interest and valuable advice in this work. The assistance of J. J. hIcKeown with some of the experimental Ivork is gratefully acknowledged. LITERATURE CITED
(1) Am. Public Health A~soc.,New York,
“Standard Methods for the Examination of Water, Sewage and Industrial Wastes,” tenth ed., 1955. (2) Bikerman, J. J., “Surface Chemistry,” p. 71, 2nd ed., Academic Press, Sem York, 1958.
(3) Blaedel, W. J., Todd, J. W., ANAL CHEM.30, 1821 (1958). (4) CamDbeU, H. S., Trans. Faradau SOC. . 50, 1361 (1954). ‘ (5) Coe, R. H., Rogers, L. B., J . Ain Chem. SOC. 70, 3276 (1948). (6) Frumkin, A., “Electrical Pfimomena and Solid/Liquid Interface, p. 58, Academic Press, Kew Torli, 1957. ( 7 ) Kemula, W.,Roczniki Chem. 29, 1153, /,rice\
\lY&JJ.
(8) Mann,
C. IC., ANAL. CHEJI. 29, 1385 (1957). (9) McKinney, R. E Sewage and I n d . Wastes 29, S o . 6 , 6h4 (1957). (10) Muller, 0. H., J . Ani Cherrz. P O C 69,2992 (1947).
(11) Rand, 11, C., Heukeleliian, H., Sewage and Ind. Wastes 23, 1141
(1951). (12) Randles, J. E. B., Discussions Faraday SOC.1, 11 (1947). (13) Randles, J. E. B., Somerton, X. R., Trans. Faraday Soc. 48,937, 951 (1952). (14) Schmid, R. W., Reilley, C. X., J . Am. Chem. SOC.80, 2087 (1958). (15) Spoor, R. A., Science 108, 421 (1948).
RECEIVEDfor review July 13, 1959. hccepted October 20, 1959. Work supported in part by a grant, RG-3720 from The Kational Institutes of Health of the Public Health Service.
Modifications of Kjeldahl Digestion for Organic Nitrogen R. B. BRADSTREET United States Testing Company, Inc., Hoboken,
b Where an internal reducing system is indicated, the use of sucrose in place of salicylic acid-thiosulfate is recommended, because reduction can be carried out a t much lower temperatures. By adding phosphoric acid, with sucrose, pyridine ring compounds may b e determined. Pyrazolone, antipyrine, benzotriazole, and nitroaliphatics in which the nitrogen is attached to a secondary or tertiary carbon cannot b e determined.
T
Kjeldahl inethod has been applied to nearly all fornis of organic nitrogen through various pretreatments and modified procedures. Where a reducing system such as salicylic acid and sodium thiosulfate is used, the Kjeldahl digestion consists of treating the sample with sulfuric acid containing salicylic acid or a similarly related compound, t o which is added sodium thiosulfate for reduction, heating until carbonization takes place, cooling, adding potassium sulfate and catalyst. and digesting a t boiling temperature for a t least an hour after clearing of the mixture. If it is assumed that the method is adaptable to all fornis of organic nitrogen, this procedure, in principle, should be applicable to all types of organic nitrogen compounds. However, certain weaknesses are apparent in the method. Salicylic acid is added to the digestion mixture primarily to supply free carbon, so that a reducing medium can be sustained through the reduction of sulfuric acid. The only other apparent function is that it fornis adducts ivith nitro compounds-and probably with other types of nitrogen as well. I t HE
