1446
ANALYTICAL CHEMISTRY
determine phosphorus in alloys containing up to 28% chromium. The purpoae of the n ork reported here was to develop a method which shows no interference from alloying elements in concentrations which would be expected to occur in commercial titanium alloys. S o interference was observed with titanium samples containing 15% chromium, 10% molybdenum, 5% aluminum, 2.570 vanadium, 1% tungsten, 20% iron, 3% silica, and 3% nickel (Table 11). Synthetic alloy samples xvere prepared by adding the various elements to the titanium in the form of their soluble salts. In order to obtain a suitable working range, a curve was constructed at 650 mp. The molybdenum blue compound was found to follow Beer’s law a t this wave length for the concentrations of phosphorus plotted on the cali1,ration curve (Figure 1 ) . Good color stability has heen observed up to 1 hour, after which the blue color begins to deepen owing to reduction pf excess molybdenum. The concentrations of reagents specified in the procedure may be varied somewhat. The limits established by Hill ( 3 ) were found to be applicable to titanium. Variations up to 10% in acid concentration, 2.5% in fluoride concentration, and MYo in sodium nitrite concentration may be tolerated, while wider variations are permitted in ammonium molybdate and potassium permanganate Concentrations.
The sample may be diluted with either sulfuric acid or nitric acid. The molybdenum blue color appeared to be somem-hat more stable in sulfuric acid, although excellent results can be obtained with either acid. LITERATURE CITED (1) Epperson, A. IT.,J . Am. Chem. SOC., 50,332 (1928).
(2) Hague, J. L., a n d Bright, H . A , , J . Research .Vatl. Bur. Standurds, 26, 405-13 (1941). (3) Hill, E. T.,- 4 9 . 4 ~ . CHEM., 23, 1496-7 (1951). (4) Katz, H . L., a n d Proctor, K. L., I h i d . , 19, 612-14 (1947). (5) Imidell, G. E. F., and Hoffman, J. I., I n d . Etu. Chena., 15, 44 (1923). (1;) Lundell, G. E. F., a n d Hoffman, J. I., J . Research .VatZ. B w . Stnndards, 19,59-64 (1937). ( 7 ) Snell, F. D., a n d Snell, C. T., “Colorimetric Methods of dnalysis,” Vol. 1, New York, D. Van Nostrand Co., 1936. (8) Woods. J. T., and Mellon, hf. G., IND.ENG.CHEM.,. ~ N A L . ED., 13,760 (1941). (9) Toe, J. H., “Photometric Chemical Analysis,” Vol. 1, New York, J. Wiley & Son-8, 1928. RECEIVEDfor review September 20, 1952. Accepted December 22, 1982. Presented before the Pittsburgh Conference on Analytical Chemistry and Applied Spect,roscopy, Pittsburgh. Pa., March 2, 1953.
(Methods j o r Analysis of Titanium Alloys)
Colorimetric Determination of Small Amounts of Boron in Titanium Alloys M.4URICE CODELL AND GEORGE YORWITZ Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, Pa.
