~~
Table V.
Electrolyte UFd-BaFg-CaFz UF,-B aF2-C aFz UFa-BaFn-SrF? UFi-BaF;SrFi UF4-BaF2-LiF UF4-BaF2-LiF CeF3-LiF-BaF2 CeF3-LiF-BaF2 YFX-LiF-BaFv YFlLiF-BaF2LaFs-LiF-BaF2 LaF8-LiF-BaFz
Accuracy Tests on Mixed Fluoride Standards
Residual 0.189 0,189 0.321 0.321 0,227 0.227 0.180 0 243 0 128 0 128 0 324 0 324
Oxygen, % Added 0.100 0.500 0.100
0,500 0,100 0.500 0.050 0.300 0 100 0 500 0 100 0 500
Rel. error Recovereda 0.095 0.500 0.106
0.iii
%b
-5.0
0.0
+6.0 i2.2
0.101 $1.0 0.504 $0.8 0.052 +4.0 0.290 -3.3 0 102 +2.0 0 508 t-1 6 0 104 $4 0 0 504 +O 8 Values have been corrected for residual oxygen, average of 10 determinations. Relative error = 100 (mean - known)/known.
high affinity of La203for mater, it was necessary to prepare the lanthanum standards in a dry atmosphere. The weighed oxide was ignited a t 1000° C. and, while red hot, was placed in a controlled atmosphere chamber. The standards were mixed and fused a t 1000” C. in this chamber. Even after long exposure a t room atmosphere, the fused standards did not show any detectable increase in oxygen content. The oxygen content of most ternary electrolytes, as well as the individual metal fluorides, usually exceeded O.lY0. Consequently, no special handling techniques were required t o achieve a suitable degree of accuracy. T o analyze
fluorides containing lesser amountct of oxygen, i t would be necessary to grind, weigh, and encapsulate the powder in platinum before removing the sample from the inert atmosphere chamber. One of the ternary fluorides was subjected to special electrolysis conditions to deplete deliberately the oxygen content. Because of the extreme hardness of the sample, contamination from the alundum mixing vial occurred during pulverization causing the oxygen results to be abnormally high. =Ifter this problem was eliminated by using a stainless steel vial, an oxygen analysis of 0.04y0 was obtained. Prolonged
pulverization in alundum and stainless steel vials showed that, for samples pulverized in excess of about 15 minutes, there was a gradual increase in oxygen owing t o the alundum contamination whereas the stainless steel had no effect. However, in both cases, pulverization increased the oxygen content unless the samples were oven dried prior to analysis. LITERATURE CITED
V., O’Laughlin, J. W., Kamin, G. J., -49.4~.CHEM.32, 1613 (1960). (2) Glassner, A , U. S. Atomic Energy Commission ANL-5750 (1959). (3) Glasstone, S., “Textboo_k of Physical Chemistry,” 2nd ed., p. (13, Van Nostrand, New York, 1948. (4) Goldberg, G., hleyer, A. S., Jr., White, J. C., ANAL.CHEW32, 314 (1960). (5) Hansen, W. R., Mallett, M. W., Trzeciak, M. J., Ibid., 31, 1237 (1959). (6) Horrigan, V. M., Fassel, V. .4., Goetzinger, J. W., Ibzd., 32, i87 (1960). ( 7 ) Kubmchewski, O., Evans, E. L., “Metallurgical Thermochemistry,” pp. 36, 336, Pergamon, New York, 1958. (8) Morrice, E., Darrah, J., Brown, E , Wyche, C., Hedrick, W., Williams, R., Knickerbocker, R. G., Bur. Mznes Rept. Invest. 5549 (1960). (9) Porter, B., Brown, E. A., Ibzd., 5878 (1961). (10) Sloman, H. A , , Harvey, C. A,, J . Inst. Metals 80, 392 (1951). RECEIVEDfor review June 8, 1962. hccepted September 1962. Reference to specific makes or models of equipment is made to facilitate understanding and does not imply endorsement by the Bureau of Mines. (1) Banks, C.
