Determination of Germanium Colorimetric Determination of Microgram Amounts with Oxidized Hematoxylin HASNA NETCOMBE, Pi;. -4.E. MCBRYDE, JOHN BARTLETT, University of.Toronto, Toronto, Canada For the determination and separation of bery small amounts of germanium, a method was desired which Mould avoid some of the disadvantages of present methods and be applicable to zinc refinery byproducts and other industrial sources. .J. new colorimetric procedure for determining 0.08 to 1.6 p.p.m. of germanium is based upon the formation of a purple color w-hen germanium solntions are treated \+ ith oxidized hematoxylin solution. Germanium
T
HE most generally used colorimetric method for the determination of germanium is t h a t based on the production of “molybdenum blue” (6, 9-11, 16-18); another uses the yellow color of germanomolybdic acid ( 3 , 14). These methods suffer from a serious interference by phosphate, arsenate, and especially silica. A systematic search of organic reagents led to the discovery of the color react,ion of oxidized hematoxylin with germanium. This reaction forms a purple compound, of the formula Ge(Ht)z, as determined by the method of continuous variations. The product is highly insoluble, and therefore the nieasurement is carried out on a suspension stabilized with gelatin. The reaction is carried out a t a pH of 3.2. APPARATUS AKD REAGENTS
Apparatus. The absorbancy measurements were obtained with a Klett-Summerson photoelectric colorin~eter, using the test tube and Photovolt “narrow band” filters. Absorbancy
Dropping
Funnel
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fl I1
FIG. I 01STILLAT ION APPARAT‘US Thermometer
n
Fig. I b -
,
I
I
Separa tdry
Funnel
I
I c
_____--
First,
>
Second and Third Rece+er
~ N DF.
E. BEAMISH
can be separated from numerous other elements by distillation from hydrochloric acid, and extraction from the distillate into carbon tetrachloride. In view of the increasing industrial importance of germanium-for example, in the manufacture of some types of electronic equipment-and the realization of potential supplies of the metal in industrial by-products, this method may fill a definite need, because of its sensitivity and specificity.
measurements for the determination of the stoichiometry of the color reaction were made in a Becksnan Model D spectrophotometer adjusted to band widths of about 1 mp. A Coleman pH meter wv&s used for pH measurements. The distillations were carried out in the apparatus illustrated in Figure 1. Germanium Dioxide Solution. Spectrographically pure germanium ‘dioxide (1.456 grams) obtained from the Eagle-Picher Co., Cincinnati, Ohio, was dissolved in water and made up t o 1 liter. This solution should contain 2.019 mg. of germanium per ml. A later standardization, by evaporation of an aliquot in a platinum crucible, ignition a t 900” C.! and weighing as the dioxide, gave 2.031 * 0.003 mg. of germanium per ml. as the average of four determinations. From this, standard solutions were made containing 203, 20.3, 10.2, and 2.0 micrograms of germanium per nil. Oxidized Hematoxylin Reagent. Hematoxylin (3.0 grams) obtained from the British Drug Houses Co. was dissolved in a solution of 500 ml. of water and 200 ml. of 9 5 q et,hyl alcohol, 20 ml. of 5 7 , hydrogen peroxide were added, and the solution was heated in a boiling water bath for 15 minutes. .ifter cooling, t,he solution was made up t o 1 liter with water. This solution was stable enough to be used indefinitely. The action of hydrogen peroxide on hematoxylin may produce hematein; hematein gave a similar color with germanium solutions. Buffer, pH 3.2. The buffer was prepared by mixing 250 ml. of 0.1 potassium hydrogen phthalate and 73.5 ml. of 0.1 N hydrochloric acid and making the solution up to 500 nil. with water. The pH was checked with a pH meter. Gelatin. One gram of gelatin was dissolved in 100 ml. of water, and a crystal of thymol was added as a preservative. Carbon Tetrachloride. The C.P. product was redist’illed. Concentrated Hydrochloric Acid. Five hundred milliliters of the C.P. product were extracted with 100 ml. of redistilled carbon tetrachloride, It, is not known what impurity was removed, but t’he treated acid gave more consistent results. Sodium Silicate Solution. A solution containing 2 mg. of silicon per ml. was prepared by dissolving 0.8; gram of sodium silicate in water and making up t o 100 ml. Reagents for Molybdenum Blue Determination were prepared according to the instructions