Reaction of Carbon Disulfide with Primary and Secondary Aliphatic Amines as an Analytical Tool F R A N K E. CRlTCHFlELD and J A M E S B. J O H N S O N Carbide and Carbon Chemicals Co., Division o f Unicn Carbide and Carbon Corp., South Charleston, W. Va.
Nebbia and Guerrieri (10) recently reported a titrimetric method for secondary amines which is based upon the formation of a nickel-dithiocarbamate complex. I n these laboratories it wns found that, under certain conditions, these dithiocarbamic acids can be titrated directly and quantitatively with standard sodium hydroxide. This principle has been utilized for the analysis of mixtures of aliphatic amines, inorganic base-amine, acid-amine, or carboxylic acid-anhydride.
The reaction of primary and secondary amines witti carbon disulfide to form dithiocarbamic acids is utilized as the basis of several analytical methods. In these methods the dithiocarbaniic acids are titrated witli standard aqueous sodium hydroxide using either phenol phthalein or thymolphthalein indicator. Methods are presented for the analysis of mixtures ofprimary, second ary, and tertiary aliphatic amines, and ammonia. For the analysis of these mixtures tlic reaction of primary amines with 2-ethylhexaldehydc to form the corresponding imine also has been utilized. Secondarj amines do not undergo this reaction and are determined by conversion to the corresponding dithiocarbaniic acids. The total primary and secondary amine content is determined by utilization of the carbon disulfide reaction. Tertiary amines and amiiionia do not interfere in this procedure. By combining these methods with a tertiary amine and total amine determination a coinplete resolution of amine mixtures can be effected. Methods are also presented for the analysis of inorganic base-amine mixtures and acid-aniine mixtures. These methods are of particular interest because a single sample is used. A method for the analysis of carboxylic acid-anhydride mixtures is described, which also uses a single sample. The anhydride in the sample is made to react with a measured excess of morpholine and gives one mole each of amide and acid. The acid formed plus any free acid present in the sample is titrated with standard sodium hydroxide t o the thymolphthalein end point. The excess morpholine and the amide are neutral in the titration medium used. Excess morpholinc is determined by conversion to the dithiocarbamic acid by reaction with carbon disulfide and titration with sodium hydroxide. From these two determinations the free acid and anhydride content of the sample are calculated. Potentiometric titration curves for the determination of several amines, inorganic base-amine mixtures, acid-amine mixtures, and carboxylic acid-anhydride mixtures are presented. Data obtained using indicators are presented for each method described. In general the accuracies obtained by the methods are within the Interferences and compounds that order of &O.l%. fail to react are discussed.
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DETERhIINATION OF PRIAfARY AND SECONDARY ALIPHATIC AnIINES AND ANALYSIS OF ARIINE-ACID AND ARIINE-STRONG BASE MIXTURES
In general, the determination of primary and secondary amines in the presence of tertiary amines and ammonia has heretofore been accomplished by use of specific methods for the latter compounds. Acetylation procedures for tertiary amines (8, 11, 14) and the sodium cobaltinitrile method for ammonia (9) have been used. The primary and secondary amine content of the samples has been determined by difference. Recently McIntire, Clements, and Sproull ( 7 ) reported a colorimetric method, based upon the reaction of l-fluor0-2,4-dinitrobenzene, for these amines. This section describes a specific method,for the direct determination of the total primary and secondary amine content in the presence of tertiary amines. In this method an excess of carbon disulfide is reacted with the primary or secondary amine in an essentially nonaqueous medium such as isopropyl alcohol or a pyridine-isopropyl alcohol mixture. The dithiocarbamic acid formed in the reaction is then titrated with standard sodium hydroxide using phenolphthalein indicator. Potentiometric curves for the titration of the reaction products of carbon disulfide and three amines are shown in Figure 1. The method is unique in that primary and secondary amines are converted to acids and titrated with a base. Because of this fact, these amines can be determined in the presence of strong inorganic bases, ammonia, tertiary amines, and most acids. APPARATUS AND REAGENTS
All potentiometric titrations were performed using a Leeds & Northrup line-operated p H meter equipped xvith glass and calomel electrodes. Carbon disulfide, reagent grade. Isopropyl alcohol, commercial grade, Carbide and Carbon Chemicals Co. Pyridine, redistilled. This material should contain less than 0.0005 meq. of primary and secondary amines per gram. The amines are determined by the procedure described below. Sodium hydroxide, 0.5N. Hydrochloric acid, 0.5N. Phenolphthalein, 1.0% pyridine solution. Thymolphthnlein, 1.0% pyridine solution.
I
N T H E methods presented here the reaction of primary and
secondary amines with carbon disulfide to form dithiocarbaniic acids has been utilized ( 4 ) .