Action of Perchloric Acid and Perchloric Acid Plus Periodic Acid on Ammonia and Amino Nitrogen FRANCIS B. MOORE’ and HARVEY DIEHL Department of Chemistry, lowa State University, Ames, lowa Ammonia is not oxidized b y boiling perchloric acid (the 203” C. boiling azeotrope, HC104.2HzO) nor is it oxidized by boiling perchloric acid plus periodic acid plus vanadic acid. Earlier troubles in the application of perchloric acid to the Kjeldahl digestion have been traced to the action of chlorine (hypochlorous acid) on the ammonia after dilution with water. By making provision for the prompt removal of chlorine, quantitative recovery of ammonia can b e obtained. When the perchloric acid-periodic acid digestion was applied to the determination of nitrogen, good results were obtained on acetanilide, trishydroxymethylaminomethane, and wheat, and with suitable modifications, on Nbromosuccinimide and azobenrene. Good results were obtained on urea
1638
ANALYTICAL CHEMISTRY
and chloroacetamide only when the periodic acid was omitted. Results on 8-quinolino1, linseed oil meal, and nylon were unsatisfactory. The perchloric acid-periodic acid digestion is an excellent method for the wet oxidation of organic materials but it does not furnish a generally applicable method for the determination of nitrogen.
T
HE VERY NUMBER of the papers dealing with the Kjeldahl determination of amino nitrogen attests the difficulties inherent to its application and the qualified precision and accuracy of the results. Recent success ( 7 ) in the destruction of organic matter with a mixture of periodic acid and perchloric acid led us to adapt this mixture to
the determination of amino nitrogen, presuming as in the classical Kjeldahl procedure that all of the nitrogen is retained in the minus three state and yields ammonia on treatment with fixed alkali. Perchloric acid has been used previously in the Kjeldahl method, both alone and mixed with sulfuric acid t o effect the digestion (Q), and also with questionable success when added dropwise during the last stage of the conventional digestion n-ith sulfuric acid as an aid in completing the destruction of organic matter (3, 8); a review of the subject is presented by Bradstreet ( 2 ) . The action of periodic acid on aliphatic amino compounds yields ammonia al1 Dept. of Chemistry, University of Minnesota, Duluth, Minn. On sabbatical leave 1961-62.
belt in something less than quantitative yields (4). The combination of periodic and perchloric acids offers for the digestion of organic matter the great solvent action of perchloric acid, the fragmentation action of the periodic acid in the initial stage of the digestion. and the great oxidizing power of perchloric acid a t the closing stage of the digestion. The questions to be anqmered are: Can conditions be found under which nitrogen is retained as ammonia and complete breakdown of the organic compound is assured' What catalysts for the final, perchloric acxid stage of the oxidation can be tolerated (j)?Do othrr side reactions affect the retention of nitrogen? Does the iodate formed in the digestion interfere in the subsequent distillation of the ammonia? We carried out first the obvious, direct assault on the first of these questions b y stewing u p perchloric acid of various concentrations with known amounts of ammonium sulfate. The problem of maintaining a constant composition of perchloric acid during the digestion had already been solved by the apparatus devised b y Bethge ( I ) and used later b y Smith (6) and b y others. The Bethge apparatus provides total reflux and a sump for the collection of condensate and is so arranged that the condensate may be either returned t o the reaction mixture or withdrawn. A digestion time of 30 minutes was arbitrarily adopted here; this is more drastic than would prevail at, the close of a digestion of organic matter, but the result? (Table I) indicate that even in this extended period no losq i q observed at concentrations of ptwhloric acid below 67.5% HClO4 (b.p. 190' (2.). Thisis quite in contrast to Willard and Cake (Q),who concluded that with perchloric acid alone or with perchloric acid plus sulfuric acid, oxidative destruction was never complete before oxidation of ammonia occurred. The ~ u c ( e s shere is due simply t o the use of an ample volume of acid and the application of total reflux to maintain a constant concentration of perchloric acid and thus a specified and limited oxidizing pon-er. Repetition of this experiment, with 67.5% pprchloric acid, and with a few milligrams of vanadium present also gave quantitative recovery of ammonia. Kor did the addition of 0.5 gram of sodium iodate, the reduction product of periodic acid in the oxidation of organic matter. affect the results. These experimentq with ammonium sulfate were then extended b y adding periodic acid to the mixture of ammonium sulfate and perchloric acid. After the digestion and on diluting with water and adding sodium hydroxide, a white precipitate formed. This was trisodium dihydrogen paraperiodate,
