ANALYTICAL CHEMISTRY
1764 422 to 440 m$. The absorbancies of dichromate solutions are relatively insensitive to slight variations in wave length over this range, as can be seen from the spectral absorption curves shown in Figure 2. Experimental evidence showed that both dichromate and trivalent chromic ion concentrations under these conditions were directly proportional to the measured absorbancies of their solutions. Discussion. The analytical procedures in the absence of chloride ion as outlined were as precise as the various reagent measurements. The dichromate reagent was measured from a 5ml. microburet; the reagent was allowed to flow from the buret a t the same rate and over the same range of the volumetric scale for each sample, to eliminate drainage errors. The exact quantity of reagent added was not defined, but each sample received exactly the same quantity. The permanganate titration was also performed with a 5-ml. microburet and corrected for indicator blank. The precision of measurement attained for the over-all procedure was about zkO.02 ml. of the 0.1N potassium permanganate, 1 0 . 0 2 mg. of starch, or AO.01 mg. of laurylamine acetate in the samples of their pure solutions taken for analysis. Sodium oleate responded similarly to the laurylamine acetate in wet combustion tests. Only small amounts of chloride could be tolerated in the highacid reaction mixtures, and then suitable blanks were necessary. Samples containing more than 0.02 meq. of chloride ion were un-
suitable because of the uncertain amount of dichromate consumption through oxidation of the chloride in the strongly acid reaction mixtures. As satisfactory results were obtained without employing catalysts in the oxidation step ( 5 , 6 ) , the effect of silver and other metallic ions on the oxidation reaction was not investigated. ACKNOWLEDGMENT
This work was supported by a fellowship grant of the Corn Products Refining Co., and administered by the School of Mines and Metallurgy, University of Minnesota, Minneapolis, hfinn. LITERATURE CITED
(1) Ingols, R. S.,and hlurray, P. E., Watm and Sewaue Works, 95, 113-17 (1948). (2) Isimatu, M,, Mitt.med. Akad. Kioto, 25, 376-86 (1939). (3) Medalia, A. I., ANAL.CHEM.,23, 1318 (1951). (4)Moore, W. A.. Kroner, R. C.. and Ruchhoft. C. C., Ibid.,21, 953 (1949). (5) Moore, W.A , , Ludzack, F. J., and Ruchhoft, C. C., Ibid., 23, 1297 (1951). (6) Muers, M. M., J. SOC.Chem. Ind., 55,71T (1936). (7) Randall, R. B., Benger, M., and Groocock, C. M., Proc. Roy. SOC. (London),A165, 432-52 (1938). (8) Shaw, J. A , , ANAL.CHEM.,23, 1764 (1951).
RECEIVED for review January 26, 1953. Accepted September 28, 1953.
Determination of Primary Fatty Amines in Amine Mixtures Potentiometric Titration Techniques JOHN E. JACKSON' Chemical Laboratories, General Mills, Znc., Minneapolis, Minn. in nonaqueous media with perchloric acid has been found to be more satisfactory than the Van Slyke procedure for the determination of the amino acids ( 1 ) . This procedure has been adapted to the determination of the amine number of mixtures of fatty amine products of high molecular weight (2). The perchloric acid titration, however, does not differentiate between the primary amines and the secondary and tertiary amines in a mixture. To effect such a differentiation between primary amines and secondary and tertiary amines, Wagner, Brown, and Peters (3) potentiometrically titrated methanol solutions of aromatic and short-chain aliphatic amine mixtures after treatment with salicylaldehyde to neutralize the primary amines as azomethines. Under these conditions the secondary (plus tertiary) amine content of the amine mixtures is equivalent to the acid used in titrating t o the first end point, Primary amine is then calculated by difference from a eeparate determination of total base. This procedure is not directly applicable to the fatty amines of high molecular weight because of their poor solubility in methanol (and isopropyl alcohol). However, these products are soluble in chloroform, even after the addition of some water in isopropyl alcohol. By this modification the method can be used to determine the primary amine content of a mixture of long-chain fatty amines. The experimental results shown in Tables I and I1 illustrate the scope and accuracy of the Wagner, Brown, and Peters method when adapted to the analysis of fatty amines of highmolecular weight as described here. This method is particularly effective for the analysis of long-chain fatty amine derivatives which contain both a primary and a secondary amino group in the same molecule. Such compounds are not completely soluble in the Van Slyke reaction mixture, and by the Van Slyke proceITRATIOS
1
Present address. University of Minnesota, St. Paul 1, Minn.
