ORGANIC GRAVIMETRIC ANALYSIS JOHN F. FLAGG Knolls Atomic Power Laboratory, General Electric Company, Schenectady, IV. Y .
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HE subject of organic elementary analysis, with emphasis on
the determination of carbon, hydrogen, nitrogen, sulfur, halogen, and oxygen has been reviewed briefly but critically ( 7 ) . I n the same article there appears a description of the adaptation of the carbon and hydrogen method of Tunnicliff el al. (17) and Wagnian (19) for 50- to 75-mg. samples. The material is burned in air rather than pure oxygen; the advantages claimed are great>erdegree of control and freedom from the risk of explosion. Air for the combust,ion is passed over copper oxide a t 850" C., through Ascarite and Dehydrite, and thcrice to the combustion train. The sample, contained in a quai t z combustion tube, is distilled at such a rate as to requirp "0 to 30 minutes for burning; a heater kept a t 700" C. betweeii t!ie sample and t,he air-purifying section of the train prevents hack-distillation. The t,ube is packed with copper oxide (850" C.), silver vanadate (400" C.) for removing sulfur and halogen, and lead peroxide (190" C.) if nitrogen oxides are present. ThP :tbsorption train consists of a Dehydrite-filled U-t,ube, and an Ascarite-filled vertical absorption tube. It is claimed that no substance has been encountered that cannot be burned, and data on hydrocarbons, substituted aromatic acids, and some miscellaneous compounds show an attainable standard deviation of *0.2y0 on carbon and * O . l % on hydrogen. The empty tube combustion method continues to attract study, and results of a critical examination of the method have been published (10). Satisfactory results were obtained using the method as originally described, but on certain research compounds, particularly volatile liquids and hydrocarbons, incomplete combustion occurred. Low carbon and slightly high hydrogen values were traced to a fine carbon fog which passed through the combustion tube and was retained in the water absorbent. Lengthening the combustion tube to 30 cm. from the recommended 25 em., and insertion of a plug of quartz wool to trap carbon fog, make the method applicable to all types of compounds, and as efficient as the standard Pregl method. Among the 25 compounds for which data are given are picric acid, dimethylglyoxime, p-diacetoazoxybenzene, DDT, tetrabromodiphenylamine, sulfonal, and N-acetyl sulfanilamide. The effectiveness of an external absorbent for nitrogen oxides is confirmed, although the rapid deterioration of the permanganatesulfuric acid solution used makes renewal necessary after every two or three combustions, and points to the need for further investigation of such absorbents. The empty tube method, as investigated under other conditions ( 5 ) ,was considered to be too narrow in range of application to meet the requirements of general routine analysis. Accordingly, the method has been modified, and the sample is burned in a twelve-turn silica helix 24 cm. long, 4 cm. in external diameter, and heated to 800" C. through which oxygen is passed at the rate of 40 ml. per minute. Oxides of nitrogen are removed by passage through lead peroxide heated to 180" C., halogen and sulfur on granulated silver at 600" C. Results are given for 35 compounds, compared with those obtained in some cases by the Pregl method, and there appears to be a slight tendency toward high results for hydrogen as found by the new method. I t is emphasized that the method is regarded as being on trial. Speed is an advantage claimed for the method, with 30 to 40 minutes needed to determine carbon and hydrogen. A simplified method for routine semimicrodetermination of carbon and hydrogen has been reported (18) for use on liquid samples, in which the sample is slowly volatilized and the vapors ere burned in a rapid (300 to 350 ml. per minute) oxygen stream a t the tip of the containing ampoule. The accuracy of the method is of the order of 0.0670 for carbon and 0.08% for hydrogen, comparing favorably with other methods.