indicating that periodic acid is not thermally decomposed in boiling perchloric acid. Low results were obtained when such periodate was present and steps were taken t o reduce the periodate remaining before adding the alkali. The results following reduction with hydrogen peroxide were then higher, and if the digestion temperature w s held to 180' C.(65 to 66% HC104). all of the ammonia in the ammonium sulfate was recovered. On a number of occasions in this preliminary work, a progressive variation in a sequence of determinations was observed. For example, see Table I, note c; this was traced to the action of free chlorine produced by the thermal decomposition of perchloric acid. I n alkaline solution, this chlorine forms hypochlorite and this in turn reacts with ammonia t o form nitrogen. Even in acid solution, the action of chlorine on ammonia is sufficiently rapid to account for loss of nitrogen; this was shown b y the deliberate addition of chlorine water to a mixture of ammonium sulfate and dilute perchloric acid, Table TI. The low result of Experiment 2, Table 11, no boiling a t all, can be attributed to osidation after neutralization, but in Experiments 3. 4, and 5, with progressively longer boiling periods, the chlorine was expelled on boiling b u t the recovery of ammonia dropped. Further confirming this are Experiments 6 and 7 , identical experiments with and without ammonium sulfate, in which a test was made for chlorine after 5 minutes of boiling b y cooling and treating with potassium iodide; free chlorine (or hypochlorous acid) was present in Experiment 7 indicating that the chlorine is not boiled away very rapidly, but no chlorine was present in Experiment 6 indicating t h a t reaction with the ammonium sulfate present had occurred. Inasmuch as chlorine oxidation of ammonia was occurring in acid solution i t was apparent that following the perchloric acid digestion, the dilution should be made with a solution of a reducing agent. Sodium sulfite proved satisfactory, a short period of boiling being sufficient then t o remove most of the excess sulfur dioxide; the remaining sulfite was then oxidized b y the addition of a little peroxide and the excess peroxide decomposed b y further brief boiling. Using this technique it was possible t o take the boiling perchloric acid to the concentration of the azeotrope, 72.5y0 HC104,without loss of ammonia (Table 111). Recovery was still quantitative when periodic acid was added, Table 111,lines 3 and 4. As a still further proof that chlorine attacks ammonia in acid solution, chlorine water was added to a mixture of ammonium sulfate and dilute perchloric acid, the solution was boiled, and the sodium sulfite quenching technique was
Table 1. Digestion of Ammonium Sulfate with Perchloric Acid
Ammonium sulfate: 50.31 mg. (5.00 ml. of stock solution); Perchloric acid: 15 ml. Bethge apparatus; 30-minute digestion; distillation into boric acid followed by titration with hydrochloric acid. (Work prior to adoption of sodium sulfite quenching procedure) Boiline HC10, temp., concn., Vol. of HC1, 0 1008N, ml. c. % ... ... 7.52 170 62.5 7.51 65,5 7.52. 7 . 5 3 , 7 . 5 1 180 185 66.5 7,51; 7.53; 7.51 D
190 -_.
195
203b
7.57
67.6 68 5 72 5
7 43, 7 40, 7 40 6 50, 6 58, 6 66, 6 76, 6 90, 7 17c
a Immediate treatment with alkaii and distillation; that is, no digestion. * Constant boiling mixture obtained without reflux. c Digested simultaneously; after standing, diluted and immediately distilled in order listed.
Table 11. Action of Chlorine on Ammonium Sulfate in Boiling Perchloric Acid Solution
Ammonium sulfate: 50.3 mg. (5.00 ml. of stock solution); 5 ml. of chlorine water; 15 ml. of 707, perchloric acid; boiling; cooling, treatment with sodium hydroxide, and distillation into boric acid and titration with hydrochloric acid Chlorine Boiling VOl. of water perjod, 0.1008N Expt. added, KO. ml. min. HCI, ml.
6 7
5 5
. . .b
5
5 (am. ..c monium sulfate omitted)
= Essentiallv same exoeriment as in Table I. * After boiling, solution n.as cooled and treated with potassium iodide; no free iodine liberated. e After boiling, solution was cooled and treated with potassium iodide; considerable free iodine formed.
applied. T h a t ammonia was lost was further evident, Table IV. The attack only occurs in dilute acid for the recovery is still quantitative from cold or hot concentrated perchloric acid solutions of ammonium sulfate saturated with chlorine gas. ' These results and the failure to remove chlorine from water on even extended boiling are probably a, consequence of the reaction Clp
+ H20
for which k
=
H+
3
x
+ C1- + HOC1 10-4.