RNH,
+ CS2
S I1
R-N€I-C-SH
(1) PROCEDURE
and
’
S
RzNH
+ CS2
R2N-&-SH
To each of two 250-ml. glass-stoppered Erlenmeyer flasks, add the solvent specified in Table I. Reserve one of the flasks as a blank. Into the other flask introduce an amount of sample that contains not more than 15 meq. of primary or secondary amine. The sample aliquot should contain not more than 15 meq. of alkali or acid, and the total tertiary amine and ammonia content should not exceed 30 meq. If the sample contains alkali, i t
(2)
Colorimetric methods of analysis for the determination of copper (16)and secondary amines (10, 13) have been based upon the ability of these acids t o form highly colored copper chelates.
430
431
V O L U M E 28, N O . 4, A P R I L 1 9 5 6
12 I
T
3
' 2
I
Z
3
II
r
z w a
g
10
9
0
IO
I
I
12
14
I rnl
I
16 18 0 5 N 5ODIU'd
I
I
I
20
22
24
I
26
HYDROXIDE
Figure 1. Potentiometric titration curves of dithiocarbamic acids from reactions of carbon disulfide with Drimarv and secondary amines 1. 0.6771 gram of isopropanolamine, solvent C 2. 0.2868 gram of ethylenediamine, solvent A 3. 1.3086 grams of dibutylamine, solvent B
P. Phenolphthalein end point
should be neutralized with standard 0.5N hydrochloric acid using thymolphthalein indicator. For samples that contain acids, neutralize with standard 0.5~3' sodium hydroxide. For samples void of alkalies or acids, use phenolphthalein indicator. If more than 2.0 meq. of ammonia is present in the sample aliquot, cool t h e contents of the flasks to approximately -10' C. Carbon dioxide from a dry ice-organic solvent bath interferes in the subsequent titration; therefore, a brine bath is most convenient for this purpose. By means of a pipet add 5 ml. of carbon disulfide to each flask and swirl t o effect solution. Titrate the contents of each flask with 0.5iV sodium hydroxide. For sample aliquots that contain more than 2.0 meq. of ammonia conduct this titration below 0" C . , by placing the flask in a 1000-ml. beaker containing a mixture of crushed ice and methanol. Stir the contents of the flask by means of a magnetic stirrer to prevent a local excess of sodium hydroxide from accumulating in the titration medium. The end point selected should be the first definite pink color for phenolphthalein, or blue or blue-green color for thymolphthalein. The color should be stable a t least 1 minute. For samples that contain alkalies, the amount of hydrochloric acid consumed in the first titration is a measure of the alkali. For samples that contain acids the amount of sodium hydroxide necessary to neutralize the sample is a measure of the acid present. The amount of standard sodium hydroxide necessary t o neutralize the sample after the addition of carbon disulfide is a direct measure of the primary and secondary amine content of the sample. 0
DISCUSSION
In the presence of a large escess of carbon disulfide the reaction of primary and secondary amines to give dithiocarbamic acids is approximately 90 to 95% complete. I n general, secondary amines are more reactive than primary amines. To adapt this reaction to a quantitative method of analysis, it is forced to completion by means of the sodium hydroxide titrant. The potentiometric titration curves shoxn in Figure 1 lyere obtained by permitting the system to reach equilibrium before the addition of each increment of titrant. Usually equilibrium is established r:ipidly, except in the vicinity of the equivalence point. An attempt was made to determine the dithiocarbamic acids by the addition of a measured excess of sodium hydroxide, and subsequent determination of the excess by titration with standard hydrochloric acid, Results obtained in this manner mere erroneous due to the incompatibility of sodium hydroxide and carbon disulfide. I n isopropyl alcohol medium and in the presence of excess sodium hydroxide, carbon disulfide reacts with the solvent t o form a santhate. Xanthate formation is even more pro-
nounced when ixethyl or ethyl alcohol is substituted for isopropyl alcohol. Because of the tendency of isopropyl alcohol t o react with carbon disulfide and sodium hydroxide, the end points obtained in this method are stable for only 4 or 5 minutes. The reaction of carbon disulfide and primary and secondary amines cannot be forced to completion by heating because of the instability of the dithiocarbamic acids a t elevated temperatures. In the case of alkyl amines, substituted ureas are formed by the evolution of hydrogen sulfide (4). Diamines, such as ethylenediamine, also liberate hydrogen sulfide but give polyalkylurea derivatives (1 ). Pyridine present in the reaction medium acts as a proton acceptor and tends to force the reaction of carbon disulfide with primary and secondary amines to completion. Listed in Table I are several primary and secondary amines that have been determined successfully by this method. In each case the purity obtained by the caibon disulfide method is compared to the purity obtained by another acid-base titration method. Also listed are the solvent mixtures recommended for the determination of these amines. Solvent A , which contains 50 ml. of pyridine, 25 ml. of water, and 50 ml. of isopropyl alcohol, is used for the determination of amines that gave dithiocarbamic acids insoluble in isopropyl alcohol. Solvent R,isopropyl alcohol, is used for reactive amines that give soluble reaction products. Solvent C, which contains 75 nil. of isopropyl alcohol and 25 ml. of pyridine, is recommended for unreactive amines that give soluble reaction products. Of the primary and secondary amines investigated, only the aromatic amines and aliphatic amines that are highly branched in the 2 position, such as tertiary butylamine and diisopropylamine, do not react quantitatively under the conditions of the method. Although the reaction of primary and secondary amines \Tit11 carbon disulfide is specific, there is a slight tendency for ammonia to react with the reagent. The extent of the reaction is shown in Table 11. I n this table data for the determination of butylamine in the presence of ammonia are given. It can be seen that a t temperatures below 0" C. the interference is negligible. A t room temperature, however, the reaction is appreciable. In
Table I.