S
MALL amounts of boron have a marked effect on the mechani-
cal strength and grain structure of titanium alloys; consequently, the determination of small amounts of boron in titanium alloys is a matter of considerable importance. Hitherto, no methods specifically developed for the determination of boron in titanium alloys have been published. In an attempt to apply standard procedures to the determination of boron in titanium alloys, various methods were considered. The volumetric methods were found to be rather untrustworthy for the small amounts of boron (O.OOO0 to 0.10%) which would ordinarily be encountered in titanium alloys. Among the volumetric methods that were considered were the titration with alkali in the presence of mannitol after a prior separation of the boron as methyl borate (8, 13), the titration with alkali in the presence of mannitol after a separation of the interfering elements with calcium carbonate (10, 18), and the potentiometric titration after prior distillation of the boron as methyl borate (20). The gravimetric method, whereby the boron is weighed as calcium borate (9),was also found to be inaccurate for small amounts of boron. Colorimetry seemed to offer the best chance for a successful attack of the problem; therefore, the various colorimetric reagenta available for boron were considered. Quinalizarin (2,19) and carmine ( 7 )seemed undesirable because of the intense color of the reagent itself. Curcumin (1, 11) and turmeric (6, 21) leave something to be desired because of their limited range. Procedures using turmeric paper (14, 16) are relatively inaccurate. The colorimetric reagent deemed most suitable waB 1,l-dianthramide. The use of this reagent for boron was first proposed by Ellis, Zook, and Baudisch (4). It was subsequently applied to the determination of boron in aluminum alloys by Brewster (3). Neither the method of Ellis, Zook, and Baudisch nor the method of Brewster is directly applicable to the determination of boron in titanium alloys because of the in-
terference from titanium and other metals that might be found in titanium alloys. Ellis, Zook, and Baudisch applied their procedure to the determination of boron in plants, a type of material that contains relatively little inorganic salts. Brewster in his method for aluminum alloys used a very small sample. This eliminates the interference due to aluminum but makes the method inapplicable to the accurate determination of small amounts of boron (less than 0.01%). In view of the marked interference of titanium with the colorimetric determination of boron by dianthramide, a prior separation of the boron was clearly necessary. Various separations were considered. Separations involving the use of sodium hydroxide, ammonium hydroxide, or calcium carbonate were rejected because of occlusion of boron by the precipitate (12). Also, filtration of gravimetric precipitates can definitely lead to high results due to boron picked up from funnels, filtering devices, or filter paper. Winsor (21) has shown, in this regard, that very significant amounts of boron can be picked up from filter paper. The use of a mercury cathode for separating boron from other elements (16, 17) was not applicable, because titanium does not deposit into the mercury cathode. In view of the unsatisfactory nature of the above separations for boron, the only feasible solution of the problem was to use the methyl borate distillation method. However, before the dianthramide colorimetric procedure could be applied to the methyl borate distillate, many problems had to be overcome. First of all, the manner of distillation of the methyl borate had to be worked out. I t n a s decided to use a sulfuric acid solution of the sample in the distillation, The use of hydrochloric acid was considered unsatisfactory because the hydrochloric acid would distill over with the alcohol and cause difficulty by making the distillate very acid. On neutralizing the excess acid prior to driving off the alcohol a considerable quantity of salts would be formed. These salts
V O L U M E 2 5 , N O . 10, O C T O B E R 1 9 5 3 would inhibit the formation of the dianthramide-boron color. Experience has shown that as little as 0.2 gram of any salt could inhibit the development of the color. The use of nitric acid for dissolving the samples was undesirable because this acid would distill over and prevent the color development. The use of hydrofluoric acid to dissolve the samples was undesirable because boron would be volatilized as boron trifluoride. Also, hydrofluoric acid would attack the glassware, and distill over and prevent the color development. Phosphoric acid could not be used to dissolve the titanium, because of the precipitation of insoluble titanium phosphate which could occlude some boron. After experimenting with various types of apparatus for the distillation of the boron, an apparatus w a finally ~ chosen-a 300ml., boron-free distillation flask, a borosilicate glass column, a borosilicate glass rondenser, and a borosilicate glass adapter. The distillate was collected in a 250-1111., boron-free beaker which was cooled in ice. Alkali was not added to the receiving beaker during the distillation since high results for boron would be obtained if the alkaline solution came in contact with the adapter. It waa considered inadvisable to have the adapter dipping below the surface of the solution in the receiving beaker because of the danger of the solution sucking back. The manner of heating the distillation flask offered a problem. At first the flask was heated by a Bunsen burner, but this led to precipitation of a solid mass of titanium salts from which the boron could not be completely recovered. The best method of driving over the distillate was to heat a beaker of tricresyl phosphate in which the distillation flask was immersed. The precipitation of titanium salts wa8 never encountered when this method of heating the distillation flask was used. Three distillations were necessary for the complete recovery of the boron. In the first distillation most of the water is driven off; in the second distillation, a little water ~villremain behind, possibly as part of an azeotropic mixture of m ater and alcohol; during the third distillation, practically no mater is present. The use of dehydrating agents to reduce the number of distillations was considered but was rejected. There is danger of distillation of a component of the dehydrating agent-the chloride from calcium chloride, for instance-and there is a definite likelihood of obtaining a high blank from the relatively large amount of dehydrating agent that would be required. There is some doubt about the effectiveness of many dehydrating agents during the distillation, since many dehydratr ing agents would lose their water of hydration even a t the relatively low temperature used for the distillation of methanol. The time needed for three distillations is short, as alcohol or mixtures of mater and alcohol distill rapidly. I t is necessary to add an oxidizing agent in order to ensure the oxidation of borides-titanium boride and hydrogen boride, for instance-to boric acid. Various oxidizing agents were considered, but hydrogen peroxide (13, 1 7 ) was deemed most suitable. I t was thought a t first that it n ould be a simple matter to destroy the excess peroxide by boiling and then proceed with the distillation. This did not prove to be the case. The hydrogen peroxide-titanium complex was so stable that the solution remained jellov even after boiling for 3 hours. After boiling for 2 or 3 hours longer, the solution became colorless. Some undestroyed peroxide, however, remained in the solution. This was evidenced by the fact that on distilling the solution, no blue color could be developed with dianthramide. Apparently the color is destroyed by the slightest amount of any oxidizing agent. Various methods nere tried to destroy the peroxide. The addition of metallic zinc to generate nascent hydrogen waa found effective for the purpose. In fact, the zinc reduced the titanium all the way to the titanous state. However, the use of zinc led to unforeseen complicatione. It was discovered that titanium in the titanous state partially distilled over with the alcohol and prevented the color development. The titanium was positively identified in the distillates by making an ammoniacal separation, filtering off the precipitate, and igniting it in a platinum crucible.