VOL 34, NO. 12, NOVEMBER 1962
The equi-
1639
Table 111. Digestion of Ammonium Sulfate with Perchloric Acid Followed b y Quenching with Sodium Sulfite Solution
Ammonium sulfate: 105.2 mg. (10.00 ml. of stock solution); 10 ml. of 709/, perchloric acid. 5 mg. of vanadium pentoxide; digestion with concentration to the constant boiling mixture (72.5Yc HClO,, b.p. 203' C,): quenching by addition of 30 ml. of water containing 0.55 gram of sodium sulfite; short boiling: addition of 1 ml. of 6% hydrogen peroside; short boiling; addition of sodium hydroxide; distillation into 25.00 ml. of 0.1008N hydrochloric acid and backtitration with O.lLV sodium hydroxide N found. % (Theoretical vilue: Treatment 21 20yc 3 ) 1. Digestion as de- 21.17, 21.15, 21,13, scribed above 21.20, 21.16, 21.18, 2 1 . 2 6 . Av.: 21.18 2. No digestion 21.23, 21.22, 21.23, 21.16, 21.18. Av.: 21.20 3. Digestion as de- 21.17, 21.15, 21.13. scribed above but Av.: 21.15 with 0 . 2 gram of paraperiodic acid added to mixture 4. Same mixture as 21.23, 21.15, 21.17. above plus 0 . 2 Av.: 21.18 gram of periodic acid, no digestion Table IV. Action of Chlorine on Ammonium Sulfate in Boiling Perchloric Acid Solution, Followed b y Quenching with Sodium Sulfite Conditions same as in Table I11 but with 5 . 0 ml. of freshly saturated chlorine water added and boiling continued for 30
minutes without allowing perchloric acid to concentrate Treatment N found, 7c Chlorine water 20.16, 20.09, 19.20 added No chlorine water 21.26, 21.18, 21.24. iiv.: 21.23. Theoadded retical value: 21,2093
librium lies appreciably to the right in dilute solutien but in concentrated perchloric acid (the 72.5y0 azeotrope is almost evactly HC104.2H20) there is essentially no free water and no hypochlorous acid. The oxidizing power of perchloric acid. when used alone, is a t a maximum a t 203' C., the temperature of the boiling azeotrope, 72.5% HC104. Even greater oxidizing power is displayed by the acid of greater concentration obtained by extractiin of water from the azeotrope by the addition of sulfuric acid. However, ammonia is lost from boiling mixtures of perchloric acid and sulfuric acid (Table V). Because water is required for the oxidation of ammonia by chlorine, this loss of ammonia is not a consequence of the increased amount of 1640
ANALYTICAL CHEMISTRY
Figure 1 .
Distillation apparatus
chlorine present but simply the result of the greater oxidizing power of the perchloric acid as it is gradually deprived of its water by the sulfuric acid. Tlie conclusions from all this are: ammonia is not oxidized by boiling perchloric acid of any concentration up to the azeotrope 72.5% perchloric acid even with periodic acid in the mixture; losses of nitrogen observed are caused by oxidation of the ammonia by hypochlorite and occur in dilutc acid solution as well as in alkaline solution; dilution of the hot perchloric acid solution with a solution of sodium sulfite obviates the trouble by reducing any chlorine present from tlie thermal decomposition of the boiling perchloric acid; and oxidation of ammonia does occur in boiling mixtures of perchloric acid plus sulfuric acid. ANALYSIS OF ORGANIC C O M P O U N D S
Encouraged by. the findings reported above. the perchloric acid plus periodic acid digestion followed by dilution with a solution of sodium sulfite was applied to the determination of the nitrogen in several well purified organic compounds : acetanilide, trishydroiymethylaminomethane, S-bromosuccinimide, urea, azobenzene, and 2-chloroacetamide. A perfectly general procedure was really not expected in vieiy of the considerable variation in the mechanisms by which organic compounds can be oxidized and the unlikelihood that nitrogen in valence greater than minus three would end up as aninionia in an attack by oxidizing agents. The periodic acid nas omitted with improve-
Table V. Action o f Perchloric Acid Plus Sulfuric Acid on Ammonium Sulfate
Ammonium sulfate: 105.2 mg.; digestion with 5 ml. of 70% perchloric acid plus 5 ml. of 95% sulfuric acid a t boiling point for 30 minutes; quenching with sodium sulfite solution, etc.