of Hybbinette and Sandell (11). Customs Zinc Concentrate was obtained from the Consolidated Mining and Smelting Co., Trail, B.C. I t consisted mainly of zinc sulfide, with small amounts of lead and iron sulfides. ilccording t o the company’s analysis, it contained 60.0% zinc, 30.0% sulfur, l.Oy, iron, 0.6% lead, 0.4% silica, 0.1% copper, 0.15y0 cadmium, and less than O.lY0 antimony and chlorine. The germanium content, disclosed only after the authors’ experiments were completed, was 0.005’%, obtained by the company by a method involving hydrochloric acid distillation and tannin precipitation of a 20- t o 50-gram sample. Zinc Oxide Fume was obtained from t,he same company. It consisted mainly of zinc oxide, with some basic lead sulfate and normal lead sulfate. The company’s analysis showed 60.0% zinc, 14.0y0 lead, 1.7% sulfur, 1.5y0sulfate-sulfur, 1.3% silica, 0.6y0 iron, 0.02% cadmium, 0.02% chlorine, and traces of silver, copper, bismuth, tin, calcium, magnesium, aluminum, fluorine, manganese, arsenic, and antimony. The germanium content, disclosed only after the authors’ experiments were 1023
1024
A N A L Y T I C A L CHEMISTRY
completed, was 0.004%, obtained by the company by a method similar t o that used for customs zinc concentrate. COLOR REACTION
Effect of Kind of Filter. A spectral distribution curve was (Figure 2) obtained for both an oxidized hematoxylin blank and a germanium-hematoxylin solution. The filter at 550 mp was chosen for future use, because the greatest difference between sample and blank occurred at this wave length. Effect of pH. Hematoxylin acts as an indicator (19); only in the p H range from 2 to 4.5 is its own color light enough not t o interfere with the colorimetric determination of germanium. By using a series of Clark and Lubs buffers (15) in this range, the greatest sensitivity was found at a p H of 3.2.
0
60
IO0
150
Amount
200
250
300
350
o f NaCl (mg.)in 2 5 ml.
Figure 3. Effect of Sodium Chloride on Color Absorption of Germanium-Hematoxylin Solutions 1 Klett-Summerson (K.S.) unit is equal to absorbance multiplied by 500
WAVE LENGTH
Figure 2.
(+)
Absorption Curves of Reagent and Colored Complex
Effect of Hydrogen Peroxide. I n the preliminary development of the method, the ratio of hydrogen peroxide t o hematoxylin in the oxidized hematoxylin reagent was varied. The sensitivity waa greatly decreased when no hydrogen peroxide, or too much, was used. The optimum ratio was found t o be 1 mg. of hydrogen peroxide t o 6 mg. of hematoxylin. Effect of Time of Standing before and after Dilution. The colors are slow t o develop, but a n interval of 45 minutes between mixing and final dilution was sufficient t o attain a constant color intensity. The solutions did not fade appreciably after dilution; nevertheless, readings were always taken immediately after dilution. Effect of Initial Volume. Small variations from the initial volume of the germanium sample were found to have no effect on the final color intensity, but immediate dilution to 25 ml. greatly hindered color development. I n view of this fact, the initial volume was always kept a t 5 * I ml. Effect of Sodium or Ammonium Chloride. Because germanium is always isolated from other elements by distillation from hydrochloric acid solution, the neutralization of the distillate to a p H of 3.2 would necessarily produce a salt. The effect of various amounts of sodium chloride on the color development and stability, using various germanium concentrations, and the
effect of centrifuging for 2 minutes a t 2000 to 3450 r.p.m. after the final dilution, were therefore studied. The results obtained, illustrated in Figure 3, showed a complex behavior. The solutions containing high concentrations of salt and of germanium were noticeably turbid. All these results can be explained by two assumptions: The colored complex is not in true solution; it consists of particles too small t o settle under gravity, but large enough to settle partially in an ordinary centrifuge. Salt causes an agglomeration of these particles, which increases the color absorption in some cases, owing t o the turbidity produced, and decreases it in other cases, because of the material settled out. Because hematoxylin itself is much more soluble in ethyl alcohol than in water, it was thought that the same might be true of the colored complex. However, the addition of ethyl alcohol was found to decrease the sensitivity without decreasing the turbidity. The use of gelatin as a “protective colloid” gave excellent results, of undiminished sensitivity, decreased turbidity, and improved precision. Calibration curves were then determined, using gelatinstabilized solutions, without salt and in the presence of 1 gram of ammonium chloride. Both were straight lines diverging from a common origin, with slopes corresponding to an absorbancy of 0.55 and 0.65 per p.p.m. of germanium, respectively. This showed that Beer’s law was followed in both cases. Range of Greatest Sensitivity. hyres ( 5 ) stated that in cases where Beer’s Ian applies the concentration of solutions could be obtained photometrically with the greatest accuracy when the transmittancy is 36.3%, but that over the range 20 to 60% transmittancy the error i- not much greater. In the present case, the points of maximum accuracy occur a t 0.788 and 0.668 p.p.m. of germanium, respectivriv, and the ranges of satisfactory accuracy are 0.4 to 1 . 2 1 and 0.36 to 1.08 p.p.m. of germanium, respectively. Whenever pobsible, the size of a sample should always be adjusted to make the final concentration lie within these limits. Stoichiometry of Colored Complex. The stoichiometry of the reaction between germanium and the oxidized hematoxylin
V O L U M E 23, NO. 7, J U L Y 1 9 5 1
1025
through the distillation train (see Figure 1) ; a part was diverted reagent was investigated by t h e technique of “continuous to sweep out the vapors in the dead space in the upper variations” (19, 21 ). Solutions of germanium and oxidized the flask. The receivers were cooled with ice. After t ~ hematoxylin of equal molarity were prepared and mixed in varimanium sample had been placed in the distilling flask, the train ous proportions such t h a t the total volume was 10 ml. This was assembled, the gas stream was started, and then 20 ml. of concentrated hydrochloric acid were dropped through the dropsolution was treated with gelatin and after 45 minutes diluted ping funnel. The distillation was conducted for about 45 minwith buffer to the prescribed volume. The oxidized hematoxylin utes at the boiling point of the system, which eventually reached solution was found to lose its effectiveness for this reaction rather 112’ C., until the level of the liquid fell below the lower end of soon after preparation a t the low concentrations used in these exthe gas bubbling tube. For samples containing several milligrams of germanium, 50 periments. It was assumed t h a t the molarity of the reagent soluml. of water were used in each of the first two receivers to collect tion could be computed from the molecular weight of hematoxythe distillate; an excess of concentrated ammonium hydroxide lin. Values of the Y function for four wave lengths (450, 500, was added while cooling in ice; the solution was evaporated and made u p to volume in a volumetric flask; and aliquots were 550, and 600 mp) showed maxima corresponding to a germanium taken for colorimetric determination. I n three determinations to reagent molar ratio of 1 to 2. These observations are interusing 5.06 mg. of germanium, the average result obtained was preted to indicate that one compound is formed in the reaction 5.07 mg. of germanium, with an average deviation of 0.09 mg. and t h a t it is of the form Ge(Ht)z. of germanium. In these and all subsequent distillations, a Comparison with Molybdenum Blue Method. INTERFERENCE blank, obtained by repeating the whole procedure without using germanium, was subtracted; and the completeness of the disOF SILICA.Molybdenum blue colors were developed according t,illation, and of the collection of germanium in the distillate, to the method of Hybbinette and Sandell (II), and read using was ascertained by tesJing the residue in the distilling flask and the 730 mp filter. By means of the sodium silicate solution, the t,he contents of the third receiver. interference of silica in both the molybdenum blue and the hemaThis method could not be applied to microgram samples, where toxylin method was studied. The results illustrated in Figure 4 t,he whole sample rather than an aliquot would have to be used show that the hematoxylin method is more sensitive, and that for the colorimetric determination, because neutralization would 0.4 p.p.m. of silicon interfered seriously in the molybdenum blue produce more ammonium chloride than would dissolve in 5 ml. of method. It was found that 80 p.p.m. of silicon was the lower water. A method was obviously needed for either using less hylimit of interference