1447 The ignited precipitate was then dissolved in a mixture of hydrofluoric and sulfuric acids, and the solution was evaporated to fumes of sulfuric acid. The solution was diluted with water, and hydrogen peroxide a a s added. A bright yellow color was obtained, indicating the presence of titanium. The best way found for destroying the hydrogen p e r o d e without reducing the titanium to the titanous state was to add a solution of a n iron salt and boil. The iron salt acts as a catalyst for the decomposition of hydrogen peroxide (6). Ferrous sulfate was chosen as the most suitable iron salt because it is readily obtained in a pure state. About 0.3 gram of the ferrous sulfate was found to be the optimum amount.
B
Figure 1. Apparatus for Distillation of Boron
After the method for the distillation had been developed, a b tention was turned to the means for the actual application of the boron colorimetric method to the distillate. First, the alcohol had to be removed and the solution evaporated to dryness, since the dianthramide color must be developed in a concentrated eulfuric acid solution. The distillate was quite acid, even though very little sulfuric acid distilled over. The acidity may be due t o some organic acid (possibly formic acid), which is formed during the distillation. In neutralizing this excess acid prior to the evaporation, sodium hydroxide or calcium hydroxide was first used. The excess sodium or calcium salts formed during the neutralization retarded the development of the boron-dianthramide color. The problem was solved by adding one pellet of sodium hydroxide and 10 ml. of ammonium hydroxide. The excess ammonium hydroxide was driven off on heating, while the pellet of sodium hydroxide furnished sufficient fixed alkali to ensure that the solution would remain definitely alkaline. Experiments were carried out to find the best method for evaporating the solution to dryness, For good color development two evaporations with intervening washing down of the sides of the beaker were necessary. The double evaporation was necessary to drive off some volatile material which could prevent color development. The material could be the alcohol itself or some decomposition product of the alcohol. Even after two evaporations, there is always some organic substance in the precipitate. This was shown by the charring obtained on igniting the precipitate. The method used for developing the boron color on the precipitate n-as essentially that of Ellis, Zook, and Baudisch ( 4 ) . APPARATUS AND REAGENTS
Apparatus. The distillation apparatus shown in Figure 1 was used. All connections were of ground glass. The 300-ml. Erlenmeyer flask and the 250-ml. beaker used to receive the distillate were made of boron-free glass (obtained from Corning Glass Works, Corning, N. Y . ) . The rest of the apparatus (10inch column, condenser, and adapter) was made of borosilicate glass. The 250-ml. beaker was cooled in ice contained in a borosilicate glass beaker. The distillation flask dipped into an 800ml. beaker containing tricresyl phosphate which was heated by a Bunsen burner.
