groups of triethanolamine react A ith acetic anhydride on standing. The extent of reaction, as well as the number of product species, vary with the reaction time. Figure 3 shows this effect of reaction time on the acetylation of triethanolamine. Within a 30-minute acetylation time, a normal titration curve is obtained, whereas the inflection point is completely obscured in 90 minutes. Evidently, acetylation of the three hydroxyl groups successively increases the chain length of the original triethanolamine molecule with acetoxy groups, whose electron-withdrawing tendencies progressively diminish the basic character of the nitrogen atom. Titration of these variouq baqic moieties
produces it result siniilar to that obtained by titrating an amine Rhich contains small amounts of other amines of varying basic strengths; that is, the titration curve does not yield a well defined inflection point. Dimethylethanolamine does not behave in this manner becausp only one Acetoxy group can be added to the molecule, and this effect is insufficient to overcome the basicity contribution. of the electronreleasing S-methyl groups. The triethanolamine problem can be avoided by restricting the acetylation time to 30 minutes. ACKNOWLEDGMENT
The authors thank E. J. Taylor, Jr.,
tor his contributions to the development and application of the procedures. REFERENCES
Blumrich, K., Bandel, G., Inyew. Chem. 54, 374 (1941). (2) Fritz, J. S., Yamamura, 3. Y., Bradford, E. C., ANAL. CHEM.31, 260 (1959). ( 3 ) Hillenbrand, E., F;, Jr., Pentz, C. A , "Organic Analysis, Vol. 111, p. 175, Interscience, Xew Tork, 1956. (4) Ruch, J. E., Johnson, J. B., Critchfield, F. E., AXIL. CHEX. 33, 1566 (1961). (5) Wagner, C. D., Brown, R. H., Peters, E. D., J . A m . Chewi. SOC.69, 2609 (1947). RECEITEDfor review April 27, 1961. Accepted July 24, 1961. Division of Analytical Chemistry, 139th Meeting, ACP, bt. Louis, Mo.>March 1961. (1)
Spectrophotometric Determination of Secondary Amines GERALD R. UMBREIT Research laboratories, The Upjohn
Co.,Kalamazoo, Mich.
b The reaction of secondary amines with carbon disulfide to form dithiocarbamic acids is utilized as a basis for their analysis. The dithiocarbamic acids react with copper(l1) to form a yellow-colored salt or complex which is measured spectrophotometrically. Since tertiary amines do not react with carbon disulfide, they do not interfere. Primary amines do undergo these reactions, but give much lower color intensities. As a consequence, primary amines in equivalent molar quantities will result in an error of 1 % or less in the determination of secondary amines. Recovery data for an alkyl- and an aryl-substituted secondary amine are given, and other reactive compounds are indicated. Data for the analysis of primary-secondary and tertiarysecondary amine mixtures are presented. A number of the variables involved are discussed.
S
for the determination of the various types of amines in amine mixtures have long been a goal of analytical chemists. The selectivity of nitrous acid reactions with primary, secondary, and tertiary amines has been utilized by Zin'kov and Pylaeva (8) for the characterization of such a mixture, and by Smales and Wilson (6) and English (3) for the selective determination of secondary amines. The reaction of primary and secondary amines with carbon disulfide has been utilized by Critchfield and ,Johnson ELECTIVE
1572
REACTIONS
ANALYTICAL CHEMISTRY