in the hematoxylin method. cli,ochloric acid during t,he distillation, or removing some of i t Final Procedure for Colorimetric Determination of Germanium from the distillate. with Oxidized Hematoxylin. T o 5 ml. of the germanium soluEarly attempts a t the latter approach failed. Precipitation of tion at a p H of 2 to 4 in a 25-ml. volumetric flask were added 1 germanium disulfide and redissolution in ammoniacal hydrogen ml. of the 1% gelatin solution and 1 ml. of the oxidized hematoxylin reagent. After 45 * l minutes, the solution was diluted peroxide ( 2 , 4,7 , 9, IS, I 4 ) , while satisfactory for amounts of to 25 ml. with the buffer, pH 3.2, and the absorbancy was detergermanium exceeding 50 mg., gave low results with smaller mined, against a blank containing buffer instead of germanium samples. Precipitation of the hydrochloric acid with silver carsolution, with the 550 mp filter. bonate, or its removal with an anion exchanger (Amberlite IR-4), was unsuccessful, because a considerable part of the germanium SEPARATION OF GERMANIUM FROM OTHER ELEMENTS was adsorbed on the silver chloride or on the resin. HydroDistillation from Hydrochloric Acid. Carbon dioxide from a chloric acid solutions of germanium could not be evaporated on cylinder, purified by passing through glass wool, concentrated the steam bath, even in the presence of oxalic acid as a complexsulfuric acid, and potassium permanganate solution, was bubbled ing agent, without almost complete loss of germanium by volatilizat.ion. Extraction with Carbon Tetrachloride. While no measurable extraction into carbon tetrachloride occurred from 4.8 hydrochloric acid, the distribution coefficient was about 0.1 in 5 iV n acid, about 1.2 a t 8 S, and about 4.9 at, 10 N . A simple calculation showed that four extractions of 40 ml. of 10 iV hydrochloric acid with 20-1111. portions of carbon tetrachloride would recover 9970 of the germanium; in practice, this series of extractions led t o quantitative recoveries, as far as. the colorimetric method could detect. The extraction from carbon tetrachloride was slow, perhaps because the hydrolysis of germanium tetrachloride to the dioxide is a necessary first step. I t was found, however, that if 80 to 90 ml. of the carbon tetrachloride extract were stirred or shaken mechanically with 100 ml. of 1%-ater,the recovery of germanium was 86% in 2.5 hours, and quantitative in 4 hours. The addition of ammonium hydroxide failed to speed up the extraction. L u c
.-E
0
s o
01.
0.4
O.G
0.8
1.0
1.1
1.4
1.6
concentration of Ge (p.p.m.)
Figure 4. Calibration Curves for Colorimetric Determination of Germanium
Distillation from Hydrochloric Acid with Carbon Tetrachloride Extraction. I n the distillation as described above, 20 ml. of carbon tetrachloride and 5 ml. of concentrated hydrochloric acid were used in the first receiver, and 20 ml. of carbon tetrachloride in the second; the third was filled with water to serve as a trap for hydrogen chloride. The separatory funnel illustrated in Figure l b was used as the first receiver. After the distillation, the contents of the second receiver and the carbon tetrachloride layer from the first receiver were transferred to a glass-stoppered flask containing 100 ml. of water. The condenser and the second receiver were rinsed with carbon tetrachloride. Then 20 ml. of concentrated hydrochloric acid were added to the first receiver in order to bring the normality to 10 S, and three more extractions ivith 20-ml. portions of carbon tetrachloride were carried out. All rinsings and extracts were transferred to the flask mentioned above; this was then shaken mechanically for 4 hours a t a speed such that the phases were
~
~
1026
ANALYTICAL CHEMISTRY
just kept thoroughly mixed a t all times. The aqueous phase was then transferred to a beaker, made alkaline with concentrated ammonium hydroxide, and evaporated t o 5 ml. Colorimetric determination followed. The results are summarized in Table I. Distillation from Sulfuric Acid. Instead of the 20 ml. of concentrated hydrochloric acid, 15 ml. of 1 to 1 sulfuric acid and 3 ml. of 1 to 1 hydrochloric acid were used in the distilling flask; 50 ml. of water were used in each of the first two receivers. The distillation was conducted for 15 t o 20 minutes a t about 105' C., until fumes became visible. The distillate was made alkaline and evaporated as before, and colorimetric determination was carried out. The results are summarized in Table I .
Table I. Ge Added
.