114
ANALYTICAL CHEMISTRY
N. 1.
is doubtful whether this fact h:t5 any direct bearing on the actual determination. To trace the action of salic! lic acid in concentrated sulfuric acid. ultraviolet curves were run using a Car>- >pectrophotometer. Salicj.lic acid. 3-nitrosalicylic acid, sn-dinitrobenzene, p-dinitrobenzene, and p-nitrochlorobenzene were used. PROCEDURE
Master solutions were made by dissolving 0.1 gram of material in 100 ml. of reagent grade sulfuric acid. Solutions used in the cell were made by taking a volume of the master solutions. m-hich, when diluted t o 100 nil. with reagent grade sulfuric acid, was sufficient for the entire curve to be recorded on the chart. I n mixtures of salicylic acid and nitro compounds, equal volumes of the master solutions were mixed and diluted accordingly. All solutions were taken through the range of 200 to 400 A. The spectrophotometer was calibrated with potassium chromate solution and balanced with reagent grade sulfuric acid. The ultraviolet curves for mixtures are shown in Figures 1 to 3. Apparently with the dinitro compounds, time must be allon-ed for the addition compound to form. In every mixture, a peak appears a t approximately 215 A. Escept for this peak, the curvps retiin the general characteristics of the curve for salicylic acid, modified slightly by the presence of the nitro compounds. Attempts t o isolate these addition compounds failed, and only the origind amounts of salicylic acid and nitro compounds n-ere recovered.
The salicylic acid modification of the Kjeldahl method does not hold for many so-called refractory compounds. There are several reasons why recovery of total nitrogen is not possible by the use of salicylic acid and thiosulfate: The sample is not completely reduced during the reaction time allowed with thiosulfate at room temperature. Subsequently, because of the comparatively high temperature existing a t the time of carbonization of the salicylic acid, the nitrogen compound or that part of it not reduced by treatment with thiosulfate, may undergo pyrrolytic decomposition simultaneously rvith reduction. It is, therefore, reasonable to assume that many of the so-called refractory compounds have been decomposed in part before total reduction was accomplished. Compounds having a high decomposition point-for example, those containing a pyridine nucleus-require a final high digestion temperature. The fact that this is sometimes not obtained, n-ith incomplete reduction, leads to low results. Another factor tending to produce lorv results is the relatively high volatility, under the influence of temperature, of some solid organic compounds. This is generally true of the niononitro compounds, although not entirely confined to them. Of the three nitrochlorobenzeiies, for example, the nieta compound can show a definite loss, as evidenced both by odor and subliniation in the cooler portion of the flask during the preliminary stage of digestion. A similar loss occurs with 2,4dinitrophenol. By replacing salicylic acid with 1-naphthol-pyrogallol mixtures (2), the total nitrogen of some hitherto refrac-
I
i
I
1
I
, I
’,
\
,
I WAVE LENGTH
300
Figure 2. a.
b. c.
a - 3 0-0 Figure 1 . a.
b. c.
Ultraviolet curves
Salicylic acid in-Dinitrobenzene, zero time in-Dinitrobenzene, 3 0 minutes
tory compounds has been obtained. A possible reason might be that the decomposition points of both l-naphthol and pyrogallol are lower than that of salicylic acid. Such reasoning might also apply to use of thiosalicylic acid ( 5 ) . The approximate temperatures a t which various compounds are carbonized, and sulfur dioxide generated are given in Table I. The points at u-hich the material carbonized, and a t which the odor of sulfur dioxide \vas noticeable in the flask mere taken as limiting temperatures, and simply indicate the temperature range within which reduction can be carried out. Sucrose and pyrogallol offer the greatest possibilities. At these decomposition temperatures, free carbon is present in colloidal form, arid is a n immediate and steady source of sulfur dioxide a t a comparatively low tempersture. It becomes unnecessary therefore, to use sodium thiosulfate as a reducing agent. The additional salt concentration contributed by the thiosulfate can be compensated for by substituting the equivalent aniount of potassium sulfate, so that the acid-salt ratio will remain the same. To test the validity of the assumptions made, the procedure outlined below for reduction and digestion vias followed. Both sucrose and pyrogallol were used as reducing agents, and selenium as the catalyst. If no more than 0.1 gram of selenium is used, there nil1 be no loss of nitrogen. One- t o two-tenths gram of sample is weighed into a Kjeldahl flask containing 25 ml. of concentrated sulfuric acid and 0.5 gram of reducing agent.