ANALYTICAL CHEMISTRY
1448
A Coleman Model 14 Universal spectrophotometer with 13 X 13 X 105 mm. matched square cuvettes and filter PC-4 was used. Reagents. Stock Solution of 1,l-Dianthramide. Make a solution of 400 mg. of 1,l-dianthramide per 100 ml. of concentrated sulfuric acid. This solution isstable for several months if stored in a refrigerator. The dianthramide used was obtained from Dajac Laboratories, Leominster, Mass. Working Solution of 1,l-Dianthramide. Dilute the stock solution of the reagent 1 to 20 with concentrated sulfuric acid. Make fresh each day. Standard Boric Acid Solution 1 (1 ml. = 0.002 mg. of boron). Dissolve 0.572 gram of C.P. boric acid in water and dilute to 500 ml. in a volumetric flask. Pipet 10 ml. of this solution into a 1liter volumetric flask and dilute to 1 liter. Standard Boric Acid Solution 2 (1 ml. = 0.01 mg. of boron). Dissolve 0.0572 gram of C.P. boric acid in water and dilute to 1 liter in a volumetric flask. Ferrous Sulfate Solution. Dissolve 7.5 grams of C.P. ferrous sulfate heptahydrate in a mixture of 100 ml. of water and 15 ml. of concentrated sulfuric acid. Dilute to 250 ml. with water. Absolute Methanol, Reagent Grade. No special treatment of the alcohol prior to its use is necessary. Concentrated Sulfuric Acid, Reagent Grade, 95 to 96%, HzSO,, specific gravity, 1.84. Ammonium Hydroxide (28%), Reagent Grade. Do not use reagent that has been stored in borosilicate glassware. Sodium Hydroxide Pellets, C.P. Keep tightly stoppered. Hydrogen Peroxide, 300/,, c.P. Tricresyl Phosphate, Technical.
Table I.
Results for Borori
Boron, yo Added Found (av.) 0 00040 0 00037 0'0020 0.0018 0.0040 0.0039 0 0060 0.0057 0,0080 0.0081 0,050 0.051 0.100 0.109
Standard Deviation. ?$ 0.00015 0,00041 0'00022 0 00098 0.00070 0,0018
N o . of
Detns. 6 6 5 5 6
5 6
0.0069
Table 11. Results for Boron in the Presence of Possible Interfering Elements Amount Added, Element Calcium Magnesium Manganese Vanadium Chromium Zinc Molybdenum Iron Silicon Copper Aluminum Phosphorus Nitrogen Carbon
%
Added As
20 20 20 20 20 20 20 20 20 20 20 20 20 0.5
ingcarbon
Boron, % Added Found 0.0040 0.0038 0.0040 0.0041 0.0040 0,0037 0.0060 0.0056 0.0060 0.0058 0.0060 0,0062 0,0060 0,0060 0.0020 0.0023 0.0020 0.0021 0.0020 0,0016 0.0040 0,0038 0,0040 0.0043 0,0040 0,0039 0,0040
0.0041
PROCEDURE
For samples containing 0,0000 to 0.008% boron proceed as follows. Transfer 0.5 gram of the sample to a 300-ml. boronfree Erlenmeyer flask with a ground-glass neck. Add 30 ml. of water and 5 ml. of concentrated sulfuric acid, and connect to a reflux condenser. Heat over an asbestos gauze with a Bunsen burner until the sample has dissolved. Wash down the inside of the reflux condenser with water, allow to cool somewhat, and disconnect the flask. Add 2 ml. of 30% hydrogen peroxide and 5 ml. of ferrous sulfate solution. Do not add the hydrogen peroxi'de or the ferrous sulfate solution through the condenser. Wash down the sides of the flask with water to ensure that no hydrogen peroxide remains there. Connect the flask again to the reflux condenser. Boil until the yellow titanium peroxide complex has disappeared and the titanium has begun to hydrolyze; then boil for an additional 10 minutes. Do not take as an indication of the destruction of the hydrogen peroxide the disappearance of the yellow color. Wash down the inside of the reflux condenser, allow to cool somewhat, and disconnect the flask.