Distillation from Hydrochloric or Sulfuric Acid Hydrochloric Acid Method hv. Ge No. of devirecovered detns. aiion
Y
Y
2.0 4 0 6.1 8.1 10.2 12.2 14 2 16.2 18 3 20.3 22.3 24 4 26.4 30.5
1.9 4.9 6.3 8.5 9.7 12.1 14.9 17.2 18.1 20.3 22.3 24.1 26.1 30.0
3
2
2 3 8 4
2 4 3 4 2 1 2 2
Ge recovered Y
0 0 0 6 0 5
. .
0 0 1 2 1 0 0
9 7 0 8 6 4 2
0 9 0 i
Table I1 shows that the sulfuric acid method gave low results with both materials, especially with zinc oxide fume, and that the differences in results obtained with salted and unsalted samples in these cases were considerable. This can probably be explained as due to the presence of lead which interferes. Spectrographic examination of the distillation residues disclosed the presence of more than 5 micrograms of germanium in the sulfuric acid distillation of zinc ovide fume, but not in the other cases.
Sulfuric Acid Method
f
0 5
germanium in the form of a standard solution, and evaporating to dryness a t 100" C. before roasting or fusing with carbonate. There was no significant loss of germanium in roasting or carbonate fusion, as shown by testing the fumes from each process from a 10-gram sample.
2 1 6.0
...
9.6
... . .
15.9
KO.of
detns.
Y
6
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...
0.2 0:4
4
0.3
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20.4
0.3
3
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SULMMARY
Av.
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Interferences of Other Elements. A series of elements was tested for interference in both the hydrochloric and the sulfuric acid distillation methods: arsenic, selenium, antimony, tin, aluminum, iron, chromium, copper, zinc, mercury, manganese, silicon, lead, bismuth, cadmium, titanium, tellurium, and molybdenum. The equivalent of 2 mg. of each element, in the form of a solution of a suitable compound, was used, and tested both alone and in conjunction with 10.2 micrograms of germanium. Arsenic did not interfere in either method, although it can be presumed to distill quantitatively, and was also shown t o accompany germanium quantitatively in the extraction with carbon tetrachloride; it does not, however, give a colored product with hematoxylin. This is another advantage of the hematoxylin method, for, although it is possible to separate germanium and arsenic by distillation (1, 8), this step is unnecessary here. Antimony did not interfere in the distillation from hydrochloric acid, but gave high results in the sulfuric acid method, probably owing to codistillation; it forms a colored complex with hematoxylin (20). The presence of tin or lead caused low results in the distillation from sulfuric acid, but not in the distillation from hydrochloric acid. I n the case of lead, this map be due t o adsorption of germanium on the lead sulfate produced. No other elements interfered. Arsenic, antimony, and tin were also tested as to their behavior in an extraction from hydrochloric acid into carbon tetrachloride. Enough difference in extractability was found to warrant hope that these three elements may be separated from each other and from germanium without distillation, h u t the subject was not pursued further. DETERMINATION OF GERMANIUM IN CUSTOMS ZINC CONCENTRATE A>D ZINC OXIDE FUME
A 0.3- t o 0.5-gram sample was weighed in a small platinum crucible. The customs zinc concentrate was then roasted a t 900" C. for 15 minutes t o convert sulfides to oxides; the zinc oxide fume was fused with 0.1 gram of sodium carbonate and 0.1 gram of potassium carbonate a t 900" C. for 15 minutes. After cooling, the crucible was dro ped in the distilling flask, and a distillation either from hy%ochloric or from sulfuric acid was carried out, as described above. A second distillation was sometimes necessary; the results from both were added to give the final result. "Salted" samples were analyzed by adding
Hematoxylin warmed with hydrogen peroxide furnishes a stable ieagent suitable for the colorimetric determination of germanium. The color reaction produces an insoluble product of the form Ge(Ht)z, but the addition of gelatin yields a stable suspension satisfactory for color measurement. The p H must be maintained at 3.2 by a phthalate buffer, The optimum concentration range for which this colorimetric procedure may be applied is 0.4 to 1.2 p.p.m. Germanium may be isolated from most other metallic elements by distillation from hydrochloric acid or sulfuric arid. In the former case the large amount of acid carried over yields a distillate too concentrated in electrolyte for the successful application of this colorimetric procedure to the whole. In this case germanium may be quantitatively extracted from 10 .V hydrochloric acid into carbon tetrachloride, and may subsequently be extracted back into water. The colorimetric procedure may be applied to this evaporated aqueous extract. Distillation from sulfuric acid proved satisfactory for many cases because only a small amount of hydrochloric acid is carried over with the germanium. In this case some interference was observed when tin, lead, or antimony was present with germanium. I I a n y other elements gave no interference in either procedure for the isolation and subaequent determination of this element.
