would be platinum

RESULTS

Titration of Uranium Standard. A standard uranyl bromide solution was made by dissolving metallic uranium in hydrobromic acid and bromine, and was used to make up the standard solutions indicated in Table 11. Ten automatic determinations were made for each sample size. The precision values in the tables are for an individual determination and are based on the standard deviation. The deviations of the average found values from the standard value were tested by Student's t test on the 99% confidence level and are not significant. Table I11 gives the results of analyses on other standards. Precisions are calculated as above. The oxide sample was prepared from carefully purifieduranium nitrate hexahydrate and was analyzed by gravimetric and volumetric macromethods to obtain the result shown as the accepted value. Weighed portions were dissolved in hydrobromic acid and bromine, and titrated. The other samples were prepared from this oxide. In all cases, approximately 1 mg. of uranium was used for the titration sample. DISCUSSION

The derivative polarographic indicator system used behaveP as expected for the irreversible uranyl-uranous couple by developing a high potential a t the start of the titration, which decreases sharply at the end point. The stoichiometric end point in the author's apparatus occurs a t 175 mv. The end point shifts for different polarization currents and size of electrodes, but can be found definitely by titration in increments across the end point. The setting of the trigger cut-off potential at the end point is not highly critical, as a change of 10 to 20 mv. will not result in appreciable change in titration time. During the development work on the method, a potentiometric indicator system, using calomel and platinum electrodes, was used. This n-as satisfactory in so far as behavior of the system was concerned, but frequent failures of the calomel electrode were caused by the high temperature of operation. The endpoint potential in this system is 260 niv. This system would be usable if a reference electrode suitable for use in the hot acid solutions were developed. Both the potentiometric and derivative polarographic indicators show a marked anticipatory behavior. This is because the ferric iron generated does not react rapidly with the uranium, and a small excess concentration of ferric ion is present during the titration. This behavior is an asset in making the titration automatic, as it prevents overtitration. Ferric ion exists as a highly colored bromide complex in the titration media. The end point can be detected by observing the permanent appearance of this dark color, if no other colored ions are present, This color formation is not suitable for precise work, however, for the appearance of a darkly colored solution at a t the end of a titration is a sign of overtitration, if the original solution is light in color. ACKNOWLEDGMENT

The author wishes to acknowledge the work on this method performed by some of his coworkers. These were D. J. Watkins,

ANALYTICAL CHEMISTRY

470

who made the precision studies, many of the interference studies, and helped in the differential polarographic end-point studies; E. bl. Kinderman, who helped with the pretreatment and interference studies and the early automatic titrations; W. W. Mills, who furnished the standards for the investigation; and Roy KO and G. Rleyer, viho established the need for iron in the generation system. LITERATURE CITED

(1) Carson, TI-,

s., Jr., k A L . CHEM., 25, 226

(1953).

(2) Carson, W.S., Jr., and KO, R., Ibid., 23, 1019 (1951). (3) Cooke, W,D., Hazel, F., and hlcNabb, W. M., Ibid., 22, 654 (1950). (4)

Reilley, C. K.,Cooke, W. D., and Furman, N. H., Ibid., 23, 1223 (1931).

(5) Shsw, J. A,, ISD.ENG.CHEM., ANAL.ED.,6,479 (1934). (6) Shipley, E. D., Atomic Energy Commission, AECD-2804 (Nov. 2, 1949).

Sill, C. W., private communication. (8) Sill, C. W,, and Peterson, H. E., ANAL.CHEM., 24, 1173 (1952). (7)

RECEIVED for review June 21,

1952.

Accepted January 8. 1953.

Direct Semimicrodetermination of Oxygen in Organic Substances Improved Apparatus and Procedure f o r Use with Carbon Reduction-Titrimetric Carbon Dioxide Method ROBERT D. HINKEL AND RAPHAEL RAYIIOKD Synthetic Fuels Research Branch, Bureau of Mines, Bruceton, Pa.