s,% Found 20 76, 20.79, 20.73, 20.64, 20.60 Calc. 21.20
ment in the results in two cases, urea and chloroacetamide; the digestion with perchloric acid alone with these materials proceeds without more than a vigorous exothermal reaction. Apparatus. T h e digestion n as carried out in a 100-ml. flask fitted with a male 24/40 standard taper joint. Fumes from the digestion werc drawn away through a glass manifold and polyethylene tubing t o a n aqpirator. The distillation apparatus, Figure 1, was a modification of the semimicro Kjeldahl apparatus commonly used; it was necessary to eliminate ball anrl socket type joints (except for the connection to the steam generator) from the apparatus as these acted as traps for condensed water which dissolved and retained ammonia. Two small Kjeldahl traps were inserted to trap spray. The traps were wrapped in aluminum foil to minimize reflux. K i t h these modifications recovery of aminonia was quantitative and reproducible. General Procedure. Weigh a sample of suitable size into a g l a v cup about 15 mm. in diameter and 15 mm. tall. A4dd about 5 mg. of vanadium pentoxide. Drop the cup down the 100-ml. Kjeldahl flask held a t a 45' angle. -Idti 3 to 4 carborundum boiling chips. 10 nil. of 707, perchloric acid and 5 nil. of a 107, solution of periodic acid. Digest over a low flame, about 1.5 cm. high, until any low temperature reaction is complete, adjusting the flame so that total condensation and reflux occurs on the upper walls of the flask and no water is lost. Then boil vigorously, expelling water until the perchloric acid azeotrope is reached as evidenced by the development of a well defined condensation ring of perchloric acid in the neck of the flask. Reflux for 5 t o 10 minutes after the characteristic yellow color of tlie fivevalent vanadium has appeared. Allow the flask to cool until it can be grasped by the neck with the fingers, snirl i t under a stream of cold tap water, and add rapidly a mixture of 5 ml. of 1070 sodium sulfite solution and 25 nil. of water. Boil the solution vigoron4y for 2 minutes, add about 1 nil. of 67, hydrogen peroxide and boil until the dark, red-brown color changes to blue. Insert the flask into the distillation apparatus and distill using a 3.5- to 4-cm. flame from a semimicro burner and a 95 to 100 volt setting on the power stat for the mantle heating the steam generator. Check the first few drops of distillate for evidence of iodine and when the distillate is clear, place a 125-ml. conical flask containing 25.00 ml. of 0 . l X hydrochloric acid under the delivery tube. Adjust the position and angle of the conical flask to cover the tip of the delivery tube to the maximum depth. d d d 15 nil. of 40% sodium hydroxide through the dropping funnel a t a rate of 2 to 3 drops per second until the solution becomes green or yellow, then a t a faster rate. Distill for 15 minutes (65 to 70 ml. of distillate), then lower the conical flask, distill for 2 more minutes and rinse the outside of the delivery tube with a few niilli-
liters of deionized watt,r. Titrate the excess acid to p H 4.1 with 0.1N sodium hydroxide. using 2 drops of methyl red and 3 drops of bromocresol green as indicator (0.04y0 solutions). 9 s end point take that volume n-hich produces the same color as that of a solution of 1 gram of potassium acid phthalate in 100 ml. of water containing the same amounts of indicator. R u n a blank in the same manner. .ICETANILII)E. Acetanilide was purified b y recrystallization from water and b y sublimation and then stored over anhydrous magnesium perchlorate. Melting point: 114' C. Found (Dumas method, Huffman 3Iicroanalytical Laboratory): 10.59, 10.36, 10.38, 10.38: Calculated: 10.36% N. I n the oxidation of acetanilide an obvious reaction with periodic acid began a t slightly above room temperature and it' was necessarj- to allow this react'ion to run its course before concentrating and bringing the perchloric acid osidation into play. The best results were obtained with the mixture: 0.2 gram of acetanilide. 0.5 gram of periodic acid. 5 mg. of vanadium pentoxide, 10 ml. of 707, perchloric acid, and 5 ml. of water. About 30 minut'es was taken for the digestion. Found: 10.35. 