Table 11. Determination of Germanium in Customs Zinc Concentrate and Zinc Oxide Fume Samnle
Additional Ge
Distillation Method
/
Customs zinc conc.
Sone 10.2
Sone 2.0 10.2
Zinc oxide fuuie
Sone 2.0 10.2
HzSO, HC1 "21
HC1
Xone 10.2
HzSOa
HISOI
HzSOI HC1 HCI HCl
4 v . Ge Found %
No. of Deins.
hr. Deviation %
0.00402 0.00356 0,00494 0,00479 0.00501
6 4 10 2 4
0.00019 0.00017
0.00267 0,00127 0,00414 0.00127 0.00410
8
0.00039 0.00060 0.00026 0.00036 0,00037
4 6 4 4
0.00017
0.00033 0,00029
.
LITERATURE CITED
(1) Aitkenhead, W. C., and Middleton, A. R.,I N D . EXG.CHmi., ANAL.ED., 10, 633-5 (1938). 12) dlimarin. I. P.. and Alekseeva, 0. A., J. Applied Chem. L'.S.S.R.. 12, 1900-6 (1939). (3) Alimarin. I. P..and Ivanov-Emin, B. N., Mikrochemie, 21, 1-10 ( i 9 3 6 ) . (4) AUimarin,I. P., Ivanov-Emin, B. N., Medvedeva, 0 . A., and Yankovskaya, C. Y . , Zavodskaya Lab., 9, 271-6 (1940). (5) Ayres, G. H., ANAL.CHEY.,21, 652-7 (1949). (6) Bolts, D . F., and hlellon, M. G., Ibid., 19, 873-7 (1947). (7) Davies, G. R., and Morgan, G., Analyst, 63, 388-97 (1938). (8) Dennis, L. M., and Johnson, E . B., J . Am. Chem. SOC.,45, 1380-91 (1923). (9) E'ischer, W., and Keim, H., Z . anal. Chem., 128, 443-58 (1948). (10) Geilman, W., and Brunger, K., Biochem. Z., 275, 375-86 (1935).
V O L U M E 23, NO. 7, J U L Y 1 9 5 1 (11) Hybbinette, A. G., and Sandell, E. B., IXD. ESG. CHEU., An.4~.ED., 14,715-16 (1942). 112) Job, P., Ann. chim., (10) 9, 113 (1928). (13) Johnson, E. B., and Dennis, L. M., J . Am. Chem. Soc., 47, 790-3 11925). (14) Kitson, R. E.; and Mellon, M. G., IBD.ENG.CHEM.,ASAL. ED., 16, 128-30 (1944). (15) "Lange's Handbook of Chemistry," 7th ed., pp. 1127-8, Sandusky, Ohio, Handbook Publishers, 1949. (16) Orliac, AT., Bull. soc.franc. mineral., 72,9-240 (1949). (17) Orliac, AI., Coinpt rend., 221, 500-1 (1945).
1027 (18) Poluetkoi, N.
S..Zacodskaya Lab., 5, 27-8 (1935); Z.anal. Chem., 105, 23-6 (1936). (19) Prideaux, E. B. R., "Theory and Use of Indicators," p. 358, London, Constable and Co., 1917. (20) Vassallo, E., Gazz. chiin. ital., 41,11, 204-12 (1912). (21) Vosburgh, W. C., and Cooper, G. R., J . Am. Chem. SOC.,63, 437 (1941). RECEIVED October 21, 1950. Work supported by a grant from the National Research Council (Canada).
Dumas Microdetermination of Nitrogen in Refractory Organic Compounds Modijfication of Zimmermann's Method PAUL D. STERNGLANZ, RCTH C. THO-MPSON, AND WALTER L. SAVELL Chemical Research Division, Remington Rand, Inc., South Norwalk, Conn.