dure appear to contain almost 200% of primary amino nitrogen. Using the procedure described here, such compounds have been titrated quantitatively both for total amino group content and for secondary amino group content. Results from the titration of such a compound, a-octadecylaminopropylamine, are included in Table I. EQUIPMENT
The instrument used for the potentiometric titrations in this study was a Central Scientific Co. titration-pH meter. However, any pH meter which can be adapted to titrations may be used. The procedure should be readily adaptable to industrial recording titrators. The electrodes were the ordinary, full-range, industrial glass and calomel electrodes obtainable from the Beckman Instrument co. A 10-mI. microburet is recommended for these titrations. REAGENTS
Chloroform, analytical reagent grade. Salicylaldehyde, free of salicylic acid. A 5% solution of distilled water in isopropyl alcohol. Hydrochloric acid, 0.5N in isopropyl alcohol, prepared by adding one volume of concentrated C.P. hydrochloric acid t o isopropyl alcohol, diluting with the alcohol to 24 volumes, and standardizing against anhydrous sodium carbonate by potentiometric titration in aqueous solution. This reagent has been used for as long as 30 days in cool weather without noticeable deterioration. In warm weather precautions should be taken t o avoid errors caused by the high coefficient of thermal expansion and volatility. PROCEDURES
Determination of Total Base. To a 1-gram Sam le of the high molecular weight fatty amine mixture in 90 ml. of ckoroform, add 10 ml. of the 5% solution of water in isopropyl alcohol, and titrate potentiometrically with 0.5N hydrochloric acid in isopropyl alcohol. (To simplify the comparison of the titrations with and without added salicylaldehyde, 5 or 6 grams of sample may
1765
V O L U M E 2 5 , NO. 11, N O V E M B E R 1 9 5 3 Table I. Experimental Results from Potentiometric Titration of Amine Mixtures of Known Composition % Composition Calpd. from Potentiometric Titration Results Primary Secondary amine amine 28.2 ... 42.7 ,.. 56.4 ... 97.4 ... 17.7 0.0 100:2
Theoretical Primary Amine Description of Sample Content, 5% Known mixturea 28.2 Known mixturea 42.8 Known mixturea 56.1 Known mixturea 47.6 Known mixturea 17.7 Dioctadecylamine 0.0 yOctadecylaminopropy1amine 97.8 98.2 99.0b ‘2’-Dodecvlethvlenediamine 97.3 97.8 98.66 3-Isopropylaminopropylamine 98.9 98.3 99.06 a Contained weighed pmounts of octadecylamine, dioctadecylamine, and X-methyldioctadecylamine. b Values represent presumed purity of samples based on source and purification procedures. C Based on theoretical total amine number as determined by perchloric acid titration.
moved and discarded before a portion is taken for analysis. Dissolved ammonia, remaining from the nitrile reduction procedure, may be present in crude samples of these high molecular weight fatty amines, To drive off any dissolved ammonia, the chloroform solution of crude amine mixtures should be boiled gently 1 or 2 minutes on a steam bath. When samples have been weighed into beakers and dissolved in chloroform, as described above, the chloroform lost by evaporation during this boiling should be replaced by adding 5 or 10 d. of additional chloroform after the solutions have cooled to room temperature.
I
Table 11. Experimental Results from Potentiometric Titration of Some Crude Amine RZixtures % Composition Calcd. from Calcd. Amine No. Amine N0.b b y HC104 Method ( 8 ) 193.6 193.4 206.1 205.9 199.7 199.6 180.1 180.7 184.0 183.8 77.4 11.10 173 3 172.7 . . 206 6 208 0 93 9 2 29 206.8 206 9 95 5 2 50 a Based on assumption that secondary amine was only basic contaminant of primary amine present. Method of preparation of these samples made this a reasonably valid assumption. b Calculated from composition of mixture as determined by potentiometric titration. Theoretical amine number of octadecylamine, 208.55. Theoretical amine number of dioctadecylamine, 107.68.