10.35, 10.32, 10.33, 10.28, 10.36, 10.35, 10.30, 10.26, 10.28, 10.33, 10.28. 10.32, 10.34, 10.25, average 10.31. Calculated: 10.36% 3. r 1RISHTDROXTMETHYLAMINOMETHAIiE , The trishydroxymethylaminomethane uscld n-as the primary standard grade obtained from the G. Frederick Smith Chemical Co. It was vacuum-dried over anhydrous magnesium perchlorate. It was used to standardize the hydrochloric acid used in this study. .Iswith acetanilide, low results were ohtained if the digestion was effected too rapidly. The best results were obtained with a 0.2-gram sample, 0.2 bo 0.7 gram of periodic. acid, 5 nig. of vanadium pentoside, 5 ml. of water, and 10 ml. of 7OY0 perchloric acid. This mixture was digested over a low flame until initial readion was complete, then heated rapidly until the yellow-orange color of the vanadic acid appeared, then cooled rapidly and treated with sodium sulfite as given in the general procedure. Found: 11.48, 11.46, 11.49, 11.50; 11.45, 11.41, 11.49, 11.45, 11.46, average 11.47. Calculated: 11.57Tc S. S-I~ROhroS~CCl~IalIDE. S-Bromosucchimide (obtained from .Irapahoe ('hcmicals, Inc., Boulder, Colo.), was uwd without further purification; although of commercial grade, the purity is st'ated t o approach that of a reagent grade chemical. 3Ielting point 180.5" to 184.0" C. (reported: 173.0" to 175.0" C . ) ; actix-e bromine = 44.7yc. Application of the general ~irocedure g i i m above yielded less than half of tliv cxpected nitrogen. prrhaps not sur1)risiiig in view of the active bromine prcscnt, in the molecule. Howrver, the addition of 0.6 gram of dextrose to t,he digestion mixture p r o x d effective in prrventing the loss of nitrogen. The quantities and conditions were otherwise the same a? with acetanilide.
7.80, 7.80, 7.61. Calculated: 7.877, X.
Found:
UREA. Analytical reagent grade material was dried in a vacuum over anhydrous magnesium perchlorate and analyzed directly. Melting point: 132" to 133" C. S o initial reaction betneen urea and periodic acid in dilute perchloric acid solution appeared to occur and the results obtained with the general procedure given above were 10% below the theoretical value for nitrogen. The periodic acid was omitted and samples of 70 nig. in size wcre put through the general procedure using 3 ml. of water, 10 mi. of 70% perchloric acid, 5 mg. of vanadium pentoxide, and moderately fast digestion (15 to 20 minutes). Found: 46.21, 46.27, 46.32, 46.12, 46.34, average: 46.25. Calculated: 46.65'% F. Hydiolysis to ammonia in the dilute acid solution was apparently not complete. for the vanadate was reduced a t one point during the concentration of the perchloric acid and the color of the five-valent vanadium was only restored after some boiling of the azeotrope. It is interesting to speculate that some urea is converted to ammonium cyanate (the Kohler conversion) during the digestion and cyanate nitrogen then is lost during the final, perchloric acid oxidation; this would explain the slightly low results reported above. 2-cHLOROACETA1LIIDE. The 2-chloroacetamide analyzed was first recrystallized from acetone and then dried in a vacuum over anhydrous magnesium perchlorate. Melting point: 118" to 119" C. The behavior of 2-chloroacetaniide rvhen subjected to the general procedure was similar to that of urea; no lowtemperature reaction occurred and the results were about 10% below theoretical. The periodic acid was then omitted and satisfactory results were obtained. Found: 14.97, 15.02, 14.94, 14.89, 14.90, 14.95, 14.90, average 14.94. Calculated 14.987, 1;. =IZOBEKZCNE. dzobenzene, melting point 68" C., was dried in a vacuum over anhydrous magnesium perchlorate and analyzed. Direct application of the general procedure failed on azobenzene as expected (actually only 107, of the nitrogen was recovered as ammonia). Preliniinary reduction with zinc dust in a benzene solution, followed by the digestion with periodic acid and perchloric acid did Lvork, hoverer. T o a sample of 0.13 gram was added 1 ml. of benzene, 2 ml. of 707, perchloric acid, 5 ml. of water and 0.1 gram of zinc dust; the mixture was warmed and agitated until decolorized; 5 ml. of water was added and the mixture was heated gently for 15 to 20 minutes; 8 ml. of 70% perchloric acid and 0.5 grain of periodic acid in 5 nil. of water were then added; the analysis was then continued as given in the general procedure aboT-e. Found: 15.34, 15.33, 15.31, 15.36, 15.33, average 15.34. Calculated: 15.37. S-QUINOLINOL. Reagent grade 8quinolinol was dried in a vacuum over