Ultraviolet curves
Salicylic acid p-Dinitrobenzene, zero time p-Dinitrobenzene, 30 minutes
The contents of the flask are kept nithin the decomposition temperature range (Table I) for 1 hour. Onetenth gram of selenium and 18 grams of potassium sulfate are added and the mixture boiled until clear. Vigorous boiling is continued for 1 hour, and the digest is cooled, diluted, and distilled in the usual manner. The technique used in the above procedure u a s as follons: The sucrose, or pyrogallol, was first carbonized and heated until sulfur dioxide was given off. The w n p l e T-,-as added after the mixture had cooled somen-hat-ca. 50” C. During the reduction period, Table 1. Decomposition Temperatures Some Hydroxy Compounds in Concentrated Sulfuric Acid
of
Compound Salicylic acid 1-Saphtho! Pj-rogallol Sucrose Thiosalicylic acid Sulfosalicylic acid
Appros. T oC. Range for Liberation of SOn 230-240 150-1 60 120-140 90-100 160-180 220-230
Table II.
Compound o-Dinitrobenzene m-Dinitrobenzene p-Dinitrobenzene 2,4-Dinitropheno! 2,4-Dich!oronitrobenzene
o-Chloronitrobenzene m-Chloronitrobenzene p-Chloronitrobenzene 3-Nitrosalicylic acid o-Nitrophenol p-Nitrophenol o-Nitrotoluene m-Nitrotoluene p-Sitrotoluene o-Nitromiline m-Kitroaniline p-Sitroaniline o-Sitrobenzoic acid m-Kitrobenzoic acid p-Nitrobenzoic acid
I
I
300
Figure 3. a.
b. c.
WAVE LENGTH
2oc
Ultraviolet curves
Salicylic acid p-Nitrochlorobenzene, zero time a n d 30 minutes 3-Nitrosalicylic acid, zero time and 30 minutes
the flask was loosely stoppered with a porous Alundum crucible which was removed a t the start of the digestion. An arbitrary reduction time of 1 hour has been used, although in many cases half an hour appears to be sufficient. Duplicate determinations were run. Table I1 compares the results obtained on nitro compounds by different modifications. These seem to offer proof that the temperature a t which reduction takes place is significant, and also that the use of thiosulfate as a source of sulfur dioxide is unnecessary. Sucrose ( 1 , 6), which has been used before, cannot be added to sulfuric acid to make up a satisfactory stock solution as can salicylic acid. Pyrogallol mixed with sulfuric acid also tends to darken within
Per Cent Nitrogen by Various Modifications
Calcd. 16.68 16.68 16.68 15.22 7.29 8.89 8.89 8.89 7.65 10.07 10.07 10.22 10.22 10 22
20.28 20.28 20.28 8.39 8.39 8.39
Salicylic ilcid
4.35
l-yaphtholPyrogallol 16.11 16.66 16.28 14.48
6.76 8.07 7.07
7.83 8.80 7.69
8.94 9.38 9.00 20.20 20.21 20.26 8.30 8.37 8.40
10: 30 10.15 10.09 20.30 20.24 20.31 8.35 8.33 8.40
... ...
...
Modified Method Sucrose Pyrogallol 16.59 16.49 16.59 16.71 16.54 16,59 14.94 15.00 6.15 6.05 8.93 8.97 8.86 8.82 8.79 8.83 -I . i l 7.63 10.07 10.01 10.10 10.04 10.23 10,27 10.18 10.21 10.20 10.16 20.40 20.31 20.27 20.18 20.23 20.22 8.36 8.34 8.32 8.42 8.41 8.37 -I
VOL. 32, NO. 1, JANUARY 1960
1 15
I
-~
tirnc maintnining a nmxinium ternpentturc.
~
Per Cent Nitrogen Extended to
Table 111.
Salicylic wid-thiosulfate Compound Pyrrole Uric acid Isntin Betaine HC1 Theobromine Brucine Piperine An t*ipyriric Phcn~lnicth~lp~rnzolonc 1,5,:i-’Renzot’ri:izoIe
Other Linkages
sucrose Xitrogen, % SnlicylicCalcd. thiosulfate Hctcrocyclic Ring Sitrogen 19.54 20.87 32.18 33.25 9 ‘ 69 9.05 9.12 8.00 LR
3!. 10 ,.lo
.