A n improved apparatus and procedure have been developed to overcome interference due to hydrogen in the Unterzaucher method, to permit accurate analyses of substances of both low and high oxygen content, and to reduce the time required for analysis. Carbon monoxide formed in the pyrolysis is oxidized with iodine pentoxide, the resulting carbon dioxide is absorbed in a measured excess of 0.05 N alkali, and the excess is back-titrated with 0.025 N acid after precipitation of the carbonate formed with barium chloride. Numerous data on pure oxygenated compounds, hydrocarbons, and miscellaneous substances indicate that accurate results are obtained over the range of 0.05 to 50% oxygen, utilizing sample weights of 15 to 350 mg. and a total analysis time of 35 to 45 minutes. The apparatus required is simple and serviceable and includes a unique style of reaction tube which permits easy and accurate handling of volatile and nonvolatile samples. Blanks caused by variation in furnace construction are discussed, and experiences with different reaction tube materials are described.

S

TUDIES related to the research and development activities of the synthetic liquid fuels program have been concerned with the development of a direct method of oxygen determination for use in identifying synthesis products. Such a method is preferred to the customary by-difference analysis, because the results are not affected by a summation of the errors made in analyzing other constituents. These errors are particularly important when carbon, hydrogen, oxygen, nitrogen, sulfur, and/or halogen are present simultaneously and when the quantity of oxygen present is small. The direct method proposed by Schutze in 1939 (21)has aroused considerable interest among those concerned with this problem. This was evident, first, in the work of Zimmerman (24) and, later, in the well-known modification of the method described by Unterzaucher (%). Asubsequent review by Elving and Ligett (11) pointed out the advantages of the Schutze-Unterzaucher technique over other methods of analysis and stimulated further interest in its use.

Briefly, the method involves pyrolyzing the sample in a closed system and sweeping the products of pyrolysis through a carbonpacked reaction tube a t 1120’ C. with a stream of oxygen-free nitrogen. In passing through the reaction tube, the oxygencontaining pyrolytic fragments are converted quantitatively into carbon monoxide, which is determined subsequently by oxidation with iodine pentoxide and measurement of the by-product iodine evolved in the reaction. To increase the sensitivity of the method, the iodine is absorbed in alkali, oxidized to the iodate form, and, finally, reduced with potassium iodide and titrated with standard thiosulfate solution. Among the first to investigate the method in this country were Aluise and coworkers ( 8 ) and Dinerstein and Klipp (9), who reported on the use of American-made equipment and reagents for this purpose. Walton, McCulloch, and Smith ( I S ) expressed . dissatisfaction with the use of iodine pentoxide, because it yielded variable blanks, and devised a modification in which the carbon monoxide is determined colorimetrically, using a specially prepared, impregnated gel. Later, Maylott and Lewis (18) described a comparison of the ter Muelen and Unterzaucher methods wherein the latter was found t o be preferable, provided that a liquid nitrogen-cooled trap was used to eliminate interfering constituents from the gases entering the iodine pentoxide tube. Recent investigations indicate rather general dissatisfaction with the Tjnterzaucher procedure, particularly when applied to the ahalysis of materials of low oxygen content, because of the reducing action of pyrolysis hydrogen on iodine pentoxide and the resultant production of iodine, which causes high results. Moreover, this action is not quantitative and, apparently, is very erratic, depending on the sensitivity of the iodine pentoxide, thus making the application of a correction factor impossible. To circumvent this difficulty, various investigators have resorted to measurement of the carbon dioside produced by oxidation of the carbon monoxide. Thus, Holowchak and Kear (14) have adopted a manometric method, while Dundy and Stehr (10) have employed a gravimetric method similar to that initially used by Schutze (91). Other investigators have also used the gravimetric method, though not specifically for avoiding hydrogen interference (6,1b, 17). An entirely different line of approach has been followed by Campanile and coworkers (6),who retained Unterzaucher’s iodometric determination, but eliminated hydrogen interference by allowing the bulk of the gas to diffuse from the reaction tube gases through a palladium thimble similar to that described by Harris, Smith, and Mitchell (IS).