anhydrous magnesium perchlorate. The best result's were obtained using the general procedure n-ith very slow digestion. However, all results were low, the optiiiium conditions yielding 8.06% S , much lower than the theoretical value of 9.65% X. It was expected that a heterocyclic nitrogen compound would not yield to this treatment but the analysis was performed with the idea that possibly the periodic acid would open up the heterocyclic ring giving an intermediate which would yield ammonia; apparent,ly it did in part. NYLON. Satisfactory results of nylon (shredded, 40 mesh) could not be obtained with the general procedure given above or various modification of it. The highest result, 11,547, nitrogen in contrast to 12.08% nitrogen obtained by the Dumas method, was obtained by very rapid digest'ion with a high initial concentration of perchloric acid; in general, the results were hopelessly low and variable. LIXSEED OIL MEAL. Satisfacbory results could not be obtained on linseed oil meal: 5.607, nitrogen was obtained by the procedure of the AO.-A.C., and 4.42 t'o 5.13 by the general procedure given aboi-e and Various modifications of i t changing the initial concentration of acid, amounts of acid, and rate of heating. THEAT. TKO analyzed samples of wheat were obtained from the State Grain Inspection Dept. of the Minnesota Railroad and Warehouse Commission. The general procedure given above failed on these samples. The best results of these samples were obtained using a 0.5 gram sample mixed with a n equal weight of dext'rose (no periodic acid), and slo~vlydigest'ed for 2 hours with a mixture of 15 ml. of 70% perchloric acid, 6 ml. of water and 5 to 10 nig. of vanadium pentoxide. Sample 1: Found 13.8S, 13.38. 13.58, 13.56, 13.43, 13.61. average 13.57% 1x0tein. Reported: 13.607, protein. Sample 2: Found 12.43, 12.39. 12.43, 12.29, 12.44, 12.49. average 12.417, protein. Reported 12.40% protein. CONCLUSIONS
It is e\-ideiit that the periodic acitlperchloric acid digestion does not provide a generally applicable method for nitrogen in organic rompounds. Hecause perchlorir acid has no detrimental action on the, ;wimonium ion. once it is formed. the low.: of n i t r o p n observed must originate in t h r attack on the organic material. mlution to the problem of t,he tl&rniination of nitrogen then mu*t an-nit thP advent of more intimate knon-lrdpe of the mechanism of the att,ac.k. 'l'lie oxidizing power of perchloric ataid can he brought to bear in ste1)iYi.e fashion and tlierc is thus oprnetl u p x fascinating field for investigation. F"r1iap too. reducing conditions should br inaintained during the early ktages of the att,ack. Obviously, the approarh to this prolilein is going t o be a piecemral affair with indiVOL. 34, NO. 12, NOVEMBER 1962
1641
vidual studies concentrated on specific compounds. This is poor comfort for the busy analyst who must perforce go on digesting with sulfuric acid and its adjuncts, resigning himself to its tardiness and uncertainties. LITERATURE CITED
(1) Bethge, P. O., Anal. Chim. Acta. 10, 317 (1954).
(2) Bradstreet, R. B., ANAL. CHEM.26, 185 (1954). (3) Mears, B., Hussey, R., Ind. Eng. Chem. 13. 1054 (19211. (4) Smith, ‘G. F.; “Analytical Applic% tions of Periodic Acid and Iodic Acid and Their Salts,” p. 100, G. Frederick Smith Chemical Co., Columbus, Ohio, 1950. (5) Smith, G. F., Anal. Chim.Acta 8,397 (1953). (6) Smith, G. F., Ibid., 17, 175 (1957).
(7) Smith, G. F., Diehl, H., Tulanta 4, 185 (1960). ( 8 ) Wicks, L. F., Firminger, H. I., IND. ENQ.CHEM.,ANAL.ED. 14, 760 (1942). (9) Willard, H. H., Cake, W. E., J. Am. Chem. SOC.42, 2646 (1920).
RECEIVED for review June 25, 1962. Accepted September 4, 1962. Work supported by a grant from the National Science Foundation, NSF-G10012.