Potentiometric Titration Result+ Primary amine Secondary amine 9.64 87.7 2.40 97.6 0.65 95.4 20.42 76.2 19.31 78.3
I
2
3
4 5 M L 0.5122 N H C i
I
6
!
7
I
8
Figure 1. Typical Titration Curves
be weighed into a 100-ml. volumetric flask, and dissolved in chloroform, and aliquots of this solution diluted to the concentrations described above. This procedure may not be feasible for some amine mixtures containing large amounts of secondary amine, as secondary long-chain fatty amines are rather poorly soluble even in chloroform. For such samples, it will be necessary to weigh the amine mixture directly into a titration beaker and dissolve as described ahove.) pH readings will drop 4 to 5 units a t the end point. The exact end point is the mid-point of this rapidly dropping portion of the curve. The end point thus determined represents the amount of 0.5N hydrochloric acid required to neutralize all of the amine in the mixture. Determination of Secondary-Tertiary Amine. To a 5-gram sample of the amine mixture in 90 ml. of chloroform, add 5 t o 6 ml. of salicvlaldehyde and allow t o stand 30 min. a t room temperature. (Samples containing large amounts of primary amine may develop a precipitate after a few minutes, but this precipitate will not affect the end point.) After 30 minutes have elapsed, add 10 ml. of the solution of 5% water in isopropyl alcohol and titrate. ildd the acid in small increments (0.1 to 0.2 ml.), as this titration may be very small. At the end point, the pH will drop abruptly 1 to 3 units. Plot the titration curve and determine the mid-point of the rapidly dropping portion of the curve. The end point thus determined represents the amount of acid required to neutralize the secondary (plus tertiary) amine in the mixture. (Figure 1 shows a typical pair of titration curves.) Determination of Primary Amine. The difference between these two titrations, equated to a common sample reight, reppresents the 0.5K hydrochloric acid equivalent to the primary amine in the miuture.
A . Titration of 50-ml. aliquot after treatment with salicylaldehyde B . 20-1111.aliquot titrated without salicylaldehyde pretreatment Sample size 5.6529 grams of crude amine mixture in 100 ml. of chloroform Data show 76 4% octadecylamine and 18.1% dioctadecylamine
When the alternative, aliquot procedure is used, add 40 or 50 ml. of chloroform to the sample weighed into a 100-ml. volumetric flask, and boil the solution 1 or 2 minutes after the sample has dissolved, Remove the flask from the steam bath, and after a moment of cooling (to avoid boiling over) add chloroform nearly to the mark on the volumetric, Complete the dilution after the solution has cooled to room temperature. The water-isopropyl alcohol solution is added to the chloroform solution of the amine sample to improve the shape of the titration curve and to improve the accuracy of very small secondary amine titrations. To avoid throwing the amines out of solution, the water-isopropyl alcohol solution must be added only after adequate dilution of the chloroform solution. When chloroform alone is used as a solvent, the pH meter reacts as if one of the electrodes were disconnected, until 1 or 2 ml. of the aqueous isopropyl alcohol titrant have been added. The titration curve then rises to a maximum, levels briefly, and shows the characteristic end-point drop and inflection as the titration is continued. Although this behavior does not affect the accuracy of large titrations, very small titrations are of doubtful value. When the water-isopropyl alcohol solution has been added to the chloroform solution of the amine, more normal titration curves are obtained (Figure 1).
REMARKS
LITERATURE CITED
Because carbonates of the fatty amines are insoluble in the solvent used here, the amine samples should be protected as much as possible from carbon dioxide in the air. If carbonates are present, they may be decomposed by heating the sample a t 100’ C. in vacuo. Samples should be weighed rapidly and if solid, in as large pieces as possible. When fresh samples are not available, the exposed surfaces of the old samples should be re-
(1) Nadeau, G. F., a n d B r a n c h e n , L. E., J. Am. Chem. Soc.. 57, 1363 (1935). (2) Terry, D. E., Eilar, K. R., a n d Mae, 0. A., ASAL. CREM..24.. 313 (1952). (3) W a g n e r , C. D.,B r o w n , R. H., a n d Peters, E. D., J . 4 m . Chem. SOC.,69, 2611 (1947). RECEIVED for review March 10, 1952. Accepted August 6, 1953. 128, Journal Series, Research Laboratories, General Mills, Inc.
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