I
Sucrose
20.62 33.00 9.42 9.1;
3:. 2 i
.
...
r.15 4.02 11.52 11.4; 11.87
3.05 8.28 9.87 11.56
4.w
14.89 13 78 35.26’ I
I’yridiiic Ring Xitrogen Pyridinc Quinolinc Bigeridinc Cinchonine Kicotinic acid Sicotinsrnide
13.10 9,74 13.55 8.87 10.36 20. G8
9.78
1T.iI
9.45 12.09
10.85
I6.4i
9.52 11.38 22.94
7.83 9.84 19.G2
Sitron1iphntic.s N itroinctlinnc
23.93
Kitrocthnne
1S.06
16.21
1-Nitropropnnc
15.71
15.03
2-Mi t.ropropnne
15,iI
9.15
0.?7
...
,
I
2 1. !IO
.
20.37 2018H 17 I66 17.3 18.70
Tris( hydrosvmcthyl)nitromethnne ( 2 - h y ~ r o s y ~ c t ~ : ~ I - 2tro-1 - 1 i i.&Propxnediol)
15.31 15.31 16.70 9.81 9,2!) 9.45
6.85 5.36 5.27
hIiscellnncous Xitrogen Linkages
Aeobcnzcnc Dime thylsminonzobe‘nzene ‘itroeobenzene o-Sitronitror;obenzene Dimethylglyoxime Cyclohesanoneosime
15.36 18.65 13.08
K
Table iV.
~
24.13 12.38
Xitrogen, % ~period, i l H ~ ,pyridine Quinoline 1 2 3
5
13,lO 13.46 13.52 13.37
Table V.
12.69 17.25 22.96 11.67
18.42
EQBcctof Boil Period
::::
10.29 9.8i
15.34 18.63 12.96 18.46 23 98 12.37
I . .
...
I
n few diiys. It is rcconimendcd thnt either comoound be added directly to the acid &fore use. The amount‘s of wid and sulfate used in the above procedure have been calculntcd BO that with normal procedure, the acid index (3) a t the cnd of the digestion will bc approsimately 1, allowing complete recowry of nitrogen while a t the same
Comparison of Digestion Mixtures Using Sucrose
Nitrogen, Compound Pyridine Quinoline Piperidine Cinchonine Siicotinio acid Xicotinamide %.Methy1-5-e thylpyridine Nitromethane Nitrmthsne 1-Nitropropane %Nitropro me Pheny lmetRglpyrazolone Antip sine 1,2,3-genzotriaeole
116
e
ANALYTICAL CMEM~STRY
Calcd. 17.71 10.85
16.47 9.52 11.38 22.94 11.57 22.95
18.6G’ 15.71 15.71 13.78 14.89 36,26
13.10 9.74 13.65 8.88 10.36 20.58 2i:h 17.60 15.31 9.81
11.47 11.52 11.81
H&OrHsPOi 17.55 10.67 10.42 9,52 11.37 22 89 11.53 21.76 17.82 15.38 9.74 11.56 11.46 11,79 I
While the viirintions bctwccn the u w of thiosulfate-snlicylc acid and SUCMS~\ (or pyrognllol) are significant, the diffcrence bctwecn thiosultatel-naphtholpyrogallol and sucrose (or pyrogallol) nre not so evident, Howcver, the outlined proccdure is less cornplicntcd, eliniinntcs thiosulfate, and a l l o ~ sthe usc of smaller quantities of sulfuric acid. The method can be further extended to include other types of nitrogen link:igcs, as indicated in Table 111. This proceduro has nlso been used succcssfully for amino acids, amines, amidrs, urcns, guanidines, thiazolcs, and nitriles. The prcscnce of Sucrose as :i reducing agent, whcther indicated or not, does not vitiate the results. The limitations of this procedure arc appnr(tiit from the rcsults obtained mitli conipounds conhining n pyridinc nucleus, pyrazolone ring, nitroaliphatics, nnd with benzotriazolc. I n compounds containing a pyridine ring, titration of tlic rlistillatc showed gcncrally poor ciitl points, and n fnint odor of pyridine was sonictimes noticcablc. This condition would sccm to indicate that pyrolytic decomposition mas not complete, and that possibly an equilibrium the follorving is cstablished: red. red. Pyridine -+- piperidine -+PIHI
-
4--
-