A Series of Procedures for Determining Boron in a Wide Variety of Organoboron Compounds RAYMOND
H. PIERSON
Research Department, U. S. Naval Ordnance Test Station, China Lake, Calif.
b Six very simple procedures for determining boron in organoboron compounds containing C, H, N, 0, Br, P, and B are presented. Results are reported for 27 compounds covering a wide range of types and a range of boron content from 1.7 to 88.5%. The average per cent relative standard deviation is 0.29 and the range for this statistic is 0.06 to 0.5770. All procedures are completed by titration with standard N a O H in the presence of a large excess of mannitol. Lack of sensitivity (sharpness) at the beginning and end points of the titration can be a maior difficulty. For the series described, adequate sharpness is achieved by selection of the appropriate degree and kind of oxidation procedures. A systematic approach to the determination of boron in an unknown organoboron compound is described. Safety matters are also considered.
T
increasing number and variety of organoboron compounds being developed has presented a problem for the analyst required to determine boron content. Because of the Ride difference in chemical behavior of these materials, no universal method has been found. I n view of this, it appeared desirable to develop a series of procedures which might be applied to any organoboron compound for which a satisfactory procedure mas not known. These procedures are, in general, modifications of existing methods of Snyder, Kuck, and Johnson (Q), Strahm and Hawthorne ( I I ) , and Cosgrove and Shears ( 3 ) . The oxygen flask method as described by Corner ( 2 ) failed in the author’s hands for several reasons: the procedure is limited to very minute samples; complete combustion was not achieved; and combustion created sufficient pres-
sure to rupture the flask or to loosen the joint thus allowing escape of solution. The method employing sodium peroxide fusion in a Parr peroxide bomb can be used as a last resort but is regarded by most analysts as undesirably tedious and not sufficiently reproducible. Recent methods of Shaheen and Braman ( 7 ) and Abbott, Liszt, and Roth ( 1 ) were not investigated because they appeared unsuitable for the coverage desired and seemed too complex and time consuming. The procedure of Dunstan and Griffiths (4)is simple and may be advantageous on certain types of compounds. However, this method failed completely with hexaethyldekazene and bis( brom0pyridine)decaborane. Colorimetric or instrumental methods were not investigated because they were not appropriate for samples of high boron content nor for the wide range of compounds for which a small series of procedures was sought. DEVELOPMENT
HE
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ANALYTICAL CHEMISTRY
I t was evident that conditions varying from no oxidant to strong oxidant would be required and that an optimum strength of oxidizer for each type of sample would be essential. Too weak an oxidizer would yield low results, while too strong an oxidizer would cause an explosion, cause the starting point (neutralization of mineral acidity or alkalinity) to be indefinite, or, to a Iesser degree, cause the final titration end point to lack sharpness. Sharpness of Beginning and End Points of Titrations. Precision demands sharpness a t the beginning point and a t the end point of the titration. Vagueness a t these points was a serious shortcoming of other boron procedures. Smith (8) provided a convenient mathematical criterion for the sensitiv-
ity (sharpness of p H change on addition of titrant). This sharpness index, 7, is expressed as change in p H per milliliter of 0.1N NaOH added. For all titrations, sensitivity a t the starting point was measured over two contiguous p H intervals-5.5 to 6.0 and 6.0 to 6.3. Sensitivity a t the end point was measured from pH 7.5 to 8.0. For pure boric acid solutions, only 1or 2 drops (0.03 ml. per drop) of 0.1N NaOH were required for a p H change from 5.5 to 6.0. Analyses proved satisfactory when the amount of alkali required for this pH change was not more than three or four times as much as for pure boric acid solutions. The Strahm-Hawthorne procedure (11) failed on some compounds, regardless of modifications made in the proportions of reagents, and, still more seriously, the titration starting point lacked sharpness. Elimination of two of the original reagents-acetonitrile and trifluoroacetic anhydride-and substitution of distilled water (in varying small quantities) and trifluoroacetic acid improved the starting point sharpness and led to the development of Procedures D and E . These changes also simplified the preparation of oxidant (no cooling required), and gave greater stability of oxidant and better control from a safety viewpoint. Some workers have attributed lack of sharpness a t the starting point (neutralization a t about pH 6.3) to the presence of residual oxidant, hydrogen peroxide. That this is not the case is demonstrated by results of a few experiments in which known quantities of 30% H 2 0 2 were added to solutions of boric acid and the resultant solutions were tested for sharpness index a t the starting point and for boron content. Some analysts have advocated titration t o constant p H to eliminate interference caused by the presence of weak