(1). The resulting dithioc~~rbairiic acids are titrated with base. Tertiary amines do not react and primary amines are masked by imine formation n i t h 2 ethylhexaldehyde to make the method selective for secondary amines. Katcher and Voroshilova (4j determined dimethylamine by tibration of the dithiocarbamate with standard copper sulfate solution. Several investigators ( 2 , 5, 7 ) have used colorimetric measurement of the copper dithiocarbamate complex, but haye mtrict'rd their invesbigations to the determinution of dimethylamine in various systems. The met'hod described here defines conditions for more general applicability of t'he colorinietric methods mentioned above for dimethylamine. Specific application is made to diethylamine and N-methylaniline. -1pplication to a number of ot'her secondary amines is indicated. Data on the analysis of primary-secondary and tertiary-secondary amine mixt'ures zr? presented. EXPERIMENTAI
Reagents. CSi-I'yiiduie-Isopi o p j 1 Alcohol. Accurately measure and mi\ 35 ml. of CS2, 25 ml. of pyridine, and 65 ml. of isopropyl alcohol. When stored in a glass-stoppered reagent bottle, i t is usable foi a t least 2 months. Cupric Chloride Solution, 0.0013 J f Dissolve 0.1 to 0.12 gram of CuC1, 2H20 in 250 ml. of nater and dilute to 500 ml. with pyridine. None of the volume or weight measurements is exceqsively critical as long
as standards are used for comparison nhen any reagent solution is replaced. Procedure. One milliliter of the sample solution is transferred t o a 15 x 150 nim. glass-stoppered test tube or other suitable reaction vessel. Four milliliters of the CSz-pyridineisopropyl alcohol reagent and 2 ml. of the r u p i i c chloride reagent are added. The niivturc is agitated, then alloned to stand for 5 to 20 minutes at room temperature. (Some of the CS2 settles out during this period) Then 3.0 ml. of acrtit acid (10% volume of glacial acetic acid in nater) and 3.0 ml. of benzene are added. The mixture is agitated by inversion several times, and the phases are then allowed to separate. From the upper (organic) phase 4.0 ml. are renioved and diluted to 5 ml. with iaopropyl alcohol. After this solution has been standing for 1 to ll/z hours (diethylamine) or 20 minutes (LVmethyhiline), the ahqorbance is measiired RESULTS A N D DISCUSSION
Table I summarizes recovery data for diethylamine and N-methylaniline. n hich were chosen as representative of Rliphatic- and aromatic-substituted secondary amines. The response in both cases is linear over the concentration ranges indicated. The application of this method to primary-secondary and tertiary-secondary amine mixtures is summarized in Table 11. Tertiary amines do not react with carbon disulfide. Thus, minute fractions of secondary amines in tertiary amines can be analyied accurately.
Priniary amines constitute a positive interference. The primary amine dithiocarbamate complexes have a n absorption maximum significantly removed from that of the secondary
Table I.
Analysis of Secondary Amines
Taken,
Found,
I@.
Pg.
Recovery. c ,c
Diethylamine 11 3
11 0
22.7 45 4 ti8 0 90.;
22.4 45 4 67 3 90.5
97 3 98.7 100.0 99 . 0 99.8
LY-hlethylaniline 38.2 7'2.4 108.6
35.3 72.2 108.6 AV.
97. G $)!I.5 100.0 99.1 5 1.0
Table II.
Determination of Mixtures C.H5NHCH3 C8H& HCH, Error, Taken, Gg. Found, pg. pg. N-methy hiline' in N,.V-Diniethylanilinea
15.2 36.4
19.0 35.4
N.Methylaniline 18.2 36.4 54,6
19.7 37.0 55.3
+0.5 -1.0
+ .%nilineb $ 1 ..i +0.6 $0.7
EtzNH EtzNH Taken, pg. Found, pg. Diethylamine in Triethvlaminec 11.3 22.7 34.0
11.5 22.9 33 7
Diethylamine
+2.0 +0.2 -0.3
+ Ethylamined
13.3 +2.0 11.3 22.7 23.3 +0.6 35 1 $1.1 34.0 0 4.779 mg. of C6H:N(CH& taken. * 100.3 pg. of CaHaNH2taken. c 3.615 mg. of EtaN taken. d 64.8 pg. of EtNHa taken. Table 111.
Reacting Compounds
Time t,0 Reach Wave Mas. Length of AbsorbAbsorption ance, e t a , Max., M p RIin. X 102
Compound Piperidine 440 30 5 37 Di-n-butylamine 440 30 7 08 N-Methylaniline 443 20 5.36 Diethylamine 440 60 9 6r Isoleucine 360 %-Butylamine 350, 4305 Aniline 355, 430b Neomycin B 390 a Based on concentration in original sample aliquot. b Secondary amine impurity is euspected.