oxid. which proceeds only slowly to the right The obvious conclusions arc that the boil pcriod is insuficicnt or the digestion temperature is not high enough. Results obtained with pyridine and quinoline indicate t h a t an extended boil period does not seem to improve the recover!. of nitrogen to any great extent (Table IV). Pyridinc decomposes at D tempcrature well above 300’ C., and because the masimum temperature obtainable with the digestion mixture used is ca. 345’ C.,it is reasonable to asaurnc that it is not sufficiently high to increase the speed of t h e reaction so that quantitative results can be obtained. To increase the temperature of digestion, a misture of sulfuric acid-glacinl phosphoric acid was used (70% H1SOr 307, HaPo,, v./v.) in place of sulfuric acid, the amount of added potassium sulfate and selenium catalyst r e m i n i i g the same. The boiling temperature of this misture is around 365” C., and the extent to which thr results are improvcd is shown in Table V, With nitroaliphatics, the recovery falls somewhat short of the calculatcd values, especially in those compounds containing a nitro group attached to n secondary or tertiary carban, although, in most cases, results have been fairly reproducible, Treatment with sulfuric acid-phosphoric acid does not improve the results, However, a reduction PO
r i d of 11/'2 hours nt slightly h o v e rooill tcmpcrrrturc give more consistent rcsults. 'L'lie firilure of the method with nitroaliphtics probably lies in the f:ict that they react msily with sulfuric. ncitl nt relativdv low tcrnpcr:tturc, :is comp a r d with riitrotiromntics which nrc appnrcntly iinrcwtivc lit reduction temperat u ru . 'i'hc p r i tn:~ry nitro p:rra ffins, whcn trc:itcrl with :I strong niincrd acid, relict to forin n cnrbosylic acid (possibly B sulfonritrtl product with sulfuric acid) crnd hydrosyltminc. Also secondary IIit ro pnr:tffins-e,g,, 2-ni troprop:incitre ciitircly decomposed by heating helow 100' C. with hydrochloric :rcid (4). Thc prob:ilic course of such n reaction iri cotiretitr:ited sulfuric acid during the reduction period cithcr :Lt rooin tvnipcraturc or slightly abovp, hns not bccn dctcrinirietl. 'Froin tlic results, lion.v.i.cr, it is cviclriit tlint coinplcte dccoinposi-
tion docs riot take phcc. \Vhen actual digestion waa started, it WLLSinvariably noticed that brown fumes were present in the Bask, and also n strong odor of nitrogen tetroxide present. This mxne condition exists with compounds containing a nitro group attached to R tcrtinry carbon. The failure of the method with pyrnzolonc and trinzole rings may be due to thcir rcsistnncc to eithcr reduction or oxidation. No sntisfactory espLinntion can be given for the failure of 2,4dichlornnitrobenzene to respond to the met hod , and repeated dcterminatione, while showing only slight vnrhtions, still fall below the calculated value. I n spite of these shortcomings it is felt that the outlined procedures offcr a method for nitroaromatics, nonrefmctor3. conipounds, and possibly azo compounds,
and also n mctliod for hetcrocyclics containing n pyridine nucleus. ACKNOWLEDGMENT
The suthor is indcbtcd to the Eastern Chemical Corp. for supplying the orgnnic chemicals used in this invcstigntiou. LITERATURE CITED
(1) hsboth, Am. Chem. J . 7, 108 (1885). (2)Bradstreet, R. B., ANAL. CHEX.26, 235 (1954). (3) Ibid., 29, 944 (1957 (4) Dunstan, W. R., kouldin E. J . Chem. SOC. 77, 1262 (1900$); Chetn. Zenlt. 2, 184 (1901). (5) hIcCutchan, Philip, Roth, W. F., ANAL.CAEM.24, 369 (1952). (6) Stebbins, Chem. Zenlr, (3) 17, 161 (1885).