Analysis of Primary and Secondary Amines by Reaction with Carbon Disulfide
Compound 2-Aminoethylethanolaniine 1%'-Aminoethylmorpholine Butylamine Butylamine, secondary Dibutylamine Diethanolamine Diethylamine Diethylenetriamine Di(2-ethylhexy1)amine Dihexylamine Dimethylamine, aqueous 2,6-Dimethylpiperazine Ethanolamine Ethylarnine, aqueous Ethylenediamine 2-Ethylhexylami ne Hexylamine Isobutylamine Isopropanolamine Isopropylamine Methylamine, aqueous blorpholine Propylenediamine
Solvent Composition
9
i:C
B C B A C B B
6B A C B B
c
C B A 4
Average Purity, Wt. %" Carbon disulfide method Otherb 100.0 = 0.0 (2) 100.0 i 0 . 1 98.6 0 . 1 (4) 98.8 i0.1 97.!) (1) 97.5 i 0 . 1 9 6 . 4 i 0 . 1 (2) 0 6 . 4 (1) 99.7 i 0.0 100.1 i 0 . 0 (2) 9 9 . 8 i 0.1 (2) 99.8 (1) 9 7 . 4 i 0.1 (3) 9 7 . 4 (1) 98.1 (1)O 92.0 i 0.0 ( 2 ) d 1 0 0 . 0 i 0.1 (2) 99.5 i 0.1 93.4 i 0.1 9 3 . 4 (1) 30.0 i 0.CI (2) 39.G (1) 99.5 ( l ) c 9 8 . 9 i 0 . 1 (2) 9 9 . 3 i 0 . 2 (3) 99.2 i 0 . 2 70.2 i 0 . 1 70.2e 9 9 . 8 i 0 . 2 (2) 9 9 . 2 (1) 99.0 (1) 98.7 (1) 9 8 . 9 (1) 99.1 i 0.0 99.3 + 0 . 1 (2) 98.8 (1) 9 9 . 2 i 0 . 1 (2) 99.2 i 0 . 0 9 9 . 1 i 0 . 2 (2) 9 9 . 2 (1) 4 4 . 9 i 0 . 1 (2) 4 5 . 0 (1) 9 9 . 3 i 0 . 1 (4) 9 9 . 4 (1) 9 9 . 0 i 0 . 0 (2) 9 8 . 9 (1)
(2)C (2)C
(2) (2)
(2) (2)
(2) (2) (2)
(2)
a Figures in parentheses represent number of determinations. b B y titration in water with standard 0.5N hydrochloric acid using bromocresol green-methyl red mixed indicator, unless otherwise specified. C B y titration in glacial acetic acid with standard 0.1N perchloric w i d using crystal violet indicator. d Sample contains 0.34 meq. of tertiary amine per gram. e Standard deviation for 8 degrees of freedom is 0.11, A = 50 ml. of pyridine, 25 ml. of water, a n d 50 ml. of isopropyl alcohol. B = 75 ml. of isopropyl alcohol. If more t h a n 2.0 m e q . of ammonia is present, add 25 ml. of pyridine. C = 25 ml. of pyridine and 75 ml. of isopropyl ;alcohol.
432
ANALYTICAL CHEMISTRY
Talde 11. Determination of Butylamine in Aqueous Ammonia Solutions (Extent of ammonia interference) Added Samplea
a 6 C
Ammonia
Butylamine
Butylamine Found
Deviation
All values are per cent b y weight. Reaction with carbon disulfide and subscquent titration a t