hydroxide to the distillate and evaporate to dryness on an electric hot plate a t moderate heat over an asbestos gauze. Do not bake the precipitate. Allow to cool and wash down the sides of the beaker with water. Evaporate to dryness. Allow to cool for a few minutes. Add 2 ml. of concentrated sulfuric acid and swirl to dissolve the salts. Add 5 ml. of dianthramide solution and heat in an oven a t 90" C. for 3 hours. Add about 10 ml. of concentrated sulfuric acid and transfer to a 25-ml. volumetric flask that has been rinsed with concentrated sulfuric acid. Dilute to the mark with concentrated sulfuric acid and shake. Compare colorimetrically in a spectrophotometer a t 620 mp with a reagent blank that has been carried through all steps of the determination. Set the reagent blank a t 100% transmittance. Convert the reading to milligrams of boron by consulting a curve in which the logarithm of the transmittance is plotted against milligrams of boron. To draw up this curve add varying aliquots of standard boric acid solution 1to 0.5-gram portions of pure sponge titanium metal and carry the samples through all steps of the procedure. The reference curve obtained by the authors is shown in Figure 2. For samples containing 0.008 to 0.10% boron proceed as above, but after collecting the distillate, transfer it to a 500-ml. volumetric flask. Dilute to the mark with water. Pipet out an aliquot of solution containing about 0.01 to 0.03 mg. of boron into a 250-mI., boron-free beaker. .4dd one pellet of sodium hydroxide and 5 ml. of ammonium hydroxide and proceed. RESULTS AND DISCUSSION
li TRANSMITTWOE
Figure 2.
Calibration Curve for Boron
Add 20 ml. of absolute methanol to the solution and connect to the distillation apparatus. Collect the distillate in a 250ml., boron-free beaker surrounded by ice. Place the Erlenmeyer flask in an 800-ml. borosilicate glass beaker containing about 400 ml. of tricresyl phosphate and heat a t the strong heat of the Bunsen burner. Continue heating until no more drops of solution come over, but do not heat to strong fumes of sulfuric acid. The solution in the flask xi11 turn a clear yellow as the hydrolyzed titanium dissolves. Lower the beaker of tricresyl phosphate and allow the solution in the Erlenmeyer flask to cool t o room temperature. Add 50 ml. of absolute methanol through the top of the column and distill again. Allow to cool to room temperature. Make another 50 ml. addition of alcohol and distill once more. Add one pellet of sodium hydroxide and 10 ml. of ammonium
Since no standard sample of a titanium alloy containing boron was available, the accuracy and precision of the method were checked by adding known amounts of boron (using standard boric acid solutions 1 and 2) to pure titanium metal and carrying the samples through the procedure. The results obtained are shown in Table I. The reagent blanks obtained were invariably very low (less than 0.000270 when compared to 5 ml. of dianthramide solution that had been heated a t 90" C. for 3 hours). The boron color is stable overnight if the solutions are stored in glass-stoppered volumetric flasks. A study of the possible interference of elements that might be found in titanium alloys was made. The results obtained are shown in Table 11. None of the elements that would be encountered in commercial titanium alloys will interfere with the method. ACKNOWLEDGMENT
The authors wish to thank Charles Clemency for his assistance in some of the experimental work.
1449’
V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3 LITERATURE CITED
(1) American Society for Testing Materials, Philadelphia, Pa., “ASTh4 Methods of Chemical ilnalysis of Metals,” p. 134,
1950. Berger, K. C., and Truog, E., IXD.ENG.CHEM.,ANAL.ED.,11, 540 (1939). (3) Brewster, D. A , , AXAL.CHEM.,23, 1809 (1951). (4) Ellis, G. H., Zook, E. G., and Baudisch, O., I b i d . , 21, 1345 (1949). (5) Foster, A I . D., IND.ENG.CHEM.,ANAL.ED., 1,27 (1929). (6) Friend, J. N., and Twiss, D. F., “Textbook of Inorganic Chemistry,” Vol. VII, Part I, p. 338, London, Charles Griffin and Co., 1924. (7) Hatcher, J. T.,and Wilcox, L. V.,~ ~ N A CHEM., L . 22, 567 (1950). (8) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 611, New York, John Wiley & Sons, 1929. (9) Ibid., p. 617. (10) Ibid., p. 619. (2)
(11) Naftel, J. A., ISD.EXG.CHEM.,~ ~ X A ED., L . 11, 407 (1939). (12) Pigott, E. C., “Chemical Analysis of Ferrous Alloys and Fourldry Materials,” p. 7 5 , London, Chapman and Hall, 1942. (13) Ibid., p. 76. (14) Scott, W. W., “Standard Methods of Chemical Analysis,” 1-01. I, p. 185, New York, D. Van Kostrand Co., 1939. (15) Scott, W. W., and Webb, S. K., IND. ENQ.CHEM.,- ~ X . \ L ,El)., 4, 180 (1932). (16) Tschischem-ski,K.,I n d . E n g . C h u . , 18, 607 (1926). (17) U. S. Steel Co., “Sampling and Analysis of Carbon and -4lloy Steels,” p. 283, New York, Reinhold Publishing Corp., 1938. (18) Ibid., p. 284. (19) Weinberg, S.,Proctor, K. L., and Niliier, O., ISD.Esc,. CHEM., ANAL. ED., 17,419 (1945). (20) Wilcox, L. V., Ibid., 4,38 (1932). (21) Winsor, W. W., I b i d . , 20, 176 (1948). RECEIVED for review September 20, 1952. Accepted December ”2, 1952. Presented before the Pittsburgh Conference on .4nalytical Chemistry and -4pplied Spectroscopy, Pittsburgh, Pa.. March 2, 19.53.