aniines (Table 111). I n addition, their molar absorptivity is significantly smaller. For these reasons, primary amines present in approximately equivalent molar quantities will result in a n error of 1% or less in the determination of secondary amines. However, because the primary amine dithiocarbamate complexes are much less soluble in both phases, they cannot normally be tolerated in aniounts greater than 100 pg. per sample. Larger amounts of primary amines result in a haze n hich interferes I? ith the spectrophotometric nieawrenients. Compounds tested specifically and nhich do not react under the specified condition> are: pyridine, triethylamine, tributylaminc, SS1V- dimethylaniline, 1,3-diplienylguanidine, diphenylamine, 3-carbonyl inlloles, and ammonia. The secondary aniines nhich do not react are all highly conjugated. Effects of Variables. T h e reaction medium, prior t o extraction, provides a sufficient excess of reactants t o permit rapid formatiwi of t h e dithiocarbamates a n d subsequent complexntion with copper. Some amines may require additional reaction time. However, after extraction there is a Fignificant period of time during which t h e absorbance increases. As indicated in Table IV, this time w r i e s
Table IV.
The necessity of adding acetic acid to accomplish the extraction supports this hypothesis. If water only is substituted as a diluent before extraction in place of the acetic acid solution, the response curve becomes nonlinear and color remains partially in the aqueous phase. The use of greater amounts of acetic acid results in the formation of haze in the organic layer. Variations in ionic strength of the sample solution using both KC1 and NaAc from 0.01 t o 0.2431 have no effect. The formation of the dithiocarbamic acids in the reaction mixture requires the free base of the amine. Although pyridine is a weaker base than most of the amines tested here, the large excess t h a t is present in the reaction mixture is sufficient t o drive the amine-amine salt equilibrium in the direction of the free amine ivhich is removed by dithiocarbamate formation. I n cases where a sample solution is strongly acidic, i t should be neutralized with sodium hydroyide or ammonia prior to analysis. Amines which are very strong bases may require addition of sodium hydroxide to attain the necessary fraction of free base. I n some cases increased reaction time is required. This must be determined individually for each amine.
Response as a Function of Time after Extraction
Absorbance at Time, t , after Extraction, Min. 10 0.335
Sample
15
N-Methylaniline" 0.350 Diethylaminob ... ... a 72.4 pg. measured a t 445 mfi. * 22.7 pg. measured at 440 mp.
20 0.360 0.225
with the amine in Question. This problem is believed td be due t o a reversible equilibrium between 1 to 1 and 2 to 1 amine to copper compounds as in Equation I.
;
2R1R2Y--C-SC~i
Ic
.
S I'
(RiRg-C-S):
CU
+ ~ C U ( A C )(1)Z
25 0.352
...
40
120
180
0:240
0:270
0:269
LITERATURE CITED
( 1 ) Critchfie!d, F. E., Johnson, J. E., ~ A L CHEW . 28, 430-6 (1956). ( 2 ) Dowden, H. C., Biochem. J . 32, 455-9 (1938). ( 3 ) English. F. L., ANAL.CHELI.23, 344-6 (1951).
(4)Katcher, E., T'oroshilova, hI., -4nzlinokrascohnaya Prom. 4, 39-41 (1934); Chem. d b s t r . 28,36897 (1934). (.5,) Xebbia. L.. Guerrieri. F.. Ch7m. e ind. ( M i l a n ) 35, '896 (1953); 'Chem. A b s t r . 48., 386% - - - - - (19.54) ( 6 ) Smales, A. A,, Kilson, H. N., S O ~ . Chem. I n d . 67, 21G13 (1945). ( 7 ) Stanley, E. L., Baum, H., Gove, J. L.. A N A L . CHEM. 23, 1779-82 (1951). ( 8 ) Zin'kov. Z. E.. Pvlaeva. L. I.. J . \ - - -
The 1 to 1 compound nould appear to be preferred in the aqueous phase nhere a significant excess of copper is available, nliile in the organic phase the 2 to 1 compound is preferred. The time required to attain maximum absorbance should be a measure of the rate of attainment of this equilibrium in the organic phase. ~h~ response curves do not become linear until this maximum absorbance is obtained.
(1960); Consultants- Bureau Englisb Trans]. 117-120 (1960). RECEIVEDfor review May 9, 1961. ilccepted June 14, 1961. Division nf
Analytical Chemistry, 139th hfeeting, hCS, St. Louis, hlo., March 1961.
END OF SYMPOSIUM VOL. 33, NO. 1 1 . OCTOBER 1961
lS7S