RECEIVEDfor review October 1, 1958. Accepted Piovembcr 3, 1939.
JOHN G. SURAK,' DALE 1. FISHER, C, L. BURROS, and L. C. BATE Analytical Chernisfry Qivision, Ook Ridge National Laboratory, Oak Ridge, fenit.
B An efficient, inexpensive pyrohydrolysir apparatus i s described for dstcrmination of fluorides in materials from which fluorides are difficult to separate by fluosilicic acid distillation. The reaction chamber is constructed of nickel. An unmodifled, standard tube furnace functions as a superheater for the steam and as the heater for the reaction chamber. In the absence of interferences due to anions of volatile acids, the hydrofluoric acid evolved from about 30 mg. of fluoride i s determined by direct alkalimetric titration. h r o n may be used for colorimetric determinationof the hydrofluoric acid evolved from samples containing microgram amounts of fluoride. For samples that contain about 30 mg. of fluoride, the precision i s within 1%. methods for the determination of halides, and of fluorides in particular, have proved effective and reliable (P,8, 6, 8-10,18,18) especially in the presence of interferences (11) which form fluoride coniplexes that are difficult to decompose by the fluosilicic acid distillation method of Willard nnd Winter (14) using perchloric YROHYDROLYTIC
1 ORINS Rewaroh Participantl Marquette University, Milwaukee 3 , W ie.
acid. The pyrohydrolytic method is especially useful tor solid samples, bec m m the fluorides can be separated directly without a preliminary fusion step or dissolution. Apparatus of several typical designs have been described (1, 6, 8, 12, IS). An efficient apparatus has been designed for the pyrohydrolysis of about 30 mg. of fluoride in complex salts, vhich is siniple in construction and inexpensive to build and operate. It is rugged, and particularly suited for continuous operation. Several of these apparatus have been built since 1955 and have given very satisfactory service. Dykes et al. (2) m d Shank et al. (10) have adapted this apparatus for the remotely controlled determination of fluoride in reactor fuels. They titrate the evolved hydro5uoric acid to a conductometric end point by means of 3 standard solution of thorium nitrate. For test portions that contained from 0.25 to 1 meq. of fluoride, Dykes el al. (a) report their "precision is 0.056 meq. standard deviation for a single determination."
*
APPARATUS
The design, materials, and constructional detaiie of the reaction chamber nnd condenser are indicated in Figure 1. The complete assembly of the pyro-
hydrolysis apparatus is shown in Figure 2. The reaction chamber is constructed from nickel (12). The condenser jacket is fabricated from copper and is silversoldered to the nickel. The nickel entryway for the steam and the nickel exit tubes for the hydrofluoric acid and steam are made integral parts of the reaction chamber by means of fusion welding. The closure to the sample port is made of Teflon. A Hevi-Duty, Type 50, 'iM-watt, 115-volt-23~volt, 12-inch tube furnace is used y t h o u t modification both for superheating of the steam and for pyrohydmlysis. The furnace mechanically supports the reaction chamber. With this arrangement no steam trap is necessary. The siniple steam generator is shown. in Figure 2.
If samples t o be pyrohydrolyaed contain only microgram m o u n t s of fluoride (of the order of 50 p.p.m.) the amount of liquid condensed with apparatus of this size is excessive for good results. A reactor of smaller dimensions would be more suitable. The volume of distillate can be reduced by introducing steam at a slower rate or by passing air saturated with water vapor rather than steam into the apparatus, However, for samples containing larger amounts of fluoride (of the order of 30 mg,) the use of steam is recommended. The temperature of VOL 32, NO, 1, JANUARY 1960
1 17