Direct Determination of Oxygen in Organic Compounds Report of the Subcommittee on Analysis of Oxygenated Compounds, Committee on Analytical Research, Division of ReJining, American Petroleum Institute W. H. JONES, Chairman Esso Laboratories, Esso Standard Oil Co., Baton Rouge, La. Cooperative work of a number of laboratories has shown that in general the total oxygen content of organic materials in the low range of 0.01 to 1.0% can be determined with reasonable accuracy by four different modifications of the Unterzaucher method. In the original Unterzaucher method the effect of pyrolytic hydrogen on the iodine pentoxide is a source of considerable error in the lowoxygen range. I n each of the four modifications, the effect of the pyrolytic hydrogen has been obviated in a different manner. Comparable precision and accuracy were obtained by methods utilizing titrimetric, gravimetric, and manometric techniques.
B
Y THE end of World War I1 the technology of petroleum re-
fining had become relatively complev and broad in scope, owing to the introduction of many new and varied processes and products. With the advent of petrochemical manufacture and with increased problems in product stability during storage and use, the role of oxygen in the petroleum industry had become of considerable importance. I n November 1946 the Committee on Analytical Research, Division of Refining. -4merican Petroleum Institute, established a subcommittee t o investigate analytical methods pertaining to oxygen in petroleum and related materials. The original subcommittee was composed of rcpresentativea from four different petroleum laboratories, and eventually qome 14 laboratories representing petroleum and othey fieldq participatediri this cooperaative work. The main objective :it the outset \vas t o develop and prove a method for total oxygen contcnt, particularly as applied to relatively low oxygen concentrations found combined in petroleum products. For such analysis, most petroleum laboratories had usually calculated osygen content b y difference between 100% and the sum of all the other elements. The limitation of this difference method is obvious, particularly when the sample is composed of several constituents and also when the oxygen content is low. Many analytical methods had been reported in the literature for the determination of oxygen by direct methods, but these are either generally considered very poor or so difficult as to be an a r t
with a particular operator. An escellent review of this literature was published by Elving and Ligett in 1944 (IO). At the time of the organization of the subcommittee, a f e u petroleum laboratories had already begun work on a direct procedure based on the thermal decomposition of the organic compound over carbon, oxidation of the resulting carbon monoxide with iodine pentoside, and titration of the liberated iodine as first proposed b y Schutze (16, 1 7 ) and further improved by Uotereaucher (18). Some consideration was also given to utilizing the conventional oxygen-type determinat,ions for carboxylic acid (3),ester (Q), alcoholic hydroxyl (15), and carbonyl ( 5 ) groups. These type determinations would give not only a value for total oxygen b u t also a breakdown which in many cases mould he Iielfpul. The limitat,ions in cases of very low oxygen contents were again recognized. I n 1918 the first cooperative work was initiated for comparing these three different methods for total osygen. TKOsynthetic samples containing I and 5% osygen, rcepectively, were prepared from mixtures of pure n-caproic acid, ethJ-1 n-caprylate. isoamyl alcohol, and methyl n-amyl ketone in iso-octane. Five different petroleum laboratories participated in this cooperative program arid the results are summarized in Table I . The conclusions reached in this preliminary phase of the work of the subcommittee iwre as follows: Oxygen by difference, ut,ilizing combustion only, gives a good average value with a relatively large sbandard deviation. The chemical methods appeared r e r y satisfactory for the