A""-
\ IEW of procedures for the Dumas microdetermination of nitrogen in refractory substances has shown the need for a simple method applicable t o both tractable and refractory compounds. Some of the existing methods which claim excellent results (9,18)are not of general applicability-for instance, the potassium chlorate method of Spies and Harris (18) and the copper acetate method of Hayman and Adler (9) were applied to pteridines without success ( 1 ) . Brancone and Fulnior ( 1 ) were able to analyze pteridines merely by raising the temperature of the movable burner to 900" C . ; oxidation aids were omitted. The results, however, show a somewhat greater spread than might be desired. Other Dumas modifications for refractory compounds have been published. One (11)involves the use of nickel oxide as combustion tube filling, a temperature of 1000" C., and a rather coniplicated setup. In another (ZZ), oxygen generated by the decomposition of 30y0 hydrogen peroxide is introduced. -411 these methods have in common an inherent drawback of the conven3
A review of procedures for Dumas microdetermination of nitrogen in refractory substances has shown the need for a simple method applicable to both tractable and refractory compounds. Some existing methods are not of general applicability, and all require a carefully controlled combustion with painstaking observation of bubble speed. Zimmermann's method, designed to facilitate the analysis of tractable compounds, eliminates concern about bubble speed. An attempt was made to simplify this method and to extend it to the analysis of such refractory substances as certain pyrimidines, purines, and pteridines. The procedure, simplified by adapting the tube filling to a boat combustion, was successfully applied to tractable compounds. Refractory compounds required the addition of an oxidation catalyst. Cobaltic oxide was found to be suitable. The movable burner was kept at a temperature of 800" to 830" C. The mixing chamber was redesigned. The automatic setup was replaced by inexpensive standard American equipment. The proposed method is quick and simple, and requires no special care on the part of the operator.
tional Dumas method : A carefully controlled combustion with painstaking observation of bubble speed is required, if accuracy and reproducibility are t o be achieved ( 1 4 ) . If the Dumas method is adapted to the analysis of refractory compounds by adding oxidation aids ( 7 , 9, 18), the changes involved may cause inconsistent results, as some investigators have reported (1, 8, 10, 17). Without certain modifications, however, refractory compounds, such as certain pyrimidines, purines, and pteridines, give extremely erratic results (1, 8-10, 12, IS, 17, 18). Concern about bubble speed was eliminated in a method published by Zimmermann in 1943 ( 2 3 ) . His Dumas modification was designed to facilitate the analysis of compounds of normal combustion behavior. In his setup, the precautions which would be required during the combustion period in the conventional Dumas method became superfluous. During the combustion period the gases are blocked a t the three wag stopcock of the nitrometer, and are conducted into a special mixing chamber located at the opposite end of the tube. Here the gases are mixed and diluted with carbon dioxide. Gases to be reduced, such as oxides of nitrogen, and gases to be oxidized, such as carbon monoxide, are now present in a lower concentration. The dilution in the chamber allows the mixed gases t o be driven through the combustion tube much faster (25) than is feasible in the conventional Dumas method. Equally important is the fact that a uniform rate of gas flow through the combustion tube is readily maintained by this procedure (3). Although Zimmermann's method is a considerable improvement over other Dumas modifications, adherence to Pregl's way of mixing the sample with copper oxide and transferring it from a mixing tube to the combustion tube is objectionable, as several investigators have stated (13, 18). .41so, Zimmermann's change of the temporary filling from the 40- to 100-mesh copper oxide (as specified by Kiederl, 1 4 ) to a fine powder seemed undesirable. Such fine powder in a 15-em. layer retards the passage of the combustion gases (16). It seemed preferable, therefore, t o distribute the sample in a combustion boat and to use coarser copper oxide. With these changes, the method was successfully applied in this laboratory t o tractable compounds. 4 n attempt was then made to extend the Zimmermann method to the analysis of refractory compounds. The results which were obtained with 4-amino-6-hydroxy-2-thiol pyrimidine monohydrate were found to be low. It was, therefore, necessary to develop a modificat,ion of the method. Introduction of an oxidation aid was considered. Because the gaseous combustion products enter the mixing chamber rather than the nitrometer during the combustion, t,he increased rate of evolution of gases could