Determination of carbon and hydrogen simultaneously with the heat of combustion of an organic compound is reported ( 3 ) . The heat of combustion is determined in a Berthelot bomb i n the usual manner, and the products are passed through an absorption train to collect the carbon dioxide and water. A high temperature gas burner has been described ( 8 )for use iri the thermal decompo,sitinnmethod of direct oxygen analysis in organic compounds. The burner may also be used in other combustion micromethods, and has the advantage of longer life than electrically heated burners a t the high temperatures used (900 to 1000" C.). The adaptation of the relativrly long (33 em.) heavy duty conibustion furnace to the analysis of semimicroquantities (15 to 25 nig.) of organic materials has been described (18); this avoids difficulties in attempting to use the shorter (18 to 20-cm.) microfurnaces for burning semimicrosamples. The lamp method as applied to the determination of sulfur h:t* been studied critically (11) and it has been shown that low and erratic recoveries of sulfur, ranging from 19 to 63% on a 0.0117~0 solution of elemental sulfur in a toluene-isopropyl alcohol mixture, are obtained, and concluded that the lamp method is not quantitative for elemental sulfur, and hence not quantitative for total sulfur unless elemental sulfur is known to be absent Complete recovery of sulfur from thiophenol, thiophene, or carbon disulfide is possible when the material is dissolved in isopropyl alcohol so as to give a nearly nonluminous flame. Results of further studies of burner modifications have been reported (20), also with respect to sulfur determination. In addition, the determination of chlorine and bromine (in gasoline). normally unsatisfactory in the conventional A.S.T.M. burner, i s possible in the modified burner described. Other modified lamp designs ( 9 ) permit the burning of petroleum fractions in the boiling point range 250" to 500" C. The accuracy of the determination of hydrogen is 10.02 to 0.03% absolute, and possible extensions of the met>hodfor the simult,aneous determination of carbon and sulfur are suggested. Adaptation of the Van Slykp and Folch wet combustion method for carbon to the simultaneous determination of total and radioactive carbon in biological materials has been reported (IS). The carbon, obtained as barium carbonate following evolution of carbon dioxide by the oxidizing mixture, is determined gravimetrically with a precision of 4 to 7 parts per thousand, and its radioactivity to approximately 2%. .MISCELL.AYEOUS DErERA\IINATIOVS
The use of picric acid as a precipitant for piperazine, (CH2)a(SH)p, has been recommended (14). The neutralized sample is treated with a saturated solution of picric acid, whereupon piperazine picrate is precipitated. Following digestion, the precipitate is filtered, washed with saturated piperazine picrate in water, then alcohol and ether, and dried. The precipitate contains 15.81% anhydrous piperazine, or 35.66% piperazine hydrate. The method is applicable to dimethyl piperazine, and may also be used in the presence of hexamethylenetetramine,provided that substance is first hydrolyzed with dilute sulfuric acid. The determination of mandelic acid, CeHaCH(OH)COOH, using periodic acid has been reported (6). Oxidation with 0.1 N periodic acid a t 100" C. converts mandelic acid quantitatively to carbon dioxide and benzaldehyde; the latter is precipitated and determined as the phenylhydrazone following steam distillation from the reaction mixture. The results are low by a constant amount, corresponding to 5 mg. of benzaldehyde
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V O L U M E 2 2 , N O . 1, J A N U A R Y 1 9 5 0 0- and p-cresols have been determined gravimetrically by the cresoxyacetic acid method (1).
The alkaline sample of the cresols is treated with chloroacetic acid, precipitating sodium p-cresoxyacetate. This salt is filtered, dissolved in hot water, and acidified with hydrochloric acid to liberate p-cresoxyacetic acid which precipitates upon cooling. The yield is determined by weighing, the purity by determining the melting point. The filtrate from the p-cresoxyacetate precipitation is acidified and extracted with ether, from which o-cresoxyacetic acid is recovered. From the weight and melting point of this acid the yield of pure aeid is calculated, making reference t o mixed melting point curves of the two acids. An accuracy of 1.5to 2% for the oand p-cresols, respectively, is claimed. Caffeine has been determined in coffee by extracting the ground sample with very dilute (eparation and determination of fluorine (110); methods for the determination of fluorine on a macro scale (110), and micro scale (143); microdetermination of iron (34); determination of magnesium nith &hydroxyquinoline ( 4 1 ) ; conductometric determination of mercurous iron (48); micromethod for potassium using cobaltinitrite ( I S ) ; determination of peroxide with permanganate (65); errors in acidimetric titrations resulting from absorption by filter paper (112); errors in hydrolytic titration of lead nitrate ( 6 4 ) ; influence of acid on the oxidimetric determination of antimony (99); determination of carbon dioxide (98); studies on complex ions of aluminum, gallium, and indium ( 7 2 ) ; studies on the reduction of uranium rom-
